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My Undersized Ducted Mini-Split Heat Pump

The Outdoor Unit For Our Mitsubishi Inverter-driven Mini-split Heat Pump System

Last year my wife and I bought a 1961 ranch-style house, and I’ve been having a lot of fun fixing it up.  I wrote about the problem with the spray foam in the attic not covering the top plates and how I fixed it when we had the soffits, fascias, and gutters replaced.  I’ve also had a lot more spray foam installed in the attic because the original attic encapsulation was pretty bad.  And I replaced the old natural draft gas water heater with a beautiful new Rheem heat pump water heater.  (I’ll write about both of those topics sometime this summer.)  

The other big thing we’ve done so far is to install a Mitsubishi* heat pump system to serve the first floor.  It’s got one outdoor unit and two air handlers in the conditioned attic.  Canton Heating & Air* installed the system and did an exceptional job.  They specialize in Mitsubishi equipment, and their dedication to high quality workmanship shows.  If you’re looking for mini-splits in the Atlanta area, you can’t go wrong with them.  (Yes, they gave us a discount, but I’d say the same things even if they hadn’t.)

You’re probably wondering how I zoned the house, what our heating and cooling loads were, and why I undersized the system, so let’s jump into the details.

Zones and loads

I did the load calculation and most of the design work for the system we installed.  (Canton Heating & Air helped flesh out the details of the pair-of-pants fittings, which I’ll discuss in the next article.)  The floor plan is shown below, and the first thing I did was divide the house up into zones.  The bedrooms on the left compose one zone (green).  The common areas on the right are another zone (pink).  And the sunroom needs to be its own zone (blue) because of all the glazing.  About 60% of the exterior wall area in that room is covered with windows (double-pane, from the 1980s or ’90s).

Bailes residence floor plan and zones

We use RightSuite Universal for HVAC design at Energy Vanguard, and that’s where the results you see here came from.  Here are the specifications I used in the Manual J load calculation:

Let me explain a little about the inputs.  Except for the sunroom and laundry room, everything else sits over a conditioned basement or conditioned crawl space.  Except they’re not directly conditioned at the moment.  After we put the new system in the encapsulated attic, we had the old furnace and air conditioner removed.  The 110,000 BTU/hr furnace and the 4 ton air conditioner served the entire house as one zone.  It wasn’t great, but the upstairs was mostly OK.  The basement was always too cold, though, as you would expect with this setup.  Now the basement is indirectly conditioned while it awaits being gutted and remodeled, which adds to the heating load of the main floor but probably helps the cooling load.

I modeled the ceiling as R-30, but it’s really more like R-40 now.  Woodman Insulation gave me more than I asked for when they added to the existing spray foam insulation up there.  And I cheated a bit on the infiltration rate.  My last blower door test, which I did after the new spray foam and my extra sealing work at the top plates, came in at 5,200 cfm50 (~9 ACH50) for the whole house.  So, the attic insulation is actually better than modeled and the air leakage is worse than modeled.  I don’t recall why I did it that way, but those two deviations probably come close to canceling each other out.

In the window inputs, DP stands for double pane and SP stands for single pane.  U is the U-factor, the reciprocal of the R-value.  SHGC is the solar heat gain coefficient.  Most of the single pane windows have storm windows, which explains the 0.57 U-factor for some of the SP windows.

I used the standard indoor design temperatures (70° F for winter, 75° F for summer) and Atlanta outdoor design temperatures of 23° F for winter and 93° F for summer.  With all those entries, the loads and floor areas for the three zones are:

Note that the heating loads in all three zones are higher than the cooling loads.  This surprises a lot of people, but it’s common around here.  The sensible heat ratios (sensible cooling load divided by total cooling load) are all above 0.8, with the sunroom coming in at 0.92.  All that glazing adds more to the sensible load than to the latent.  And, in case you were wondering, the cooling loads expressed as square feet per ton are 1,537 sf/ton for the bedrooms, 1,123 sf/ton for the common areas, and an eye-catching, rule-of-thumb-like 394 sf/ton for the sunroom (windows again).

Sizing the equipment

So, what size heating and air conditioning equipment did I choose?  First, let me tell you my thinking.  I don’t want equipment that will satisfy the loads in the house as it is.  I’ll be gutting and remodeling the basement fairly soon.  (I’ve got a book to finish first, though.)  After that, I’ll start working on the main floor, with more air-sealing and new windows and major renovations of some areas (the master bath, for sure).  Because of those plans, I want equipment that will be a bit undersized.  Besides, I know that even when I do a Manual J load calculation correctly, it still comes up with a load that’s about 10 to 20 percent higher than the actual load.

The equipment I selected includes one outdoor unit, the Mitsubishi MXZ-3C24NAHZ2 with HyperHeat, and two horizontal ducted indoor units.  Because there’s only one outdoor unit, this is just one heat pump.  The total capacity of the system is based on what that outdoor unit can do because that’s where it dumps the heat it removes from the house in cooling mode and where it pulls the heat from to send into the house in heating mode.  Our nominal total capacity is 2 tons; the 24 in the outdoor unit model number stands for 24,000 BTU/hr, and one ton of capacity is 12,000 BTU/hr.

The ducted mini-split air handlers serve all of the main floor except for the sunroom, to which I’ll be adding a separate wall-mounted ductless unit soon.  The bedrooms zone is served by the Mitsubishi SEZ-KD09NA4 (shown above), and the common areas have the Mitsubishi SEZ-KD18NA4.  Focusing just on these two main floor zones, here are the loads and the capacities of the equipment:

Since the total capacity of the system is dictated by the outdoor unit, here’s what the totals look like:

The capacities shown in both tables above are from Mitsubishi’s software called Diamond System Builder.  As you can see, the numbers don’t look too bad for total cooling, with a total capacity deficit of about 2,500 BTU/hr.  But the heating capacity is 11,000 BTU/hr lower than the load.  Can that possibly work?

I intentionally undersized the equipment installed in my house because I don’t want future improvements in the building enclosure to leave us with too much capacity.  The total cooling capacity is only 88% of our cooling load, and the heating capacity is a mere 65% of the design heating load.  Astute readers will have noticed another problem with the cooling capacity, but let’s deal first with the mode I have the most experience with: heating.

Did the house stay warm enough?

We got the heat pump installed with both air handlers operating in November 2019, so we’ve been through one whole winter with it.  Oh, did I mention we didn’t install any supplemental heat?  It’s true.  But we have lots of sweaters and blankets, so we survived.  Just kidding.  We didn’t need any supplemental heat this past winter.

Yeah, the winter wasn’t terribly cold, but we did get tested a bit.  We had about 2,200 heating degree days December through April, which is below the Atlanta average of about 3,000 HDD.  We did have a three-day stretch of cold weather when it got down close to our outdoor design temperature each night.  The night-time lows were 25° F, 24° F, and 25° F, with daytime highs of 37° F, 40° F, and 49° F.  The house stayed warm the whole time.  We woke up to an indoor temperature a bit below the thermostat setting, but it caught up when the day warmed up.  If we’d had colder weather with daytime highs in the low 20s, I think we would have needed supplemental heat.  

So, I’m not worried about the heat right now.  Manual J heating loads don’t count interior loads, so our bodies and the appliances that give off heat all help.  Once I get done improving the building enclosure, the loads should drop and be a lot closer to our heating capacity. Even before the improvements, we have some flexibility because of the two air handlers.  If it stays really cold for a few days, we can do most of our heating in the common areas zone during the day and keep the bedrooms cooler than normal.  We usually sleep with the bedrooms cooler anyway.  And we also have the Hyper-Heat model, which is supposed to provide full heating capacity down to 5° F.

The cooling effectiveness, so far

The total cooling capacity is 88% of the cooling load, so we’ll be tested on hot days.  So far, we’ve had temperatures right at 90° F, just a few degrees below our summer design temperature of 93° F.  The air conditioner hasn’t had any trouble cooling the house to the thermostat setpoint yet.

But I’ve done another little experiment to push the system further.  Since the cold weather ended, we’ve been opening up the sunroom (shown above), which is connected to the den by a set of French doors.  On the hot days we’ve had so far, the den still cools to our usual setpoint of 74° F.  To cool the den with sunroom doors open, though, the system has to run more than it was designed to run.  That leads to the living room going down to about 71° F because the thermostat is in the den.  With the sunroom doors closed, the temperatures even out pretty well.  We’re about to get a wall-mounted ductless mini-split for the sunroom, and that’ll make this problem disappear.

Now, I alluded to another cooling problem, and it’s related to humidity.  According to Mitsubishi’s software, our total latent capacity is only 718 BTU/hr whereas our latent load is 3,803 BTU/hr.  If that’s correct, our system will do little dehumidification.  I haven’t measured the amount of condensate coming out of the system yet, but I’m pretty sure we have more than 718 BTU/hr of latent capacity.  We’ve had some warm, humid days lately and the humidity in the house hasn’t gotten out of control.  Our dew points this week have gotten up as high as 74° F (Ugh!), and the indoor relative humidity is still mostly less than 60%.  If I believe the separate thermo-hygrometer over the thermostats, it’s always less than the 60%.

In short, undersizing the system hasn’t resulted in uncomfortable conditions in our house (yet).  And it hasn’t motivated me to accelerate my schedule of improvements to the house (yet).  We’ll see how it goes as we get into hotter weather this summer and what happens when we have a real cold spell.  For now, I’m happy with my skinny little undersized heat pump.

Next time, I’ll cover the duct system, including filtration, for my two mini-split air handlers in the conditioned attic.  Meanwhile, I’ve got to go see what the birds in my backyard are up to this morning.  Those bird feeders in the photo are only five of the eight I’ve set up.


Allison Bailes of Atlanta, Georgia, is a speaker, writer, building science consultant, and founder of Energy Vanguard. He is also the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.


*Disclosures:  Mitsubishi gave us a discount on the equipment and advertises in the Energy Vanguard Blog.  Canton Heating & Air gave us a discount on the installation.


Related Articles

Seizing an Air-Sealing and Insulating Opportunity

My Big Fat Oversized Air Conditioner

How to Read Manual J Load Calculation Reports


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This Post Has 72 Comments

  1. Did you run a load calc on
    Did you run a load calc on the proposed house yet? Do you have a blower door target for it?

    1. Yes, it drops the heating &

      Yes, it drops the heating & cooling loads about 12% with 2,000 cfm50.

  2. Thankls! Great article. I’ve
    Thankls! Great article. I’ve just installed a ductless heat pump as well, and also undersized it a bit. Mine’s a Fujitsu, 15,000 BTUH nominal. It’s always best to undersize these units a little based on their nominal sizing, as they actually put out a little more (mine can actually output ~18,000 BTUH heating). Oversizing can result in excessive on/off cycling and hunting, especially in the shoulder seasons, which are pretty challenging here in Nova Scotia, Canada.

    1. Jeff, yeah, that’s what I

      Jeff, yeah, that’s what I hear from other people, too.  Have you measured the 18k output?

    2. Jeff… if your system has an
      Jeff… if your system has an inveeter driven compressor, then the unit should not “short cycle”… the compressor should ramp down and maintain steady continuous operation in both heating and cooling.
      I have a Mitsubishi 3 ton Hyperheat P-Series unit connected to a 3 ton air handler in my home with no supplemental heat source-which heats my 1450 sq.ft. home to 72 degrees in winter at our design Southern Ontario design condition of 0 deg.f. / -18 deg.C. The system ramps down to 1.5 tons on cooling and maintains 72 deg.f. / 45 to 50% RH in summer (87 deg.f. summer design).
      The inverter compressor control is the solution to comfort in both summer and winter.
      Eric Shaw

      1. Eric –
        I am currently designing a 3500 sf home in the Hamilton area. I had one HVAC designer tell me that a minisplit system would not have enough power to heat my house. I read your comment with interest because it appears that the Mitsubishi system you have works just fine. Is the system still working for you? Would you recommend some form of supplemental heat?

  3. Thanks for the nice article
    Thanks for the nice article Allison – we always love to hear about real world personal experiences. I’m curious – when you spray foamed the attic, did you use open cell or closed cell? You’ve written several times about a concern for humidity/moisture rising through open cell foam to the underside of the roof deck – which may or may not be a concern depending on whether the AC in the attic is running or not – so curious how you handled that.

    1. Charles, the house had open

      Charles, the house had open-cell spray foam below the roof deck when we bought it, but not enough.  We had more open-cell sprayed over it.  I’ll be writing about our humidity control in the attic later this after I have some data.

  4. Thank you for sharing this,
    Thank you for sharing this, it is helpful to see examples where even deliberate under-sizing hasn’t been catastrophic. Shout out to Canton Heating and Air too, they put a multi-zone system in on one of our projects in North GA.

    I am curious for your thoughts now that Mitsubishi has the new SUZ exterior unit options out that can pair with the ducted SEZ indoor units as 1:1s and still get hyperheat. If those had been available last year would you have gone for those over the multi-zone outdoor unit? In my own experience I am finding that client experiences are not as easy to predict and troubleshoot w/ the multi-zone due to the complexity of the zone management.

    I’m also curious how the minimum cooling capacity of the equipment plays into your thinking, if it does. When you reported cooling capacity in your chart from Diamond System Builder I’m assuming that was the max capacity. I often hear cotnractors say “oversizing doesn’t matter with minisplits because they ramp down the capacity.” This is true but there’s still a low-end limit, and especially with the MXZ systems that lower limit is higher than I had first realized. In many of the climates where we build a hyperheat unit sized to the heating load is definitely going to have a cooling capacity that is considerably oversized for the cooling load. So then I take a crack at discovering the minimum capacity as well as the maximum and see where that falls, and try to shoot for an option, if possible, where the max capacity in heating is sized to the heating load, while the minimum cooling capacity is as far under the cooling load as I can–though sometimes I’m lucky to find it’s under it at all. So I do still worry a bit about those part-load, high humidity days.

    1. Leigha, yes, I absolutely

      Leigha, yes, I absolutely would have gotten the one-to-one Hyper-Heat units if they had been available last year.  And yes, the range of capacity definitely plays into this.  I know the actual load won’t be at or above the design load much at all, so I want the system to be operating below its max capacity as much as possible.

      1. I have been researching the Hyper Heat for a month or so. Why would you have gone with the SUZ over the MXZ?
        Forgive me but I am not clear what 1:1 means.

        1. Michael: 1:1 means each outdoor unit is matched with one and only one indoor unit. The setup I have now is a 1:2 (or 2:1) with 2 indoor units on the same outdoor unit. The SUZ is a 1:1 outdoor unit. It was not available with Hyper-Heat for my system when I got it in 2019. It’s available now. The advantage is better efficiency and better control of indoor temperatures.

          1. Thank you so much for the reply. I was quoted for an MKZ 4 ton for 1945 square foot single story home with a single blower in my utility room replacing my gas furnace that currently resides there. Would I be better off with the SVZ/SUZ combo?

  5. Nice writeup.
    Nice writeup.

    Will the sunroom ductless air handler have its own outdoor heat pump or will it be a 3rd head on the MXZ3C24?

    If memory serves, that Mits ODU will serve 3 indoor air handlers, but under that scenario connected capacity would exceed 1.3x nominal system capacity.

    The assertion that the nominal 2 ton system has just 718 Btuh latent capacity has to be an error of some kind.

    You alluded to checking flow of condensate and are probably aware of a useful rule of thumb to calculate actual latent performance – 1000 Btu per pint removed

    1. Curt, the sunroom ductless

      Curt, the sunroom ductless unit will be a one-to-one system with its own outdoor unit. 

      Yeah, I also think that latent capacity from DSB is an error.  Thanks for the rule of thumb.  I didn’t know about it.

      1. 1000 Btu/pint is not a rule
        1000 Btu/pint is not a rule of thumb, it is a good thermodynamic approximation for the latent heat of vaporization for water!

        1. Roy, you beat me to it.  I

          Roy, you beat me to it.  I realized a little while after posting my comment this morning that of course it’s about 1,000 BTU per pint because a pint is about a pound (A pint’s a pound the world around) and the latent heat of vaporization of water is 970 BTU per pound.  D’oh!

          1. I thought you knew better 😉
            I thought you knew better 😉 But there is a bigger point here. In SI units, the heat of vaporization of water is about 2300 kJ/kg which is not near as round of a number as 1000 Btu/lb. Ergo, English (American?) units are far superior to SI!

          2. Yeah, I do.  It was early and

            Yeah, I do.  It was early and the coffee hadn’t kicked in yet, I guess. 

            1,000 sounds almost metric.  A similar irony is that 0 to 100° F is pretty much the range of temperatures we humans live in.  But SI is still better (says the physicist who used electron-volts for energy when he was still doing physics).

          3. My SI comment was tongue-in
            My SI comment was tongue-in-cheek. The Fahrenheit temperature scale is interesting. It is my understanding that Dan Fahrenheit based his scale on two temperatures that he considered to cover the main range of interest and could be easily reproduced. O F is the coldest temperature at which he could get saltwater to freeze, and and 100 F was the normal temperature of the human body. Apparently, he couldn’t or didn’t worry about resolution beyond 1 F.

  6. Hi Allison, thanks for
    Hi Allison, thanks for another great article.
    I’ve always understood why one shouldn’t oversize traditional HVAC equipment. Yet I thought the variable refrigerant flow of mini-splits may mean that there is a sweet spot of approximately 30 to 70 percent run time at which the lifespan of the equipment is extended, with no comfort or energy use penalty. There is an incrementally higher equipment cost. Have I been misled?
    Second, in my thermal mass Passive House (climate zone 6), I have one air-handler per floor, not in bedrooms. Temperatures rarely differ by more than one degree F, except for a rear hallway when the heat-pump water heater is running. Yet I wonder if in a future spec Passive House if I’d be better off with a ducted-mini serving bedrooms, for those who might want different bedroom temperatures. I think there is an energy penalty compared to a single air handler, and greater complexity. Do you have a preference?

    1. I’m not aware of a ducted
      I’m not aware of a ducted mini whose supply air can be further subdivided into more than one temperature control zone…we do a fair bit of zoning, but that combination is not one I’ve come across.

    2. @Daniel, sizing a minisplit
      @Daniel, sizing a minisplit so that it operates at a 70% duty cycle (at design), let alone 30%, is a very bad idea. Aside from the questionable** efficiency argument, you forfeit some or all of the variable capacity benefit you’ve paid for.

      Let’s say your load is 24k and you install a 48k system that can ramp down to 20k. It will operate at minimum capacity probably 95% of the time. You might as well go with a single-speed 24k system (which, by the way, would likely have a higher SEER rating, as smaller systems are generally more efficient than larger systems within the same model line). So this idea that it’s OK, even preferable, to oversize minisplits makes no sense! Moreover, if we’re talking about a multisplit or a system with automatic zone control, you lose the ability to modulate capacity when one or more zones are not calling, which is the majority of the time.

      ** even a system sized to the true peak load stills run at part-load capacity for the vast majority of its operating life

    3. Dan, yeah, I like ducted

      Dan, yeah, I like ducted units for the bedrooms.  Once I get some monitoring set up, I’ll collect data on actual energy use.

  7. Have you measured your heat
    Have you measured your heat pump water heater’s impact on the cooling and heating loads? Or is it fully ducted?

    1. Patrick, no, I haven’t done

      Patrick, no, I haven’t done that yet.  I plan to duct it but currently it’s just sitting in my basement mechanical room, which is open to the conditioned crawl space and the rest of the basement, as everything is open until I gut and remodel the basement.  I don’t think the HPWH will add much to our cooling capacity because there are just two of us here and we don’t use a lot of hot water.

  8. I am curious about your
    I am curious about your windows (both current and planned). I believe the photo shows your sunroom with the double-pane (double-hung) windows. I’m guessing your single-pane windows (with storms) are also double-hung. Have you done blower door testing where you taped all the windows closed to see the difference in infiltration rates (may lead you to install either fixed-pane or casement windows)? Have you been keeping a record of how often (and how long) the windows are opened (may lead to installing fixed units to reduce cost and improve performance)? I’ve heard many building science advocates claim that you “need” to “fling open the windows” at times, so demand extremely expensive PH-rated triple-pane tilt/swivel windows. I don’t think so – especially when you consider occupancy duration/time of day (Coved-19 not withstanding), home security concerns, pre-existence of mechanical ventilation systems and install cost. I think that fixe triple-pane windows (except for mandated egress – which I don’t understand either as first responders all have axes) perform better, are locally available and more economical than any other commonly-available option.

    1. Hi David, I recently built a
      Hi David, I recently built a new home and gave some thought to which windows we would actually use for mild-weather ventilation – mostly the mandatory egress windows and several more in family/kitchen/living area for cross ventilation. All the others are fixed (although not triple pane, not much benefit here). I’m not sure I understand your logic regarding egress windows. The occupants don’t want to wait for first responders to ax their windows! And in any case, crawling through a broken window in a hurry, potentially at night, sounds dangerous!

    2. David E., the single pane

      David E., the single pane windows are mostly single hung, but that’s the same in regards to your question.  I haven’t taped the windows during a blower door test.  There’s certainly leakage through the window units themselves, but I think most of the air is leaking around the window frame.  I haven’t decided what windows we’ll go with, as that upgrade is probably a couple years or more off.

  9. FWIW, I have a Rheem HPWH
    FWIW, I have a Rheem HPWH installed in conditioned space. Back of the envelope calc suggests it provides about 1/3 ton cooling for 4-6 hours per day. It also condenses a fair bit of water vapor during humid months – up to several pints per day.

    Unfortunately, I have experienced condensate backups owing to bio fouling.

    1. Curt, is the bio-fouling in

      Curt, is the bio-fouling in the water heater itself?

    2. I understand some systems put
      I understand some systems put in a second condensate pump for redundancy.

      Would some occasional bleach help clogging?

  10. I’m surprised you didn’t
    I’m surprised you didn’t mention the fact that Mitsubishi offers a 1:6 ratio of lowest output to highest output capacity. When I was choosing a DHP system for my home I went with Daikin because it had slightly better efficiency specs and my HVAC contractor could offer better pricing on it, but only a 1:4 capacity ratio. Even though I’ve never experienced any issues relating to that (that I know of) I often wonder if I would’ve had better real life performance in our PNW long shoulder seasons based on that ratio with the Mitsubishi…

    1. Ryan, the system I have doesn

      Ryan, the system I have doesn’t go that low.  The one I have, the MXZ-3C24NAHZ, has a turndown ratio of about 1:2 for cooling (12,600 – 23,600 BTU/hr) and about 1:3 (11,400 – 30,600 BTU/hr) for heating.

  11. Couple things unrelated:
    Couple things unrelated:

    1) My read of the Mits M-Series engineering Manual (549 pages) is that very few combinations attain 6:1 turndown ratio, although one combo I routinely specify manages an astounding 9:1, which is why it is my go-to system. In general system turndown ratio is all over the map – from as low as 3:1.

    2) My understanding of egress window sizing is that it is also to allow a firefighter wearing self contained breathing apparatus to rapidly enter the structure. (Ingress, not just egress.)

    1. Curt & Allison, I should’ve
      Curt & Allison, I should’ve guessed that different combinations would produce different turn down ratios but this is what you get when you get an energy efficiency nerd doing his best to understand the complexities of the HVAC world! PS. 9:1 sounds too good to be to true! Curious what that set up is? (not which model numbers but curious if it’s multiple indoor units tied to one outdoor unit or just a 1:1)

  12. You mentioned a ductless mini
    You mentioned a ductless mini-split will eventually go in the sun room. Will that run off the same outdoor unit as the other 2 zones, or use a separate one?

    1. Steve, the sunroom ductless

      Steve, the sunroom ductless unit will be a one-to-one system with its own outdoor unit.  The current 24k outdoor unit already has 27k of indoor unit capacity.  Even if I had a larger outdoor unit there, putting a 6k, a 9k, and an 18k on the same outdoor unit might lead to trouble.

  13. I’m with you 100% on this one
    I’m with you 100% on this one, Allyson. Even if the indoor temps do rise a bit due to under sizing, the indoor humidity drop will keep you feeling just fine at the slightly higher indoor temps on those really high outdoor temp days.
    I keep preaching that every perfectly sized system is oversized 98% of the time due to it not being to the extreme design temperatures outside.

    1. Robin, the latent capacity

      Robin, the latent capacity issue I wrote about in the article makes me worry a little bit, but so far it’s been fine.  We’ll see how it goes the rest of the summer.

      1. Even though my home has a
        Even though my home has a humidity issue that would be very costly to address directly, I find that once my humidity is under control, the latent capacity is no longer condensing moisture in the air and is therefore converted to sensible cooling. Definitely a “set it and forget it” situation most times. This evening I did need to turn my stat up a degree due to it feeling a bit too cool at 36% humidity. One degree higher temperature setting allowed the humidity level to go to 40%, which is now comfortable without being chilled.
        A thermostat that had only a designated comfort zone instead of numbers, that worked from an enthalpy control, would be a great feature to prevent thermostat fiddling due to humidity changes.

        1. There are thermostats
          There are thermostats available that control a combination of temperature and humidity in an attempt to create continuous comfort over a wider range of loads. Some of the older ones would adjust the dry-bulb temperature 1 F in one direction for every 5% RH in the other direction. This turned out to be too much and didn’t work at all because lowering the dry-bulb temperature often raises the RH. If you look at comfort studies (e.g., ASHRAE Std. 55) you will see that a 15-20% RH change is equivalent to 1 F. Controlling to constant enthalpy is not the same as constant comfort.

  14. Great discussion. Three
    Great discussion. Three questions: no dedicated dehumidifier? How is ventilation handled?

    Third one is a bit harder: does anyone know whether minimum capacities can be estimated for the Mitsu equipment? the system builder software works only on maximum, afaik.

    Ok, bonus question: Did you have to convince the contractor to do this? My experience is that they are loathe to undersize. More to do with callbacks, than selling more equipment (although perhaps there is that too). So did they get you to sign something, or just assume that you know what you are doing (since you aren’t joe average homeowner and they know that?)

    1. James, good questions.  I’m

      James, good questions.  I’m running a little standalone dehu in the basement right now but nothing else at the moment.  I’m probably going to add a whole-house ventilating dehumidifier but wanted to see how the house does over this summer first.  The ventilation right now is running the exhaust fans occasionally in our 5,200 cfm50 house.  I’ll see if I can find the answer to your second question. 

      Regarding the contractor, no, I didn’t have to cajole the him or sign anything.  He specializes in Mitsubishi equipment and I’ve worked with him before.  His company is Canton Heating and Air and I recommend them highly.  I don’t know if he’d do the same thing for a regular homeowner, though.

  15. First thing I noticed, , the
    First thing I noticed, , the base is too small for the outdoor unit? I never seen an install with the legs out past the base, seems it would be unstable, and in strong winds. Second issue directly in front of a Window, that’s not too cool (pun intended). Also with windows from teh 80’s and 90’s, they are really outdated, Andersen new low e3 and all window makers have Cardina Low 3 varietys, much better than teh Low e from teh 80’s and 90’s, I replaced all my Andersen sashes and back 8 ft slider with current Low e 4 (they call it that, due to the Ti coating on the outside surface, for spotless drying) Low e 3 SUNGLASS on all my 31 sashes and back sunroom 8 foot slider, Sunglass is MAGIC, direct sun is prevented from heating the space, dramatically, SASHES and GLASS matter, and the view is actually enhanced no glare, and the blue grey tinge is great, and from outside privacy, in my 6 ft ANDERSEN Casement sashes in the back sunroom, along with teh 8ft Sunglass slider I see out, but looking in, is really diminished, it’s great, it reduces the Heat load on my 9k BTU Mitsubishi dramatically. And the modern Low e3 also in cold weather is much better than the old 80’s low e…much better sealing also in the glass, with moisture absorbent beads in the edges…thing do improve, replace all those obsolete windows sashes, with modern glass, save even more money over time..31 sashes and an 8 foot slider, big improvement in cold and hot weather. SUNGLASS MATTERS Too bad Velux only offers low e 3 glass and not any kind of sunglass, when I also replaced my 8 skylights with a new roof Owens Corning Premiere 50 year shingles that are also energy star rated they have a mineral mix to reflect the sun heat load, only the one version has that deal, great stuff…shingles also matter.

    1. ce, I asked the contractor

      ce, I asked the contractor about the size of the base and he’s confident it’ll work fine.  I’m not worried about winds because that’s on the most sheltered side of the house.  Regarding the placement in front of the window, we didn’t have a lot of good options, and that was the best.  It’s a basement bedroom that’s only going to get used occasionally so it’s not a big deal.  Window replacements are coming, with the four basement windows getting replaced when I remodel in the next year.

  16. A note: I’ve had pretty poor
    A note: I’ve had pretty poor experiences with multiple heads off one compressor, both Mitsubishi and Daikin. And I’ve found it saves me zero dollars over the 1 compressor per head option, once linesets/valves/etc are calculated.

    Having truly separated units gives all sorts of other options. After you run the numbers including install I think you will find it’s an obvious choice.

    I’ve had great luck with hyperheats, though I hate the control system. But that’s all minisplits. Next house I’m going to do all Gree, since that’s what Lennox has been rebadging, and the price point is pretty cheap for a smaller house, competitive with ducted.

    1. @Charles, your experience
      @Charles, your experience with multi-splits is not at all surprising. I avoid specifying multi-splits unless there’s an overwhelming reason to do so. First-first cost and operating efficiency tilt toward single-splits. In the mini-split world (and often with conventional unitary equipment), the smaller systems have higher SEER ratings than their larger siblings within the same model line. Also, multi-split refrigeration management (at least with 2-pipe systems) imposes an unavoidable efficiency loss. Multi-splits top out at bout 16 to 17 SEER, whereas one-to-one mini-splits top out at over 30 SEER.

    2. Charles, I’m with you on that

      Charles, I’m with you on that.  I wanted separate outdoor units, but I also wanted Hyper-Heat since I was undersizing so much.  Unfortunately, at the time I got the equipment, they weren’t yet doing one-to-one Hyper-Heat systems for the SEZ horizontal ducted air handlers.  I’ll probably change it out with two separate outdoor units at some point.

  17. Operational questions…
    Operational questions… Have checked your equipment with a current meter? How many amps is it pulling?

    Approx. what is the electric bill running, summer and winter?


    1. I also am interested in
      I also am interested in Richards questions, about your electric bills. Please answer if you don,t mind.
      I am about to install 2 Mitz systems. One is a 12K BTU cassette for common area of our small house, mid Fla., area of about 450 sq ft. Other is 9K BTU for 170 sq ft master bedroom, with a large extractor fan in attic, ducted from master to spare bedroom, 120 sq ft., wall switch , (used maybe 10 days a year). Both cassettes are single zone, with its own condenser, rated at 24 SEER. Am I barking up wrong tree, or did I bite off a good chew? Just an A/C ignorant carpenter, trying to be a handyman.
      Btw, will have a diamond-installer do final check, and startup, to certify (safety first). Paid, perhaps exhorbitantly, of course. Never cheapscate friends!!

      1. Moe, with the caveat that I

        Moe, with the caveat that I don’t know anything about the loads on your house, it seems like your systems may be oversized, even in Florida.  1.75 tons of capacity in 620 sf is 354 sf/ton, which is in rule-of-thumb territory.  If it is oversized, you don’t get much benefit from the variable capacity because it will be running mostly at the low end of its range.

    2. Richard, no, I haven’t tested

      Richard, no, I haven’t tested with an ammeter yet.  My winter electric bills ranged from 1,002 to 1,532 kWh for a total of 7,985 kWh over six months when we had 2,157 heating degree days.  It’s a 3,800 sf house, but the basement isn’t directly conditioned yet.  The main floor is about 2,400 sf.  We just got our June electric bill, the first month when we did a full month of air conditioning.  The total was 1,376 kWh.

      For context, this is a 1961 house with 5,200 cfm50 of air leakage.  I also charge my electric car at home and the lawn mower, leaf blower, and string trimmer.  I’ve done a little bit of work to make the house better, but we still have a long way to go.  The next big project is gutting and remodeling the basement, which should cut quite a bit from out air leakage.

      Here are a couple of previous articles about the house:

      My New Project:  A 1961 House With Home-Performance Angst

      Seizing an Air-Sealing and Insulating Opportunity

  18. Nice to see this hands on
    Nice to see this hands on approach with results for sizing and zoning. I’m up in Canada though and it sure would be nice to see some articles as to how to integrate back up heat into these systems when the thermometer drops. I’ve been struggling to find any reference. Any suggestions?

    1. Leonard, Mitsubishi sells

      Leonard, Mitsubishi sells auxiliary heat strips you can add to the ducted air handlers, and I think the other mini-split manufacturers do, too.  We have a fireplace that’s currently outfitted with gas logs.  We’re probably going to replace the gas logs with a wood-burning insert to use as supplemental heat.

  19. I’ve been looking into a
    I’ve been looking into a Mitsubishi Hyper-Heat system for my 1,400 sf ranch in Rhode Island for about a year. I’ve received four quotes with four widely different methods (Ductless, ducted, concealed cassette), sizing and pricing. I have not a clue which direction to go. Does anyone know of an independent engineer who could provide guidance? Secondly, because of COVID, my un-heated basement has become my work from home office. The spring was a bit chilly and have an immediate need for heating the basement before the fall and winter approaches. My second question is would it be best to get a larger outdoor unit to accommodate both the basement and the main house (with the installation of the main house heads at a later date) or having two separate dedicated outdoor units?

  20. I am planning a similar
    I am planning a similar installation for my home in buffalo, NY. I wanted to do it before I redo the siding and add exterior insulation, but I’ve been scared about the winters. My wright-j mobile block load calculation tells me I need around 70kBtu heating And I’m not about to buy that much mini-split! I’m not certain how low I want to go though. After a few beers I think “maybe a 36k Fujitsu xlth….”

  21. That unit needs legs put
    That unit needs legs put under it. I have seen a few that have had busted condenser coils do to not being able to drain and ice building up in the unit pan.

  22. Allison,

    1. Did you use the existing ducts in side of the house?
    2. What was the end cost installed Btu, all in?
    3. Do you have and sophisticated filtration on the indoor unit?
    4. Wildcard Question: I mainly use open web joists now, is there any reason that I wouldn’t use a supply to semi condition (supply) the interstitial spaces between floors?

  23. Have 500 sq.ft. family rm with many windows addition on slab with cathedral 10 ft ceiling that’s very cold even tho it ducted thru living rm. My regular hvac has Trane. What specific ductless system(s) would you recommend?

  24. Allison,
    Thank you for your helpful post! I am thinking about installing a Mitsubishi ducted mini split but I am concerned about cleaning the system. One contractor told me the wall and ceiling mounted units will be easier to clean (make sense because they will be more accessible) and the ducted systems are very difficult to clean (you have to take off the inlet and outlet plenums to access the mini air handler). Can you share your experiences in performing maintenance on the ducted mini split? Also, has the mini split ducted unit been able to keep up when you have company over at the house–I am concerned that when there are more people in the house the mini split ducted unit might not be able to keep up. Thanks and Happy New Year!

  25. I do believe this article highlights the big divide in the building science and hvac worlds. No one seems to be able to get beyond their own box.

  26. Wife and I redid an Tenn farm house, from the studs, doublings the sqft about 20yrs ago (before MSHPs came on the scene). Avg R in walls ~20, in ceiling ~R 35. Careful to build tight and low vapor penetration, thermopane windows. 3,300 sf (two stories), plus 1700 sf basement all conditioned with one ducted water source HP. Most of the duct work inside the envelope. When I got the ductwork finished, I had a used hotel room water source heat pump, 10,000 btu! I hooked it up to see what it would do. It ran for 5 to 6 years and was all we had during that time (6,000 sf/ton!). We did have a wall mt gas heater (18,000 btu – also is our power-outage emergency heat). When the temps got below 30 F we would turn on one notch of the gas heater (6k btu). A few times we did kick it up to 12k. In the summer when it got above about 90F it would fall behind. In upper 90s outside it might be 80 inside BUT, it was DRY and with a little circulation, it was quite tolerable. When it gave up (water coil leak) we installed a 2T unit (2500 sf/T). I think 1.5T might have been a better match (I got the near new 2T for $200). What we did notice is that while the humidity control is very good now, it was noticeably dryer with the 10Kbtu unit. On the average it did quite well. My point is, I think the fear of under sizing pushes many to spend too much. The industry has a bent to push over sizing. I say some amount of under sizing is a good option for many. I would like to see some houses designed to operate using a one head MSHP. I suspect this could be done by upping the insulation (wall, ceiling and windows) and installing air circulation blowers, etc. into areas not directly conditioned (like bedrooms).

    1. If your glass is 20 years old, its already way behind what new stuff can do. I swapped out 33 year old andersen low e and low e sunglass from 33 years ago, just the sashes, the frames and everything are still fine, 33 units in total about $12,500 for all the sashes, all casement..BIG improvement front of house gets blasted with intense sun at 3 PM in the Low e 4 SUNGLASS by Andersen(Cardinal glass actually) is a better color blue grey than the older bronze, room is brighter and whiter, and less glare and less heat than the old sunglass, plus the argon does go away over the years, so in 20 years there really is no argon left….and the new low e 4 glass has a titanium coating outside to keep it cleaner..and the new casement sashes have desiccant now in the thermal edges ..always improving. The new Andersen SUNGLASS is much better, I did all sunglass this time, I like the privacy it does in the back sun room, clear looking from inside, but a nice dark tinge, looking in from outside, can’t see me, and the 8 foot andersen slider is all new Sunnglass, that side got blasted with sun in the moring E rising, magic, don’t feel anything, open the door WHAM, you feel that sun, technology in thin coatings, look up cardinal glass, lots of interesting glass facts, if your glass is over 20 years, it’s obsolete. My 5 Mitsubishis mini splits heat and cool the entire house in every season NEVER need the furnace to come on when it’s 5 degrees outside,they are actually rated to -15 F to give like 60% full heat, amazing stuff they are fantastic…newer ones again, are even BETTER my newest one is no 3 years old, already they have a more efficient, better in colder air…Mitsubishi teamed with trane now…Mitsubishi mini splits are the best of any out there. I did all the sashes myself, easy on casements, get some new straps from andersen, for the non operating ones, the operating ones you reuse all the original hardware,stuff is fine order from no shipping charges for 33 sashes is pretty cool, everyone arrived in perfect shape, directly from the Andersen warehouse, use the Andersen part number on the screen tells you which window size you have, not one mistake perfect ..they order sunglass as special order, since it’s not regularly used in the northeast, came in 2 weeks or less,I did it in 3 orders cus I wouldn’t have had space for 33 casement sashes. And my roof is now Owens corning premium duration with heat reflective granules 50 year roof, new glass and hardie board siding, factory baked on paint 30 years warranty on material, 15 year warranty on the paint, get a hardie Board elite installer, they know what they are doing, that stuff is HEAVY!

  27. @P, you have an interesting project. I have a one-ton Carrier 18VS for 3,300 ft2 (ranch over finished basement) and it works well. It has a small (3k) strip kit for coldest days, but it holds its own between about 25F and 102F.

    The biggest reason I didn’t go with a 12k mini is the necessity for zoned air distribution, especially for basement vs main level. I haven’t been impressed with efforts to zone ducted mini’s, and my room loads are waay too small to use a multi-split (also, 2-pipe air-source multi-splits give up too much in efficiency).

    So how are you managing air distribution with a single system servicing 4,000 ft2?

    1. No, I haven’t measured that, but I’m sure it’s not much. For one thing, it’s in the basement, which has little sensible cooling load. For another, we don’t run it enough to get a lot of cooling or dehumidification from it.

  28. I purchased a 1964 home and found something quite interesting in if the rooms. I wanted to check the venting as I noticed that there wasn’t much air flow coming through. When I looked in the vent I could not believe what I seeing. There was no stack, I coul touch the Tyvec from inside the vent opening as well as the siding to the house. What can I do to rectify this disaster?

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