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My Undersized Mini-Split Heat Pump in a Heat Wave

My Undersized Mini-split Heat Pump Performed Admirably In A Heat Wave

You may recall that I wrote about my undersized mini-split heat pump a couple of years ago.  It’s undersized compared to both the heating and cooling loads, as you can see in the table below.  I’ll discuss the heating performance in a future article.  (It’s been fine for three winters).  Today, though, I’d like to show you how well the heat pump has done in our recent heat wave, when we had three really hot days in a string of six days at or above our design temperature.

Mini-split heat pump heating and cooling loads

Cooling performance in the heat wave

You can see from the numbers above that my house has a cooling load of about 21 thousand BTU/hr and the heat pump has a cooling capacity of a bit less than 19 thousand BTU/hr.  According to those numbers, my heat pump has only 88 percent of the capacity it needs to keep my house cool on a design day.  And our 1% design temperature is 92 °F here in Atlanta, Georgia.

So how did we do?  The graph below shows the outdoor temperature (blue) during that heat wave, the week of 20-25 June 2022.  The red line is our outdoor design temperature of 92 °F.  And down below, the orange curve shows our indoor temperature.  I leave both thermostats set to 74 °F in the summer, and you can see that it stayed pretty darn close the whole time.  If I showed you only the indoor temperature, I think you’d have a hard time picking out the hot days.

Temperatures indoors and outdoors during a heat wave

Let’s take a closer look at just the indoor temperature data.  The graph below shows them along with a linear fit to the data.  As you can see, the indoor temperature rose only a tiny bit over the course of these six days.  The temperature varied from a low of 73.0 to a high of 76.1 °F.  And the average was about 74.4  °F.  I’d say that’s pretty impressive.

Indoor temperature trend during a heat wave

Heat pump power use

I didn’t plan it this way, but I installed my Emporia Vue electricity monitor on 19 June.  Thus, the first day included in the graphs above and below was my first full day of tracking the heat pump power use.  How lucky!

Power use of my undersized heat pump during a heat wave

The graph of power use shows a number of interesting features.  Most nights, for example, the power use bottoms out at about 65 watts.  I thought that was the absolute bottom, when only the blowers in the air handlers were running.  But then I saw the data for the morning of 25 June and realized it could go even lower.  It ran at a mere 20 watts for 26 minutes

Of course, the most interesting feature is that the power ramps up and down as the cooling load goes up and down.  That’s the beauty of a variable capacity inverter-driven mini-split heat pump.  There’s pretty much no short-cycling of the equipment, the bane of oversized fixed-capacity air conditioners.  The power level ranges from about 620 watts up to nearly 1,400 watts when it’s cooling.

Also note what happened even after three days of outdoor temperatures well above our design temperature.  The power level still dropped to the (main) bottom end of 65 watts by the next morning.


What’s going on here?  Do I really have an undersized mini-split heat pump?  Judging by the data, it looks like I don’t.  I think two things are happening here.  First,  ACCA Manual J load calculations inflate the loads 10 to 20 percent even when you do them correctly.  And of course, I assumed a certain amount of internal load in the calculations, but we probably didn’t have that much on those days.  So, the load calculation could account for all or most of the discrepancy.

Also, the ACCA Manual S standard includes a range of total cooling capacity that’s allowed.  The low end in that range is 90 percent.  With our capacity-to-load ratio of 88 percent, I’d say we’re in compliance with that standard.

But there’s another reason that my undersized mini-split heat pump doesn’t seem so undersized.  The performance data of my Mitsubishi equipment (described in more detail here) may not be accurate.  OK, I’m being generous when I say “may” there.  Look at that table at the top of the article, and you’ll see that Mitsubishi says the latent capacity (for humidity removal) is a mere 718 BTU/hr.  (For the HVAC geeks, that’s a sensible heat ratio of 0.96.)  I don’t believe that for one second.  The sensible capacity may be off, too.

Now don’t take this the wrong way.  I’m not saying you never have to worry about undersizing your cooling equipment.  But if you have an accurate load calculation and good cooling equipment, it may turn out better than it looks on paper.  Also, I intentionally undersized my heat pump because I’m working on the house and reducing both the heating and the cooling loads (though I haven’t done much yet).

How does your air conditioning equipment do in a heat wave?


Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and writes the Energy Vanguard Blog. He also has a book on building science coming out in the fall of 2022. You can follow him on Twitter at @EnergyVanguard.


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This Post Has 62 Comments

  1. I just wanted to add that another inherent benefit of variable speed multi zone ductless systems is that they adapt to the load on the house. This is another reason that a slightly undersized system could be do okay.
    If one zone is the east and the other is on the west, then as the sun migrates across the sky, the loads change on the structure. In the afternoons, the east side of the structure will require a higher load than the west side. If the indoor units are sized appropriately, then one indoor unit can run at full capacity and the other unit with a decreased load can be running at a lower capacity, thus never placing a full load on the outdoor unit, and allowing the entire structure to maintain set-points, even when the total outdoor capacity shows to be undersized.
    Just another thought process to think about.

    1. Jasper: Yes, that is indeed another advantage for homes that are zoned. I have two zones for this heat pump, but most of my windows face south or north. The three facing west are well shaded by trees.

  2. I really appreciate this posting because I can use it to show my contractors and HVAC students why oversizing is the worst thing we can do for an HVAC system.
    Since a “properly” sized system is based on the extremes in weather, a properly sized system is actually oversized most of the time when those extreme conditions are not a reality.
    What is referred to as an undersized system is actually a system that is closer to properly sized under more conditions than not. My home system is also considered to be undersized, and I love it. I may like 74 degrees at 40% or less humidity, but on extremely hot days when the inside temps go as far as 78 degrees, I am still comfortable because the constant operation of the cooling system (with an oversized indoor coil and variable speed blower set to 280 cfm per ton) reduces the humidity level even more. For the heating mode, I just put on a sweater during the extreme cold spells.

    1. Robin: Yes, one of the benefits of sticking close to the result of the cooling load calculation is better dehumidification, as you know. Mini-splits, as I’ve mentioned before, don’t have great latent capacity but by sizing them like this, you get decent performance on the hot days. My indoor relative humidity over those six days ranged from 51 to 64 percent, with an average of about 56 percent.

      1. Allison, have you tried setting your mini-split units to the dry mode at the temperature you want? Doing this will increase the latent while decreasing the sensible to the same capacity. Even if doing this tops out the capacity, your RH should drop enough to make a degree or two higher temperature more comfortable than a lower temperature at a higher RH. Most comfort charts I have read put 72 degrees at 40% humidity as the average comfort range. Personally, I prefer 74 degrees at 40% RH.

  3. Last year I replaced a 20-yr old 2-ton AC serving the 2nd floor in my house with a 1.5-ton unit. I also had the knee attic that contained the indoor unit encapsulated with foam insulation. The fairly detailed Manual J I did showed that that at 77 indoor temperature (where we usually keep the thermostat) and the 92 design temp, a 1.5 ton unit (exactly) was required. If I was doing the Manual J for a customer who may want to keep their house at 75 or below, I would have thought 1.5 ton unit would be too small and may have recommended a larger unit. It turns out that when it reached 97 degrees here in June the unit had no trouble maintaining 77 degrees, or a 20-degree delta-T. In fact the unit was still shutting off 6 minutes each hour so it probably could have handled a 22 degree delta-T.

  4. I live in a 2 story house in Atlanta with a 3-ton variable capacity 2 zone system (1 zone per floor). My system is undersized about one half ton because we modified our second floor addition after PV installed the system. Controlling set points by zone based on occupancy plus a tight envelope for our 1926 renovated house (3 ACH50) allows our system to main optimal comfort. It also helps that PV replaced and commissioned the new duct system based on a Manual D.

    1. Howard: Thanks for corroborating the article with your experience as well.

      By the way, for anyone confused by “PV” in Howard’s comment, he’s referring to PV Heating and Air, a really good HVAC company in the Atlanta area. Here’s their website:

    2. I too live in a 2 storey house in Atlanta. Having moved from San Diego last summer in the middle of the horrible heat and humidity and being an avid reader of Allison’s blog, I decided to put his principles into practice – replaced the 30 yr old furnace and AC without a filter in the basement with a 3.5 ton variable ducted heat pump. The attic with ridge vent was reaching 120 degrees but after OC spray foaming and adding a small vent fan, max temps came down to the outdoor temp (max so far 99 deg), so there are some leaks for sure.
      The 13 yr old 2.5 ton heat pump in the attic then failed so I got PV to put in another 1.5 ton Daikin variable ducted heat pump – but above 97 deg, it struggles to cool the top floor below 77 – which I’m guessing from this article is because of design temp?
      Humidity as not been an issue so far in the attic (62% max briefly and 58 max on the second floor), so PV plans to add a 4 in duct from a vent to condition the space. I wonder though, if I would have been better off getting a slightly larger capacity HVAC for these extreme heat days ?
      Thanks to Allison and everyone here for all the information – can’t wait to get your book!

  5. According to Robert Bean’s group, air temps are a poor gauge for human comfort (but I’m guessing you already know this since you have previously mentioned him). So why continue to use it? What about mean radient temps, air flow or humidity? It’s a chance to lead by example to shange the HVAC world from selling boxes/ducts to change air temps to what the business should really be about – selling human comfort.
    What I saw in your stats is that your current system is greatly undersized at removing latent heat (in Georgia?) So why no separate dehumidification system?

    1. David: Yes, of course thermal comfort is about more than air temperature. The six factors in the ASHRAE comfort standard (#55) are:

      Air temperature
      Air speed
      Relative humidity
      Mean radiant temperature
      Metabolic rate
      Clothing insulation

      I’ve written about that many times and have a chapter and more about it in my book. But this article is about heat pump sizing and performance more than comfort.

      Yes, mini-splits aren’t great at removing humidity, but the undersizing of mine makes it acceptable. As I wrote in a couple of other comments, our indoor RH ranged from 51 to 64 percent, with an average of about 56 percent over the six days discussed in the article.

      Now, when I get some of my enclosure improvements done, I’ll post some data on radiant temperature.

  6. Our utility has paid rebates on thousands of systems over the past decade. The average home era is about 1978 and the BTU average of installed systems comes to an average of 725 square feet per 12,000 BTU performance after insulation and minimal infiltration sealing. So if you are a contractor on the 400, 500, or 600 square feet per ton merry go round, you are way off. A 1978 home sealed at the top plate will perform at 800 squared feet + per ton level . Here we have a 99 degree ACCA ambient.

    1. In the Mid-Atlantic region, those rebates and tax credits have caused a lot more harm than good. In order to achieve the ratings for so many cooling systems, they had to utilize oversized gas furnaces. As a manufacturer’s rep, I can’t count how many oversized gas furnaces had to be replaced after initial installations of oversized furnaces required to meet rebates. So many furnaces were operating more on limit switches than on thermostats.

  7. I love reading your AC geek articles, even though many times there over my head. I have a very different opinions about right or under-sizing AC units especially in the Texas heat. I have two AC heat/pumps, one for each floor. The downstairs is a 4 ton and the upstairs is a 3 ton properly sized by Austin standards. The downstairs works great, but the upstairs cannot maintain 74-76 after about noon when outside temperature is above 100. I will try and upgrade it to a 4 ton next time which should be soon since that unit is 20 years old (wow). I get the under-sizing argument, saves power consumptions, but if it won’t maintain coolness and runs constantly most of the day I wonder if the power savings is justifiable. Again I love your articles, thought I would share my perspective under the HOT Texas sun. 🙂

    1. When people in the HVAC sizing business refer to an “undersized” unit they are usually referring to undersized per Manual J load calculation, not undersized as to actual cooling load. Try getting a Manual J load calculation done for your 2nd floor or better yet, encapsulate your attic if that is where the ducts are, then get a load calculation and then decide what size unit you need.

    2. John: Before you invest in a larger air conditioner, there are so many less expensive things you could do that might address the real problem. As David Wasserman mentioned, ducts in the attic can be the source of a lot of heat gain. Are they sealed properly? Are they insulated properly? Are any ducts disconnected? (That’s way more common than you might think.) Are you getting the right amount of air flow from them? And then there are all the other sources of heat gain: attic insulation, window shading, infiltration… See this article for more:

      Simple Steps to Improve Air Conditioner Performance

      The bottom line is that you shouldn’t blame the AC before you know what all is going on.

      1. Agreeing w/ Allison and David: John, don’t throw good money after bad. Really… how big is this house that you think it needs 8T of cooling? There is something fundamentally wrong w/ the upstairs zone for sure. Hire an energy tester/rater/auditor before you make the leap to 4 tons of cooling that appears to be mostly wasted.

    3. Since you have a 3 ton system for your second floor, may we assume this is an attic installed system? If so, there are many things you can most likely due to allow the existing 3 ton system to operate more effectively. Adding another 25% capacity to a duct system will cause even more problems. It is not unheard of for an attic system to be wasting 25% of it’s capacity into the attic. Two major things to look at are the insulation of the entire system, including refrigerant lines, and areas of possible leakage of air.
      Air leakage can be from the air handling unit, air filter access, ductwork and/or condensate lines. A condensate line on a positive pressure indoor coil may only be allowing air to escape into the attic through a 3/4″ hole, but that is compounded by the same amount of air now causing more of a negative pressure in the home for outside air infiltration. Seems like a little thing, but it all adds up. If your air filter access is in the attic, be sure the opening is completely sealed. Replacing a piece of tape each filter change is no big deal.
      Both refrigerant lines, the liquid (small) line and the suction (larger) line should be well insulated with a minimum closed cell insulation of 1/2″ with absolutely no bare copper showing. This is even more important if a filter/drier has been installed in the attic near the indoor coil. Make sure that filter/drier is also wrapped with 1/2″ closed cell insulation.
      Everything that contains the air volume of your system should be well insulated with a vapor barrier. This includes the air handling unit. I encase air handlers and furnaces in ductboard prior to installation when I am forced to put the HVAC system in an attic.

  8. One thing this performance review doesn’t address directly is the ability of the cooling system to catch-up when lowering the thermostat. I just installed a hybrid heat pump in my house a few weeks ago and was very excited to downsize the AC side of things from my slightly oversized 2.5 ton system to a smaller variable capacity system with the benefits of better managing humidity and better comfort with longer slower runs. But I found that it takes a long long time to bring temperatures down from our “away at work” to “home and comfortable” settings on hot days. I am having to reconsider how much I want to let the temps increase when I am away and may need to leave it steadier like Allison.

    The use of heat pumps with programmable thermostats and smart self-adjusting thermostats seems to be a significant problem that isn’t discussed much. When there is a call for more heat because the thermostat has been bumped up 4 degrees for your evening schedule or your thermostat self adjusts when it sees you are home and the user pushes it even higher because they are cold, many systems will immediately go to backup / secondary heat which is often heat strips. When I turned on the heat a few weeks ago on an unusually cold day and my psychotic Ecobee thermostat suddenly dropped the current temperature by 3 degrees I watched in horror as my backup heat kicked in – on a 50 degree day.

    From my corner of the world the HVAC industry is doing a horrible job of setting up intelligent thermostats with heat pumps and explaining how they work to the customers. I’m confident I found the best heat pump guys in the region and they didn’t say anything. Part of that explanation should be to avoid large jumps in your heat or cooling settings and not setting it back so much.

    1. Albany Landlord: No matter what type of system you put in, if you size it close to the load like I did, you’ll need to give up the idea of using setbacks. As you’ve discovered, recovery just takes too long. And yes, the HVAC folks who install and service these systems need to understand and explain that, too.

      1. Allison – can you expand on your comment a bit more? Are you saying a cooling system sized close to the load won’t be able to handle letting the temp creep up during the day and then bring it back down in a reasonable amount of time to the desired temp, and the best solution is to keep the target temp steady?

        In my case, keeping the temp would mean the unit works hardest when it’s hardest for it to work (highest outdoor temp, highest attic temp, highest solar load). I actually prefer setting my thermostat to as low as I can tolerate (68F) early in the morning, and then letting it creep up to 75F or so, by which time the attic temp would have started dropping again and keeping it at or below 75F or lower in the evening is easier.

        Admittedly, a main reason I would love to switch to a variable speed system is how much I hate the thing blowing on me at max speed. Pause a while and then keep cycling, as I have to cycle between sweater and no sweater.

  9. Thanks for a great and informative article Allison. Are you tracking interior humidity levels as well? That would be another great data point for your graph.

    1. Charles: Of course I track relative humidity, too. During the six days covered here, it ranged from 51 to 64 percent, with an average of about 56 percent. Mini-splits don’t have great latent capacity, and my house is still on the leaky side.

  10. Great article, Allison, and some great comments also. Congrats on how well your system is working.

    1. Thanks, Bill! I was worried when I installed it, but now that we’ve had our first hot weather in the three summers it’s been running, I know it works just fine.

  11. Allison

    Our close monitoring of a number of loads on houses showed that the actual sensible load at design conditions was pretty consistently around 67% of Manual J7. This is in a couple of our reports. Manual J8 overstates the sensible loads by even more.

      1. Allison, I have never looked at Manual J, so I have a question. Does it assume steady-state conditions for the design cooling load? If so, I would claim that is why it overpredicts the design load since cooling conditions are always highly transient and thus damped by the thermal capacitance of the structure and its contents.

        1. Roy: Yes, Manual J is a steady-state calculation. The thermal lags and varying temperatures of course play into it, as you say. I think the idea of Manual J, though, is that when you’re at the design conditions (ignoring the thermal lag), the AC should be running continuously at peak capacity if it’s sized to meet the load. In my case, with a 12% deficit in capacity compared to what’s needed at design conditions, I shouldn’t be able to keep up.

          1. Manual J is not really a steady state calculation. It assumes that there is a lag in the indoor temperature effect due to conduction gains. That is the reason that they specify a lower design temperature than the actual maximum temperature expected. I recall a illustration of the outdoor temperature vs. the indoor heat gain but I cannot remember where it is. Maybe a reader has knowledge of that illustration.

  12. I have often wondered why they don’t create an app that uses the weather forecast to anticipate future weather (from forecasts) and our input of what temperatures we want at each hour, and the amount of time it would take the system to get to the desired temperatures. If we allow, it could even raise the thermostat a few degrees when the sun is warming the outside unit especially when the forecast is extreme cold later that night. We could tell the thermostat how far off ideal we would accept if it would reduce energy costs. (I actually do this manually as a game. I gradually bump up the thermostat as high as my wife can stand it when the sun is on the outside unit, and when the sun stops shining on it, I turn the thermostat way down to 60. With our extremely efficient Appalachian Mountain home with lots of thermal mass, we coast all the way through the night and arrive at the morning above 60. Our annual heating costs are less than $100 for 460 square feet ICF, spray foam roof, passive solar, earth bermed on the north side home.

    1. I just replied to another comment about how far lower than ideal I manually set my thermostat to cool down the house in preparation for the day’s heat..

    2. No matter what we do, we are going to be at the mercy of the elements. This means that the very best we can do is to determine what the extreme highs and lows in temperature will be and work from there. I am not one to take a systems capacity to either extreme because it just doesn’t fare out in the long run. By being a bit undersized on the extreme temperatures, we are still going to be oversized for most days. The narrower we can make our design temperature differences, the more effective and more efficient HVAC systems are going to operate.

  13. Looking at you KWH usage, it doesn’t appear that you have reached maximum capacity of the system. If it was maxed out I would expect to see flat spots on the peaks as it tried in vain to achieve your set point. You are only pulling around 6.3 amps at peak. What tonnage is your unit? Mine is a 3 ton Mits and I pull about 6.7 amps at midday on the whole house with TV running, etc. I wonder what is max draw by design in amps?

    1. I noticed the same thing Thomas. I would like to see Allison turn down the thermostat to 60 and have it run flat out for an hour to see what the maximum draw is.

  14. Sure:
    Monitored In-Situ Performance of Residential Air-Conditioning Systems
    SF-98-30-4 ASHRAE TRANSACTIONS 1998, V. 104, Pt. 1.
    Proctor, J.P., T.D. Downey, M. Blasnik, and G. Peterson.
    1997. HVAC pilot project for Nevada Power Company.
    Final Report. Palo Alto: Electric Power Research Institute, Inc
    There was also a monitoring project in Southern California that there is a report on that I am searching through archives for.

  15. There is a term in the utility peak shaving efforts called “Drop and Drift” The concept is to make a tight well insulated home that can be turned off during peak load events. The home is so efficient that it can coast along without HVAC until the peak load resides. I think your home allows for some cushion in the heating/cooling requirements in much the same way.

  16. My question is how much cooking did you do during that “heat wave”. Here in Crematoria (AKA Texas) most people don’t do any cooking on the stove or oven in much of the summer because the AC is already running non stop and will never catch up with the cooking you are doing.

    Citys here are finally requiring contractors to do the calcs for sizing etc on new houses and we have had not thousands but tens of thousands built recently. I can’t tell you how many of those new houses right now are running on undersized equipment. To the point of many of them have purchased window units to aid in cooling. Fortunately the design temp was finally raised to 101* here as I had been complaining for years the old temp was way too low according to local historical data. That number could easily be 102-103 but they are at least slowly fixing their numbers to reflect actual weather data instead of the chamber of commerce numbers which are lower in the summer and higher in the winter…

  17. My mini split system can’t keep up with a 25 degree differential from outside air to inside desired temp for heating, which is show stopper in the winter. I doubt it’s the unit’s fault. It’s in a sunroom on the north side of the house with outdoor windows covering every inch of three sides of the room and a vaulted ceiling with two skylights. The sunroom is elevated so that the floor just has some fiberglass batts to insulate it. My beginner level building science knowledge suggests even a correctly sized unit could not overcome the lack of insultation and air sealing involved. I don’t think my contractor, who was a good guy, ever thought of a Manual J type calculation.

  18. I have a story worth adding to this:

    I built our home 5 years ago. I wanted it to be super-insulated, but still wanted a traditional-looking home. Spent a lot of time on this site and GreenBuildingAdvisor asking questions and trying to learn as much as I could. I live in a small community, and this was the first project of its kind in our area, so no one believed that my HVAC loads could be so low, even after getting a professional Manual J calculation. Fortunately, my builder trusted my research and we got everything installed per my specifications, which were two ductless mini-splits servicing the two sides of our home.

    Fast forward to this summer, and (long story, short) one of my ductless mini-splits isn’t working. And, as luck would have it, we have had several weeks in a row of high-90s weather with high 90s humidity, which is above the normal design temperature for our area (Central KY).

    However, even while being down to 1/2 capacity, and not having the ductless minisplit in a central location and/or blowing towards the center of the home, we have been able to keep our indoor temperature right where we normally keep it.

    Granted, it is easier to cool a house with a temperature difference of ~20* than it is to heat a house with a temperature difference of 70*+ (harsh summer vs harsh winter), but I think it is safe to say that I have plenty of capacity, maybe even too much, even though it is a fraction of the capacity that everyone around me thought I needed.

  19. Is there reason to believe that the efficiency of mini-split heat pumps will reduce as years go by?

  20. Good article Allison. This re-affirms some of the lingering questions I had in my head prior to our discussion this week. Living in a part of the world where residential heat loads do not exist, it is reassuring to know I’m headed in the right direction.

  21. Allison, just wanted to comment on the importance of having real data as you obviously do. With the availability of low cost data acquisition tools it is (relatively) easy to really know what is going on. By continuously tracking runtimes, temperature/humidity throughout the house and electrical use you don’t have to ‘think’ , you ‘know’ exactly how your systems are performing. Nice work!

    1. Thanks, Michael. Yes, the availability of good, low-cost monitoring tools like the Emporia Vue, the Awair Element IAQ monitor, and Onset’s HOBO data loggers makes it a lot easier to know what’s going on.

  22. I was going to insert my usage monitor data from this past month, but cannot paste. My total usage for the last 30 days is 700kwh. That is less than 25kwh per day. I have 3 ton Mitsubishi ducted mini-split with a 1 ton head on the lower floor and a 1 1/2 ton on the upper floor. 2200sf with vaulted ceiling top floor. I confess that the waterheater and dryer are LP. I live on coastal NC, which is hot and humid at this time of year. My highest consumption runs less than 1.5kwh with an occasional spike from defrost cycles and elevator usage. The point being that outside of cooking usage, our consumption is very low. During hurricane outages, I have pulled the main breaker to the street, bridged all 110v circuits together and run the house using an inverter in the trunk of my hybrid Avalon for everything except 220v branches that include the HVAC. If my inverter would give me 220v, I could run the HVAC as well.

  23. We’ve noticed that many of the Mitsubishi heat pumps we’ve installed have power outputs that exceed the stated maximum output on the submittal sheets. For example, on the 1.5 ton hyper-heat in my house, the maximum power input at 95F is listed at 1440W, but I’ve seen above 1700W regularly when in the mid-90s. I haven’t tried ramping down the thermostat to see if the power draw would go even higher. Same story in the winter – listed at 2740W max input at 17F, but I’ve seen it around 3800W when in the teens (also data from a Vue). Based on readings in the supply & return ducts, it looks like capacity was indeed elevated beyond the stated maximums as well.

    Long story short, it seems like most of the single-splits we’ve installed have both heating and cooling capacities that exceed their listed specs. Curious if that may be the case with your multi-split as well.

  24. YES!! I agree with the 72 degrees with 40% humidity. I also set my thermostat to 73 and my unit pulls a TON of moisture out no matter what. I also set it down to 70 at night as we like it cold and if its much hotter I wont sleep well.

  25. Maybe this was covered earlier? Do mini-splits actually de-humidify at all. Do the head units have drain lines? Thanks.

    1. John: Yes, ductless and ducted mini-splits DO remove moisture from the air. I haven’t measured the condensate at my house, but for a while I had the line draining onto the sidewalk in my backyard. It produced a lot more water than I expected, and it did so at times even when I didn’t expect to see much.

    2. I have found that because ductless systems control the evaporator coil temperature to maintain below dewpoint that ductless systems do a good job with dehumidification. When a ductless unit is set to “dry”, the evaporator maintains the coil just above freezing by modulating the air volume. This causes the evaporator to use most of its energy changing the state of the water vapor in the air to liquid condensate and less energy removing relative heat from the air. In this way, a ductless unit can dehumidify much better than a conventional split system without overcooling the area.

  26. Thanks Allison and Robin,

    Are there any dual-outlet mini-split systems (or any systems) with the Heating mounted or ducted low on the wall and the cooling mounted or ducted up high?

    –John Weil

    1. John, I am not aware of any indoor units that do what you are asking. However, there are many variations of indoor units that can be mounted low wall on the floor or ceiling mount depending on physical aspects of the area needing to be conditioned. All of the indoor units take into consideration that the air louvers need to direct the air upward for cooling and downward for heating. A ducted indoor unit can also be installed where we can have the return into the unit up high and duct the supply low on the wall. Pretty much any situation can be resolved with the various indoor unit types.

  27. I’m trying to right-size a replacement mini-split heat pump for my house, but I’m not sure how exactly to interpret the “rated capacity” and “capacity range” figures that manufacturers specify. Rated capacity seems to be actual measured output under some standard test conditions (here in Australasia, that’s usually “H1”, which is 7℃ dry-bulb, 6℃ wet-bulb outside and 20℃ inside). But how should the capacity range be interpreted? Is it the available range at those same test conditions? Or is it the minimum and maximum under any (likely different) conditions? Is the “rated” output chosen to maximise the COP or other desirable specification for marketing? It seems manufacturers want you to use the rated capacity when sizing the unit, but I want to be sure that that’s the right way to do it! The old unit was definitely oversized.

    (Background: I have a good idea of the 99% Winter design temperature at this site. I’ve measured the heating load on a cold night and determined the thermal resistance of the rooms, and I’m comfortable with how to determine the balance point. I’m aware of the need to derate the heat pump’s output for the design temperature, and I have supplemental heating available and don’t expect that the heat pump will never defrost.)

  28. Equipment ratings have become more confusing due to manufacturers having to appease government regulations in most countries. For a mini-split system with inverter technology, you want to focus on rated capacity and sensible capacity at full capacity of the range. If the total capacity is 36,000 Btu and the sensible capacity at full operation is 31,000 Btu, the difference in capacity between the two is the latent capacity of cooling that will go into condensing moisture from the air. If Australasia is as arid as I am thinking, you can lean on the total capacity more so than sensible since there won’t be as much latent capacity used to remove humidity. Hope this helps.

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