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What Exactly Is a Low-Load Home?

Passive Solar House Richard Levine Kentucky 600

I found out last month that the Air Conditioning Contractors of America (ACCA) is working on a new design manual.  You probably know of some of their other manuals:  Manual J, Manual S, Manual D…maybe even Manual T.  They have quite a few others as well (P, H, Zr…) but now they’re working on one that will address a part of the market we regularly discuss here.  I’m talking about low-load homes, of course.

ACCA put out a call for volunteers to work on the manual in the spring of this year.  (Applications were due in April so it’s too late to apply now.)  The objectives of the new manual, at least as specified in the call for volunteers, are:

  • Defining low load homes characteristics (i.e., low infiltration, sealed combustion appliances, ducts in conditioned space, low CFM exhaust fans, etc.).
  • Resolving ventilation requirements (for occupant health and safety) while maintaining moisture control.
  • Addressing ancillary dehumidification equipment for humid locations.
  • Offering air distribution strategies for occupant comfort; especially problematic when a 2000 square foot home (and larger) may only need one-ton of air conditioning (hence, only 400 CFM of total airflow is available).

At the ASHRAE conference in Houston, I spoke with someone who’s on the task force and was told they’re defining a low-load home as a house that has a house-size-to-load ratio of 1,500 square feet per ton or more.  That sounds about right to me.  That means a 2,000 square foot house would need smaller than a 1.5 ton air conditioner or heat pump. That translates to about 16,000 BTU per hour of heating or cooling capacity. 

And that’s just the starting point for low-load homes.  We’ve done load calculations for homes that are in the 2,500 to 3,000 square feet per ton range.  (See my 2016 article with data from our results.)  Now we’re talking about a 2,000 sf home that needs less than a ton of cooling.  As the objectives above point out, heating or cooling 2,000 square feet with 400 cubic feet per minute of air flow or less is a challenge.

This is an important issue because manufacturers have been slow to make low-capacity equipment for low-load homes.  Just try finding a 16,000 BTU/hr furnace. 

Back in 2012, Professor John Straube gave a full-day presentation on mechanical systems for low-load homes at Building Science Corporation’s Experts’ Session.  He’s a Canadian so he gave only a number for heating but he defined a low-load home as one having a peak heating load of 15,000 to 30,000 BTU/hr. 

Passive House is the ultimate low-load home program.  They’re all about increasing the insulation and airtightness while decreasing the heat transfer through thermal bridges.  Their requirements result in homes with heating and cooling loads in the range of 2,500 to 3,000 sf/ton. 

How do you define “low-load home”?


Related Articles

A Day with Professor Straube at the Building Science Experts’ Session

Why a New Standard for Passive House?

Air Conditioner Sizing Rules of Thumb Must Die


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This Post Has 24 Comments

  1. Wouldn’t it be more realistic
    Wouldn’t it be more realistic to define the load relative to the environment? 30,000 btu/hr in Vermont would be below 5 btu/sf/dd. In Georgia, it would be twice that.

    1. Jonathan, yes, climate is a

      Jonathan, yes, climate is a big factor.  I’m not sure what the task force is going to do with that but this is the HVAC contractors trade association trying to figure out how to design heating, cooling, and ventilation systems for homes with difficult constraints.  As I mentioned in the article, trying to heat or cool 2,000 sf with 400 cfm or less of air flow is very different than what contractors are used to doing.

  2. I’ve talked to manufacturers
    I’ve talked to manufacturers for many years about developing appropriate condensers, air handlers and heat pumps for high-performing homes, and usually the answer is that until the market forces them to develop those units, they won’t do anything about it. Sad but true… I wonder if they could talk to Microsoft, Apple or Amazon about planning for the future!

    1. Armando, the market doesn’t
      Armando, the market doesn’t “force” manufacturers to do anything, they mostly respond to it as needed. The government does force manufacturers to do things like raising minimum efficiencies and switching to more environmentally friendly refrigerants. In the meantime, houses keep getting bigger, so the trend has been towards larger capacity units, not smaller ones. Talk to the home builders, designers, and installers who specify one 5-ton unit instead of two 2.5 ton units or three 1.5-ton units that would better serve these houses with shorter ducts and better zone control. As for the “high-tech” companies, keep in mind that they mostly sell convenience or services. Most of their products are practically disposable. HVAC manufacturers meet a basic life necessity (indoor environmental control) with products that are expected to last 15 years with no maintenance in most cases. Computers and cell phones get software updates on a monthly basis and are not supported at all after a few years.

  3. When “low-load” homes become
    When “low-load” homes become a significant (or even noticeable) market size, manufacturers will provide more and better equipment to meet their needs. My crystal ball says that it won’t simply be scaled down versions of our current equipment. If all you need is a 15,000 Btu/hr gas furnace, why bother even having natural gas brought to your sight? The meter fee is probably higher than the energy costs. You will probably need air conditioning anyway, so using a heat pump might make more sense. Even if you are in a cold northern climate and can’t wait for global warming, a heat pump might make sense, even if you need strip heat since the required strip heat would also be quite small. As for cooling requirements, since ventilation will become a significant part of the load, the AC unit will probably have mechanical ventilation integrated into it. So don’t blame the current equipment manufacturers for not meeting your needs until you show that there really is one. Yea, I am getting a bit defensive here.

    1. RoyC, I think low-load homes

      RoyC, I think low-load homes are already a noticeable part of the market.  I believe even some production builders are hitting 1,500 sf/ton.  I do agree with you on using heat pumps instead of furnaces.    The equipment issue is one thing; the air distribution side is another.  There are contractors out there who can hit 400 cfm in 2,000 sf. Some of it even do it intentionally and make it work.  But it’s a different ballgame than they’re used to.  Understanding air flow will no longer be optional.  My two cents.

      1. Allison, I checked the AHRI
        Allison, I checked the AHRI Central AC and HP shipment (5 ton and smaller) data for the U.S. Over the last 5 years, shipments of systems with less than 1.5 tons of capacity were about 3% of the total units sold and were quite flat during that time period. I was surprised it was that high, and I don’t even know what companies sell those units, but that is not really enough of a market to get the large manufacturers to take seriously.

  4. Whatever you do and whatever
    Whatever you do and whatever you call it – you START with the BUILDING ENVELOPE – a great wall system with minimal (or no) thermal bridging by structural members and fenestration (windows, doors, etc.) suited for the climate and orientation (don’t forget depending on the orientation of the house some windows may have a different solar heat gain coefficients (SHG) than others. Then after you have created this envelope and understood its potential for both passive and active gains and losses, then you look at how you are going to manage interior comfort and indoor air quality/health most efficiently and effectively.

  5. With heat and cooling loads
    With heat and cooling loads that low, I would assume the industry could use 2″ PVC pipe for ducting?

    1. Gary, that’s a great question

      Gary, that’s a great question.  If the industry does go that route, it would eliminate a lot of the stuff we all complain about with flex duct.  But proper duct design becomes even more important.  And there may be an issue with fire codes, too.

  6. LOW LOAD can be built in a
    LOW LOAD can be built in a factory. We recently built a 1st ever On or Off-The-Grid (Manufactured) HUD code home with a 18000 BTU CREE mini Split and eliminated all ducts. The idea was to showcase an instant house for Fire Victims in Northern California to return to their burnt lots to rebuild. The outside Feb 23rd daily temperature went from 31 to 52 degrees during the event. We performed 5 Blower Door tests in front of 2000 visitors for a Green Expo. No utilities were connected to it.
    Average Test result 371 CFM @ 50 pascals. We were shocked and dismayed by being short of 191 cfm to meet PASSIVE criteria.
    It appears low load reduces the size of an HVAC unit given the fact it is not leaking to outside air

  7. I guess I am a true nerd as I
    I guess I am a true nerd as I think this is really exciting stuff. If they are truly trying to move the industry, I think they should start at 1000 sf per ton in their definition. It would get builders a more reasonable target to move toward instead of simply dismissing the whole concept.
    My latest house is 1600 sf single story and the blower door test on it was .54 ACH. We can now reach this number without doing anything excessive, just building as we do (good casement windows, closed cell foam in attic, simple mastic on questionable joints and, of course, precast insulated concrete wall panels from Superior Walls.
    What I would also like to see added is what I call “small, tall and tight” houses. Beach area lots are quite narrow so the house has to be small per floor. To get the space, you have to go up multiple floors. Now add a really tight envelope and the load PER FLOOR is really small at around 1000 sf. As space is a premium, vertical ducking is avoided. A 1-1/2 ton conventional system on each floor is way too much. Ducted mini splits work pretty well but are still quite pricey. 1/2 ton air handlers would be a great start.
    My real challenge is determining if adding any more insulation really is worth the money once you have a really tight envelope? The chart “The Diminishing Returns of More Insulation”, shows that after about R12 or R14 you may never see an ROI. Can these Passive Houses prove that a 12″ wall of insulation is really worth it?

    Thank you Allison. Great topic.

  8. My 2-ton system your engineer
    My 2-ton system your engineer Andy Bell designed is loafing along, doing a great job on our 2800 SF house. And we have 4 zones (not counting the basement zone, which is also not counted in the 2800 SF). Yes, we are in Massachusetts, but it is 95 degrees out. The contractor swore we were making a big mistake with such a small system. Most family members feel comfortable setting their zone to high seventies or even 80, I guess because of the great dehumidification the system provides. It’s quiet and not drafty. I feel sure 105 degrees would be no problem for it. Let’s hope we don’t get to find out.

  9. James, you hit it square on
    James, you hit it square on the head. Dehumidification allows people to feel comfortable with settings in the high 70s. We run 77-78 degrees all summer, but our RH is in the mid 40s. This is why I am so interested in this new ACCA manual. Humidity control can be a real challenge with tight houses.

  10. I would also like to see much
    I would also like to see much more research and development into dehumidification and air conditioning equipment to make those processes much less costly. It costs far more to maintain indoor air at a comfortable level when the outdoor air is 30 degrees warmer than it does when the outdoor air is 30 degrees colder.

  11. Long, long overdue. Hank
    Long, long overdue. Hank (primary author of ACCA residential design manuals) contacted me about this project in early 2013, and the task force is just now getting started? Maybe by 2020 we’ll see an actual book.

    To my knowledge, this new manual is not intended as a standard but rather design guidance, so the definition of ‘low load’ has little practical consequence. OTOH, I agree with Thomas that it might get on more folks’ radar had low-load been defined @ 1,000 ft2/ton, especially considering that the 400-600 ft2/ton rules-of-thumb is still prevalent in the market.

    More importantly, the idea that ‘low load’ is still a niche market is flat out wrong. Many of the issues being addressed by the task force, in reality, should apply to any home built/verified to comply with today’s energy code.

  12. I’m in the process of
    I’m in the process of building my own (hopefully) last home. Before giving the first thought to insulation or windows, I optimized site, orientation, room layout and overhangs to minimize cooling loads. In particular, I bought a north-facing lot and put garage on west side (house only has one west facing window). Most importantly, half of the 3,100 ft2 floor area is below grade.

    Basements are rare in my area (SE Arizona) but this was non-negotiable for me. In addition to reducing loads, this brings ducts inside and allows me to install a suspended ceiling with lightweight tiles for future easy access to all pipes, wires, ducts, etc.

    Above-grade walls are 2×6 24-oc with BIBS (R23 cavities) + 1″ EPS outside (typical stucco system). Roof is ‘flat’ with R32 rigid foam on top. Windows are vinyl with 366 glass (0.27/0.19 for the most part). Furred out basement walls are R13 cavity + R2.5 XPS behind studs, and R4 EPS outside to protect waterproofing system. Floor trusses are hung inside basement wall with 6″ spray foam between trusses. No sub-slab insulation. None needed. Anticipate 1.5 ACH50.

    The total cooling load is only 0.9 tons @ 100F outdoor, which works out to 3,500 ft2 per ton. Indoor design is 76F with nighttime setback to 73F. Heat load ‘by the book’ is ~18k @ 28F/69F but I estimate MJ overstates my heat load by 100% given the particulars of the house & locale (e.g., I estimate true design heat load will be less than 9k).

    Whole-house ductless isn’t an option in cooling dominated climates, and with only 0.2 ton cooling load for the basement, I gotta have zone control. Unfortunately, I haven’t been impressed with zone control options for ducted mini’s. Given my zone sizes, a single stage 1.5 ton system won’t work, and 2-ton variable speed equipment doesn’t have nearly enough range to handle single zone calls. What to do??

    Fortunately, Carrier added a 1 ton model to its Infinity 18VS (5-speed inverter compressor). Given that Carrier’s zone control system is arguably the best on the market, this system is ideal for my project. Normally I’m agnostic when it comes to brands, but to my knowledge, no other manufacturer makes a 1-ton (non mini-split) heat pump. Kudos to Carrier for responding to the market!

    1. I’m not sure you should get
      I’m not sure you should get too excited about the apparent unique characteristics of Carrier’s “1 ton” 5 stage split heat pump system. A 15 minute browse of Carrier product data suggests that the 1 ton is simply a factory de-tuned 2 ton. I base that assertion on Product Data showing the 5 stages of cooling capacity percentage: 1 ton: 58,72,81,90,100 2 ton: 35,56,69,87,100

      The min speed for 1 and 2 ton systems are both set at 1500 RPM, likely for oil return concerns.

      The smallest air handler available is a 2 ton, and the lowest allowed air flow for the one ton system is 300 CFM…not much turndown.

      1. @Curt, I’m totally familiar
        @Curt, I’m totally familiar with the specs for that model. I realize the compressor is a de-tuned 2-ton model.

        300 CFM minimum works for my situation since my design CFM is 550 CFM/ton (there’s no actionable latent load). The stage-1 capacity @ 95F/75F and 57F EWB is 6,420 BTU/h @ 300 CFM, which works out to 560/ton. I don’t think the 2-ton model would work because of the way the Infinity Zone control is programmed when matched with a two ton outdoor unit.

  13. I believe that the HVAC
    I believe that the HVAC systems of the future will be load matching systems. I think sizing conundrums of current market will fade once the dumb downed single stage on or off HVAC systems of yesteryear are eventually done away with. It might take another 10 years before we might be able to predict this further.

    A load matching system can solve a number of problems outside of that of structure sizing concerns. For one it can reduce the number of pieces a manufacture has to make, the number of parts needed to repair it when it breaks. Everyone knows when it come to sprockets and gizmo’s the less you have to stock the more profitable you become in that the less parts you must make to sit on a shelf until the day it may or may not be needed.

    So what is a load matching system? Inverter is the name of the game. They already exist but they are all high end machines when it comes to central duct type systems. While you can buy them in mini split configuration these options tend to be even more pricey than a central ducted system as mini splits are mostly designed for small structure spot cooling and heating applications.

    The larger the structure the less likely a mini split will adequately cover the structure without additional more costly equipment and more maintenance and eventual breakage.

    How a ducted inverter AC works: Typically most HVAC manufacturer’s pull this off by modulation of a variable speed compressor in 5 stages of cooling. So in a space that only needs 1 ton of cooling the 2 ton model would be chosen, due to the ramping of the 2 ton model with 5 stages the system would most likely never hit the 2 ton mode or 5th speed of the compressor in this case. Worst case scenario it would probably hit stage 3 occasionally.

    2 tons of cooling = 24,000 BTU / 5 = roughly 4,800 BTU of cooling per step of the compressor. The air flow required is typically run by algorithmic control scheme handled by the communicating thermostat or the control logic of the main board in the furnace.

    With that said some manufacturer’s make these ducted inverters full variable. Which means the compressor is almost always running and searching for the optimum amount of cooling needed to maintain the heat / cool load of the structure.

    The sizing these AC systems come in currently are 2, 3, 4 and 5 ton sizes.

    However, all of this could soon change. There is a Chinese manufacturer that could disrupt all of this. They have a full variable machine that can run with almost any indoor equipment and any basic thermostat. No real special indoor equipment etc.

    Basically how they pull it off is that the Inverter controls the compressor via constant Evaporator temperature. There are some requirements but because of the nature of how the compressor is controlled opens up a whole lot of new capabilities with less initial costs in many cases as you don’t need special indoor equipment and communicating thermostat to make it work.

    Also, get this the unit comes in 3 ton and 5 ton configurations. If you need a 2 ton you simple convert the 3 ton via a switch of some kind on the inverter. If you need a 4 ton, you convert the 5 ton via a switch of some kind on the inverter. So as you can see, the revolution is coming….

  14. Owing to our experience with
    Owing to our experience with fairly large and complex homes that often come in at 1200 – 1800 SF / ton in an exceptionally humid climate, I responded to the call for volunteers for this committee last winter. I got a few initial organizational emails and nothing more…not sure what exactly happened…

  15. It’s all about air
    It’s all about air distribution and dehumidification all while using a very small amount of conditioned air. For proper air distribution (mixing) either use low air volume high velocity jets, or introduce additional circulation air. Ideally, the supply air in cooling should be 50 to 55°F to minimize the need for a dehumidifier. This comes from studies made by Florida Solar Energy Center, IBACOS, NREL, and NIST.

    It’s been available for years, known as small-duct high-velocity. Sometimes people get hung up on the lower SEER but that’s an inherent aspect of the slightly extra fan power needed to create the jets and make the air colder. Interestingly, it’s not much of a penalty, especially for low load homes and, in many cases, no penalty at all if you factor duct efficiency. The ducts fit inside the conditioned envelope and are extremely tight.

    I’ve seen studies that use smaller ducts with nozzles with conventional blowers but you still get a fan penalty because nozzles have more pressure drop than grilles in order to create the jets. You can reduce the air friction using PVC but this doesn’t pass fire codes. And hard pipe is not as easy to install as flexible ducts. In addition, conventional blowers don’t provide cold enough air unless you use less than rated airflow — which then makes it similar to an SDHV system except it wasn’t designed for lower airflow.

    You might also want to know that we have a 1.5-ton inverter heat pump with a really small air handler (and it can be zoned).

    1. @Craig, I have to push back a
      @Craig, I have to push back a bit on your pitch. I specialize in designing mechanical systems for purpose-built low-load homes and I can’t imagine using a high velocity system in that niche market. First of all, almost by definition, the ducts will already be inside the envelope, so comparing high velocity favorably with systems that have attic ducts doesn’t make sense in this context. There are multiple ways to get the ducts inside without resorting to high velocity distribution.

      Beyond that, the blower energy penalty for high velocity is not insignificant, and looms even larger as we ratchet down a home’s overall energy profile, as is always the case with clients who build low-load homes.

      But the biggest drawback of high velocity in my opinion is the degree to which sensible efficiency suffers when you run supply air in the low 50’s. You deftly fashion that handicap as a selling point but in my world, enhanced dehumidification should only be done on-demand. Otherwise the system wastes energy as it reduces RH lower than necessary to maintain optimal comfort.

      For those who aren’t aware, high velocity systems operate at an extremely low CFM-per ton (250 or even less!). This is done so blower energy doesn’t eat your lunch. At rated conditions, this, plus the higher blower energy is what reduces the SEER (federal minimum for small-duct, high-velocity carve-out is 11 SEER, 6.8 HSPF).

      In my climate (desert southwest), I typically design to 500 to 550 CFM per ton. This increases system capacity and efficiency beyond rated values — but only if ducts are low-static (i.e., low velocity). On the other hand, high velocity will operate at less than its already low rated efficiency when there’s no latent load to speak of.

      That said, the majority of my clients over the years are in humid climates where there may be a need for enhanced dehumidification (DH) from time to time. However, a well-sealed house that’s not over-ventilated should only experience occasional DH calls. Conventional variable capacity systems are designed to reduce blower speed on demand to address DH calls. That makes MUCH more sense than running the system at low speed (cold coil) throughout the cooling season!

      Don’t get me wrong… high velocity distribution has it’s place — for example, for retrofitting A/C in an older building that doesn’t have a central duct system and no other way install conventional ducts. But I would argue that it has no place in high performance, low-load homes. At least not that I’ve ever encountered.

  16. Hi David, good comments. Your
    Hi David, good comments. Your climate is the most difficult to show an energy benefit for SDHV, however I don’t think the differences are as great as you might think.

    Not meant to be a pitch actually. I can understand that it might look that way though since I work for an SDHV company. Im basing it on recent studies by NREL, FSEC, IBACOS, NIST, and a many of our customers. The latest is FSEC at I would be happy to share what I’ve read.

    There’s a misunderstanding about blower power. SDHV is rated at 300 com/ton at 1.2 IWC with an ec motor. The confusion is because people talk about the airflow per nominal (nameplate) ton of the outdoor unit. SDHV uses the same outdoor unit as conventional and the SDHV rated capacity is typically 15 to 20 less than the nominal. We rate at 250 cfm/nominal ton. The minimum for good operation before you start to have problems is 200 cfm/nominal ton and we don’t encourage this. Anyway, you get the idea.

    Measured Fan efficacy at rated conditions is 0.45 w/cfm compared to 0.20 w/cfm for a 400 cfm/ton conventional system at 0.2 IWC (and it is actually higher in practice because static pressure is always higher than the rated value. DOE is correcting this to be 0.50 in 2021). Using the DOE value, conventional fan efficacy for ec motors is more like 0.25 w/cfm. Translated it is 135w/ton vs 100w/ton. In a low load home at 1500 sqft/ton, that isn’t much difference at all. Starting in 2015, the minimum required SDHV seer is 12 and now you can get 13 or 15 Seer with an inverter outdoor unit.

    I agree if all ducts are in conditioned space there is no energy benefit to either system from the ducts but it is easier to find room for smaller ducts. So, there is a construction benefit.

    Low load homes require smaller systems (naturally) and this means less airflow is available. IBACOS found that high velocity jets did the best job of air distribution for low airflow systems. They developed their own system using PVC for ducting and a conventional AHU. Not quite SDHV but very similar.

    The loads are so low in these houses that the internal loads are more significant then the envelope. This means humidity from showers, cooking and breathing impart a significant latent load. Maybe in a dry cool climate you could introduce an economizer function but most climates don’t allow that. Thus, space shr is typically 0.75 even in climates you might think are dry so you may need to run at normal cfm/ton.

    As you point out, it would be great to run at 500cfm/t when no latent load and 300cfm/t when you do. This would be ideal. Unfortunately the rating methods don’t consider this so you wind up with over reporting efficiency (conventional systems running at lower airflow) or under reporting efficiency (SDHV). The truth is in the middle. It’s up to us engineers to figure that out.

    I’m not saying SDHV is always better but certainly it isn’t always worse either. Bottom line, use the system that is best for the application factoring performance, comfort, cost and aesthetics. You will find that the two systems are not that much different, especially for low load homes.

    Anyway, this is a good discussion and I welcome your thoughts especially for your climate which is probably the least favorable for SDHV although not terrible either.

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