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Green Building Blog

It’s Air Conditioning, Not Air Cool-ditioning

Tight homes need dehumidification more than cooling, and SEER ratings should recognize latent as well as sensible cooling

This 640-square-foot cabin with radiant floor and spray foam insulation hardly needs any cooling, but it needs dehumidification provided by the 11,700 BTU/h vertical PTHP that fits in the bottom of the display cabinet. At 10.7 SEER, its doesn't meet code minimums despite the efficiency of the envelope. A little unit like this would be perfect for a number of superinsulated situations, except it's illegal.

“Why should anyone bother to ‘right-size’ an air conditioner — especially when it costs $350 to hire an engineer to ensure that the Manual J and Manual D calculations are performed properly?”

The reason I always pay a third-party engineer, not employed by my HVAC contractor, to do accurate load calculations is because my mixed-humid climate here in NC makes it very critical to get the sizing right in order to enhance dehumidification. As we build smaller, better and tighter envelopes, especially with exterior shading, it becomes increasingly difficult to size equipment small enough to run long enough to remove the latent heat (moisture) without removing too much sensible heat (over-cooling). The result far too often is a well designed envelope with summer humidity and air quality issues.

Right sizing generally doesn’t save money; a larger unit on the same duct system will actually cost only slightly more than a right-sized unit, and the cost of hiring the engineer to calculate the load and CFM per room for a tight, well-insulated and shaded, passive solar home may wipe out that savings. But on any given day, the right-sized equipment will run longer and thus dehumidify much more efficiently.

It’s tough to find a small, efficient, ducted air conditioner

Unfortunately, it can be difficult to find affordable, very small ducted central air conditioners (less than 2 tons cooling) with high energy-efficiency ratings, and it’s really a struggle to convince building inspectors to allow us to use simulated performance offsets to permit low-performing equipment in homes with a really great building envelope. In air conditioning we compare equipment by EER and SEER ratings. The code minimum where I live is 13 SEER (14 for Energy Star), but I can’t buy a ducted AC smaller than 2 tons that meets that standard.

In the past, we’ve responded by installing variable-speed and zoned-bypass HVAC solutions with longer, more complex duct systems to allow us to combine zones to use larger 2-3 ton equipment with higher SEER ratings. But even very well-designed complex duct systems are much more prone to installer error than simple duct systems with multiple air handlers. So we end up using much more expensive ductless minisplits hung on the walls (which incidentally have overly complex wireless thermostats that our clients have to hide away from kids and guests).

The problem with EER and SEER ratings

The larger issue in my mind is the fact that code-minimum SEER ratings are calculated to prioritize sensible cooling and disregard latent cooling (dehumidification).

The Energy Efficiency Ratios (EER) of a cooling system (AC or heat pump) is calculated by comparing the sensible cooling ability with the electricity consumed on a BTU per Watt-hour basis. One BTU is the amount of energy (252 heat calories) needed to change the temperature of a pound of water one degree Fahrenheit.

One ton of cooling is defined as 12,000 BTU/h. (Don’t ask me for metric equivalents, please.) If a cooling system supplies 12,000 BTU/h of cooling capacity while consuming 1,000 watts of power, it is rated as a 12 EER system (12,000 / 1,000 = 12 EER.)

SEER ratings are set by NAECA, a Federal Regulatory Agency independent of the code authorities, and are intended to represent cooling efficiency over the course of a cooling season. These measurements only take sensible cooling into consideration. Minimum HVAC efficiencies are set by the code by SEER ratings, not EER ratings. It takes energy — 1,000 BTU per pint — to remove moisture from the air. But if that energy doesn’t contribute to changing the temperature of the air, it is excluded from the SEER ratings.

As Alex Wilson mentioned in his earlier piece, the industry has responded by providing equipment that cools air with less dehumidification. That way the units get higher SEER ratings — a way to be competitive in the marketplace.

A one-ton unit (12,000 BTU/h sensible) which is dehumidifying at 3 pints/hr is using 3,300 BTU/h latent for dehumidification and is removing 15,300 BTUs in combined sensible and latent heat. If it is consuming 1,000 watts, then it’s actually removing 15,300 BTUs combined sensible and latent for an effective EER of 15.3. But in this example the rated SEER is still 12 and fails to meet the minimum code standard.

So if the house I’m building is well enough insulated it may be illegal to install the right sized equipment as the smaller equipment isn’t available in the necessary efficiency unless you use variable speed to reduce the capacity of oversized ducted equipment or use minisplit units, both of which are much more expensive solutions than smaller ducted equipment that would actually be sized correctly. My clients often go to great lengths to avoid using the AC by opening windows or setting the thermostat at a very warm setting, so we are forced to install very expensive equipment that may be used for three or four weeks per year, despite the fact that these homes are HERS 50 already without the fancy equipment.

Technical solutions to boost latent capacity

There are some manufacturers who have built equipment that can dry air without cooling it very much. One way to do this is to use a “run around coil” where a heat exchange coil in the return air line upstream of the air handler is connected to a coil in the supply manifold downstream (and typically above) the air handler. The cooled air leaving the unit cools the fluid in the exit “re-heat” coil which drops to the intake “pre-cool” coil and pre-cools the air entering the unit. This results in warmer air returning to the house and a longer run time, with resultant increased dehumidification.

A more efficient way to do this, called “sub-cool reheat,” is to divert the warm liquid refrigerant from the outside compressor and pass it through the re-heat coil on its way to the air handler. This warms the exit air but also pre-cools the refrigerant and improves the performance of the air conditioner. Either of these add air resistance at the second coil and increase energy usage at the blower, resulting in a lower EER as well as a MUCH lower SEER.

Yet another way is to install a valve that can divert some of the expanded gas to a compressor that actively heats the re-heat coil in the same way a conventional dehumidifier would and sends some of it to an outdoor compressor where it is cooled in the fashion of a conventional heat-pump. This is how the Daikin minisplits work.

Adding run-around coils or the valves and controls to manage sub-cool reheat is expensive and, since it’s not reflected in the SEER rating, is pretty much reserved for larger capacity high-end equipment. If the SEER standard could take the combined latent and sensible BTUs into account, manufacturers would be more motivated to produce equipment that would offer better dehumidification at all sizes.

Smart thermostats can be set to lower the setting if they sense a high humidity condition in the house. If the owners set it to 80 degrees and the humidity rises above 60%, the thermostat can reset itself to 75 degrees until the humidity is reduced. But our customers are so proud of their energy-efficient homes that they gleefully report that the “turned their AC off almost all summer” and added significant risk of mildew and indoor air problems.

Small is more important than efficient when the envelope is done right

Designers of small, super-insulated buildings in mixed-humid and hot-humid climates may choose to prioritize right-sized equipment over high SEER equipment, largely due to the fact that advanced dehumidification is just not marketable in the smaller tonnage sizes that are appropriate for the small demand buildings. The minisplit systems that have these capabilities in small capacity sizes are very expensive and may be seen as unsightly and overly complex by many clients. (Our clients think they are ugly and hate the multi-function wireless remote thermostats).

On our very small (840-1,100 sq. ft.) buildings with radiant floor heating, we’ve been using a 10.7 EER GE ducted vertical Packaged Terminal Heat Pump (PTHP) unit (Zone-Line AZ75H12DAC for $1,454 delivered — see photo), which gives 11,700 BTU/h cooling and 3.6 pts/hr (3,960 BTU/h) dehumidification for 1,093 watts and can supplement the radiant floor heat if necessary with a 3.3 COP. The unit can be easily installed in a 24″x24″x32″ space. It’s a great, if somewhat noisy, dehumidification solution for affordable, small, very well insulated homes in a mixed-humid zone.

With the code minimum now at 13 SEER and Energy Star at 14, it’s very hard to get equipment sized small enough for the load without serious energy modeling and offset negotiations with the inspectors. If energy efficiency ratings could give credit for the dehumidification capacity as well as the sensible cooling capacity, it would give incentives for manufacturers to offer equipment in all sizes and price ranges with better dehumidification. As I understand it, Amana’s ActivDry PTAC is currently withdrawn from the market largely due to this issue.

The affordable gut remodel we are building now hits a projected HERS 59 by REM/Rate (8.5% better than code by REScheck) with 8,330 BTU/h sensible and 1,204 latent. We can choose to forgo air conditioning all together, but if we want a 2-ton, 14 SEER unit to meet Energy Star, we’ll be oversized by a factor of two at significant cost and even with variable speed will be likely to run into humidity issues, regardless of how little or how much the owners actually use it.

Code minimums are ever increasing, and 16 SEER seems not far away, perhaps a new class of dehumidifiers or through-wall mini AC units recently proposed by James Morgan will lead the way. It would be fun for the Americans to be leading the pack again.


  1. user_963668 | | #1

    16 SEER 2/1.5 TON
    There is another option. A 16 SEER 2Ton/1.5TON 2 stage condenser by Goodman This is what I installed in our small home.

    It will run on 1.5ton most the time and then on 2Ton if needed on those 100+ days.

    Ducted is the only way for a small house 1300 sqft 2 floor with a non open floor plan. Other wise would need 2 or 4 ductless minisplit indoor units. Now that gets expensive.

    I sized mine based on past years of running a 15000 BTU window mounted. Kept it below 80 with windows shut on a 100 degree day but 1.5 ton would be a little nicer.

    Ideally a variable speed condenser would solve small home needs. Nordyne has some under the Maytag brand. This would be perfect for everyone, saves cost for homeowners and installers can oversize and not worry like they do now.

  2. 5C8rvfuWev | | #2

    Another ignorant question
    Michael, you don't say, but in a small home wouldn't it all be simpler to use a stand alone dehumidifier? I know it uses power ... as do the solutions you mention ... and like them it costs money but perhaps not so much?

    Or is this just a bad idea for some reason I haven't considered?



    Stand alone de-humidifier
    I've tried that Joe, the problem with stand alone de-humidifiers, and the ducted April-Aire versions, is that the remove the heat by running the air over a cooling coil but they cool that coil by running the exhaust air over a warming coil so they add a very significant amount of heat to the air as they dry it.

    I think it was James Morgan who had a great idea of putting a massive block of insulation through the middle of a dehumidifier so it could be built into an exterior wall with the heating coil outside the envelope and the cooling cooling/drying coil inside the envelope. it would add an extra fan but could be built into the thermal envelope.

  4. homedesign | | #4

    It Costs money to Dehumidify........
    Adding a stand alone dehumidifier is often recommended by Joe Lstiburek and Armin Rudd.
    True that it adds heat and may cause the air conditioner to "kick-on".
    If the air conditoner does start up it will also remove more moisture.
    Lstiburek says "you don't get somethin for nothin"

    I used my stand-alone a few times recently when outside AND inside conditions were mild and extra humid.

    It was better (more comfortable) than lowering the thermostat to reduce the interior humidity.

  5. user-659915 | | #5

    De heat, de humidity.
    Thanks for the credit Michael but I'll have to disclaim authorship of that particular idea. I did suggest something similar, which was improving both the thermal and acoustic qualities of a package unit through-wall heat pump by splitting the unit to install back to back on either side of a well-insulated wall. I agree we definitely need some better solutions for small well-insulated buildings. And I hope to see US ducted system manufacturers matching the low-temperature performance of the best of the minisplits before too long. 18°F ambient without heat strips: that would be a game-changer. The Prius of HVAC systems.

  6. user-600754 | | #6

    I have spoken to a rep


    I have spoken to a rep from a large Japanese mini split manufacturer who claims that they will be coming out with a low temp ducted 1 ton mini split by the end of the year. My fingers are crossed!

  7. user-659915 | | #7

    Nick -
    Sounds good!

  8. bbSGYnAJUU | | #8

    Pre-existing Homes in Humid Houston
    So I am the poster child for this frustration. My 2500 sq ft home was built in 1982 and sized for a 5-ton, appropriate for insulation levels and typical leakage in that era. Now after a few years of my attentiveness to leaks, more insulation, double-paned windows, some duct replacement, conditioning (insulating the rafter bays) of the air handler's attic area, adding more air returns in hotter locations, and installing a thermostat in my single-zone home that has a 'circulate' feature to better equalize the temps upstairs and down without a call for cooling, my system is now oversized for cooling (9+ months out of the year our demand is for cooling). I could lower the thermostat do dehumidify, but that defeats the energy savings. I could cut into my walls and semi-permanently install a dehumidifier, but that would look uglier than a Passive House. I could spend $7000 to replace my 5 ton system with a 3 ton system, and reap the savings after a gazillion years. What's the practical solution, other than to move to a less humid climate?

  9. c3hdQkcong | | #9

    Great Article!
    Thanks for writing this article, Michael. As a fellow North Carolinian with a green house (and a previously oversized geothermal unit) I'm semi-obsessed with home indoor humidity. I was not aware that it was hard to find right-sized equipment that would meet code. I'd love to find a way to explore this topic on if I could talk to a homeowner -- since our content is all written from the homeowner's perspective. If you can ever hook me up with a homeowner who faced this problem and succeeded (or failed) at resolving it, I'd love to talk with them.

  10. servant74 | | #10

    Houston Home
    I had a home in Houston for about 20 years. It was the early 1980's two story,one zone. Yes, it had various 'hot spot' issues internally.

    My wife wanted vinyl siding (not my favorite, but wifes opinions carry lots of weight on such things :) that added some insulation to the house. When we replaced the roof, we put on as light colored roof that our subdivision 'building committee' allowed.

    The house already had 'double pane' windows, but we did put in attic blanket insulation, and when the 5 ton unit went out, we had our HVAC people put in two units. 2T downstairs and 3T for the upstairs. We also had ceiling fans throughout that we used year round.

    Our power bills were about half of most of our neighbors but it wasn't without a lot of effort over time.

    Dehumidification would have allowed us to keep the house at higher temps in the summer (Houston heat season is 2-3 months, and AC is 9 months or so. Only a couple of weeks of 'open window' weather for us (partly due to allergies). Also power (5+ years ago) was about $0.12/KWH so the price of cooling was the major component of the power use.

    As we are 'downsizing' and moved from the area, eventually we hope to build a energy efficient home. (We moved for my wifes job, and the employer provides house and utilities ... I would hate to pay the bills here ... old house, leaky, electric portable heaters, central air is not sized well ... bummer)

  11. user-622614 | | #11

    Mini splits
    Some of the newer mini split heat pumps from Japan come in small sizes and have SEER ratings as high as 26 and can heat at 100% capacity down to 5F outside air temperature (at least according to their literature). It is difficult to beat this performance with other technology right now, although if you have a series of tiny rooms the non-ducted feature may be a problem.

  12. loracekim | | #12

    Water heater Heat pump
    What about using the newer Water heater heat pump. It could heat the water, help cool the home and dehumidify the air. Although it would not be able to run on demand fro cool air, it would however run more if hot water is being used which adds to the interior humidity levels. Just a thought!

  13. user-934694 | | #13

    SEER does include latent heat
    SEER and EER are both based on the total cooling capacity of the system, not just the sensible capacity. The systems are rated under indoor conditions where latent capacity is significant, typically, 20 to 25% of total. Most manufacturers will provide latent capacity information for your equipment so that you can size appropriately for your climate. Some of the higher efficiency equipment use higher indoor airflow rates which result in less latent capacity. You can usually set the indoor air flow rate to a lower value during installation if you need more latent capacity. Some high-end humidity-sensing thermostats can do this automatically when it is needed.

  14. oldhouseguy | | #14

    Zone-Line AZ75H12DAC
    Product NO longer being manufacturered Great Idea but a non starter ... why buy something which may be irrepairable in the future?

  15. pu98otc3SH | | #15

    Stand a lone dehumidifier
    I use a stand a lone dehumidifier during the tween seasons (fall and spring). Sure it puts heat into the structure, makes noise and is a bit of an eyesore, but its effective.

    Another solution would be to use an independent ducted system that draws air from high humidity zones (bathroom, kitchen, laundry), dehumidifies it, filters it and returns the air to the living area. This system would run independently of the HVAC system. Sure it costs money to do this, but nothing you want is ever free.

    Enjoyed the article. Good luck.

  16. Isaac_Savage | | #16

    Manual-J - The Proverbial Lightbulb
    I think that this is a great discussion. In our area (southeast), we've been providing Manual-J training classes to folks in the industry, including HVAC contractors. It's always interesting to see how surprised the students are when they print that final report - "Wow, that's a small load!" Many HVAC professionals don't realize how important it is to properly select heat pump equipment - based on the cooling loads. The Manual-J exercise is worthless if proper action isn't taken based on the results. I'm glad to see there's a growing need for smaller, more efficient equipment. It means the industry is doing something right!

  17. gypsydawg | | #17

    Turns out 1 ton is a equiv.
    Turns out 1 ton is a equiv. to 3.5kw.; so a 1 ton airconditioner using 1kw has a metric EER of 3.5/1.0 = 3.5. It strikes me that an EER of 3.5 is far more than believable than this efficiency ratio of 12 :-)

  18. 8p7FqB4Bqb | | #18

    Split System Advantages
    I think you all need to review split system costs again. Not only are they much less expensive to install in existing buildings but allow for much more flexibility in moisture and cooling control in old or new ones. Twin or Tri-zone splits can manage a 'normal' sized, house more efficiently and with better upfront costs. The units are individually controlled so you can go from the equivalent 1/2 ton to 2 or more tons very easily, which can be programmed in for anyone who can use and cell phone. Mount the controllers on the wall at a thermostats normal level and you'll keep little hands away from them.

    HVAC contractors have been playing down these units for years in the USA. Most energy 'expensive' countries push the split systems in all but the larger commercial applications and extremely large houses. The elimination of sheet metal work and long duct runs makes Splits the way to go, unless you've got money in an HVAC company.

  19. MICHAEL CHANDLER | | #19

    COP vrs EER
    Andrew - I think you must be confusing EER with COP. If a heat pump delivers one ton, or 1,200 BTUh, equal to 3.412 kw of heating or cooling while consuming 1 kw of electricity it is performing at a coefficient of performance (COP) of 3.412 in that it delivers 3.412 kw of conditioning for every 1 kw of electricity used. It is still rated as a 12 EER system

    As I noted the 10.7 EER unit we use on very small accessory buildings delivers 3.3 COP It doesn't have a published SEER rating as that is a different calculation controlled by the federal government and is not usually provided for non-residential equipment such as the hotel units like this. While it may take latent heat into consideration it is not interchangeable with EER

    Douglas, While the specific model number I quoted may no longer be in production has the roughly equivalent GE Zone-Line Vertical PTHP AZ75H12EAC for sale at $1,055.56 plus shipping as of today. Once you add the cord set, wall sleeve, grill t-stat, etc the material cost is probably in the $1,600 range for one ton of cooling/dehumidification at 10.7 EER with backup heating capability at 3.3 COP. Installation involves setting it in a closet w/ a hole in the wall to the outside and connecting the t-stat, drip tube, and minimal 10" duct-work and plugging it into a 15 amp 220 volt wall outlet. Not very much harder than installing a window air conditioner.

    Sure this is nowhere near as efficient as a $4,000 mini-split or a $7,000 1.5 - 2 ton variable speed ducted heat pump with aux de-humidifier but really makes sense to me for smaller, well insulated, solar-radiant heated homes with very little cooling in comparison to de-humidification requirements.

  20. MICHAEL CHANDLER | | #20

    Playing games with mini-split prices?
    Joel I remember when spray foam first hit our market, the same job bid to three companies returned prices of $6.500, $7,500 and $13,500 We saw similar price premiums when JM Spider first hit. I think you may have a point that the guys who are tooled up for sheet metal may not see much advantage in bidding mini-splits competitively. Perhaps there is room in our market for someone who sells mini splits exclusively at a price that doesn't include overhead for the sheet metal shop...

  21. homedesign | | #21

    PTAC vs DIY minisplit
    First....I believe that a SINGLE ductless mini-split would not work very well in a house with more than One Bedroom.... there would need to be multiple heads or otherwise some method to distribute conditioned air to bedrooms with doors closed.
    If I had to choose a "single source" for a small "cabin" or One Bedroom Home... I would prefer a mini-split to the PTAC that you are recommending.
    Just a little browsing shows that DIY minisplit "kits" with precharged lines are not-so-expensive.
    Have you considered the kits?

  22. gypsydawg | | #22

    Michael, I see why you didn't
    Michael, I see why you didn't want to go near metric ;-)
    From 1-ton is 12000btu/h or about 3.5kw (1000btu/h is about 3.4kw). In metric, the only difference (as best I know) between EER and COP is the context in which it is used; for instance
    While you say "COP", I'll say [metric] "EER".

  23. MICHAEL CHANDLER | | #23

    COP vrs EER
    Andrew, in Australia COP and EER are interchangeable, in America they are not. In America EER and SEER are also not interchangeable. I assume there are different units of measure in Asia and Europe as well. Is this logical? I think not.

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