I'm in sunny humid Florida and have had great luck with air source heat pump water heaters, particularly the GE models.

80+% of water heaters here are in garages, so HPWH work well.

Now I'm faced with a client with a completely detached garage, and their water heater is in a big master walk in closet. I don't dare go with a GE HPWH owing to concern that the Master WIC would get mighty chilly immediately after bathing, particularly during cooler weather.

I have read that the Air Generate units can have their exhaust ducted, which would allow a WIC install.

Does anyone have any firsthand or solid hearsay experience with AirGenerate?

Do they work well?
Any early failures



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We are running the Heat Pump Water Heater study in the Pacific NW (Smart Water Heat program) and have experience with both non-ducted and ducted HPWHs.  We typically refer to them as Tier 1 and Tier 2 units (Tier 2 being the ducted model).


To date we have installed 30 AirGenerate ducted units.  By and large, the homeowners have been very positive about the new unit.  So far, homeowners comment that they are not having issues with running out of hot water (likely due to the fact that this unit is 66 gallons and not 40 or 50 that they may have had before).  We have also had positive reactions to the design and functionality of the units and early feedback of reduced bills.  We will have more information on the billing data later in the program.


There have been a few negatives that we have learned along the way.


  • Plumbing connections are on the side, which differs from most water heaters
  • AirGenerate is very heavy (dry weight is 200lbs) and can take 2 people to move
  • Height of unit is around 70”, which makes it hard to transport on some existing trucks
  • Height of unit also comes into play when contractor is onsite during pre-site visit and the space might not allow for the installation.  This could certainly be an issue in a closet install. 
  • Operating noise level is about 48 decibels, which is about as loud as a high efficiency dishwasher
    • Homeowners aren’t used to water heaters generating noise and could be sensitive to noise in general
  • Installation of earthquake strapping requires the contractor to remove a panel in the front
  • 2 out of 30 homeowners from the field study reported feeling vibration after installation that required contractor follow-up
    • Our Smart Water Heat Best Practices recommends that contractors install dampening mounts to mitigate callbacks
  • Ducted units:
    • Certain types of ducting amplify the sound of the exhaust.  Installing duct insulation can minimize this.
    • Condensation can occur on the outside of non-insulated duct
    • Depressurization is a concern and caused smoke from the wood stove to be pulled back into the home in 1 of the 30 homes from the field study (the unit exhausts 400 - 600 cfm typically.  We recommend a Constant Air Regulator to reduce exhaust flow to ~280 cfm and have not seen a reduction in efficiency)
    • CO detectors should be installed in all installations with ducted units when combustion appliances are in the home.
    • Make up air may need to be provided depending upon the tightness of the home.  For a closet install, a louvered door is almost a must.

We also recommend providing contractors and homeowners with best practice/information guides.  You can find them here:



I hope this is all helpful!


Dan Wildenhaus

Sr Building Scientist


503.808.9003 | fluidms.com

We change the way people use energy


Have you measured the drop in efficiency/capacity caused by reducing the airflow? 

Why are you recommending exhausting the discharge outside of the house? 



One of our field studies is currently evaluating the performance with the CAR in only.  Another project will be running them for several months without the CAR and then several months with for just this reason.


Since we are in a heating dominated climate, the energy and comfort penalty associated with addressing the cold exhaust makes venting to the exterior almost mandatory,


 Our recommendations are for the "Northern Climate".



Great Info - thanks so much!

48 db is less than the 55db level specified for GE, and those are quiet enough for a garage or utility room but not anywhere near a bedroom. Are there any AirGenerate installs fairly near noise-sensitive rooms (living / family / sleeping?)

Did either homeowner reporting vibration have the HPWH on a concrete slab?

Have you measured delta-T on the airside? I would expect that to rise when you regulate the airflow from 500 down to 270, along with a slight drop in COP, although throttling airflow probably causes blower to unload by a tenth of an amp or so, maybe more.

500 CFM strikes me as very high, given that the GE and Geyser operate at 100 CFM or so (guesstimate). AO Smith seem to operate around 200-300 CFM (another guesstimate)

Given that the AirGenerate moves quite a bit more air AND at much higher static pressure (as required for ductability) I infer that AirGenerate blower power must be substantially higher than the GE and may well represent a significant fraction of total system power. That's an interesting tradeoff. Do you have compressor and blower wattage data?

I'm intrigued by the possibilities of ducting HPWH, since it opens up a wider range of install locations AND has the additional benefit of providing useful cooling and dehu in my hot humid climate.

However, I'm having trouble accepting the benefit of ducting HPWH exhaust outdoors, considering the house as a system in ANY climate. Aside from the obvious issues of depressurization and backdrafting, the thermodynamics don't work for me:

Setting humidity / enthalpy aside to keep this simple...suppose (pending data) the AirGenerate airside delta T is 15*F dry bulb. For discussion's sake, the unit is installed within conditioned space in a home with a heating set point of 70. If the unit takes in 270, 400 or 600 CFM of 70*F air, cools it to 55*F and blows it outside, that exahust air flow will be made up, whether actively or passively, from outdoors. That seems to be a losing proposition for the homeowner anytime outside air temperature is below 55*F, which is pretty much all of the time during heating season in northern climates.

While I know that heating degree days are figured on average outdoor temp of 65*F, I strongly suspect that a combination of tighter homes, internal gains, solar gains and thrift results in very few folks reaching for the thermostat until outdoor temps are at 55 or less.

In other words, exhausting air at 55*F and replacing it with outdoor air at 54 or less is a net loser, getting progressively worse as heating season deepens. At the point in the season where water falls from the sky pre-frozen and then persists in that state on the ground (that anyone tolerates that for more than a few days is truly bizarre to Floridians, but I digress...), humidity becomes a concern, since subfreezing outdoor air is uncomfortably dry when heated to 70.

While there has been quite a bit of concern and criticism of HPWH in northern climates owing to the heating season heating penalty, dumping the exhaust outdoors would seem to exacerbate the penalty. With AirGenerate's high air flows, I suspect the airside delta-T is closer to 10*F, worsening matters.

If one could supply the unit with outside air only, that would deal with the heating penalty and depressurization concerns, but COP and recovery would plummet during cold weather, and frosting / icing / sweating of ductwork would have to be very carefully managed.

Perhaps an option would be to equip HPWH with an outdoor temperature sensor and have a setup menu option to switch the unit to resistance heating at a selected outdoor temperature, and Best Practices would recommend a range of settings for that switchover based on individual circumstances. (install room volume, type and operating cost of space heat)

I plan to dump exhaust into home's return air ductwork, spreading any undesirable cooling effect so thinly as to escape concern and enjoy bonus cooling and dehu 8+ months per year.

FWIW, when my fireplace is inactive, operating either the range hood (~200 CFM) or the clothes dryer pulls the odor of woodsmoke into home. Our house leaks 1600CFM50 or about 2.5ACH50.

Please keep the data coming!


Good to hear you're evaluating the reduced airflow form the CAR's. I'm interested to see the results. 

Regarding the discharge airflow - comfort issues are relatively easy to mitigate with location, placement and type of supply grille.

Regarding energy penalty - as Curt says above, it depends on the difference between the discharge air temperature and the outdoor temperature. I don't know what the average discharge temperature for the AirTap units is for a typical water heating cycle, but I suspect it's somewhere around 50-55F. If the average outside air temperature during a water heating cycle is lower than the average discharge temperature, then you're creating an energy penalty by exhausting the discharge air to the outside.    


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