Borrowing from solar thermal system - Why not use two tank systems with heat pump water heaters?

We are starting to roll out heat pump water heaters in the Pacific Northwest (Oregon, Washington, Idaho, Montana, and Wyoming).  One of the concerns we have in colder climates is the recovery time when the ambient air is cold. The lower the ambient temperature the less effective the heat pump water heater will be in generating hot water causing issues in households with more people, more bathrooms, and in colder months if the heat pump water heater is installed in garages or basements.

Thinking about this specific issues and discussing it with home performance contractors we have decided to explore the possibility to employ a two tank heat pump water heater system:

The first water heater in the two tank system acts as a pre-heater (this would be a solar thermal heater or in our application a heat pump water heater).

The second water heater collects the water from the first tank and boosts it if necessary to the 120-135 degrees Fahrenheit. Most of the time the booster tank is a standard electric water heater. The two tank system adds a back-up, a booster, and more thermal capacity opening up hybrid electric water heaters to larger households.

We have not seen any issues so far with this solution. Any input, thoughts, experiences that you have with two tank systems? Please share.

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Tags: AO, AirGenerate, GeoSpring, Rheem, Smith, Voltex, electric, heat, heater, heaters, More…hybrid, pump, water


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Comment by Curt Kinder on September 13, 2012 at 7:26pm

In my neck of the woods 400 or 200 CFM of cooled dried air would be welcome 8 months of the year and a minor nuisance during "winter" if properly ducted.

So if we had a ductable HPWH within the pressure and thermal envelope of the home but located in a small room such as a closet or laundry room, we could dump the exhaust air into the duct system and be better off for it.

If the HPWH is in the garage, we can't duct its exhaust into the HVAC system, but garages satisfy minimum room volume specs.

If I had an HPWH in a small room within the home not feasibly ductible into the HVAC I'd look at a thermostatically controlled fan to ventilate the room to keep its air temperature safely above drywall mold formation level, perhaps around 68-70 in my area.

Comment by Roch Naleway on September 13, 2012 at 6:59pm

The manufacturer is aware of the requirement.

I can't comment on this requirement...someone may read what I am thinking and that may not be good. The constant airflow specs are for 150-200CFM regulators.

Here is what the rebate form says:

DUCTED INSTALLATIONS: Must be installed in accordance with Smart Water Heat Installation Best Practices, including installing a constant airflow regulator and ensuring home has a functional CO detector installed near the primary sleeping area.

Link to the rebate form.

Comment by John C. Semmelhack on September 13, 2012 at 6:48pm

Are you kidding me? That's the worst thing I've heard all month. Cutting the airflow across the heat exchanger of a heat pump system by 50-60% will significantly reduce capacity and efficiency. I'm glad I'm not a rate-payer in that program. In addition, the reduced airflow may lead to excessively cold discharge temperatures. I'm not surprised there have been cases of condensation on the duct work. 

These systems are not designed to be exhaust appliances and should not be installed as such. If they are installed in conditioned space, the discharge air should stay in conditioned space. If they are installed in an unconditioned basement, the discharge air should stay in the unconditioned basement.

Can you post or send a link to more information about the rebate program and requirements? I know the manufacturer is aware of the rebate program since they promote on their site. Are they aware of the install requirements?

semmelhack (at)

Also, yes, there is a louvered door in the install I was discussing earlier.

Comment by Roch Naleway on September 13, 2012 at 6:20pm

Do you have a louvered door in the small room?

Comment by Roch Naleway on September 13, 2012 at 6:14pm

The Northwest requirements for qualified utility rebated installation is to reduce airflow to 150-200CFM for the unit. Contractors need to install a so call continuous flow regulator at the duct hook-up. There is concern of back drafting gas equipment if it is left with 400CFM. It's like 5 bathroom fans working at the same time. The unit is not sealed, so the make-up air must come from the imminent space the water heater is surrounded by.

The installation manual does not talk about length of duct work allowed or permitted. The duct must be insulated in certain instances. I have heard about condensation around the duct work in a basement installation with laundry equipment right next to it. They had to swap out to insulated duct.

All very interesting to say the least. I think it is good technology, but the contractor has to think before acting.

Comment by John C. Semmelhack on September 13, 2012 at 5:58pm

The ATI-50 and ATI-66 have a 6" diameter discharge collar on the top of the unit that can be ducted in the case of installations in small spaces. In the particular scenario I've been discussing, the discharge is ducted about 15' over to the kitchen, near the refrigerator. The closet the HPWH is installed in has louvered doors.

In my opinion, the 6" discharge is woefully undersized given that the discharge airflow is about 400cfm. It might give an installer/designer the impression that they should continue with 6" ducting. Doing so on a long run with multiple bends could significantly reduce the airflow across the heat exchange coil, reducing the overall efficiency of the unit. The manufacturer needs to provide better guidance in their install manual regarding ducting. Good duct design and installation practices should be followed: right-sized pipe, short runs, minimize fittings, use smooth fittings. For these units, I would recommend increasing the duct size to at least 10" (gradually). I'll try to get more information from the manufacturer regarding the fan that might help folks with duct design in these kinds of situations.   

Comment by Curt Kinder on September 13, 2012 at 5:39pm

What is done with the discharge air from the ATI-66, given that it is in a small volume room?

Comment by John C. Semmelhack on September 13, 2012 at 5:24pm

The ATI-66 for the seven-person household is in conditioned space (hallway closet between kitchen and living room), so the ambient temperature is pretty consistent. I'm sure HPWH's have a harder time keeping up with demand in colder months in "tempered" spaces (garages or unconditioned basements), especially if the incoming water temperature is also a little colder.

Comment by Curt Kinder on September 13, 2012 at 5:15pm

It shouldn't have taken too much investigation to determine why a 66 gallon water heater might run short when serving six people.

AirGen has not yet seen fit to answer my query concerning the nature of the refrigerant / water heat exchanger.

Comment by Roch Naleway on September 13, 2012 at 9:08am

John - very good. It is great when folks can get the job done with the smaller AirGenerate hybrid electric water heater.

I have seen a situation with a household of 4 grown ups + 2 occasional visitors (during weekends) that caused an AirGenerate ATI66 to run out of hot water. This was in January/February with cold air temperatures.The home performance contractor went back with a plumber twice. Finally the home owner said: get this darn thing out  of here. They unfortunately burned through 2 service calls without involving the factory or someone qualified.

Your recommendation regarding high performance or low flow water fixture is really good. I would have recommended the same thing in the situation above. The newer WaterSense fixtures by Delta work well without compromising comfort or performance. It beats the heck out of lower quality products. Unfortunately, the ended up being a train wreck with the contractor having to eat "labor charges".

I recommend an AirTap ATI50 for households with 2-3 persons, which is pretty much standard occupancy for most households. I would say that the ATI50 acts more like a standard 40 gallon electric hot water heater. The 66 gallon version (ATI66) is equivalent to a 50 gallon heat pump water heater. This is for colder climates that actually do get close to freezing temperatures in the fall and winter months.

Anywhere else I'd say that a any of the 50 gallon, 66 gallon, or 80 gallon heat pump water heaters should do their job quite well.

John - you must have done a really good job with the household of seven. This is great. Thank you for sharing.

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