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 Roch Naleway on September 20, 2012 at 10:12am

This is good information. I was aware of UL listing requirements for electrically powered consumer products in the US. Without UL listing manufacturers run a huge risk of getting into lawsuits.

I was not aware the ENERGY STAR requires UL certification and double wall heat exchangers for their ENERGY STAR heat pump water heater category.

Thank you for sharing.

Comment by Curt Kinder on September 20, 2012 at 5:04am

UPDATE:

I finally spoke with someone at AirGenerate regarding the nature of that brand's refrigerant heat exchanger. I believe the representative's name is Dong Do. I had some difficulty understanding him, but he assured me that AirGenerate HPWH are UL listed, and for UL listing the refrigerant heat exchanger must be double walled.

I have since educated myself - AirGenerate HPWH are Energy Star listed. One of the criteria for Energy Star is UL Listing, specifically UL 1995. I looked online and UL 1995 includes a provision that refrigerant to water heat exchangers be double wall and vented or otherwise redundantly protected.

Having raised the issue, I wrote this update to advise that my concern is resolved.

Comment by Curt Kinder on September 17, 2012 at 4:32am

Personally, If the HPWH is in the conditioned envelope, and ductwork is present I'd prefer to dump the air into the return system. Dumping it outside could over-ventilate the home and drive up costs and problems associated with that.

The strategy would vary widely across climate and situation specifics, but in general I agree that the ductability of the discharge is a plus.

Mum's the word regarding the heat exchanger. In addition, I haven't figured out how they sell - just one vendor, GP Conservation, appears in a Google shopping search.

Maybe I'll call them later today.

Comment by Roch Naleway on September 16, 2012 at 8:22pm

Ok. In other words: Keep the air moving and (if concerned) go ahead duct the exhaust to the exterior of the home. This is the nice feature of the AirTap heat pump water heater. It allows for ducts to be attached.

Has the factory ever gotten back to you regarding the details of the heat pump used in the AirGenerate water heaters?

Comment by Curt Kinder on September 15, 2012 at 8:08pm

Humidity in the room isn't the issue, rather the risk lies in that room's drywall - if the HPWH cooled a small room to, say, 60*F, warm moist air in surrounding rooms could drop water onto the backside of the drywall in the HPWH room - that's never good.

Comment by Roch Naleway on September 15, 2012 at 7:35pm

The mold risk would only be there if there was quite a bit of cooling and excessive moisture in the room, right? In general it should be pretty minimal.

Comment by Curt Kinder on September 15, 2012 at 8:06am

If the ambient humidity is high enough that the HPWH's evaporator coil is cooler than ambient dewpoint, the evaporator will condense water from the passing airflow. That's why HPWH installation includes condensate drain line.

Basically an HPWH acts like a small window air conditioner whenever it runs, except that rather than transferring heat outdoors, it puts the heat into the tank of water

Comment by Roch Naleway on September 14, 2012 at 8:33pm

The heat pump water heater also acts as dehumidifier though....I have to think about it overnight....

Comment by Curt Kinder on September 14, 2012 at 7:17pm

10x10 is probably conservatively large, but that's the nature of abundant caution in our litigious land.

Advanced Energy in North Carolina authored an extensive study of a GE HPWH deliberately installed in a small 225 CuFt room to guage the effect of such an install. The study found that the HPWH gives up only about 10% energy factor in such a situation, and noted in passing the room temperature depression that follows from such an installation...down to 55*F.

While it is laudable that an HPWH tucked into an out-of-the-way laundry or utility room suffers only minor loss of efficiency, it seemed to me they overlooked the mold risk of cooling a room to well below ambient dew point for hours at a time.

Comment by Roch Naleway on September 14, 2012 at 10:17am

Most HPWHs if not all have a requirement of a 10x10' room in order to have sufficient air supply (love that band, haha). The cold air needs to go somewhere. If it staid in the small room it will reduce the performance as the temperature drops. At minimum a louvered door or a few pressure balancing ducts to allow for some ventilation into the remaining conditioned space (if that was desired for the specific climate). 

I think that it is great that John's system for 7 people is working out. It's like driving one of these original VW beetle with 5 passengers over a Mountain pass. If it works out that's awesome. It takes really courage.

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