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 9, 2012 at 8:50pm

Agreed. I am sure that Rheem will come around at some point. With DOE changes regarding minimum efficiency requirements for electric water heaters in 2015 they will be forced to have decent hybrid electric water heating equipment in place for storage tanks with capacity of 50 gallon or more.

I like the GE GeoSpring for large scale retrofits. They are easy to install and can installed without having to redo the plumbing connections. I like the AirGenerate AirTap Hybrid water heaters for advanced energy efficiency applications (net zero homes, Passivhaus, etc).

Not sure about whether this is a single or double wall heat exchanger....inside or outside the tank. Plumbing code varies all over the place. I like the food grade refrigerant solution (referred to as "salad sauce") used in Rheem solar water heating system. It is non-toxic and poses no risk.

Comment by Curt Kinder on September 9, 2012 at 6:54pm

I simply don't agree with Rheem's tradeoffs, so I never installed one. That there are other issues you cite ices the cake on my decision.

Rheem has some homework to do to catch up with the rest of the field.

Comment by Roch Naleway on September 9, 2012 at 6:41pm

The Rheem HP50 model had a lot of problems. I had my hands on the models that came out in 2008/2009. Lots of issues regarding delivery of hot water, noise, vibration, etc. I am waiting for a better Rheem model to come out.

Comment by Curt Kinder on September 9, 2012 at 5:58pm

Yes - I'm aware of the tube wrapped tank design of the Geospring models. State uses that as well. It raises EF by maximizing metal to metal contact area and avoiding parasitic loss of a pump as well as the cost of a heat exchanger external to the tank.

OTOH Rheem uses a water pump and separate heat exchanger, picking up about a 5% improvement in first hour rating in exchange for a 15-20% loss of EF as well as additional complexity.

I'm quite curious to learn the nature of the AirGenerate heat exchanger. I hope they respond to my inquiry, and I will post what I learn.

Comment by Roch Naleway on September 9, 2012 at 5:37pm

The Geospring units does not have the copper coil submerged in the water. It is wrapped around the actual tank and can never touch the water. Not sure about the AirGenerate AirTap hybrid water heaters. It may or may not be directly in the tank.

With solar thermal water heaters I have seen food grade refrigerant getting used to keep the toxicity out of the system.

Comment by Curt Kinder on September 9, 2012 at 5:33pm

I browsed their site and could not confirm the nature of their heat exchanger. I attempted to inquire via their contact form only to have it insist that my website is not a "valid URL"...complete nonsense, of course.

That they incorporate a reversing valve, allowing operation in sub freezing ambient air as well as provision for ducting 400 CFM out of the unit, yet still maintain an EF of 2.40, suggests they may have succumbed to the siren song of higher heat transfer and better recovery available via single wall heat exchange.

I've written them a conventional email...we'll see

Comment by Roch Naleway on September 9, 2012 at 2:46pm

They have a water heater conversion model - the A7. It gets added to an existing electric water heater. This is the one you may refer to. I would stay away from the retrofit.

The AirGenerate hybrid water heater I am referring to is an all integrated water heater. You can google AirGenerate ATI50 or AirGenerate ATI66 and you will find the new model without the issues.

Comment by Curt Kinder on September 9, 2012 at 1:55pm

If memory some early AirTap products had refrigerant tubes in direct contact with potable water. That seems a bad idea, and, I believe, violtes some plumbing codes requiring double wall and vented heat exchangers between refrigerant and potable water.

Do new AirGen models address that issue?

Comment by Roch Naleway on September 9, 2012 at 1:41pm

The HPWH we use in Pacific NW climates is made by AirGenerate. It works down to 20 degrees ambient temperature. It runs at 48 decibels...a lot less noise than anything on the market. The AirGenerate AirTap Hybrid water heater works well in households with up to 4 people in the Pacific Northwest. The two tank system is intended to be a solution for large households coupled with colder temperatures. I don't think that you need it in Florida. I would go for a larger tank like the 80 gallon heat pump water heater you are suggesting. I am not sure that this would work closer to the Canadian border. :-)

The GE unit cuts over to electric resistant at 40 or 45 degrees ambient temperature. It does work well in Southern climates where it never gets that cold. The GE product may lose 2-3 months of operating time in Northern climates if installed in garages. The jury is still out. We will see. I like the way the GeoSpring unit is installed. There is no need to rework the plumbing connections.

Comment by Curt Kinder on September 9, 2012 at 10:37am

I'm a huge fan of HPWH, particularly in my AO (Florida) They are an absolute no brainer in the typical Florida garage.

That said, I wonder if they are less of a no-brainer up north. Does the heat they need add to heating costs?

Would you be better off simply specifying a larger HPWH? I particularly like the GE, but it only comes in a 50 gallon model. Smiths, marketed under about 10 names, come in 60 and 80 gallon models. I tried one, but the unit is very noisy

OTOH, that extra noise comes from higher air flow, which may help the unit in cold climate applications.

GE is supposed to come out with an 80 in January - looking forward to it.

We use two tank systems with heat recovery desuperheaters working off heat pumps, air source or geothermal.


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