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.

Views: 1205

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

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Comment by John C. Semmelhack on September 13, 2012 at 8:23am

It seems like a better solution would be a larger tank for the HPWH in these situations, such as the Steibel Eltron or the AirTap ATI-66. Certainly, both of these are more expensive than the 50-gallon models from GE, Rheem etc., but they also have improved features (such as better efficiency, stainless steel tank, less noise, more control options and longer warranty on the ATI-66) over the "commodity" models.

In addition, demand-side management (low flow fixtures, water efficient clothes washers and dishwashers) will  help in all cases. A recent client with a household of seven people (three adults, five kids) installed an ATI-50 in his new house. Once he installed low flow showerheads, the ATI-50 has been completely satisfactory to him and his family. And they run it in heat-pump mode only (electric resistance heater disabled).    

Comment by Curt Kinder on September 13, 2012 at 4:24am

My Nyle is an early model, so it had a few serial # 1 issues which I'm told have been improved. It is noisy and looks something of a Frankensystem owing to external plumbing. It seems to operate at a COP just above 2.0, but It doesn't get "easy" chilly incoming water as it would in a conventional system, so no worries there. In summer months when it only has to overcome standby losses, it costs about $3 per month. In spring and fall that rises to $10-12.

I think the sweet spot for Nyle is folks with a Marathon who would like to stop firing the resistance elements

I'm old school - Excel spreadsheets for logs

Water source heat pump is simply conventional central heat pump with the substitution of a water loop or source for the air blown through outdoor coil.

Comment by Roch Naleway on September 12, 2012 at 11:13pm

I definitely read your "Double 50" to mean two HPWHs. Haha.

How do you like the Nyle product? I talked to them a few months ago. The utilities in the Pacific Northwest do not endorse the add-on heat pump water heaters at this point. I read about product quality issues for their competitors (AirGenerate A7) heat pump water heater. Not sure what to think. I have not seen an add-on unit life and in person so far. I have seen plenty of integrated ones for the better or worse.

You should be able to use your Ipad or Iphone to log the data. :-) There should be an app for that.

Solar PV (after utility incentives) is a no brainer in Oregon. Great incentives, rebates, tax credits. My home is pretty efficient for its age and I guess it is not overly challenging. I do not know what's holding me up. Perhaps I am looking for complicated solutions so I can be a mad scientist.

Your watersource central heat pump also sounds like a project to figure out a  challenge for sure.

Comment by Curt Kinder on September 12, 2012 at 5:49pm

You read something into my "Double 50" post that I did not intially intend - two HPWH in series. I was thinking along the lines of your orginal post - HPWH feeding standard tank. That said, the idea is intriguing for truly large households, 6+ perhaps. The cost delta between HPWH and a decent (12 yr) resistance tank has fallen and will continue to drop.


My dual 80 setup is essentially two HPWH. The upstream tank is unpowered but heated by the desuperheater on a watersource central heat pump. It is active during our 7 month summer and also during 3 months of cool weather during which we need some space heating (can't really call it winter since it never features frozen water outside a drink cup) The upstream tank top temperature averages 105+ all summer and most of December through February.

The finishing tank is an identical unpowered 80 except that it is heated by a rev 1.0 Nyle North Road Geyser external HPWH. It is plugged into a Kill-A-Watt monitor, reset monthly on meter reading day, so I have several years of high quality water heating cost data for a family of 4.5 (one part time kid)

In a dual HPWH setup, if the upstream tank is set 5-ish degrees warmer than the downstream tank, then it should shoulder most of the standby losses and meet them in pure heat pump mode.

Instead of clinging to solar thermal, why not instead embrace solar PV? Electrical energy is so much more versatile and valuable than thermal.

Comment by Roch Naleway on September 12, 2012 at 3:05pm

Double 50...would not be bad. The first one set up in heat pump only mode. The second in hybrid mode.

Some of the HPWHs only have one electric back-up element though. This could be troublesome with those models.

I am a solar thermal enthusiast. It will not pencil any longer with HPWHs on the market. With that said I am a purist and love the idea of solar thermal. I would do just for myself and won't bother to do the payback calculations. It's pretty much never if one does not take into account utility rebates....etc.

Comment by Curt Kinder on September 11, 2012 at 6:40pm

I would be willing to bet that a double 50 would be a winner in all but the coldest climates - HPWH should make 75-90% of the demand at a COP > 2.0 most of the time.

I'm no fan of solar thermal domestic hot water - the numbers just don't pencil out unless a family is huge.

Comment by Roch Naleway on September 10, 2012 at 10:40pm

Yes, we are on the same page. No problem in the South. Some potential problems in the North. Two tank systems should make for a safe choice. I talked to a builder last Friday. He is contemplating to choose HPWHs in new construction instead of a solar thermal set-up. He is worried about a standard 50 gallon model and its ability to keep up with demand when the going gets tough.

The good news is that new HPWH technology is actually an acceptable solution for efficient hot water these days. You do need a good plumber, contractor, or home performance contractor as a home owner for sure. Otherwise it is still possible to get trapped in an underperforming set up.

Comment by Curt Kinder on September 10, 2012 at 10:07pm

Your understanding of EF as it relates to larger tanks mirrors my own.

If the second tank is to be a "silent partner" it would have to have its setpoint 5-10 degrees below that of the active HPWH tank. Artificially Increasing the setpoint of a HPWH to accommodate such a silent partner imposes both an efficiency and a durability penalty on the HPWH since they are highly effected by setpoint (refrigerant pressures, compressor power and COP)


If the house's plumbing is such that a HPWH set for 120 and a finishing tank set for 115 provides acceptable bathing / showering conditions, then life is good. If, however, the HPWH setpoint has to be 135+ feeding a finishing tank set at 125+, efficiency suffers.

You do raise an interesting alternative for consideration given market conditions. The "commodity" product is a 50 gallon HPWH, often available, on sale for under $1k. That capacity is suitable for 3 maybe 4 people down south, 2, maybe 3 people up north.


Households of 3+ up north, 4+ down south, likely need a larger tank. Though the manufacturing cost of an 80 vs 50 gallon tank is likely no more than $100-$200, as matters now stand an 80 gal HPWH is priced $1000 more than a 50.

That begs the question of whether a "double 50" solution consisting of a commodity 50 gal HPWH ($~1000 on sale) feeding a basic 5 year 50 gal standard tank heater (~$300) is a more cost effective alternative than a premium priced 80 gallon HPWH costing $2k or more.

Comment by Roch Naleway on September 10, 2012 at 9:57am

Larger tanks have higher stand-by losses to begin with. Small Marathon water heaters with lets say 50 gallon capacity come with an EF of 0.94. Your heat loss per 24hr cycle is roughly 5%. If you get a Rheem Marathon water heater with 85 or 105 gallon capacity the EF drops to roughly .92 EF. It is an additional 2-3% stand-by loss simply by having a larger tank. Larger tanks have more surface hence you will end up with higher stand-by losses anyways.

I am proposing the two tank system to make the heat pump technology available for applications in homes with 3-4 bathroom or more than 4 people living in a household. An 80 gallon hybrid electric water heater may work for that matter in Southern climates. If you go North you have to overcome colder water inlet temperatures and lower ambient air temperatures (at least in garages) so the actual capacity of an heat pump water heater (operating in heat pump mode) is compromised.

I am thinking of the booster tank to be a "turbo tank" that really just kicks in when it needs to. Otherwise it is a "silent" partner. :-)

Obviously, the tanks should be positioned side-by-side to prevent unnecessary line losses. Similarly, one should insulate the pipes running from one tank to the other with an added 6 ft of insulation on the first 6 feet of pipe running from the booster tank towards any of any of the hot water applications.

Also, one should not use a crappy electric water heater with an EF of 0.88 or 0.89. This is not good enough. We can do better.

Good consideration regarding line losses for sure. Thank you.

Comment by Curt Kinder on September 10, 2012 at 5:36am

I don't envision a food grade refrigerant / compressor lubricating oil ever being deployed. Worse yet would be the result of a compressor burnout - truly nasty stuff is born when winding insulation, refrigerant and oil burn together.

A specific concern of a two tank system (I have two 80s in a mech room that averages 70-75 year round) is standby loss from tanks and interconnecting pipe. Tank bottom is a hidden area for loss since it may not be well insulated - most water heaters do not heat the water at the very bottom, so loss is low in that direction. A dual tank arrangement puts hot water in the very bottom of the downstream finishing tank.

Standby loss is low, typically 10% or less, but it will add up with two tanks and even more with tanks in much lower ambient air temps coupled with higher setpoints typical of northern climates.

In the south we can deliver 105+ for bathing with setpoints of 115. Up north it sometimes takes a setpoint of 125 to deliver 105 owing to increased line loss during hot water draw events.

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