Heat Pump Water Heaters -- no free lunch

Jamie Hall

Since I like to rock the boat, at least gently, now and then...

There has been a lot of discussion and enthusiasm about heat pump water heaters. Unfortunately, at least some of it has overtones of free lunch.

Consider. A heat pump water heater works by extracting heat from the surrounding air and boosting the temperature of that heat and delivering it to water, thus heating the water. So far so good. It uses considerably less power to do that than fired or electric water heaters.

However. Problem. Where does that low temperature heat come from? The surrounding air, and it cools the surrounding air by a certain amount. Rather small, but it does. This is not a consideration if that surrounding air is outside air, or the water heater is in an unconditioned space. However, if it is in a conditioned space -- that is one that a furnace or boiler is striving to maintain at some set temperature -- then the furnace or boiler will have to add some additional heat to compensate for that subtracted by the heat pump.

How does this affect the overall energy efficiency of the heat pump water heater, as compared to other methods of making hot water? Rough noodling suggests that, considered from an overall system standpoint, the end result is that the heat pump water heater, if located in a conditioned space, will actually use the same amount (or even a bit more) total energy to provide the hot water than other approaches might...

There's no free lunch.

Note: this is a think piece, not an advocacy piece!

Comments?

Hot_water_fan

I don’t think people consider it a free lunch. Caleffi has the equation in one of their journals. I think COP of ~2 during heating season in a space heated by a heat pump and ~3.5 during summer plus cooling/dehumidification during the summer. So pretty efficient still. 

ethicalpaul

Mine is next to my boiler. In the summer, the heat is unwanted so it is indeed a free lunch.

In the winter, there is excess heat adjacent to my boiler that is just going to ooze its way out of my basement walls and windows anyway. The water heater gets it before that occurs. Free lunch.

I do pay to move the heat of course, but as we know, that is way cheaper than creating it.

Trust me Jamie, my water heater is not noticeably cooling the area.

hot_rod

I'm not sure any of the pros here mentioned "free" dhw? The numbers are definable.

Getting the "heat" from an an air to air heat pump that is heating the space does change the equation some.

We talk about that in I-33

https://idronics.caleffi.com/sites/default/files/magazine/file/idronics_33_na.pdf

EBEBRATT-Ed

Same thing as your refrigerator. It helps you heat the house in the winter, but you pay to get rid of the heat when running the ac in the summer although the compressor is much smaller than a HPWH.

My problem with HPWH is when they go bad (it's like a window AC or a refrigerator or freezer) no one will fix them, or they will not be worth the cost to fix them, and they will be discarded. Then you won't likely get a rebate to replace it.


@ethicalpaul @Jamie Hall specified that he was talking about a water heater in a conditioned space not a basement

Larry Weingarten

Hi, As long as we're gently rocking the hypothetical boat, it is possible to duct the inlet and/or outlet of the HP, both to prevent mixing of the air-streams, and to take advantage of potential cooling and de-humidification. So, there could be benefits aside from hot water, particularly if the air-stream could be switched with dampers, to get winter and summer modes.

A big potential downside is short service life, in part because anodes in the glass-lined tanks can be very difficult to access... and too few people are aware of the need to check them.

Yours, Larry

ethicalpaul

You are right, Ed, he did say that. I completely missed it in the middle of that paragraph. Who would put it in a conditioned space? Down south they put them in the garage!

I will say that I bought mine with an extended warranty, and supposedly they will send a technician to work on it if/when it fails.

PC7060

ethicalpaul said:

I will say that I bought mine with an extended warranty, and supposedly they will send a technician to work on it if/when it fails.

A coworker has a AO Smith HPWH with extended warranty from a big box store. The unit began having the same problem with periodic compressor shutdown requiring a power cycle as this thread. 

https://forum.heatinghelp.com/discussion/193173/ao-smith-ecc-code

He files claim for repairs and after a short time gets a gift card for original purchase $ from the box box store.  Turns out there is no way economic way to repair that particular problem.  On the plus side, power resets, while annoying, are cheap. 

Intplm.

These water heaters will surely work best in warm climate areas. Many water heaters are installed in garages in Florida. This is a great place for a hybrid to be at its best savings.
The hybrids that I have seen have a sticker/menu on the jacket for settings best suited to your needs.
Some have four selections others have five different selections.
The one I most recently installed was a Bradford White with five different selections. (Heat Pump, Hybrid Standard, Hybrid Plus, Electric and Vacation mode.) There is also a standard for cold climates on some models.
If your area is offering a rebate for the installation. This rebate can offer a lot of free lunches?
It depends on where and how it is used.
Here is a picture of one I recently installed. It is quite impressive.
I'm beginning to warm to these things, and never thought that I would.

ChrisJ

EBEBRATT-Ed said:

Same thing as your refrigerator. It helps you heat the house in the winter, but you pay to get rid of the heat when running the ac in the summer although the compressor is much smaller than a HPWH.

My problem with HPWH is when they go bad (it's like a window AC or a refrigerator or freezer) no one will fix them, or they will not be worth the cost to fix them, and they will be discarded. Then you won't likely get a rebate to replace it.


@ethicalpaul @Jamie Hall specified that he was talking about a water heater in a conditioned space not a basement

I don't think I can agree with the fridge analogy.

A fridge is keeping it's interior cool vs the inside of your house primarily. The added heat it may reject comes from cooked food etc, but the rest of it is primarily just trying to keep it cooler than the space it's in. When you open the door the heat rushing in comes from the house, so it's simply putting it back.

The water heater is, at least in a cold climate trying to heat what can be very cold water from an external source and could often seem never ending.

Others have mentioned it uses waste heat from a boiler. The problem I have with that is the cooler the space my boiler is in, the more waste heat it's going to create. And during the winter that boiler would be the only source of heat. So now I'm using a heat pump to pull waste heat from a natural gas fired boiler and the more it pulls, the more the boiler is going to supply (greater temperature difference).

In the summer months it may absolutely be beneficial to me. Especially since my basement runs warm and I need a dehumidifier.

In the winter, I feel it never will.

So, this is probably another example of one size cannot fit all. Not in usage or location.
It could very well be an excellent solution in southern states but less than great in northern.

I've also said, personally I'd rather an electric tank heater over a tankless coil. The HP WH gives me another better solution to that as well if I didn't have natural gas.

ethicalpaul

Don't overthink this. I have never been able to detect a difference in the ambient air temperature where my heat pump water heater sits. It's a rounding error.

ChrisJ

ethicalpaul said:

Don't overthink this. I have never been able to detect a difference in the ambient air temperature where my heat pump water heater sits. It's a rounding error.

I respectfully disagree.

ethicalpaul

I look forward to hearing from someone complaining that their heat pump water heater has cooled off the space it's in, but I'm not holding my breath.

Jamie Hall

In most cases it will be pretty much a rounding error. My sort of hidden objective was to help people remember that whatever we put in for heating and cooling and hot water -- and refrigerators and freezers or even ourselves -- does not stand alone all by itself, but is part of an overall system which needs to be considered as such.

ChrisJ

ethicalpaul said:

I look forward to hearing from someone complaining that their heat pump water heater has cooled off the space it's in, but I'm not holding my breath.

In a situation like mine or yours where is that heat coming from if not the natural gas boiler?  It's not beaming down from Mr Scott.

JMWHVAC

Can't resist commenting on this one. I have an old, stone foundation, farmhouse where the water runs across the floor when it floods. When winter gets real, my basement can drop into the 40's. Soo, my water first goes thru an indirect water heater, which is heated (sometimes) by the oil boiler. From there it goes into the wonderful heat pump wh. I set the boiler system up so that the indirect never gives a call to the boiler, but the return from the heating zones goes thru the indirect. Works fantastic! Dehumidification and "free" hot water all summer, then, as the basement cools with advancing winter an increasing percentage of my hot water comes from oil. I have done some similar setups for other people. Many, many benefits if you are willing to spend for two water heaters.

EBEBRATT-Ed

HP water heater are great until they break. Unless you have all the refrigeration tools to work on one they will become like a window AC or a refrigerator.

It's not that they can't be fixed....they can. If you can do it yourself that's fine. If you can't the cost of anything except a minor repair will quickly exceed the cost of the unit.

Teemok

There's a big difference between big box tank HP water heaters, Sanco2, Spacepak, Arctic, Chilltrix and Refrigerant split types. The type, the loads, where it's used geographically and where and how they are installed are critical differences.
HP's should not be talked about as if they are just one thing.
The tech is far from new.
COP's will vary based on design and application. Jamie is right, no free lunch but I think the desired is more like a COP of 2-3.6. He's also right that the conditions are easily created to be robing Peter to pay Paul. A huge unfinished earth contact basement masonry can give up the 3,500 BTU's an hour to a heat pump no problem. No near by boiler or furnace needed. Problem is they get stuffed in places by people who firmly believe in all types of dumb myths.
I have a customer who, along with his expensive architect, insisted that the two fully louvered doors to the outside on his little mechanical room closet was enough ventilation for their big box 80 gallon AOsmith HP. It was a big heated ego involved debate. They made zero provisions for the duct I recommended. Imagine the cost of those expensive steel doors vs two ducts. I put my opinion in writing, noting I will not be involved in any remedy. I closed the doors after the install and set a temp. tracker. The closet temp. dropped fast to 36F the over night ambient was 54F. The unit ran for 11 hours straight to hit 125F set-point. It would take 13.3 hrs @ 3,500 btu/hr. The COP is not great if 36F is the normal inlet air temperature. They currently leave the room doors open and the rain is coming soon.
This kind of thing will happen over and over until the physics are respected. How robust are these little tank top snail of a recovery rate units? I have good reason to believe the lawyers and accountants made them just well enough to genuinely hate them and buy another. I bet they can earn their expense in ideal conditions. Will they be set up to rob Peter and pay Paul? Will they be expected to magically perform when and where they can't? Will they add to the junk pile and chemicals in the atmosphere? Yup!

Teemok

An 80 gallon tank at 55F heated to 125F will need 46,704 Btu's. How long it take's and where the BTU's come from is the question. The temperature split off the big box HP's heat collection coil is small but it's a fairly large volume of air being moved. If the surrounding space can give up the BTU's to the slightly cooled air, from an unpaid for source, at the typical snails pace needed than all is well. If not the COP will suffer or your likely robbing heat from a previously paid for source.

fentonc

I just had a Bradford White 50 gallon HPWH installed (replacing a 40 gallon AO Smith gas-heated tank) a month ago, so hopefully I'll have some good data on overall fuel + energy usage for this season. It's located next to the gas boiler in an unfinished, concrete-block mechanical room (with open doors to an otherwise finished, heated basement), and the boiler is way oversized so it typically runs for pretty short cycles.

So far, the heat pump is only running for a 2-3 hours a day, so it's had a barely perceptible impact on temperature in the basement (maybe a temporary half-degree drop for a few hours after recovering from morning showers). From a total energy-use perspective, I at least considered the following:

• The AO smith tank was only nominally ~60% efficient at turning gas into heat, whereas my boiler is at least nominally 80+% efficient.
  
• The gas tank heater had included an always-open flue pipe with no damper that's now sealed off.
  
• The mechanical room is uninsulated, so the frequent short-cycling of the boiler would mean that the residual heat in the boiler would gradually leak into the surrounding environment via the uninsulated concrete walls/floor, with some small amount making it into the rest of the house. Now it seems like at least some of that heat will get stuffed into the insulated water tank. How much of a net benefit (if any) this is? No idea.
  

At least for our situation with fairly low water use (~3 showers a day), I think @ethicalpaul mostly has it right though - it's probably been averaging something like 800 BTU/hr through the day (nearly compensated for by the 2 kwh or so the standup freezer in the same room uses), so it's just a pretty minor effect overall.

hot_rod

Doesn't it always come down to the proper product properly applied It is not that hard to understand how a heat pump "pumps heat" from one location to another.

If it plugs into a wall outlet you could rest assured it is not "free" heat.

Unless you have it connected to a PV array.

In California the programs for power have you load the tank to 140F on off peak power, then use it during the day, via a 115 set mix valve. So an 80 gallon tank thinks it is a 105 gallon.


So leverage the off peak power, which in some grids can be 1 cent per KWh, that with the COP add makes DHW fairly inexpensive.

$500- 1000 rebate in some areas, starts to make the cost look pretty attractive.
If you have warmer incoming water, or a larger tank, maybe a 100 gallon the stored DHW, looks pretty good for typical home owners.

The tank I have, the compressor looks an awful lot like a refrigerator compressor. I'd expect 15- 20 years.

If the tank fails due to lack of anode maintenance, that is on the owner, not the tank.

JMWHVAC

In certain cases it IS free with a hpwh. In my case the basement would be damp and musty without dehumidification, so my hp water heater is my dehumidifier and the byproduct is hot water! How neat! The evaporator on my Rheem unit runs about 20* below ambient so it will not dehumidify enough for some people. Also depends how much hot water is used. For me, my basement is pleasant and dry. Average a gallon of condensate per day over summer.

Derheatmeister

ethicalpaul said:

You are right, Ed, he did say that. I completely missed it in the middle of that paragraph. Who would put it in a conditioned space? Down south they put them in the garage!

I will say that I bought mine with an extended warranty, and supposedly they will send a technician to work on it if/when it fails.

My brother which resides in Florida installed it in his unconditioned garage.
In his situation this is working out great.

kenjohnson

JMWHVAC said:

my water first goes thru an indirect water heater, which is heated (sometimes) by the oil boiler. From there it goes into the wonderful heat pump wh. I set the boiler system up so that the indirect never gives a call to the boiler, but the return from the heating zones goes thru the indirect. Works fantastic! Dehumidification and "free" hot water all summer, then, as the basement cools with advancing winter an increasing percentage of my hot water comes from oil.

I have done the same type of install with the incoming cold water supply running first through the heat exchanger in my Stiebel Eltron radiant floor heat storage tank, then out to the input of the HPWH. Works great summer/winter - I could also essentially have unlimited hot water for big gatherings if I planned ahead and set the radiant floor heat controls to create 120 degree water for the big storage tank.