I’ve watched the “Ask This Old House” episode, where one of these heat pump water heaters is installed, twice now and something has been eating at me each time.
I’d consider installing one (when the price [hopefully] comes down) – but right now, Lowes wants $1599 for the GE model.
Here’s what’s eating me: If you install one of these units, the heat pump will extract heat from the space it’s installed in; for example your basement. That’d be fine when the unit first starts up. But as it runs for a while the air coming out of the heat pump will be pretty cold – the same as an HVAC heat pump works. So … after it’s been running for a while, your basement will get colder and colder. Not only will it, sooner or later, and especially in the winter, get pretty damn cold in your basement, but at that point there’ll be less and less heat for the heat pump to extract from the air in the room, so that it can heat your water.
Wouldn’t the unit be less and less efficient the longer it runs? Obviously, if the air in your basement is 70 degrees, after the heat pump has been running for a while, the basement temp might be down to 60 degrees. Then if it keeps running (like in a high demand period), because it still needs more heat to transfer into the water, it would seem to me that it would keep running longer-and-longer each cycle to get the heat it needs out of the colder-and-colder air.
Anybody installed one of these yet?
Replies
I like the idea of the heat pump water heater. A couple of my concerns are the maintence requirements such as fan replacement, electronic controls, compressor, and the payback period.
I'm going to say that you are right about the issue of cold environments reducing the efficiency of the heat pump. I also assume the heat pump will only operate at suitable temperatures, so it probably is not a really big concern unless you live in a heating environment.
Where is the heat going to come from in the winter? Your heating plant. If your heating plant consumes electricity for fuel, I don't see where the heat pump water heater would be more efficient than resistance heaters.
I believe the heat pump water heater excells in warm environments. It reduces air coinditioning load, while substantially reducing energy used to heat water. All the while dehumidifying the air, which is a plus in humid environments.
I doubt that they can compare to my utility costs using natural gas though.
The way I understand it, if the compressor can't deliver the amount of heat needed to bring the water to the set temp, or to deliver the water in the required period of time, the electric back-up elements kick in.
I DO live in a heating environment, and I've only installed one heating register in the basement to keep everything at a reasonable temp during the winter. If I'd run one of the heat pump units, I'm assuming that within a couple of cycles, my basement would be at about 40 degrees, instead of the 58 - 60 that I keep it at now. I know from my air-to-air unit outside, that somewhere around 34-35 degrees it starts losing efficiency pretty quickly. I can only figure the water heater unit would do the same thing.
You're right about the furnace or other main heat source producing the heat to run this thing - I can't decide if that's a good thing or not.
It seems like the heat pump compressor would only function like a pre-heater in colder climates - definitely not worth $1600 for me! Maybe if they come way down in price.
During a web search, I did find a couple of companies that make an external unit that works with your existing water tank. I'm sure it would function the same way, though (cooling the basement).
Wego, you are right about the basement getting colder. HPWH's are much better suited to the Gulf Coast climates which can take advantage of the cooling and dehumidifying. When the manufacturer makes claims to a certain Coefficient Of Performance, he is basing it on certain prescribed ambient conditions of temperature, humidity and room size. Your COP will change based on how your conditions change. Obviously the hotter the enveronment, the better it will perform.
To deal with this problem, I saw a website where the guy added louvers for summer / winter operation. In the winter he vented outside.
Yeah - the more I learn about these units, the less I think I'd be able to use one here in northern WV. Maybe if the price came w-a-y down, I'd consider making some modifications to install one. As it is, I like the temp of my basement just the way it is in both summer and winter, and I don't think I'd want to cut any of the foundation block out just to run some sort of ducting or louvering system to the outside. I rarely have to use any air conditioning since I live at around 2000 feet. Maybe once or twice in the summer do we flip our current air-to-air heat pump on to A/C.
Maybe I'll look into solar water heating, although the way this winter's been going, I'm not sure I'd recognize the sun if it came out!
Thanks for the input, guys.
The unit doesn't get less efficient as it runs longer, but, as it runs longer, the area where it's sited goes into a larger and larger "heat debt". Sooner or later you can't make the payments and foreclosure is required.
The units really only make sense where heat is free (the south) or cheap (heating with nat gas), or where the cold can be used for some purpose (eg, the first units of this sort were sold to dairies -- the heat was needed for cleaning and the cold to chill the milk).
Any heat pump is exactly that -- a heat pump. It does not create heat, but rather pumps it from one place to another, in this case from the room air to the tank water.
The performance will always be dependent on temperature differentials. Maximum performance will occur when the water in the tank is coldest, and the utility room air is the hottest. As the utility room air drops, and the water temperature in the tank rises, the Coefficient of Performance will continually deteriorate. The same thing happens to a whole house heat pump in Minnesota -- it just can't "pump" across a temperature differential which is too great..
What DanH said. If you're already paying to move heat out of your house, you might as well move it into your domestic hot water.
Otherwise, you'll pay once to take the heat out of your interior air, then you'll pay again to put heat back into it. These units would never be feasible in our climate.
If you are payingh to get heat out of your house (the AC is on) why not just get the heat recovery uint made for the purpose that takes heat from the HVAC condenser? That is pretty cheap compared to the price of a heat pump water heater..
Hi, You might take a look at the info here:
http://www.builditsolar.com/Projects/Wat...
The last one "Wiinter use of Heat Pump Water Heater" describes a setup that helps with the issue of the heat pump water heater fighting with your furnace in the winter. It involves a little ductwork, and you have seasonally change a damper, but it seems workable to me.
The part on the heat pump water heater is a ways down in the document.
The video from Tom Gocze on installing a heat pump water heater is interesting and fun.
Gary
Probably a reasonable solution farther south, but I'm guessing that the HPWH doesn't operate too well on the 0-30F air they'd get from a crawl (if there was one) around here.
Hi,
That's a mighty cold crawl space -- I'm in Montana and mind runs in the 40 to 50F area all winter.
Gary