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UNCSoc 09-04-15 10:49 PM

Heat pump for water heater

It seems that there are downsides of the heat pump water heater as well as the tankless water heaters. But why not combine them?

So the first source of hot water would be a standard heat pump water heater in my basement utility room. I would need one that would only use the heat pump--no other source of heat (would have to shut off any backup heat in this unit).

From there the heated water would run through an inexpensive (propane) tankless hot water heater. If the water was not hot enough, the tankless hot water heater would come on as needed, and the fully heated water would be ready for use in the house.

In the summer, we would set the heat pump water heater at a higher temperature to remove the heat and humidity in the basement. In the winter, we might set the temperature on this unit lower. And maybe we could move some air through a door to a large insulated (ICF block) crawl space, which under some conditions may offer another source of "geothermal" heat. And that way the utility room wouldn't get overly cold from the heat pump (cold) air and "leak" that cold into the rest of the basement. Could insulate the walls of the utility room to keep that relative cold enclosed. Or just use mostly the tankless hot water heater in the cold winter month or two, and keep the heat pump hot water heater on a very low temperature.

Does any of this make any sense? I am not that familiar with these systems so would be thankful for any advice. Thanks.

Daox 10-07-15 10:45 AM

I think the dual water heater setup would be quite cost prohibitive. One or the other can easily be used as a stand alone heater. What downsides are you trying to overcome?

UNCSoc 10-07-15 11:51 AM

Yes, the cost is an issue. And as someone else pointed out the variables involved are a bit overwhelming.

1. I read a study that said that tankless water heaters in use are not saving energy at nearly the levels implied by the government tests, which assume long draws. In practice, homeowners are taking a lot of little draws to wash their hands or a dish, which are not efficiently handled by a tankless water heater. Might as well have a well insulated tank for these small draws.

2. Some tankless now actually have a small tank to overcome the problem of short draws and cold water sandwiches. But one study showed that these can be very inefficient unless implemented properly, so you lose any energy efficiency in having a tankless. Perhaps better to get an inexpensive gas tank model to handle this.

3. Do-it-all tankless units are very expensive, but cheaper units can be had (with some decent reviews). Wouldn't want to rely on smaller and cheaper for all my water needs but should be fine in combination with something else (eventually solar but not quite there yet).

4. There is energy a wastin' in my basement air, partly because of summertime heat and humidity and partly because I can link to a large and insulated crawl space area, which is probably 54 all year round (guessing on that temp).

5. Would be nice to dehumidify the basement (and crawl space) in the process of getting heat in the summer, but would not like to do as much of this in the winter. Seems inefficient to take heat out of the air to heat water and then pump in more heat to keep the basement warm. So would crank down the hpwh in the winter and rely more of the tankless.

Again, I am just guessing on lots of this stuff. Thanks for any expertise you can offer.

Daox 10-07-15 12:04 PM

That explains the downsides of the tankless units. What are the downsides of a heat pump water heater (that would prevent you from only using it)?

UNCSoc 10-07-15 12:16 PM

Good point. Here are my guesses on why I don't want only an hpwh:

1. The tank has to be insulated well, especially if the temperature is set high. But I can set the temp lower if I have the tankless ready to do the heating for a shower or other long high temp draws. (Some technical issues here in terms of how the units work together--might kill the plan.)

2. I don't want to invoke the inefficient electric backup should the heat pump not be able to keep up to demand for a lot of (very) hot water. I want to shut that capability off entirely. My backup will be the tankless water heater.

3. There are times during the winter months when I would use the hpwh in my basement at very low temps, if at all. I would rely on the tankless during those months because I don't want to take heat out of my basement during those cold months.

Thanks for any help on this.

UNCSoc 10-07-15 12:21 PM

A question on hybrid water heaters:

Do any companies make an hpwh with a gas or LP backup (rather than the electric backup that is very inefficient)? I haven't seen one.

I think some of you have essentially built your own version of this though. Maybe that is the only solution.

jeff5may 10-07-15 12:29 PM

Depends on your hot water usage habits. Like Daox said, both of these heaters are much more expensive than their conventional counterparts and are built to handle what they do quite sufficiently on their own.

In another thread, you were asking about an 80 gallon HPWH tank. Unless you are using that unit for hydronic heating of some kind, or some other heavy usage scenario (commercial or rental units, family of 15, laundromat, etc.), it would handle some pretty serious practical usage with ease. The unit cited already has a backup source: a resistive electric heating element. The HPWH tanks are known for a 3-to-1 energy advantage or better over straight resistive electric tanks.

The propane burner heater is the polar opposite of the above unit. Where the HP units have relatively slow and low energy usage, these things operate like a hot air baloon: lots of usage fast then stop. The advantage of these units is perpetual hot water with no storage tank losses. These units need to be sized for your maximum flow rate, so generally more flow is more expensive. If you exceed the max flow of the unit, you get lukewarm water instead of hot water. Modulating condensing burners are an option which add serious expense to the purchase price. That being said, if you are running a laundromat, it could keep up with that after-work rush every day of the year if you bought a really big one. If you were a germophobe, it could fill up your hot tub every time you used it, so you wouldn't have to re-use the old water from last time. The heat pump unit couldn't do it: you would run out of tank before the hot tub was full.

If you were looking for a doomsday prepper machine, you could buy both and install a solar electric PV panel to the built-in resistor element in the HPWH. When the sun was shining, you would get essentially free hot water from the PV panel. If the power grid went out, you would still get reserve hot water in the tank every sunny day, and you could run the gas burner when that tank got depleted. Neither would cool your basement at all.

FWIW, the HPWH unit is not going to freeze you out of your basement either, check xringer's thread on his overly complicated water heating project for realistic detailed measurements. I believe he only uses his fossil fuel backup to verify that it still operates. If ultimate energy savings is a concern, it would not be a major undertaking to direct the cool HP exhaust air towards your crawl space. Once the air is blown in that general direction, a cold-retaining knee wall would keep the air where it belongs next to the warm ground to gain calories.

I see you mentioned propane as a fuel source, not natural gas. In this light, the HPWH would be way less expensive to run. Bottled propane costs a lot more than piped in natural gas.

pinballlooking 10-07-15 01:32 PM

We have a tankless natural gas hot water heater. When you go into a bathroom and turn on the light switch I have the water start circulating so by the time you need to use the hot water it is there or very close.
Our kitchen switch light does the same thing. This almost eliminates the sandwich.

I had a tankless feeding a 40 gallon natural gas tank before similar to what you are asking about. (Except the HP part)
The new setup works better than the old one for less energy usage and faster hot water at the sink. The equipment was less money also but not cheap.

we have endless hot water.

Last month usage was 4 therms $12.49 this is a family of 4 with one teenager.

Seafoam 10-20-15 07:31 PM

Hydronic Heating - Indirect Tank AWHP
I have an indirect hot water tank that is linked to an electric hot water tank. The indirect tank gets heat from my Noritz propane boiler that heats a zoned water radiators. The price of propane is now very expensive.

I am considering an air to water heat pump that I would link into the indirect tank and the heat exchanger would pick up heat for the circulation system for the radiators. I also have a large wood burning space heating stove on the first level. I can capture heated air and blow it down to the basement to improve the COP of the AWHP. The heat pump would eliminate the need to dehumidify the basement.

In summary I would use the AWHP in the summer and shoulder season when the propane boiler is not in use. There is company in Maine that sells the AWHP at a reasonable price.

Has anyone designed a similar system?


Ice 11-12-15 06:55 AM


Originally Posted by UNCSoc (Post 47305)
I don't want to invoke the inefficient electric backup

Hi All,

My first post.

I wonder what UNCSoc is using for a tankless water if his electric element backup is inefficient? If it is gas, the best you could expect is 80% efficient whereas electric element would give 100% efficiency. Now cost may be a different story but efficiency is a different story.


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