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I'm going to buy a small heat pump dedicated to pools (around 3.5 kW total output). The pump would not be used for 6-7 months a year (autumn/winter); can I plug it into an existing water heating system to pre-heat water before it gets into the gas boiler (to use the pump year round and save on the heating bills)?

Yes, there are tangible savings from switching to a heat pump (it's around 2x cheaper than using gas boiler in my location). If you're unfamiliar with heat pumps: they're NOT electrical boilers, these are sci-fi devices that give out more heating power than their electrical consumption. The unit that I want to buy uses ca. 0.6 kWh of electricity but provides ca. 3.5 kWh of heat.

Please note that there is a dedicated heat pump unit that essentially does what I want (work in conjunction with an existing boiler), and it is supposed to be installed inside the boiler room (with an option to vent the cold air outside). I don't remember the brand so couldn't find it online; but this dedicated HP is rather expensive and, most importantly, I'll invest in a pool heat pump anyway that won't be used for 7 months in a year.

Those are my concerns / ideas why it might not work / possible problems, am I missing something obvious but critical?.. :)

  • Pool heat pumps [PHP for short] work when air temperature is higher than ca. 12C/50F — I will run it in the house, in the boiler room
  • PHPs are designed to heat water up to max. 35C/100F — not a problem, I'll use it for additional heating only (e.g. wire the heat pump instead of a recirculation pump)
  • Maybe PHP will never be switched on because the water in the tank will always be hotter than 100F?.. — the boiler is programmed to run during the day only, I think the water gets quite cold during the night
  • PHPs are built to handle corrosive pool water — hmmm that doesn't sound like a problem, more like a pro ;)
  • BUT maybe PHPs won't be able to handle water heated to 50C/120F?..
  • Maybe PHPs won't be able to hold/run water at around 2 bars pressure (what I have now in the tank)?

Anything else I might be missing?

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    What do you gain by pre-heating the water? You need to pay for every bit of energy that goes into heating the water, one way or another. Electric boilers typically cost more to operate than gas ones, so using an electric pre-heater to do work that would otherwise be done by a gas boiler will likely cost more, not less. As long as the gas boiler alone is keeping up with your energy needs, why would you want to use a less cost-effective setup to do the same job? Commented Feb 3, 2021 at 14:29
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    I take it you are talking about an air-source heat-pump? These are designed to harvest 'free' heat out of the atmosphere. If you put it in the boiler room, you will be taking heat out of your house, which you are paying to put in there, mostly. I can't see that's going to be efficient.
    – SiHa
    Commented Feb 3, 2021 at 14:55
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    @NuclearHoagie, heating water using a heat pump is about two times cheaper then heating water using gas boilers (I've already ran the calculations for my local energy prices).
    – Alexander
    Commented Feb 3, 2021 at 15:01
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    Thanks everyone for the comments, but I beg to differ with the sentiment ("heat pumps are useless for pre-heating usage water"). THERE IS a dedicated heat pump that essentially does what I want (work in conjunction with an existing boiler), and it is supposed to be installed inside the boiler room (with an option to vent the cold air outside). I don't remember the brand so couldn't find it online; but this dedicated HP is rather expensive and, most importantly, I'll invest in a pool heat pump anyway that won't be used for 7 months in a year.
    – Alexander
    Commented Feb 3, 2021 at 15:12
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    "but I beg to differ with the sentiment" You asked, "am I missing something obvious but critical?" and folks answered with their concerns. My opinion is; a pool heat pump is for a pool and using it in an application it is not designed for may not result in the efficiency you are after.
    – Alaska Man
    Commented Feb 3, 2021 at 17:50

2 Answers 2

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There's no free lunch. Really.

Saying a heat pump is an over-unity engine ignores why it works: It takes something from something else; i.e. it creates an externality. Ignoring those is a common error of over-unity believers.

Heat energy is the level of excitation of atoms. It generally stores on a per-atom basis not a per-gram basis. What's relevant there is that water is the densest thermal media known, because it has 3 fairly lightweight atoms in a small space. Raising 1kg of water 1 degree requires reducing a great deal more than 1kg of other stuff 1 degree. 1kg of air is a lot of air.

Also heat pumps aren't always one particular efficiency. Their efficiency varies based on conditions. For instance if you have 10C/50F pool water on a 35C/100F day, the heat pump is "pumping downhill" as I like to say, and it will be quite efficient there. Raising 20C pool water to 23C/75F on a 11C/52F day is a great deal harder and less efficient. The 6:1 efficiency you claim is only valid in certain test conditions.

"To keep the heat pump in its working range, I'm placing its condenser inside the utility room".

The heat pump will steal heat from the room and place it in the water. That gives you 2 choices, neither one good:

  • allow it to chill the room, reducing the heat pump's efficiency badly, until it gets to 10C/50F, at which point the unit shuts off, or reaches 1.0 efficiency at which point it amounts to electrically heating the water "the hard way". Or
  • divert additional heat into the utility room, which you then have to pay for. Now if your regular hot water heater is gas, and your house's furnace is gas, then it's six of one, half dozen of the other. Except you are paying the pumping costs also. So it's a net lose.

"I'll just eject the chilled air outside, then. SOLVED!"

Except when you push air out of your house, you draw a vacuum (negative millibar) inside your house, and air simply leaks in somewhere else. This "makeup air" now needs conditioning. Now you have 2 problems.

  • You are "heating the great outdoors" by intaking -5C air and ejecting 10C air.
  • The makeup air is much drier than the air that went into the heat pump. Here we must do a deep dive into latent heat of vaporization, but the upshot is that water vapor in the air stores a great deal of energy - 2,258 J/g or 1 megajoule per pound. When your process has the effect of reducing humidity, it's throwing away this latent energy, which must be made up for real. Effectively, "heating the great outdoors" and "humidifying the great outdoors" is the same thing.

Some of this latent energy goes into the water - the heat pump's evaporator (of freon) condenses water, and that 2,258 J/g went into the hot water, which at least isn't "humidifying the great outdoors", but still presents the same problems I discussed in the first part. Your humidifier must make up for this lost humidity, by boiling water into the air, again at 2,258 J/g. Even if it's a sparger humidifier (which sprays a fine mist which spontaneously evaporates, there's still no free lunch; that evaporation happened by stealing the 2,258 J/g out of the air, reducing its temperature and the heater must work harder to compensate.

If you want a heat pump water heater, just buy one.

They are readily available. However they solve none of the above problems. They're not the panacea one might make them out to be.

However... if the heat pumps could be water sourced.... (i.e. their interchange fluid is water instead of air)... different deal. Now you can pump water out of the aquifer and put the heated/cooled water back in (at a different height obviously, so you aren't trying to chill water you just chilled).

And if you're where I think you are, you have a second option, as long as you don't mind a house that is 7' or 13' wide (depending on if you're willing to limit yourself to broad canals). You can interchange to canal water (uptake from one end of the boat, discharge to the other). Or simply run a bunch of glycol loops before you pour floor insulation, and use the boat's entire steel bottom as your heatsink, without ever having to clean a clogged filter! Covering an entire boat roof and upper sides with walkable solar panels has not been done. So much opportunity for innovation!

Other complications

Placing the heat pump on the intake line to the water heater is a lost cause. Water only moves through that pipe while hot water is being used. (cold water pushes out the hot water, that's how the pressurization works; the volume of actual water in the tank remains the same). Doing that would be like having a "tankless on-demand heat-pump water heater" and that would require a positively enormous freon engine and evaporator, we're talking 20,000 joules/second (aka watts).

Having the heat pump recirculate through the water heater (out the drain valve, in the cold) won't work either, because its maximum temp is below the water heater's minimum temp.

"It will work at night when the water heater gets super cold" Actually, any competent, modern tanked water heater will hold usable temperature for at least 12 hours after power loss; and if it doesn't because it's old, they make rockwool insulation kits for them. Of course, hot water won't be replaced as it's used, but yeah, you can get a shower before the system turns the heat back on.

"Storing water around 100F, what could possibly go wrong?" Actually it'll breed all sorts of bacteria, including legionella. (you may not have heard this because it's relatively new science, partly arising from the fiasco in Flint, Michigan. Tiny amounts of bacteria were in water pipes because of a botched water-supply changeover [from the regional lake-water system shared by most suburbs, to obsolete plant drawing from the Flint River]. That, plus the "petri dish" effects of water heaters kept at 40-55C, created a perfect storm.)

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We’ve used heat pumps for residential hot water for ~15 years, the first from Air Generate, and our current one a Geyser-R unit from Nyle. Neither is currently made. These are in our basement, and work best if one has a large surface area slab, in our case, ~1200 sq ft. They work by taking heat from the air and transferring to the water, with the cold air sinking and being warmed by heat transfer through the slab from the soil. In our case, summer air temperatures are in the low 60’s, & winter, mid 50’s. Running the unit does little to change the air temperature and in the summer, we get free dehumidification. In our installation, these are 2-4x as efficient as electric. I fear that many install these in very small areas, or in areas that are heated/cooled by the house HVAC. They won’t work nearly as well in these situations.

The Nyle Geyser didn’t have a national support network, so I had to figure out problems myself with remote assistance from their technicians. Ours failed when a power relay got stuck. I replaced it with a $7 solid state relay, which does the job. Nyle seems to have switched to making large industrial units, abandoning the residential market.

To answer the original question: I’d guess that a pool heater could work, though there may be UL code requirements to make sure that refrigerant doesn’t contaminate the domestic hot water. The Geyser uses a clever double wall tube that both injects the hot water and returns cool water through the hot water tank drain valve, but it would be possible to inject the hot water through the drain valve and get the return from the cold water supply line. The lattter is shown as an alternative means to plumb the Geyser into the system.

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