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I've recently become aware of the existence of 'hybrid water heaters' which use a heat pump to heat the water in the tank. The thing that is appealing to me about this is that the water heater also works as a dehumidifier. That should, at the very least reduce the need to run a dedicated humidifier and are supposedly even cheaper to run than gas heaters (this probably depends on the cost of electricity and gas)

The examples I've seen all use an electric resistance heater as a backup which is really expensive to run. Are there hybrid water heaters that have gas backup? I've seen some discussions about using a tankless heater to augment the hybrid heater but even if that works, it's a lot of up front cost.

Do such units exist? If not, is there a something about this idea that wouldn't work or is it just that there's no demand or perceived demand?

For context, I've been watching the usage on my dehumidifier for the last 65 days and it's used about 170KWH. This is probably the highest demand period for that, though.

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  • Probably no demand. But if you have gas, then a generator might be a better investment than a tankless heater.
    – longneck
    May 29 '20 at 19:30
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    @longneck, the "backup" is for when you need more water than they hybrid system can heat. They are slower to heat than a standard water heater. Just like a home heat pump has "aux" heat when its extra cold outside.
    – JPhi1618
    May 29 '20 at 19:33
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    The issue with your questions is that you are assuming the total cost of the heat pump and electricity as backup is too expensive. Where is the facts behind "electric resistance heater as a backup which is really expensive to run." The backup electric usage would only come into play at very low temperature. The unit is located in the house or garage, not outside. Your home heating is also providing some of the heat in the air, that the heat pump is extracting to heat the water.
    – Programmer66
    May 29 '20 at 19:53
  • @Programmer66 I'm not sure if you are asking me why I think running electric heat is more expensive than gas but I think that's pretty much common knowledge. But the other part of your comment is interesting. That was something I am unsure of. If the backup doesn't come into play in normal use when capacity is exceeded, then maybe it's more a question of whether these can keep up with our demand.
    – JimmyJames
    May 29 '20 at 20:39
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    You could go the whole year and never use the alternate electric mode. Stay within the 40-50 gal usage and electricity will only be used to run the heat pump.
    – Programmer66
    May 29 '20 at 22:27
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The problem I see is that a gas water heater has a big pipe/flue running through the center of it which takes away a lot of room that the hybrid system probably needs. It also vents very hot gas out of the top which is where the compressor is mounted in a hybrid. The electric backup elements are tiny in comparison and just need to stick into the side of the heater. People with gas, which is normally cheaper than electric, don't have as compelling of a reason to choose a hybrid heater, so I doubt there is much demand.

So, very complex and low demand... Probably doesn't exist. I've never seen one.

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  • Yeah, that was kind of what I was thinking. I found this paper which has a section titled 'Condensing Gas "Hybrid"' but I'm not sure if that's something else.
    – JimmyJames
    May 29 '20 at 19:42
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    @JimmyJames, looks like those are called "Hybrid" because they are basically the main parts for a gas tankless heater attached to a small tank with a few other features to give better, "buffered" water output. Condensing just refers to high-efficiency gas heaters where even the exhaust gas is used in the heating process and the output is air that is cool enough to be vented through PVC. They also require a drain line, because burning natural gas produces water vapor, and cooling the exhaust "condenses" the vapor into liquid water.
    – JPhi1618
    May 29 '20 at 19:49
  • "Condensing just refers to high-efficiency gas heaters where even the exhaust gas is used in the heating process and the output is air that is cool enough to be vented through PVC" Right, that thought occurred to me after I added the comment.
    – JimmyJames
    May 29 '20 at 19:51

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