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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, 2020 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, 2020 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. May 29, 2020 at 19:53
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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. May 29, 2020 at 22:27
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    @JimmyJames -- split-systems don't provide that dehumidification, but do have the advantage that they don't "parasite load" the heating system during winter (which can be a significant issue in a high-performance envelope). Furthermore, the SanCO2 system has no need for electric backup heat Jun 2, 2020 at 14:33

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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, 2020 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, 2020 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, 2020 at 19:51
  • Gas as an auxiliary to an electric heat pump in one tank would be a nightmare. I cannot believe it exists. Gas fired tank water heaters heat through the bottom and have a flue through the center of the tank. This would be a large route for heat leakage through those surfaces. Electric resistance heating elements project into the tank from the side and have very little heat leakage. Feb 29 at 0:50
  • Where is your water heater located? In a tight closet or in a moderately open room such as a basement? Feb 29 at 0:53
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If you get past the pure economics of a hybrid water heater running in electric resistance mode when it's cold, there's still another consideration. Keeping the heater out of resistance mode when it's cold outside is going to make the room the heater is in cold. That's great in the Summer, but it's the last thing one is going to want in the Winter...producing hot water at the cost of a cold room. Set the heater to pure resistance heating in the Winter though, and the utility bill is going to skyrocket. Thus, the idea of gas as a secondary energy source is a good one from an economics standpoint. Many HVAC manufacturers offer "dual fuel" options allowing a heat pump to automatically switch to gas when the outside temperature gets cold, so the engineering is plausible. The heat pump being on the top of the water heater may constrain the design though, and perhaps this is why don't seem to be around.

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  • Also think about just how much parasite loss a gas heater has up the flue when the burner isn't firing...and just how few tanked condensing gas heaters there are on the market. Feb 29 at 3:34
  • Welcome to Home Improvement, please take the tour. Then, feel free to edit your answer to include some formatting - that will make it easier to read and understand. "Wall 'o text" isn't a good communication format...
    – FreeMan
    Feb 29 at 13:21
  • @ThreePhaseEel, the availability of a certain type of heater depends very much on the local market. Over here in the Netherlands, tanked condensing gas heaters are rare (we typically have tankless condensing gas heaters), but in Germany they are almost the norm. And then I am talking about two neighboring countries. Feb 29 at 15:06

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