[Ayoob]

I saw something about these water heaters that made me think of a friend of mine. They work on the same principle as a air conditioner heat pump. Suck the heat out of the surrounding air and put it into something else. If you could suck the heat out of an overheated house all day long and use it to raise the temp of 80 gallons of water, it would be pretty fucking hot by nighttime. Feed that water through a copper coil zip tied to the face of one of those big stainless steel three-legged fans that move huge amounts of air. That plus a thermostat to measure how much heat is left in the reservoir vs how much you need to maintain current temperature.

I'm not an engineer, but that just sounds like an interesting challenge to me.

Would there be a way to use the hot water to create electricity to be used for recharging batteries or something? Sell the power back to the grid?

[Tanada]

I saw something about these water heaters that made me think of a friend of mine. They work on the same principle as a air conditioner heat pump. Suck the heat out of the surrounding air and put it into something else. If you could suck the heat out of an overheated house all day long and use it to raise the temp of 80 gallons of water, it would be pretty fucking hot by nighttime. Feed that water through a copper coil zip tied to the face of one of those big stainless steel three-legged fans that move huge amounts of air. That plus a thermostat to measure how much heat is left in the reservoir vs how much you need to maintain current temperature.

I'm not an engineer, but that just sounds like an interesting challenge to me.

Would there be a way to use the hot water to create electricity to be used for recharging batteries or something? Sell the power back to the grid?

I looked into these in 2004 when I bought my house, I assume the technology is better now. The two most common applications are in laundromats and commercial kitchens where a lot of waste heat is generated and a lot of hot water is needed for washing. Installed properly they can cut the energy demand for a laundry facility by 50 percent and over 25 percent for a restaurant that does a lot of both cooking and cleaning .

The ones I looked at had supplemental heating elements so that if the ambient heat was not sufficient they would still supply very hot water early in the day when waste heat was less available.

[Pops]

I bought one because the utility had a rebate of $500. My plan is to locate it in the utility room just off the kitchen next to the washer/dryer and fridge and some nice south facing windows (maybe french doors)

The big question is the noise. I'm thinking a little enclosure made of louvered doors and some sound absorbing material?

[KaiserJeep]

Pops, the key to keeping down noise is isolating the floor and walls from vibrations from compressor and blower motors. Sound insulation no matter how good, mostly suppresses the higher frequencies, it is ineffective for lower frequency mechanical noises. That is why conventional space heating/cooling heat pumps put the biggest motor outside and on a concrete pad.

In the case of a water heater, get the best quality unit with the lowest acoustic signature, and learn to live with it. The Europeans have been building these for decades, and their current designs are quite good. Here in America, the manufacturers are still trying to reduce the price penalty vs. a conventional FF heater, and it shows in noise and reduced reliability.

Point-of-use or tankless water heaters are a good compromise on a grid home with a gas line, but unsuited for most off-the-grid applications because of the sudden intense energy consumption. If you size your system correctly or are confident of LP supply availability, it's a different story.

Then there are always these:

...water-to-water heat exchangers are really efficient, so if you are burning wood anyway, why not?

[Smurfs1976]

I've had one of these water heaters for 6 years now. No problems with it and the energy saving is 65% over 12 months (measured).

It's running fine on off-peak electricity supply in Tasmania (Australia), power supply being on between 2pm and 4:30pm of an afternoon and also overnight from 8pm to 6am each day. The system has no booster heat source of any kind.

Tank capacity is 327 litres and input power rating is 1.4kW (so about 4.2kW of heat input typically when running). In Summer it generally runs during the afternoon period only, in Winter it runs for a while in the evening as well.

Cost - I needed a new water heater anyway and around here an electric one is about AUD $800 plus installation. This one cost $2650 installed but the savings on power have recovered the additional cost after about 5 years so I'm quite happy with that. System has a 10 year warranty on the stainless steel tank.

I looked at the options when needing a new water heater and this seemed like the winner. Solar was problematic since there's already a lot of PV panels on the roof (grid-connected system, 6.69kW all up) using the best roof space. No natural gas and LPG (propane) is fairly expensive at about AUD 2.75 per kilogram (equivalent to about USD $4 per gallon at the moment). So the sensible choice was some form of electric system.

So I can verify from 6 years experience that these systems can heat water reliably with no booster element needed. That said, the weather here doesn't generally go below 0'C (32' F) so it might be a problem in a cooler climate.

[PrestonSturges]

The old coal fired water heaters were about the size of fire hydrant. A friend in old house found one in his junk room, hooked back up in its spot in the basement and used it for heat. it would hold one large chunk of kindling.