this post was submitted on 22 Nov 2023
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I've been considering replacing the central a/c in my house with a heat pump to handle home heating as well, but never thought about heating water that way also. That's interesting. Can you get a single heat pump system that does both?
Where water heating is concerned, I do know a guy who set up a solar water heater, and he made some compelling arguments for it. Like he considered rooftop solar panels initially to generate electricity, but opted for water heating instead. He pointed out that while electric panels are maybe 20% efficient, water heating is nearly 100% efficient, and his system works so well that it even needs to shut off every now and then to prevent overheating. Anyway, I'm not vouching for this personally since I have no experience with it, but I'm just throwing it out there another possible approach?
I don't think there's much for consumer single heat pump systems that do both. I've seen a few, especially with geothermal systems, but mostly it's just a tiny heat pump built into the cap of a traditional water heater.
Worth pointing out that the nature of a heat pump is that the housewide heat pump is first pumping warm air into the house to make it available for the water heater, which then pumps that warm air into the water. So it is just one big machine, fundamentally. Or, if your air conditioner is running, the water heater heat pump is adding some cooling to the space.
The criticism of the heat pump water heater: they're loud. A high frequency compressor buzz while operating. If you are switching to one, make sure it is located somewhere where the noise won't bother you. Mine is in a mechanical room in the middle of my house and it is annoying when operating -- I program it to run at night and close doors when going to bed. If I could do it over again, I'd put in in the (insulated) attic in spite of all the risks involved in that. More hot air available for it to use up there anyway.
at my parents house the is a 14kw heat pump that does heating and water and it's not an industrial heat pump. you can get them from daikin, but they need three phase ac.
That's a lot of good info. Thanks!
I hadn't considered the noise issue. I often wear noise-cancelling ear cans while I'm working. I wonder if they might help?
Can confirm the solar water heating works great, if it's sunny you get 60°c water by noon.
My concern is legionella. If you don't get enough sun and the water temperature stay at ~45°C for extended amount of time, isn't the water tank became an ideal breeding ground for legionella?
Powered solar heat pumps sanitize themselves once a week if temperature doesn't reach 70C at least once. Same for heat pump boilers, they are usually set to 50C but goes up to 70C weekly.
On solar, there is always a downstream heater that can heat cold water to 60C+ and must be set at least that high for legionella. My setup is like that, unpowered solar tank for free heat if available, and gas boosts it up to safe temperature and does all the work in the winter.
Chlorine is also pretty effective at killing Legionella.
It is helpful to have backup heat when using solar is sunny days are not plentiful.
https://en.m.wikipedia.org/wiki/Legionella
The tanks can be set to bring the temp up to 60° specifically to prevent this.
That's great! Good to know. I remember staying at a hotel in rural Mexico where they had a cistern on the roof. Basically just a big metal tank, and even that thing heated enough that it was comfortable to shower when I'd come back from the beach in the afternoon. So it doesn't surprise me that a purpose-built water heater could do quite well.
We have a geothermal heat pump that we put in when we built our house. It uses water (closed loops) to exchange heat/cold instead of air. But that unit also has a desuperheater that does supplement hot water. The ideal setup is to have a non-powered water heater that is warmed by the output of the desuperheater, that feeds a powered water heater. This creates a situation where the water entering the powered water heater is already pretty warm requiring less work for it.
unfortunately, my knowledge at the time wasn't where it is today, and our installer was also not well versed. So we just have a single water heater, but it is plumbed into the desuperheater at least. Ours just cycles water through the desuperheater into the powered tank. Probably still helps some but I suspect not as much.
Unfortunately in most countries in Europe the incentives for PV are much higher than solar water, which means we spend an awful lot of money supporting the least efficient of the two.
[citation required]
Unsubsidized PV is around €1 to €1.5 per Wp, IKEA offers 4200 Wp for €4321 right now. That saves at least €800 off my power bill.
A solar boiler starts at €2500 at minimum and easily goes up to €4000. It saves me €250 per year in gas. Or I could get a heat pump boiler for €1500 and save 80% of the €250.
Thermal solar was a thing when PV was stupid expensive but it makes no sense in ROI now, with PV you can power your hot water, home heat and everything else for about the same price as 2-3 solar heat collectors and a boiler.
You’re talking ROI. I’m talking efficiency. SHW is about 65%-70% efficient. PV is about 20% efficient.
Efficient with money means you get more of that world and environment as savings per dollar.
Would you choose paying €4000 to replace 600 kWh of heat pump boiler hot water with solar boiler, or 4000 kWh of basically a whole home's energy usage including hot water?
The solar boiler doesn't even heat well during all seasons, mine still uses gas to back it up at least half the days.