General > General Technical Chat
What is the real story around heat pumps?
zilp:
--- Quote from: Siwastaja on February 24, 2024, 03:39:56 pm ---And this is definitely safer than than the "official" way of doing it offered by heatpumps, namely heatpump maintaining 50-55degC in DHW tank, which is a tad too low, with a 60-65degC "legionella" program running once a week. The risk is this program being too short or failing somehow. Even then, I'm not aware of any actual problems.
--- End quote ---
Well, the preferred way is to not have a DHW tank at all, but only a storage tank with a heat exchanger for DHW, so that there is no significant volume of DHW at problematic temperatures, and what is there is cleaned out completely whenever you use hot water.
--- Quote from: Siwastaja on February 24, 2024, 03:39:56 pm ---You probably did not understand what I suggested from my description in the first place, and being lazy I'm not going to try to draw a picture of it right now so just reread carefully my (possibly poor) explanation. Single storage for house heating -> pre-heated lukewarm DHW through existing heat exchanger -> small electric boiler in series to final temperature. This small boiler sits at high temperature all the time.
--- End quote ---
I am still not entirely sure what exactly you mean, but also, my response wasn't necessarily relevant to your specific setup, just a warning of something to consider.
If you mean a storage tank filled with non-DHW that is kept warm by the heat pump and a DHW heat exchanger in that tank (I think that that is what you are describing?), then that should be fine because there is no significant (dead) volume of DHW kept at problematic temperatures for a long time, so, with regular use, you shouldn't ever get any persistent high legionella concentrations.
The problem arises if you have a storage tank with DHW in it, which you could possibly keep at 30 to 40°C with the house heating heat pump as well, but then I would think that an electric boiler at the output heating the water to 60°C would not be sufficient to kill the legionella if they were to get established at high concentrations in the tank.
NiHaoMike:
--- Quote from: Siwastaja on February 24, 2024, 03:39:56 pm ---And this is definitely safer than than the "official" way of doing it offered by heatpumps, namely heatpump maintaining 50-55degC in DHW tank, which is a tad too low, with a 60-65degC "legionella" program running once a week. The risk is this program being too short or failing somehow. Even then, I'm not aware of any actual problems.
--- End quote ---
In the US, the solution (at least for the majority on city water) is the water has chlorine or chloramine added. The recommended hot water temperature is 120F as the risk of burns increases dramatically beyond that.
Another solution would be to have a UVC light inside the tank. My guess as to why that's not common is simply because there's no need for it with city water.
Siwastaja:
--- Quote from: zilp on February 24, 2024, 04:39:14 pm ---Well, the preferred way is to not have a DHW tank at all
--- End quote ---
DHW tanks are super popular here, during the era when electric radiator heated houses (with no hydronic distribution) were built starting from 1980's (after the oil crisis). Most are made of stainless steel and it was really popular to automagically time them to use nightime tariffs for which 300 liters was a usual volume and 3kW the usual power. Smaller volumes for continuous operation. No one here considers them problematic at all. They are clean from biological growth because of the temperature, and last easily for 30-40 years. But clearly different solutions are popular in different countries. The small ones cost just a few hundred €. It's easy to put one in series to get to high enough water temperature if you have an "easy" (and high COP) source of preheated but not warm enough DHW, which is often the case in monoblock air-to-water retrofits in existing systems which did earlier use higher temperature in a tank with heat exchanger as explained by you.
nctnico:
--- Quote from: NiHaoMike on February 24, 2024, 04:58:45 pm ---
--- Quote from: Siwastaja on February 24, 2024, 03:39:56 pm ---And this is definitely safer than than the "official" way of doing it offered by heatpumps, namely heatpump maintaining 50-55degC in DHW tank, which is a tad too low, with a 60-65degC "legionella" program running once a week. The risk is this program being too short or failing somehow. Even then, I'm not aware of any actual problems.
--- End quote ---
In the US, the solution (at least for the majority on city water) is the water has chlorine or chloramine added. The recommended hot water temperature is 120F as the risk of burns increases dramatically beyond that.
--- End quote ---
That is too low to protect against legionella. And there can be other parasites in the water as well which are chlorine resistant AND can make you quite sick. So I'd keep the water temperature high. However, I can highly recommend getting a thermostatic tap for shower(s) and bathroom sink(s). They are limited to a safe temperature. And it saves quite a bit of water because it reaches the setpoint temperature quicker compared to manual mixing.
Siwastaja:
--- Quote from: nctnico on February 24, 2024, 06:56:23 pm ---That is too low to protect against legionella.
--- End quote ---
This underlines how cultural differences affect engineering. In the land of freedom and lawsuits, you use chemical warfare against the threat, but you want to avoid hot water being hot because someone could get burns and sue. I'm sure it works there. Here we just try to carefully adjust our mixing valves to 60degC which is just enough to kill legionella and just low enough to at least avoid serious burns even though someone who is very careless could get minor burns. And we think it's parent's job to keep children away from dangerous stuff.
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