Hi, this week is forecast for sunny days and frosty nights. Usually I heat water overnight on cheap power, but that triggers defrosting craziness, so for the last few days I’ve overridden hot water cycle and run it midday from solar instead. (Pity there’s only one schedule - having to keep editing it.)
Legionella cycle is due tonight. Will it run even if I have the hot water operation turned off via MMI ? (I’m happy for it skip a week.)
Does legionella use immersion only, or both HP and immersion? I tend to schedule HP-heating prior to the legionella anyway, so it starts from a relatively hot place.)
It runs even when hw is turned off. For this reason, added to the fact that I am away for extended periods, I turn the legionella off and instead schedule one comfort cycle every week where comfort is 60c. I am not sure if this is strictly correct but my reasoning is that its better to let the tank go cold than have a once per week legionella cycle which leaves it luke warm most of the time I am away.
…and in Summer the heat pump can just about make 60c without or with minimal immersion but in Winter the immersion is used more at the top of the temperature range.
What’s the general opinion about needing to run the legionella cycle? At present I have mine disabled, but I’m happy to be convinced that it is both necessary and effective.
I’m thinking:
I get that the Daikin immersion element can raise the temp of the top third of the tank hot enough to kill the bacteria, but is that a wide enough scope? What about the water that’s already in the pipe work closer to the taps? What about the water that is in the bottom third of the tank? Additional precautions would surely be required beyond running the heat pump’s Legionella cycle once a week.
In a normal occupied household, that uses enough hot water to need daily heating of the tank, is there sufficient stagnant water in the DHW system to allow enough bacterial contamination to create a problem?
Ultimately it’s your call. Personally I heat to 43 and only bother with a legionella cycle if I’ve been away for a couple of weeks with no hot water heating, and feel comfortable with that.
Is the legionella cycle anything more than “heat it to 60 degrees once a week” ?
If you’re not using the water, won’t it spend quite a lot of time at legionella-growth temperatures? Perhaps better to just let it get cold and stay cold, and disinfect once when you get back ?
Sorry if I have misunderstood your response but the comfort cycle is part of the normal schedule so switches off fully when I switch hw heating off… so it all goes cold when i am away.
It needs something like 50c for 20 minute to kill 100% of the bacteria. As the immersion heats the water next to it at say 80c, that water moves to the top of the tank and pushes the water from the top down the sides of the tank to the bottom.
The heatpump monitors the tank temperature with a sensor that is about 1/3 from bottom and keeps the immersion on until it have been at 60c for a considerable length of time.
This does not work for large tanks, hence hospitals etc have a pump fitted from bottom to top of tank and keeps heating until the water leaving the bottom is over 60c.