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Legionnaires and heat pumps

Air Source Heat Pumps (ASHPs)

Last Post by prjohn 5 years ago

4 Posts

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Justin

(@justinsb)

Estimable Member Member

Joined: 5 years ago

Posts: 100

Topic starter 10/12/2021 6:54 pm [#218]

I'd like to find out what people think about the Anti-Legionnaires settings on the Ecodans (& probably other ASHP's too). Look at @rv3's power usage graph a few posts back. Look at the orange DHW (Hot Water) readings. Every 15 days, look at the Anti-Legionnaires peak. I'm not advising anyone to do anything that might pose a health risk, but I am interested in an honest discussion about it. Particularly since the insulation on those tanks means that it keeps heat in beautifully. Does anyone know how long & at what temp we have to keep it to be safe? Also, how often?

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Kev M

(@kev-m)

Noble Member Member

Joined: 5 years ago

Posts: 1276

10/12/2021 9:54 pm

Good topic.

I think the risk of catching Legionnaires from an ASHP heating/DHW system is very, very small. It's primarily an airborne pathogen and since most ASHP systems are unvented, it's hard to see how it would get into the water. It could concievably enter at the water treatment plant but these are quite sterile environments and a lot of our water has stuff added to it that is likely to kill off any nasties.

If any make it into your DHW, they won't be very happy. This from Wikipedia.

Temperature affects the survival of Legionella as follows:^[3]

Above 70 °C (158 °F) – Legionella dies almost instantly
At 60 °C (140 °F) – 90% die in 2 minutes (Decimal reduction time (D) = 2 minutes)
At 50 °C (122 °F) – 90% die in 80–124 minutes, depending on strain (D = 80–124 minutes)
48 to 50 °C (118 to 122 °F) – can survive but do not multiply
32 to 42 °C (90 to 108 °F) – ideal growth range
25 to 45 °C (77 to 113 °F) – growth range
Below 20 °C (68 °F) – can survive, even below freezing, but are dormant

So the Legionnaires cycle decreases the risk from very, very small to vanishingly small. If your DHW is at 50C constantly anyway, 60C for 3 hours once a week isn't going make a lot of difference.

Should you do it/keep doing it?

Only you can decide. 😉

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Mars

(@editor)

Illustrious Member Admin

Joined: 6 years ago

Posts: 4809

10/12/2021 11:08 pm

@kev-m, that’s all bang on. I didn’t realise Legionnaires died so quickly at over 60C. Interesting.

Our DHW is set to 44C. The heat pump was set to take it to 60C for a least an hour once a week. I changed that to 10 days. We have an iBoost that heats our hot water to over 60C using excess solar - today we produced loads of solar so the ASHP system has reset its 10 day countdown.

The likelihood of getting Legionnaires via your tank is slim, but I still keep our settings activated and let the iBoost do its thing, as we’d rather be safe than sorry. In a world that’s going more and more bonkers with each passing day, we’ve opted to be cautious.

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