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Why run a disinfectant cycle ?

 
Air Source Heat Pumps (ASHPs)
Last Post by mjr 4 years ago
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 heat-pump-newbie
(@heat-pump-newbie)
Reputable Member Member
Joined: 4 years ago
Posts: 170
Topic starter 16/03/2022 2:00 pm   [#361]

My heat pump has been dutifully running the disinfectant cycle every week, but I read and hear that the jury is still out on whether this is really necessary. What do other people think ? 

I have to go into Installer mode to change it, and I can only run it at the same time every week, or not at all. As it uses a significant amount of energy I would have liked to be able to control the timing eg run it when the grid is producing lower co2 electricity, or when the outside temperature is warmest.

If it's not really necessary I'm going to switch it off and get a much better COP.



   
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DHW disinfectant legionella
 Kev M
(@kev-m)
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Joined: 5 years ago
Posts: 1276
16/03/2022 2:25 pm  
Posted by: @heat-pump-newbie

My heat pump has been dutifully running the disinfectant cycle every week, but I read and hear that the jury is still out on whether this is really necessary. What do other people think ? 

I have to go into Installer mode to change it, and I can only run it at the same time every week, or not at all. As it uses a significant amount of energy I would have liked to be able to control the timing eg run it when the grid is producing lower co2 electricity, or when the outside temperature is warmest.

If it's not really necessary I'm going to switch it off and get a much better COP.

If your water is already at 50C then you're going to have to heat it 10 deg to 60.  For a 200l tank that's circa 8kwh at a COP of 1.  So a couple of quid a week at the most.  You could always have a really hot legionella-free  bath to make up for it.

On the other hand the risk is extremely low (1) of legionella getting into your unvented cylinder and (2) you catching it from said cylinder.  

It also doesn't like even moderately hot water (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

 

I would have thought the risk with HW at 48 deg is vanishingly small and that you're in more danger from scalding yourself with your 60 degree, guaranteed legionella free water once a week. 



   
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 mjr
(@mjr)
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Joined: 4 years ago
Posts: 301
16/03/2022 4:17 pm  

I think it is probably unnecessary but we will never hear the end of it from boiler fans if a heatpumped tank hosts an infection, so I have the standard controller set to still run the cycle the least often as a safeguard (monthly I think, at 0100GMT so cheap electricity and before the coldest part of night) and my add-on smart controller heats a tank of 52˚c water on any moderately warm late afternoon (so cheaper heating) which should kill most of it.

For best risk reduction, remember to run any seldom-used hot taps every so often, especially if the pipe to it is in a warm place such as next to heating pipes. Still water in pipes is probably a far bigger risk than the tank.



   
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