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Does legionella cycle also heats up radiators?

 
Air Source Heat Pumps (ASHPs)
Last Post by Mars 9 months ago
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 bbmorales
(@bbmorales)
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Topic starter 08/06/2024 7:17 am  

Hello. Beginner's question:

I have a system with a ASHP and a hot water tank. 

When the legionella cycle (Friday 1pm for 1 hour) kicks in my radiators also heat up. Is this supposed to happen?

I am suspecting the valve that controls where the hot water from the pump goes (diverting to DHW or Heating) is not working well, and DHW is leaking to the radiators.


   
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Mars
 Mars
(@editor)
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08/06/2024 8:04 am  

Welcome to the forums @bbmorales

No, that shouldn’t be happening. It sounds like you’re on the right track with your diagnosis though. During the legionella cycle, only your hot water tank should be heating up, not the radiators. This suggests that the diverter valve may be faulty and allowing hot water to flow into the heating circuit. A photo of the diverter valve would be helpful.

The other question this poses is that I assume your heating is off at the moment and you’re only using your heat pump for hot water production. So why is your system coming on at all and distributing heat to your rads. Does your circulation pump come on when the legionella cycles starts? 

This post was modified 9 months ago by Mars

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 bbmorales
(@bbmorales)
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Topic starter 09/06/2024 7:47 am  

Hi @editor Mars, many thanks.

Regarding the images, I've uploaded two, hopefully this gives you an idea of the system.

Regarding your other question 

"Does your circulation pump come on when the legionella cycles starts? "

Apologies for the question, but where is this pump and how to I verify this?

I have not disabled completely the heating, but I can try doing so next Friday (disable completely the heating, leaving on only DHW).

Thanks for your help!


   
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Mars
 Mars
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09/06/2024 8:24 am  

@bbmorales let's backtrack a little bit. Can you please share more information about the system you have (brand and model of heat pump would be a good starting point)? We'll need to ascertain whether your heating is in fact off.

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Morgan
 Morgan
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09/06/2024 3:30 pm  

@bbmorales 

There is no need, imo, to run a legionella cycle as often and it would be better run during the night on cheaper rate electricity. The ASHP heats to 50c and the immersion takes over to 60c. Expensive on day time rate on a weekly basis. Needn’t be for as long as an hour either so far as I know.

Retrofitted 11.2kw Mitsubishi Ecodan to new radiators commissioned November 2021.
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 HughF
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15/06/2024 7:24 am  

Faulty 3 port diverter….

Easiest fix is to disable the legionella program.

Off grid on the isle of purbeck
2.4kW solar, 15kWh Seplos Mason, Outback power systems 3kW inverter/charger, solid fuel heating with air/air for shoulder months, 10 acres of heathland/woods.

My wife’s house: 1946 3 bed end of terrace in Somerset, ASHP with rads + UFH, triple glazed, retrofit IWI in troublesome rooms, small rear extension.


   
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 bbmorales
(@bbmorales)
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Topic starter 15/06/2024 8:19 am  

Hi All, thanks for your answers.

@editor 

I have a Vaillant AroTherm plus 5kw.

Regarding the legionella cycle:

@hughf I had the impression it is important to do the cycle to keep bacteria from growing. Why would you disable it completely?

Thanks


   
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Mars
 Mars
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15/06/2024 12:57 pm  

@bbmorales it's a personal preference and depends on the likelihood of bacteria proliferating in your tank. We run a Legionella cycle every two weeks. Since we work from home and use hot water frequently, the water in our cylinder is constantly being drained and replaced, preventing stagnation. This routine works for us. However, some homeowners choose to turn it off completely.

Buy Bodge Buster – Homeowner Air Source Heat Pump Installation Guide: https://amzn.to/3NVndlU
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