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Ecodan - Legionnaires Cycle

Air Source Heat Pumps (ASHPs)

Last Post by HJD 2 years ago

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HJD

(@hjd)

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Joined: 4 years ago

Posts: 40

Topic starter 05/12/2023 7:57 pm

Does anyone else observe this behaviour with their Ecodan?

When the Legionnaires cycle kicks in, the ASHP raises the LWT to around 60 degrees very quickly but as soon as it gets there it peaks and then starts falling again, at which point the immersion kicks in.

However, I never really get the tank temperature up to 60 degrees. I would have thought the ASHP could have operated for longer at 60 degrees LWT and use that to do a longer job of heating up the tank.

Is it something to do with how long I have set the ‘hold at temperature’ setting for? I thought this was referring to holding the tank temperature but perhaps it means holding the LWT temperature?

Thoughts?

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Kev M

(@kev-m)

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Joined: 4 years ago

Posts: 1276

06/12/2023 8:53 am

Posted by: @hjd

↑

Does anyone else observe this behaviour with their Ecodan?

When the Legionnaires cycle kicks in, the ASHP raises the LWT to around 60 degrees very quickly but as soon as it gets there it peaks and then starts falling again, at which point the immersion kicks in.

However, I never really get the tank temperature up to 60 degrees. I would have thought the ASHP could have operated for longer at 60 degrees LWT and use that to do a longer job of heating up the tank.

Is it something to do with how long I have set the ‘hold at temperature’ setting for? I thought this was referring to holding the tank temperature but perhaps it means holding the LWT temperature?

Thoughts?

I hadn't looked at this before but I did and this is what mine does. This was one of mine in October. I don't know if the immersion is used; I didn't think it was but I have never looked. Looks similar to what you describe.

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SKD

(@skd)

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Joined: 3 years ago

Posts: 55

06/12/2023 10:10 am

Posted by: @hjd

↑

Does anyone else observe this behaviour with their Ecodan?

When the Legionnaires cycle kicks in, the ASHP raises the LWT to around 60 degrees very quickly but as soon as it gets there it peaks and then starts falling again, at which point the immersion kicks in.

However, I never really get the tank temperature up to 60 degrees. I would have thought the ASHP could have operated for longer at 60 degrees LWT and use that to do a longer job of heating up the tank.

Is it something to do with how long I have set the ‘hold at temperature’ setting for? I thought this was referring to holding the tank temperature but perhaps it means holding the LWT temperature?

Thoughts?

With a 60degC legionella set point, my ASHP takes the DHW temperature up to 50degC and then goes to standby, and the immersion heater takes over for the rest of the legionella cycle. I've seen it work the same way at a 65degC set point too. The immersion seems happy to support whatever duraiton I specify for holding the tank temperature. The immersion is the 3kW Ecodan element that came with their water cylinder.

What rating of immersion element do you have?

Mitsubishi Ecodan 11.2kW R32 ASHP; Ecodan DHW cylinder; UFH+rads
20x430W Jinko TOPCON Tiger Neo solar; Luxpower 6+4kW hybrid inverter; 20kWh LFP battery storage

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Rod

(@rod)

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Joined: 3 years ago

Posts: 24

06/12/2023 12:08 pm

@skd I think the pump will raise the LWT temp to the setting for the maximum heat pump flow temperature. I increased that to 60C and during the legionella cycle it raised the tank temp and handed over to the immersion. My actual normal Hot Water tank temp is 46 - we have a big tank and low usage.

Unfortunately my issue it that I have two thermostats: one on the immersion and one measuring temperature for the heat pump. The immersion is in the middle of the tank, not the bottom and stratification happens quickly when it's running so the tank immersion cuts out even though it's set at it's maximum thermostatic setting because the layer of water it's in is over the temp, however just a little below it is the heat pump temperature sensor where it can be 5-20c lower temp, and even lower towards the bottom of the tank.

Result: Heat pump gets the whole tank well over 50 then stops, immersion takes over until it cuts out when the top layer of the tank is maybe over 65 but I can't heat the whole tank to 60, I can get most of it well above 50 and then I try and make sure it's left there for at least 4 hours which is a fair legionella killer.

In an unvented system, I really don't feel the legionella risk is high and this is reasonable mitigation once every 30 days.

I need a destrat pump.

👍

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SKD

(@skd)

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Joined: 3 years ago

Posts: 55

06/12/2023 5:57 pm

@rod thanks, yes that figures. My max ASHP temp is currently set to 50degC. I need to increase it too

Mitsubishi Ecodan 11.2kW R32 ASHP; Ecodan DHW cylinder; UFH+rads
20x430W Jinko TOPCON Tiger Neo solar; Luxpower 6+4kW hybrid inverter; 20kWh LFP battery storage

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HJD

(@hjd)

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Joined: 4 years ago

Posts: 40