Ecodan unable to hit legionella target temp - what's the consensus?

Air Source Heat Pumps (ASHPs)

Last Post by benson 1 hour ago

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9jwr9

(@9jwr9)

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Topic starter 17/12/2025 8:43 am

Hi all,

We have an Ecodan 14kw (PUHZ-HW14VHA2-BS) paired with Joule Cyclone Platinum 300l cylinder and a buffer tank. Heating is all working great, however the hot water cycle defaults to the legionella cycle every day, because the temperature maxes out at 59 degrees, and cannot reach the required 60. Temperature gets up to 50 degrees within 30 mins via the HP, then HP disengages and immersion kicks in and slowly (around 1hr) gets it up to 59 degrees. It seems as though the immersion then cycles, keeping the temp at a constant 59.

Others have reported similar issues and it seems it was often an issue with the immersion thermostat having a max of 60 degrees, with a higher-rated replacement thermostat solving the issue. Others have talked about software within the FTC2 controller somehow instructing the immersion to cut out at 59, but that's a little technically beyond me.

The thermostat for the immersion is a Thermowatt T105 RTS 3 (pictured) which, from what I can tell, should be able to well-exceed 60 degrees on the max setting (which I've moved it to).

Does anybody have any further thoughts on this, or perhaps had the same issue and found a solution?

Thanks in advance.

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benson

(@benson)

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17/12/2025 8:50 am

I think the key question potentially is why you are trying to hit a target temp of 60, all of the time? Why don't you just set it at 50 which the ashp can manage, and be done with it?

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9jwr9

(@9jwr9)

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Topic starter 17/12/2025 8:56 am

@benson Thanks for the reply. Ecodan has a pre-programmed legionella protection cycle where it takes takes the hot water temperature up to 60+ degrees once every 14 hot water cycles to kill any bacteria in the cylinder. It then reverts to our regular hot water target temp of 46 degrees. But because it's unable to hit the magical 60 degrees, the legionella cycle kicks in again the next day and tries again (and fails to hit 60), and the next day, and the next... Our installer says very difficult, if not impossible, to fully disable the legionella protection cycle.

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benson

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17/12/2025 9:05 am

@9jwr9 oh ok. There will almost certainly be a way to turn it off?

Your installer will likely just be reluctant to advise on it as it will be a requirement of the install to set it up I'd imagine. We had the same (albeit not an ecodan), and the first thing I did was turn it off when they left. I am guessing you have an instruction manual, or just search on youtube.

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