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Debugging my legionella immersion heater

Prunus · 2026-04-08T08:52:06Z

My legionella cycle has never worked, and I'm trying to work out why I have a Grant Aerona3 13kW and I've wired it so the ASHP controls the legionella cycle (disconnecting the spurious Grant timer box they install). The way this works is the ASHP runs up the hot water to the maximum temperature it can manage, then hands over to the immersion heater for the last few degrees to get it to 60C. On the standard setup it has 2 hours to do that - there's a service manual parameter to extend it to 3 hours, which I've done. After 3 hours the water is only at 50C and the ASHP flashes 'LC' meaning legionella cycle failed. Next week comes around and it does legionella again and the same happens... every week. To cut a long story short of pondering various control system things (eg what if the pump is running during the immersion cycle, won't that be using the DHW coil to cool the cylinder?), I turned off the ASHP completely and just manually powered the immersion (it goes via a contactor which I can enable independently). I have a power meter on it so I can see the current. And this is what I see: It seems the hot water temperature is plateauing at 52C. I confirmed this with two thermometers in the drawn water. I could also see that the current had dropped away, ie the immersion had cut out even though the contactor was still enabled. I took off the back of the immersion (Thermowatt 340160900000 aka 3401609) and I saw this: A bit hard to see but the dial was about 90% of the way anticlockwise towards the + symbol. I turned it anticlockwise all the way: then restarted the immersion and I got: so 57C is the max it will go. According to some calculations, 3kW (confirmed on the meter) into a 200 litre cylinder should cause a rise of 12.9degC per hour, or roughly a degree per 5 minutes. The immersion is at the bottom of the cylinder as you'd expect and the pocket with the thermistor that the ASHP is reading is halfway up (the immersion is not being externally controlled here, just using its own thermostat). The hot water is being drawn off the top of the cylinder.I can't find any manual for this immersion, but the only downloadable manuals on the Thermowatt website say the factory set should be 65C or 75C for various models. That's also what the label seems to imply: It seems to be really struggling to heat the water. I think this points to a faulty thermostat on the immersion, but before I go draining everything down to change it did I perhaps miss something?

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JamesPa

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17/05/2026 7:50 pm

Posted by: @prunus

↑

Now it doesn't actually manage 70C, it only gets to 66, but that's enough for the ASHP to be happy. It's also rather hot - I never worried about having separate taps instead of mixers until now! I'll probably back it off a bit, but for now seems to be much improvement - £23.25 well spent!

I notice this (66 is rather hot) with my legionella cycle, which in my case is done natively on the ASHP. In the accompanying ASHP app there is even a warning about it!

Who said ASHPs werent capable of producing very hot water!

4kW peak of solar PV since 2011; EV and a 1930s house which has been partially renovated to improve its efficiency. 7kW Vaillant heat pump.

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