Search with Wattson

Notifications

Clear all

Joining the Renewable Heating Hub forums is completely free and only takes a minute. By registering you’ll be able to ask questions, join discussions, follow topics you’re interested in, bookmark useful threads and receive notifications when someone replies. Non-registered members also do not have access to our AI features. When choosing your username, please note that it cannot be changed later, so we recommend avoiding brand or product names. Before registering, please take a moment to read the Forum Rules & Terms of Use so we can keep the community helpful, respectful and informative for everyone. Thanks for joining!

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?

Page 2 / 2 Prev

Domestic Hot Water & Cylinders

Last Post by JamesPa 4 months ago

13 Posts

2 Users

1 Reactions

1,552 Views

RSS

JamesPa

(@jamespa)

Illustrious Member Moderator

Joined: 3 years ago

Posts: 5370

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.

ReplyQuote

Page 2 / 2 Prev

27 Forums
2,760 Topics
65 K Posts
2,212 Online
7,167 Members

Forum Icons: Forum contains no unread posts Forum contains unread posts

Topic Icons: Not Replied Replied Active Hot Sticky Unapproved Solved Private Closed

RE: Where heat loss surveys seem to go against practical reality

It's not there because it is January data (was clear in...

By cathodeRay , 28 minutes ago
RE: Yutaki M (RHUE-3AVHN) + XEK23232A System Controller — DHW reheat randomly fails to start, no fault code, fixed only by power-cycling controller

There arent many of these hitachi pumps around here so ...

By JamesPa , 2 hours ago
RE: Next steps on my ASHP journey: Radiators and Pipework

I think trying to deduce flow rate from an instantaneou...

By JamesPa , 2 hours ago
RE: 7kW EV Charger Options Advice

I’ve had an EV with wall charger for over 3 years and t...

By AndrewJ , 4 hours ago
My ASHP is finally installed!

My Adapt is finally installed and running! Almost a yea...

By tomasmcguinness , 4 hours ago
RE: Vaillant Arotherm – Using Heating

Batpred thank you

By trebor12345 , 3 days ago
RE: Being screwed over by Volkswagen Audi Group

Thanks, @batpred. That integration is the one I have in...

By Majordennisbloodnok , 3 days ago
RE: How Accurate Is My DIY Heat Loss Calculation?

@batpred I suspect it will be more this year. I had 9 ...

By AndrewJ , 3 days ago
RE: Surface mount or bury primaries for heat pump installation?

@jamespa It's rusting 🙂

By tomasmcguinness , 3 days ago
RE: Glass lined Cylinder for private water supply

Thank you, once again, for your replies. Yes...

By ian33a , 3 days ago
RE: Heat Pump Books For Beginners: The Ultimate Guide to Heat Pumps, Bodge Buster, From Zero To Heat Pump Hero & Britain's Heat Pump Shitshow

It’s been out a few days and Amazon has already attache...

By Mars , 3 days ago
RE: Missing flow temperature in Heat Geek proposal

Yes, and this is another point I agree. We did find a f...

By Batpred , 3 days ago
RE: Problems with sizing a replacement for a Viessmann Vitocal 200-S

A very good point. In effect, the DHW pipework is an em...

By cathodeRay , 3 days ago
RE: Solar Power Output – Let’s Compare Generation Figures

@tim441 Here are those same three month periods for 202...

By Toodles , 5 days ago

Renewable Heating Hub Heat Pump Forums

Debugging my legionella immersion heater

SPONSORS

Join Us!

Installer Finder

Degrees of Separation

Latest Posts

SPONSORS

Members Online