Possibly a dodgy immersion heater thermostat? An immersion heater element would normally run (the 3 kW power level is right) until it reaches the set temperature then cutout; looks as though your thermostat needs attention (or replacement?). Toodles.

I've had a closer look at the immersion heater. It seems that this model (Thermowatt RTS 3 ERP) is available in 3 different versions. I managed to find the proper instructions, and the different models gave different colour adjustment knobs, deniting different temperature ranges. Mine has a yellow knob, and is only adjustable between 52 and 60 degrees!

I've turned it up to full whack, and set the Legionella cycle to start at 11:00....

@clockworks Sounds like you may have your answer there then! Regards, Toodles.

Hi Clockworks. Have a look at this existing discussion about  Ecodan - Legionella Operation Time and Target Temperature.

Not only is there heaps of technical detail, but that topic was the second occasion on which members here suggested that the Mitsubishi software is flawed in respect of the anti-legionella target temperature.

Posted by: @clockworks

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Immersion runs normally for about 20 minutes, then starts switching on and off.

Good observation. That doesn't sound right.

@editor -- can we move this discussion to the end of that other topic please?

Reading the thread linked to, very similar setup to mine.

To me, it seems like the installers have made an error by fitting an immersion that will cut off at 60 degrees, and then setting the heatpump controller to run the legionella cycle for 2 hours with a target temp of 60 degrees (as measured by a pocket sensor in the cylinder).

Chances are, the sensor will never actually read 60 degrees, so the cycle will just run for 2+ hours with the immersion cycling every few minutes. During all this time, the heatpump will be in "standby" doing nothing, and the house will cool down.

They should really be fitting immersion thermostats that enable running at above the maximum legionella setting of the heatpump controller. That way, the controller would actually be in control, not waiting for an event that will never happen, and then timing out.

Assuming I'm correct, the answer in my case will be to set the Mitsubishi's Legionella temperature to 58 degrees for now. It should then actually reach 58 degrees, hold for 10 minutes, and revert to heating mode.

I'll raise my concerns with installer.

Seems like I can't force it to run a Legionella cycle, and setting it to run at 11:00 daily didn't work, presumably because it knew it had already run a cycle overnight?

I have set it back to the usual time of 01:00, but daily, so I'll see what happens tonight.

I've also realised that I can't set a temperature lower than 60 degrees!

I'll contact the installer, try and get them to fit an immersion heater that will operate at over 60 degrees.

Posted by: @clockworks

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Assuming I'm correct, the answer in my case will be to set the Mitsubishi's Legionella temperature to 58 degrees for now. It should then actually reach 58 degrees, hold for 10 minutes, and revert to heating mode.

Yes. You've got it.

And you should definitely throw the problem back at the installers.
You musn't have the thermostat in the immersion heater and the heat-pump control fighting each other around 60°C

As for your 'workaround':-

A. I doubt the Mitsubishi software is that accurate.
You may need to drop lower than 58°C before you see any difference.

B. Others have reported that the Mitsubishi code itself is flawed.
Be prepared for the system still not working.

C. I also understand the biology of bacterial growth.
If you drop the temperature then you need to elongate the time at which the peak is being held.

And, on another issue, would you mind moving your anti-legionella cycle start-time away from it coinciding with a half-hour interval?

That's when there are surge-demands on the electricity grid because tariffs get changed at half-hourly intervals in UK.

It's 'grid-friendly' if you choose a more random time, such as 01:17

The Mitsubishi controller only allows for setting the Legionella cycle start time in hourly increments.

All the other timers/schedules are adjustable in shorter increments.

The immersion thermostat is available as a "blue knob" version, which is adjustable between 63 and 71 degrees. That's the one they should have fitted.

@clockworks It rather sounds as though the heat pump manufacturers (or rather their software programmers) are not taking on board the need to lessen surges on the grid yet😒 Toodles

Posted by: @toodles

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Posted by: @clockworks

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The Mitsubishi controller only allows for setting the Legionella cycle start time in hourly increments.

It rather sounds as though the heat pump manufacturers [...] are not taking on board the need to lessen surges on the grid yet

I'll raise that with the DESNZ committee who are creating the 'Standard' for Demand Side Response de-coupling of heat-pumps from the grid.

There are representatives of the ENA who are members,
and they have an interest in the avoidance of demand surges.

To be fair, the total demand for the period was a maximum of 3kW - the ASHP goes idle while the immersion is running.

In contrast, my electric car's demand was 7.4 kW, but the charger is controlled by the energy supplier (Octopus). They tell the charger when to run, and can also knock the max current back if needed.

I was given a charging slot of 05:00 to 07:30, so I got an extra 2 hours at 7p for the whole house.

I think I read somewhere that Octopus will be able to do similar demand matching with their "Cosy" range of heatpumps?

It might be a good idea if some "randomisation" of schedules and timers was standard on electric heating systems though - just 5 minutes either side could make a difference in surges, and be transparent to the consumer.