I want to sanity check that my installation is wrong and to warn others of the possibility that theirs might be too.

The Legionella cycle that is run periodically on the water cylinder is supposed to heat the water up to 60C or 65C (you can choose). I don't know, but suspect that the system uses the ASHP to get most of the way to that temperature, but then uses the immersion heater to do that last 10C - 15C. The immersion system rightly has a safety cut off thermostat to avoid overheating the unvented cylinder (serious issue if that goes wrong).

Surprisingly, the Ecodan FTC6 doesn't throw and error if the Legionella temperature is not reached. Based on the immersion heater being involved, there are two simple failure modes that can cause this to happen:

1. RCBO/MCB off: the immersion heater has its own breaker in the consumer unit (fuse box). This could have been left turned off or could have tripped.

2. Cylinder's Immersion Thermostat Set wrong: The immersion heater has a companion bimetallic thermostat that is there for safety and has no software connection to go wrong (or that allows monitoring). This thermostat has a safety cut off which is understandably likely to be mandatory for an unvented cylinder. If this thermostat is set too low, it will stop the immersion heater heating the water to the required temperature. Also, the resettable safety may have tripped.

Below is an photo of my immersion heater's thermostat which looks to be set to (3) = 40C. Feels like this should be set to (5) = 70C [SAFETY WARNING: there are obvious issues here playing with these settings]. Can somebody please check their installation to see it they have the same thermostat and what it is set to.

Ecodan FTC6 Installation Manual:

COTHERM TSE thermostat:

My Thermostat:

My Ecodan's attempt at Legionella cycle:

Based on the setting on my immersion you are just set to low. Turn it to max setting or just below, switch immersion on and see how hot your cylinder gets, adjust down a little at time.

That's a bit poor if that was installed and set to 40 Deg C - surely your ASHP gets the tank to a higher temp than that already??

If you are turning over the water on a regular basis legionella should [edited to add "not":] not be a problem in most systems

I normally heat my tank to 48 Deg C daily and only raise it to 55 Deg C for Mrs Alien's Bath night

Edited 21 hours ago21 hr by jack
Typo

This immersion heater thermostat setting is not just an Ecodan issue. ANYONE with an ASHP, HW tank and immersion heater should check this.

None of them have calibrated dials. Mine is turned up to maximum (for maximum surplus solar PV storage) and trips out at 76 degrees. I do not use any normal or timed legionella measures. HP heats it to 48 degrees if solar PV has not go there first.

1 hour ago, JohnMo said:

Based on the setting on my immersion you are just set to low. Turn it to max setting or just below, switch immersion on and see how hot your cylinder gets, adjust down a little at time.

I'm going to turn it up to (5) = 70C. I emailed the installed to double check and they confirmed I should.

1 hour ago, marshian said:

If you are turning over the water on a regular basis legionella should not be a problem in most systems

Missing not :-)

It's crazy that the Ecodan doesn't log an error though since it knows the temperature didn't get high enough. My cylinder has been like that for well over a year with no water being used so it's more likely a Legionella risk than in most cases.

Sorry typing on a phone - yes I did mean "not"

59 minutes ago, marshian said:

Sorry typing on a phone - yes I did mean "not"

I edited the "not" in to avoid any confusion.

44 minutes ago, jack said:

I edited the "not" in to avoid any confusion.

Thank you Jack

Successfully ran a Legionella cycle. I had both of the failure modes I had identified since I had left the RCBO turned off over the long period without using hot water, not knowing that the immersion was essential for the Legionella cycle, and my Immersion thermostat was set wrong.

Couple of changes that Mitsubishi really should make to the FTC6 controller firmware:

• Reporting an error if the water temperature didn't reach the target temperature during the Legionella cycle

• Having the ability to manually trigger a Legionella cycle. The only way I could do this was to set the Frequency to 1day and the time to a time in the near future. I then came up against it having already done a cycle today so had to set the date to tomorrow to trick it.

Cycle run and pipes flushed.

I've put the settings back to where they were (7days, 3am, 65C) and it's going to run it again tonight which is no biggy since the tank is still over 60C. If you assume the immersion heater does all the work raising the temperature from 50C to 65C, then for a 275L tank that takes about 5kWh of electricity which is pretty costly (£0.70 at 3am). £36.50 per year though I may change the frequency to 21days.

Why do you need to heat to 65 degs? 60 is more than adequate, if indeed you need to do it all.

May be worth completing a risk assessment.

4 minutes ago, JohnMo said:

Why do you need to heat to 65 degs? 60 is more than adequate, if indeed you need to do it all.

May be worth completing a risk assessment.

Fair point, probably worth reducing it to 60C.

I would go with JohnMo on the 60C point. If the cycle is actually reaching temperature and the water is being turned over regularly, I would not chase a higher setting just for the sake of it. The useful check for me is the one you found, making sure the immersion is actually able to finish the cycle rather than assuming it has.

Is the legionella cycle really doing all the job by just heating the tank what is happening in all the dead legs such as expansion vessels. I read this a couple of years ago which sets out the challenge, mainly for commercial buildings but the same effects must be present in domestic situations.

3 hours ago, MikeSharp01 said:

Is the legionella cycle really doing all the job by just heating the tank what is happening in all the dead legs such as expansion vessels. I read this a couple of years ago which sets out the challenge, mainly for commercial buildings but the same effects must be present in domestic situations.

I thought this video did a good job explaining all the risks

One of the interesting points in a article on the web was it wasn't just the tank you should sterilise but if you are away for a period of time you should run the hot taps for a period to purge the lines