It appears that your current Legionella cycle is quite lengthy. Typically, achieving and maintaining a water temperature of at least 60°C for about 30 minutes is enough for Legionella prevention. Given that your system is struggling to reach this temperature within a reasonable time frame, some adjustments might be necessary.

Typically, in low temperature units (like ours) the heat pump gets us 50C, the immersion takes over and gets us to 60C relatively quickly. I’m not sure why your immersion is “struggling”.

There are a lot Ecodan users in the forums that can provide their insights.

Ecodan user here and I have disabled the Legionella cycle..it is just not necessary. Heating water to 50C is more than adequate to mitigate Legionella bacteria growth, and as nearly all domestic settings do not have dead legs and do not have aerosol producing equipment and will turn over the water capacity in the pipework regularly so the risk is negligible. 

I also have Solar Thermal which raises the stored DHW temperature to 60C in any case and as the DHW is heated via the Ecodan's AlfaLaval heat exchanger there is no stagnant usable water in the 40C-45C state.

The 60C hype is an abundance of caution.  Your mileage may vary!

@abernyte Thank you for your reply and I agree that it's debatable as to whether a legionaires cycle is necessary. However, we are classed as vulnerable so maybe better safe than sorry.

Regardless, this is a new system so, at this time, what I am doing is simply proving that the system can do it if required. Based on what I have seen so far, it would appear something is not quite right so I was seeking information on what timings are to be expected.

The decision as to whether I continue with a legionaires cycle or not will come later.

Understood. My 14kW Ecodan does the DHW cycle from cold 25Cish to 52C in about 30 minutes. I am not sure what yours is dawdling around doing! Just checked this mornings report: it started heating DHW at 1006hrs with flow temp of 28C and tank temp of 28C and completed it at 1035hrs with flow temp of 62C  and tank temp of 52C. No solar gain due to 100% cloud cover.

@richard24738 I think the issue with your system, which requires investigation, is why it's taking so long and not ultimately getting to 60C. Like @abernyte, our heat pump (18kW) takes about 30 minutes to get the hot water over 50C, and then the immersion takes over. Since your system is new, I'd call your installers and also speak to Mitsubishi as everything is still under warranty.

@editor Thanks.

I, of course, thought something was wrong with the timings but wanted to be sure what is normal before contacting my installer or Mitsubishi.

@Richard24738 - how did this pan out?

Hi. Unfortunately on-going.

Mitsubishi agreed that the immersion was taking too long and also pointed out that my software was out of date.

Installer adjusted the immersion thermostat but I couldn't see a difference.

Last Test:

The heat pump gets DHW to 50 degrees within 30 minutes, switches off and then the immersion switches on.

It then took 1 hour 48 mins to go from 50 to 59.5.

Stayed at 59.5 for 42 minutes with the immersion sign on until the cycle timed out at 3 hours. It never reached my desired 60 degrees.

No errors recorded.

Agreed with the installer that we would wait for the software update and see if it fixed the problem.

Further Action:

Mitsubishi updated the software last week and I have a scheduled legionella cycle for tomorrow so will report back afterwards. 

UPDATE

Mitsubishi changes made no difference and they want an engineer to visit.

Waiting for an appointment.

Can you read the model number of your immersion heater @richard24738 ?

We are used to all immersion heaters being rated at 3kW and supplied via a 2.5mm² cable.
But that may not be what your installer has fitted.

Is it possibly a 1kW unit?

Posted by: @abernyte

↑

I have disabled the Legionella cycle..it is just not necessary. Heating water to 50C is more than adequate to mitigate Legionella bacteria growth, and as nearly all domestic settings do not have dead legs and do not have aerosol producing equipment and will turn over the water capacity in the pipework regularly so the risk is negligible. 

That is generally true.

But it also depends whether your aim is prevent Legionella pneumophila bacteria growing within the water in the first place, or to quench a colony which is already established.
The latter situation might occur if your heat-pump system has been left idle whilst occupants are on vacation, for example.

If your mains water supply meets the required standards, then the purification processes are adequate to eradicate Legionella pneumophila bacteria and the protozoa they use for replication.

However, public trust in water authorities isn't particularly high at the moment.
Even when staff pick up breaches in regulations, these are often being recategorised internally to avoid notifying relevant authorities.

Residents in the Brixham area of Devon became seriously ill in May/June'24 due to the presence of cryptosporidium in the water supply.
This entered through a broken air-release valve after the water had been through the treatment works.

It took South West Water three months to flush through the pipework and eradicate the contamination.

I suggest that a householder in Brixham would be responding here very differently to @abernyte
Some of them are so traumatised that it's unlikely they will ever drink tap water again.

[aside] It's interesting to me that heat pump manufacturers seem to have universally chosen heating via an immersion as their method of deactivating pathogens.

The water authorities use UVc radiation (200nm - 280nm) which is extremely effective.

Since it is now possible to buy LEDs emitting 265nm, I wonder why these aren't being used for DHW tanks?
The running costs would be much less, even after factoring in the need to replace them at regular maintenance intervals.

What does @editor think?

@transparent Hi. Thanks for the comments.

Just had a look at the immersion and it's a "THERMOWATT 3KW".

In fact my 14 day cycle for Legionnaires was yesterday so I have up to date results.

Ecodan > 50c = 29 minutes

Immersion 50 > 54.5 = 1 hour

Immersion 54.5 > 58.5 = 31 minutes

Cycle timed out at the 2 hours set by Mitsubishi. It never reached the target 60c for 5 minutes.

So it would seem that the start up of the immersion is where time is excessive.

I am aware of the discusions as to whether a legionnaire cycle is necessary in a domestic setting and I think it really depends on your circumstances and usage.

If I was a fit 30 year old then I probably wouldn't run it. I am however older and considered vulnerable so, to play safe, will run it.