@richard24738 

We have our legionella cycle once a week set at 55deg as we run it midday when our Elec is cheap (cosy tariff) so its at 55deg for several hours.

It runs shortly after our usual DHW cycle which we have set at 48deg and the heatpump takes approx. 1hr to get to 48deg and it then takes the immersion (3kW) a little over 1 hr to go from 48 to 55 

I found this graphic from heat geeks useful on deciding my temperature.

I think the clue to this problem is in the first post here from @richard24738
He has a 250-litre tank...
... and is presumably running a milking parlour!

The specific heat capacity of water is 4,200 joules per kilogram per degree Celsius (J/kg°C)

The energy required to raise 250l by 10°C (from 50 to 60°C) is therefore 4200 x 250 x 10 which equals 10,500,000 J

• 1 watt is one joule per second
• 1kW is 1000 joules/sec
• 1kWh is 3,600,000J

10,500,000J / 3,600,000 = just under 3kWh

So a 3kW immersion heater will take an hour to raise the temperature from 50 to 60°C

Either Richard's immersion heater isn't being turned full on during the heating cycle
or else there's a significant heat loss somewhere.

Can someone else please check my maths?

@transparent 

My Tank is 250L

and looking at the last cycle it took 1hr 10mins to raise the temp 7deg so it looks like your math is not far off maybe we are not getting exact 3kw from the immersion heater.

Edit

Just checked and the last cycle took 3.297 kWh and was 1hr 8mins to be precise.

Looks like my immersion never quite hits 3kW

@transparent Thanks. You have confirmed my belief that when running the immersion it should reach the desired 50 > 60c after 1 hour confirming my gut feeling that 1.5 hours is too long.

@trbob had a similar problem and I have asked how it was resolved.

By the way, it's a Telford Tempest Heat Pump Cylinder and I spoke to them. They were adamant that it's a Mitsubishi software issue.

@richard24738 Perhaps I need to re-visit mine for a closer inspection, thanks. I would not be surprised at all if there were gremlins in the Mitsubishi software.

Posted by: @richard24738

↑

it's a Telford Tempest Heat Pump Cylinder and I spoke to them. They were adamant that it's a Mitsubishi software issue.

In future, please try to use email rather than phone calls when pursuing 'issues' with manufacturers, suppliers and installers.
The text in an email is clear evidence, timed and dated.
It can be used in the event of a claim made under the Consumer Rights Act.

On the other hand, you can freely make guesses and incorrect assumptions here on the Forum.
If you get the maths wrong, it doesn't matter because someone else can step in and correct it.
Try to imagine this topic rather like a conversation in a pub, where everyone is able to chip in with what they think.

If I were in your house, I'd be doing a couple of test investigations.
For example:

1: What happens if I disconnect the supply to the immersion heater from the Mitsubishi controller, and instead use a 13A mains socket to power it?
Does it then operate at the specified 3kW rating and heat the water faster?

2: Whilst the mains is off, undo the cover of the immersion heater and have a look at the wiring.
There should be two thermostats in series, one of which is usually adjustable.
The live wire then goes to the heating element.
But what if your particular model has two elements in parallel, each rated 1.5kW?
If only one is functional, then that would account for the symptoms you're reporting here.

@transparent Thank you for your comments.

Unfortunately what you suggest I do is beyond my technical abilities so I will have to wait for the installer/mitsubishi.

That's fine @richard24738

We always try to leave technical suggestions like mine in topics.
Others will soon come across this discussion, and some might have the tools and experience to try what I've posted.
This is very much a shared investigation.
Not everyone here can attempt everything; we pool the knowledge.

Perhaps, however, there's a model number on the end of your immersion heater which you could post here.
There are different versions of the Thermowatt.
I can then look it up and see what its specification is.

@transparent the details on the plate are:

000003402550 RT IH IN1 14/RTS-ERP A 3KW 240V

Thanks Richard. That's the detail we need.

Here's what I've found.

RT IH IN1 14 tells us that you have an immersion heater with a 1¾-inch threaded flange.
There's a single heating element within an Incoloy metal outer sleeve.
The sleeve is doubled over to present a length of 14-inches (350mm) within the tank.

The element is rated 3kW for a supply voltage of 240v AC.
If you have only 230v, then it operates at 2.7kW.

The thermostat component is the higher-quality version of the usual RTS.
It is certified to the ErP Standard which requires both the live and neutral to disconnect in the event of over-temperature.

One of the two internal thermostatic disconnectors is adjustable for different temperatures.

Possibly your installer hasn't set that thermostat to at least 60°C.
Thus it could be switching off before reaching the target temperature.

The Mitsubishi software might be able to detect that, but I don't know.
Someone else with an Ecodan is better placed to comment on that.

... and now you see the benefit of posting model numbers here 😀 

I managed to partially keep an eye on mine today for it's cycle (200L tank btw);

13:27   Tank temp 49.5°C    ASHP goes off

3kW load in addition to baseline

13:40  Tank temp 52.5°C 

13:58   Tank temp 55.5°C 

14:20   Tank temp 59.5°C (I was out and unable to manually verify this one, so it's taken from the MEL Cloud app graph which is not great).

Just checked my settings on the controller and the target temperature is 61°C for 5 minutes. Once it hit 59.5°C the element stops drawing power, but I'm pretty sure that the program continues running until 15:00, preventing the heating from coming on.

Not needed today, but when it gets cooler it is annoying, even more so, because surely the heating should be made available anyway as the ASHP is not doing anything when the element in the tank takes over right?

@trbob Very interesting.

It took round about an hour for your immersion to get from 49.5 to 59.5. Probably acceptable

It looks like the Mitsi software switched the immersion off at 59.5 instead of going on to your set 61 degrees for 5 minutes and simply waited to time out at 2 hours.

Yes, annoying as it doesn't release back to heating and I think a bit more worrying in that you haven't really been running your full legionnaire cycle at all!

Something is definately wrong with the software. It's not just my Ecodan.

Thanks for following up.