I think the key question potentially is why you are trying to hit a target temp of 60, all of the time? Why don't you just set it at 50 which the ashp can manage, and be done with it?

@benson Thanks for the reply. Ecodan has a pre-programmed legionella protection cycle where it takes takes the hot water temperature up to 60+ degrees once every 14 hot water cycles to kill any bacteria in the cylinder. It then reverts to our regular hot water target temp of 46 degrees. But because it's unable to hit the magical 60 degrees, the legionella cycle kicks in again the next day and tries again (and fails to hit 60), and the next day, and the next...  Our installer says very difficult, if not impossible, to fully disable the legionella protection cycle.

@9jwr9 oh ok. There will almost certainly be a way to turn it off? 

Your installer will likely just be reluctant to advise on it as it will be a requirement of the install to set it up I'd imagine. We had the same (albeit not an ecodan), and the first thing I did was turn it off when they left. I am guessing you have an instruction manual, or just search on youtube. 

There's a lot of misconceptions about legionella and 60degC. Similarly a lot of misunderstanding about how a stratified [non-mixing] domestic hot water cylinder works.

First; Legionella bacteria, this chart from Heat Geek is informative. Legionella bacteria is dormant above 45degC, and killed above 55degC. Temperatures of 60degC or higher kill bacteria almost instantaneously, but water stored at 55degC for several hours will also kill bacteria.

There's far greater risk of contracting Legionella from a mixer tap outlet or shower head outlet, as these often have microbial growth on them, are in contact with air, retain some water at room temperature and supply water in the ideal temperature range for Legionella growth. If you're really worried about Legionella, or in a vulnerable group, sterilise your taps and your shower head periodically, and run taps and showers full hot for a few minutes a week. The risk from an unvented sealed and pressurised DHW cylinder is very low.

Second; a DHW cylinder with internal heating coil does not store water a constant temperature. Not all of the volume in an internally heated hot water cylinder is utilised. It uses stratification, hottest water at the top where the outlet is, cold water at the bottom where the mains inlet is. In between is a boundary layer between hot and cold. As you draw hot water off, that layer rises, eventually reaching the hot outlet whereby the hot water suddenly runs cold. Nearly all domestic how water cylinders without secondary return pumps or external plate heat exchangers are like this.

Similarly, an electric immersion does not heat all the cylinder contents up to whatever temperature the immersion thermostat is set to, but about 2/3 to 3/4 of the cylinder volume. Below the level of the immersion heater element the contents will remain cold. The heating coil in a cylinder is more effective at evenly heating the contents of the tank, because is has a much larger surface area than a small 3kW immersion element, and often extends almost to the bottom of the tank.

@allyfish thank for taking the time to reply, and I agree with all of the points you make. If it were easy to disable the legionella cycle altogether (or lower the target temp to 59!) then I'd happily do that, but I'm being told by our installer/Mitsubishi that this is impossible/near impossible, and so the only way to proceed is to satisfy the Ecodan's 60 degree temperature requirement. Or at least satisfy it in the immediate vicinty of the temperature probe! So figuring out why the immersion is seemingly cutting out at 59 is the objective, for now 🙂

I've just had a quick search on the forum and found @abernyte reported that he'd disabled the legionella cycle so it's neither near nor actually impossible. If @abernyte doesn't swing by and let us know how he did it, I'll dig a bit further to see if I can find the details.

Edit: removed info relating to later FTCs (not FTC2) after realising I didn't read the OP carefully enough... *groans*

Posted by: @9jwr9

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So figuring out why the immersion is seemingly cutting out at 59 is the objective, for now

The built-in thermostats on most immersion heaters are very approximate. They are more of a safety thermostat than a control device. Mine is maxed out at an indicated temperature well above 60degC, but doesn't heat the DHW contents beyond 57degC as indicated on the cylinder stat before it starts cycling on/off on the thermostat.

(irrelevant content removed)

Posted by: @rhh2348

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FYI all of the below is in the Mitsubishi user and installer manuals - download them yourself, they are very useful for reference.

 

It's easy to disable legionella cycle if you want to.  Press the Menu button on the main remote controller (which is the button most left and bottom).

Hot Water should be selected - press the Tick button to select it.

On the right hand side you should see the Legionella box with a select box - press F3 to toggle between selected (on) or deselected (off).

Press Tick again to save settings then reverse arrow ("back") button until you're back to the status screen.

 

If you want to edit Legionella settings, press *and hold* the Menu button *for four seconds* instead.  Select Hot water and then you should see a second option for legionella - edit (press the fourth button underneath to enter).  It will allow you to change a few things, such as target setpoint, frequency, start time, etc.

Note: the minimum legionella setpoint is 60C.

Also note: setting the legionella schedule is a pain - you need to complete a cycle successfully before you can set a frequency other than 1 day, unless you deselect Legionella first (I think you then have to wait over 24 hours so the next start is expired).  Alternatively, an undocumented method is to adjust the system date but I can't recall exact steps / details as I type.

Just to say that, whilst @rhh2348 was posting this I took a look on my controller and was about to write something similar. I'll also say that there's a youtube video about legionella settings for an Ecodan showing all this in action.

As a result, your installer has set quite a low bar for their definition of "nearly or actually impossible".

@rhh2348 thanks for the reply. I think the process you describe is for newer units (we have an old FTC2 controller, see picture), which doesn't present any settings for the legionella cycle. I've read the manual top to bottom and it doesn't even mention legionella! So I'm wondering whether easy changing of legionella settings is something which only came in with newer FTC units?