Changing the target temperature via MelCloud should be of no interest to the FTC then as it just using the curve in its calculations. If I go to the app and change mine now to below actual temp it doesn't stop the heating function. But something is triggering it to reset to 14, perhaps one of the 'control disputes' that @morgan alludes to with the Heatmiser thermostats.
Mitsubishi EcoDan 8.5 kW ASHP - radiators on a single loop 210l Mitsubishi solar tank Solar thermal 3.94kW of PV
@harriup I am looking for any issues that may be causing the bigger issue of that the system doesn't work when outside temp drops to 7deg or below but is fine above that. I believe (although i am no expert) that the ASHP is not big enough to cope with the demand, this is why when it has to work harder for the demand it cant provide the flow temps. Trying to get an engineer or company who can diagnose such systems is proving very difficult, I have even had a Mitsubishi approved engineer visit with no success.
@harriup I am looking for any issues that may be causing the bigger issue of that the system doesn't work when outside temp drops to 7deg or below but is fine above that. I believe (although i am no expert) that the ASHP is not big enough to cope with the demand, this is why when it has to work harder for the demand it cant provide the flow temps. Trying to get an engineer or company who can diagnose such systems is proving very difficult, I have even had a Mitsubishi approved engineer visit with no success.
Could you post a picture of your compensation curve? What flow temperature is it trying to achieve at, say, 7 degrees outside?
Do you have a heat loss calculation for your house? It's hard to say for sure but I would have thought a 14kW Ecodan with ufh should be able to heat even your quite large new build house at 7 degrees ambient quite easily.
Do you have a schematic for your system showing the main pipework, flows, buffers (if present), etc?
I only have a photo of the target temp for the Flow & DHW. The compensation curve is set at 55deg to -30deg if that makes sense. I have also attached the heat loss calcs.
@harriup I am looking for any issues that may be causing the bigger issue of that the system doesn't work when outside temp drops to 7deg or below but is fine above that. I believe (although i am no expert) that the ASHP is not big enough to cope with the demand, this is why when it has to work harder for the demand it can't provide the flow temps. Trying to get an engineer or company who can diagnose such systems is proving very difficult, I have even had a Mitsubishi approved engineer visit with no success.
If the system isn't big enough then you will just get a bit colder as it fails to generate the flow to match the house heat loss, but at much lower external temps – a complete hissy fit at a relatively warm 7C and partial/complete stop (?) must be a different beast. You have had visits from trained engineers so we ought to be able to assume it is not a mechanical problem with the outdoor unit, or a miswiring of the FTC unit.
The house heating arrangement and its controls are such an important factor in the system (and usually unique) that I would tend to suspect some issue with that relationship. Following Morgan's comments I would be minded to get an engineer to disconnect the Heatmiser control of the FTC, which I think is no more than on/off commands, and connect a temporary, different thermostat to see whether that makes any difference to the working of the unit. You need to narrow down the source of the problem somehow.
Mitsubishi EcoDan 8.5 kW ASHP - radiators on a single loop 210l Mitsubishi solar tank Solar thermal 3.94kW of PV
@derek-m I did try to find, but have just done some more reading and found what I can locate. Is there a particular setting this should be?
What is the present setting?
Try lowering the setting and see if it resolves your problem. Since it is designed to prevent the water from freezing, I suppose the setting would be dependent upon if you have an anti-freeze mixture and the percentage mix.
The freeze stat setting has the flow temp of 20C which is not alterable, but an activation temp from 3-20 or ** which turns the setting off, which you would do in a glycol-filled system. Why you'd want to set it to come on at 20 beats me, mine is set to 3. You could try turning to off completely and see what happens – even if outside temps do drop to freezing while you are experimenting there'll be enough heat in insulated pipes to keep the water from dropping low enough to trigger anti-freeze valves opening for quite some time I would have thought.
Mitsubishi EcoDan 8.5 kW ASHP - radiators on a single loop 210l Mitsubishi solar tank Solar thermal 3.94kW of PV
I only have a photo of the target temp for the Flow & DHW. The compensation curve is set at 55deg to -30deg if that makes sense. I have also attached the heat loss calcs.
there are two obvious things wrong here. Firstly, the system has been set to run at 55 degrees flow at 0 degrees in a ufh system. That's far too hot for ufh. It also looks like your design flow temp is 50 degrees, which is also far too hot. You wouldn't be able to walk on your floor without shoes on if it were that hot.
Secondly, your ASHP isn't achieving what looks like (from your WC curve) about 45 degrees flow at 7 degrees ambient. If it's designed and working properly it should do that easily (not that it should have to)
The good news is your heat loss of 12kW is well within the capabilities of your ASHP and it should be able to heat your house all the way down to design temperature and below.
Something that occurs to me is that asking the ASHP for 55 degrees flow at 0 deg ambient is going to cause a lot of defrost cycles. In fact, I'd confidently predict that your average flow is going to be a lot less.
When you say the system doesn't work at less than 7 degrees do you mean that the ASHP stops or the flow just doesn't get any hotter as ambient falls? If the latter then there is a max flow temp setting in one of the engineer's menus that (I think) overrides the WC curve.
Given the age of the system and your house you really need to get the installers/builder back to make the system fit for purpose because it definitely isn't at the moment. With a 14kW Ecodan, a heat loss of 12kW and a well designed ufh system in a new build you should be toasty warm with plenty of hot water and reasonable (I won't say low!) bills.
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