@editor I have now been allocated a complaint handler by NAPIT. The email advising me of this states 'In accordance with our formal complaint's procedure, we have now referred this matter to our technical team for an initial review. This is to determine whether any technical issues are present that may have contributed to the concerns raised.

@jamespa thank you again. I know using the offset to try to raise the room temperature isn't ideal, but I was desperate! It was one of those times when the LWT wasn't reaching near what it should according to the WD curve. However I have adjusted the curve very slightly to 50 at -2 instead of -3. I've had a look at the installer's manual which I was given a copy of. It's as badly written as the user's manual. But interestingly there is a table showing 'space heating setpoint range' which is maximum 55 for underfloor heating and maximum 60 for radiators - which I have. My system is set to 50 maximum but I've had a look at the installer settings and can see how to increase that to a maximum of 60. Is that worth trying? I have always been puzzled as to what happens when as you mention it reaches the low temperature at which the curve flatlines and can't respond any further to falling temperatures. I'm thinking on the very cold days it may well have been -5/-6 or lower overnight and the house has dropped in temperature and the ASHP is just scrabbling all day trying to reverse that? No doubt it won't be good for my SCOP but my priority is being comfortable over cost.

As far as I can see there is no way to push the system to use purely the immersion heater for DHW, and I'm afraid my skills wouldn't stretch to the kind of modification that you have made. I would likely have to speak to the service company about that.

I wonder about initially changing the setpoint to 55 or 60 and seeing how I get on in the next cold snap. I would greatly appreciate your thoughts on that. Many thanks.

Posted by: @mairia

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My system is set to 50 maximum but I've had a look at the installer settings and can see how to increase that to a maximum of 60. Is that worth trying? I have always been puzzled as to what happens when as you mention it reaches the low temperature at which the curve flatlines and can't respond any further to falling temperatures. I'm thinking on the very cold days it may well have been -5/-6 or lower overnight and the house has dropped in temperature and the ASHP is just scrabbling all day trying to reverse that? No doubt it won't be good for my SCOP but my priority is being comfortable over cost.

In recent cold weather the curve might be your problem.  If its 50 at -2 then below -2 the flow temp will flatline when you need it to continue increasing to deliver enough energy from your emitters.  Most of the time that doesnt matter because its unlikely to be lower than -2 for more than a couple of hours a day, but in the past weeks thats not been the case.

The thing to do to deal with this is NOT to increase the target temp at -2, its to change -2 to (say) -5 or even -10 and set a higher target temperature for that OAT.  The the target FT should keep rising even when it very cold outside.  If the heat pump doesn't keep up with the target for some reason thats a separate problem, but if the target isn't high enough then you have no hope.

I did say this earlier but perhaps you didnt pick up on it or I didnt make it clear.

If you tell me the flow temperatures you are in practice operating at and at what OATs to keep warm or your current WC settings I can do a quick calculation on what you should set it to at -5 or -10. 

@jamespa thank you again. Apologies I missed you pointing this out earlier, but it certainly makes sense. My curve is currently set to LWT 25 at OAT 14 and LWT 50 at OAT -2. It will be great if that resolves the issue of heating in very cold weather, if not then will give me more information at least. I do wonder why the system was set up like this, temperatures in Glasgow do fall below -3 most winters for brief periods.

Posted by: @mairia

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@jamespa thank you again. Apologies I missed you pointing this out earlier, but it certainly makes sense. My curve is currently set to LWT 25 at OAT 14 and LWT 50 at OAT -2. It will be great if that resolves the issue of heating in very cold weather, if not then will give me more information at least. I do wonder why the system was set up like this, temperatures in Glasgow do fall below -3 most winters for brief periods.

Thats great thanks.  TBH setting the low end of WC curve according to the design temperature has been very common (possibly even universal) practice so far as I can tell.  I confess that, until the recent cold spell, thats the advice I have been giving (I have now changed).

So what you want is a linear projection of the curve to a lower OAT.  This will keep the curve the same at OAT-2 or above, but extend it to lesser OATs.  A linear projection to -10 takes us beyond 60C so that wont do if its the limit on your machine.  59@-8 is pretty close, so I would go for that.  Alternatively 55@ -5.

@jamespa thank you. I think I'll go for 59 at -8, we can get temperatures below -5 on occasion but I would say below -8 is incredibly rare. I'll report back once we have some colder weather - nothing forecast in the next couple of weeks.

Posted by: @mairia

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I'll report back once we have some colder weather - nothing forecast in the next couple of weeks.

Another good bout of cold weather should test whether the heat pump has sufficient capacity or not.  That said, if the 'design temperature' was -3, then you would, in ordinary circumstances, have no contractual right to expect it to cope below that figure!  Your specific circumstances are different though so you would have an argument. 

Mine was designed for -2 and pretty keenly sized (I knew and wanted that).  It did cope in past days, but was clearly getting something of a work out, so I suspect that any less and it would have been inadequate.  Foir the very rare occasions that happens some supplementary heating is the way to go.  In our case thats to heat DHW on the immersion instead of the heat pump and fire up a 3kW log burner, but an electric heater is the obvious alternative.

I've now had an interesting email from a Complaints Resolution Specialist at MCS. She is part of a newly formed complaints team, and advises that ''your case has been picked up for further review under our new scheme framework, which sees MCS taking a more active role in complaints handling and accreditation oversight.'' She seems to be taking the situation much more seriously than the original response and has given details of the 3 MCS certificates issued, the recommended design room temperatures in place at the time my system was designed which are not the same as those used by the installer (21 degrees for lounge vs 20 degrees stated by the installer), how any changes to the design should be handled. She will be contacting HIES and NAPIT to check on progress. It all sounds quite positive, fingers crossed.