@jamespa 

The flow temp is a tricky one, because it is on/off constantly the heat pump ramps up and is only operating at max flow temp for a couple of minutes before it stops. So it increases in temp rather rapidly, then stops, this makes correlation with 

In the 5 months it has been installed, I’ve only seen it stop to defrost once. It’s never running long enough to get a build up of ice. Mine stops because the homely says it has reached set point, the node is in the lounge and the temp never deviates  from -0.2C to +0.5C of set point, so in my case it starts heat pump at 17.8 and stops at 18.2 which overheats to 18.5, before decaying for 40min before restarting. In homely defence it gives good economy, my system uses 50% less energy than other identical systems on heat pump monitor. But I have cold bedrooms!

The installers offered to remove the homely free of charge last week. I’d want them to remove it and refund the installation cost. I’m holding fire on this until I’ve sorted out the rads first and potentially downsized the heat pump, depending on the new heat loss.

Yes most of Switzerland is dry, but we were in the Rhine valley, which has a wet microclimate. Golf ball sized hail and massive electrical storms every 4 days in the summer, crazy hot and humid. Wish I’d never left now, but at the time and without irony I missed the crapness of the UK, Switzerland was too boring!

Posted by: @papahuhu

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The flow temp is a tricky one, because it is on/off constantly the heat pump ramps up and is only operating at max flow temp for a couple of minutes before it stops. So it increases in temp rather rapidly, then stops, this makes correlation with 

That really, really shouldn't happen and suggests that the heat pump may be well oversized or there is some other problem eg a badly set up buffer/LLH (if you have one) or blocked system (cold rads suggest it is blocked/needs bleeding/lsvs have been turned down too much/TRVs are stuck)

Posted by: @papahuhu

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In homely defence it gives good economy, my system uses 50% less energy than other identical systems on heat pump monitor.

Are you sure that isn't because your heat pump has twice the capacity it needs and the pumps you are comparing with are running closer to capacity?

The more you tell me the less certain I am that the rads are too small, although I grant that they may be.  The other system issues need to be sorted first before you can decide the cause, a run time of 2 mins is just crazy.  A heat loss survey wont sort these system issues (although it might point at oversizing) and until they are sorted everything about rad sizing is just speculation.

Posted by: @papahuhu

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Yes most of Switzerland is dry, but we were in the Rhine valley, which has a wet microclimate. Golf ball sized hail and massive electrical storms every 4 days in the summer, crazy hot and humid. Wish I’d never left now, but at the time and without irony I missed the crapness of the UK, Switzerland was too boring!

Difficult one!

@jamespa 

Thanks, yes. 
it’s even more difficult as only they have access to the homely data.
The installer is coming out next week, a repeat full heat loss survey should sort out whether the unit is oversized and any necessary remediation should follow on from that. I’m considering it as starting the install process afresh, whether they share my perception is yet to be determined. I will get them to put it right though, at their dollar, even if it means ultimately going to the SCC and getting a full re-installation.

All this is complicated by the fact that I moved my very sick and elderly parents in with us during covid. So they are freezing cold at the moment. Heat pumps are not my favourite things at the moment, although I know it is the installer rather than the technology.

Posted by: @papahuhu

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Mine stops because the homely says it has reached set point, the node is in the lounge and the temp never deviates  from -0.2C to +0.5C of set point, so in my case it starts heat pump at 17.8 and stops at 18.2 which overheats to 18.

I find it impossible to believe that your house heats by 0.7C, or even 0.2C, in 2 minutes, unless you live in a tent.  IMHO something else is causing the heat pump to shut down unless the Homely is directly over a radiator (in which case move it!) or has been set up with a hysteresis sub .1C

My strong advice to you is to find out whats going on before the installers turn up.  A new heat loss survey is no better than the previous one, just different, so isnt going to give a definitive answer.  Given what you now say there is no evidence so far that the upstairs rads are too small and there is strong evidence of an underlying system problem.

I would recommend you start by whacking up the set temperature on the Homely to say 30C.  If its the Homely causing the heat pump to switch off, this will stop it doing so.  You can then monitor flow temperature and rad temps whilst the house heats for an hour or two.  That will give an insight.  If the heat pump still switches off after a few minutes, then the Homely (unless its seriously misconfigured) isn't the problem, something in the system is blocked (could be as simple as a filter) or the heat pump is grossly oversized.  Simple experiment to start with, which will tell us a lot.

You are right to insist its put right at their expense, but the fact its got to this stage and they swapped out an oversized radiator tells me they haven't a scooby what they are doing so you may well be best off doing a bit of self diagnosis if you dont want this to drag on for years.