Posted by: @cleardene-ashp

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So expecting it to also hit 50 - 55 for DHW seems a stretch to me.

You cant infer the temperature it will do when heating DHW from the temp it does when space heating because the load is different.  When its heating the DHW it is only heating the DHW so should reach the max temperature that the ASHP is capable of, most likely 55. 

As a concrete example my 7kW ASHP reaches 75 when heating the DHW, but runs at <40 most of the time.  On the coldest days it just gets to 45 when doing space heating, but it will still do 75 when its doing the DHW.  Mine has R290 refrigerant so is inherently capable of higher flow temperatures.  Yours is R410 so will be capable of 55.

Posted by: @cleardene-ashp

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All of this discussion I’ve insisted here is possibly a bit premature because I first need to reach the point of the company formally agreeing what problem I have.

Agreed.  

Posted by: @jamespa

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When its heating the DHW it is only heating the DHW so should reach the max temperature that the ADHP is capable of, most likely 55. 

Ahh!

ok thanks for that. It’s a small but very significant detail that had escaped me.

Posted by: @cleardene-ashp

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Posted by: @jamespa

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When its heating the DHW it is only heating the DHW so should reach the max temperature that the ADHP is capable of, most likely 55. 

Ahh!

ok thanks for that. It’s a small but very significant detail that had escaped me.

Indeed.  Once you know whether its fixable or not please post back with more detail about the system if its not.  Dont be suckered into replacing more hardware than you need to for the convenience of the installer, from what you have said about how you operate it its quite likely that there is 10% spare capacity which will be sufficient for your DHW unless you have a very high demand.  The collective intelligence here will help minimise the probability that you pay more than you need to.

Posted by: @jamespa

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please post back with more detail

Will do! Thanks for everyone’s thoughts so far, it’s already helped understand things better.

@cleardene-ashp  

Thinking about this the worst case scenario, should the existing system not be repairable, is very probably: plumb the DHW to the heating ASHP; use the immersion to supplement on the few days each year when ASHP has insufficient capacity.    The great thing about this solution is you can 'suck it and see' without doing any calculations.  If the ASHP does have sufficient spare capacity you just never use the immersion! 

Obviously we cant be absolutely certain that's feasible without more detail, but very likely it is.

@jamespa I think that's a good interim solution James. The point I was making about using the immersion on cheap rate electricity is that the ROI of buying a heat exchanger and plumbing it in compared to just using the immersion is many years.

Posted by: @bontwoody

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@jamespa I think that's a good interim solution James. The point I was making about using the immersion on cheap rate electricity is that the ROI of buying a heat exchanger and plumbing it in compared to just using the immersion is many years.

I couldn't agree more.  This is a case where you buy as little as possible unless it's taking you in the long term sensible direction which imho and based on the limited info we have is very likely a single unit

@cleardene-ashp just thinking about your issues here. You use ‘the supplier’ as if he did the original work. If that’s the case don’t rush to use him again. The reason I’m saying this is he’s designed your ufh as if it was a gas system. For a heat pump you should put the thermostat just above the temperature you want in the room and then open/close the manifold adjusters to get the room temperature right. You also might have ‘automatic adjustment valves’ on your manifold which again is a gas boiler technique. It will take longer to optimise because floor slabs have a large thermal mass. But that will give you the lowest flow temperature which should run continuously. Read the article on balancing radiators for the principles (mentally substituting manifold adjustment for radiator valve). But it can take ~2days for an ufh system to heat up so you should not be operating it in an intermittent manner. The exception to this is if your heat pump is too over-sized, which doesn’t appear to be the case.

The long time to get the balancing right is why an installer wouldn’t bother and would fit thermostats.

So your heating ashp may be plenty big enough when used right!

Your pump heating your DHW on overnight very cheap EV rates will be fairly low cost even with immersion top up, and it will have switched over from heating for an hour or so to do that, just the same as a set-back overnight.