@derek-m Thanks again for the feedback, what you describe makes perfect sense to me and what I have believed to be the case for some time.

The WC settings I have never altered as didn't want to mess with the system assuming it had been set up correctly and in answer your query on this I believe this is correct. 

I am in agreement that the pump is at its maximum and unable to keep up with demand during the colder periods, would this be the reason for the Compressor to have failed on the original heat pump?  Mitsubishi replaced with a new heat pump in March as the original was only 1 year old.

The problem I have is finding a reputable installer/engineer who can rectify, even it means a total removal of the current system and renew as my trust in the expertise's of this technology is very low due to the number of engineers/designers who have been to look at it costing me thousands.  

@harriup Thanks for the feedback, to my mind I have exhausted all options now and think the system is not fit for purpose and its time to look at a complete new system as the costs in electric out weigh persevering to find a solution.  I could have had new Gas a main installed from 1km away at what it has cost me the last 2 years.

Out of curiosity would you expect to find some sort of immersion & temperature gauge on the buffer tank?  Unless I'm missing something I don't understand what the buffer tank is doing on my system as the flow/returns just pass through.  I don't see anything indicating it fills, stores & heats water.  I was of the understanding it would do this to help with demand when the stats call for heat.

Posted by: @leftbacks

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@derek-m Thanks again for the feedback, what you describe makes perfect sense to me and what I have believed to be the case for some time.

The WC settings I have never altered as didn't want to mess with the system assuming it had been set up correctly and in answer your query on this I believe this is correct. 

I am in agreement that the pump is at its maximum and unable to keep up with demand during the colder periods, would this be the reason for the Compressor to have failed on the original heat pump?  Mitsubishi replaced with a new heat pump in March as the original was only 1 year old.

 

The problem I have is finding a reputable installer/engineer who can rectify, even it means a total removal of the current system and renew as my trust in the expertise's of this technology is very low due to the number of engineers/designers who have been to look at it costing me thousands.  

It is difficult to provide precise advice without knowing all the details, so instead I will merely make some suggestions.

Carry out, or have carried out, a full heat loss calculation, and then compare this with the original one from which the size of heat pump was decided.

I'm no lawyer, but I suspect that the heating system provided is 'not fit for purpose', and that the heat pump installed is undersized for your home.

Confirm if your system contains anti-freeze and if so the concentration, since this will probably reduce the thermal energy output of the heat pump.

Look for ways to improve the insulation of your home.

I may be barking up the wrong tree here, but looking at the heat loss calculation 70% of your floor area was designed at 18C, this has determined that a 14KW heat pump was sufficient and this is confirmed by your temperature plot showing the hallway at 19C when it was 5C outside. 

When its -2C outside your heat pump was sized to supply enough heat to keep the majority of the house at 18C, therefore, the heat pump won't be able to get rooms to 21C as the heat loss would be much greater than the 14KW heat pump can supply.  When its milder outside and there is less heat loss the heatpump will be able to get the house to 21C.

The heat pump was sized correctly for the design temperatures, but the design temperatures are different from the temperature you want the house to be at.

Posted by: @derek-m

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Posted by: @leftbacks

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@derek-m Thanks again for the feedback, what you describe makes perfect sense to me and what I have believed to be the case for some time.

The WC settings I have never altered as didn't want to mess with the system assuming it had been set up correctly and in answer your query on this I believe this is correct. 

I am in agreement that the pump is at its maximum and unable to keep up with demand during the colder periods, would this be the reason for the Compressor to have failed on the original heat pump?  Mitsubishi replaced with a new heat pump in March as the original was only 1 year old.

 

The problem I have is finding a reputable installer/engineer who can rectify, even it means a total removal of the current system and renew as my trust in the expertise's of this technology is very low due to the number of engineers/designers who have been to look at it costing me thousands.  

It is difficult to provide precise advice without knowing all the details, so instead I will merely make some suggestions.

Carry out, or have carried out, a full heat loss calculation, and then compare this with the original one from which the size of heat pump was decided.

I'm no lawyer, but I suspect that the heating system provided is 'not fit for purpose', and that the heat pump installed is undersized for your home.

Confirm if your system contains anti-freeze and if so the concentration, since this will probably reduce the thermal energy output of the heat pump.

Look for ways to improve the insulation of your home.

 

It's almost unbelievable that a 14kW Ecodan can't heat a new build house, albeit quite a large one. In order for a 14kW Ecodan not to be able to heat your house at 7 deg C OAT, your heat loss at your design temp of -1.6 deg C would have to be over 20kW.  I've never heard of a domestic property with that big a design heat loss, let a alone a new one, although I'm sure they exist.  I don't think building regs would permit that high a heat loss.

I think something fundamental is wrong with the design, installation, the setup or the ASHP itself.  Unfortunately I don't know what but UFH does seem very tricky to set up properly and buffers and LLHs often cause issues. 

Posted by: @gary

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I may be barking up the wrong tree here, but looking at the heat loss calculation 70% of your floor area was designed at 18C, this has determined that a 14KW heat pump was sufficient and this is confirmed by your temperature plot showing the hallway at 19C when it was 5C outside. 

When its -2C outside your heat pump was sized to supply enough heat to keep the majority of the house at 18C, therefore, the heat pump won't be able to get rooms to 21C as the heat loss would be much greater than the 14KW heat pump can supply.  When its milder outside and there is less heat loss the heatpump will be able to get the house to 21C.

The heat pump was sized correctly for the design temperatures, but the design temperatures are different from the temperature you want the house to be at.

 

 

That is indeed a valid point, since I did not look too closely at the details, mainly the total heat loss figure.

Having different room temperatures within a home is a very subjective matter, and is it even feasible to have an en-suite at 22C and the adjoining bedroom at 18C.

If I remember correctly the data I used for the calculation of thermal energy output from the heat pump was at an OAT of 4C, not the design -1.6C.

The other factor to consider is the British weather, temperatures can go below -1.6C, and also we have a very damp climate, hence frequent defrost cycles under certain operating conditions.

All these factors should be taken into account when assessing the size of heat pump required, including realistic IAT's.

@derek-m

Posted by: @derek-m

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Having different room temperatures within a home is a very subjective matter, and is it even feasible to have an en-suite at 22C and the adjoining bedroom at 18C.

I agree it isn't, but with 70% of the area designed at 18C and 30% at 21C this averages out at 19C, so the house is going to struggle to get to 21C is the heat loss assessment is accurate.

@Leftbacks what I haven't seem is what are the flow rates for your UFH circuits, I do know from my own experience if they are set too high then my heatpump will struggle to maintain its flow temperature during the colder months.  Do you have all the circuits open and calling for heat at the same time, as this will only exacerbate the issue. 

I think Derek suggested just opening one circuit and see what the heat pump flow temperature get to, if the heat pump can get to the set flow temp then its working fine, you could then open up more circuits and monitor flow temp and see how many you can have active at the same time.

@gary This makes perfect sense, surely 18deg is low for these areas?

@gary Sorry, in answer to your suggestion of trying one zone/stat at a time, this has been done but nothing really improves but does get worse when more are activated.

@leftbacks unfortunately it does look like the heat pump is just too small for your property then

Thanks both, I believe I have my answers and although a costly one I need to get the system redesigned, calculated and installed correctly by a reputable and knowledgeable installer.

Posted by: @leftbacks

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@gary Sorry, in answer to your suggestion of trying one zone/stat at a time, this has been done but nothing really improves but does get worse when more are activated.

Please explain 'worse'. If the heat pump is producing almost 14kW, but the present heat demand is greater than 14kW, then the net effect will be for the IAT to fall, which in turn reduces the heat loss, until at some IAT the system will balance.

Temporarily shutting down one of the UFH zones should have the effect of reducing the heat demand on the heat pump, which should then cause the LWT to increase.

I would also suggest measuring the temperatures around the buffer tank, before and after shutting down a UFH zone.