@bart Best of luck moving forward, I look forward to hearing when all is settled and corrected for you. Please let us know once you have everything sorted.

By way of an update: NAPIT's initial overview is more than a little disconcerting advising that PAS2035/PAS2030 only applies to RIWI/ventilation and concluding that ASHP oversizing and inappropriate use of volumiser is not evidenced by design calculations.

Specifically regarding ASHP sizing given a 3.6kw property heat loss and the provision of a 6kw system this seems unconscionable however I can't find any MCS requirement better than the below (ie. 'design calculations' require its bigger than 3.6!) - does anyone know of a more robust requirement regarding appropriate sizing please as NAPIT's 'impartial' investigation would seem disinclined to help? 

My current combined SCOP is 2.3, which broadly matches manufacturer's data. Whilst I would hope that "reasonable skill and care" would require a more appropriate selection (ie. a 5KW system) given that manufacturer data (1) at my capacity confirms the system cannot meet MCS ASHP SCOP predictions (2) when a 5kw system would and a volumiser wouldn't appear to be required (3).

1.

2.

3.

Regarding accuracy of predictions the best I've found from MCS documentation is that +/-30% would not be unreasonable (ie.-30% ASHP SCOP at design stage would seem unreasonable) but a more robust requirement for either efficiency or sizing would be greatly appreciated.  

@bart, I think you're right to focus on that section of MCS MIS 3005-D Issue 2.0, which says:

“A heat pump shall be selected that will provide at least 100% of the calculated heat load taking into consideration the flow temperature at the heat pump and without input from any supplementary electric heater.”

At your local design outdoor temperature, the heat pump must, by itself, cover 100% of the heat loss at the design indoor temperatures with no help from the backup heater. The heat pump’s output should be calculated at a flow temperature of 55°C (unless the design is specifically lower temp with low temp emitters and properly justified).

So if your house has a 3.6kW heat loss at your location’s design temp (let’s say -3C for example), then the installed heat pump must deliver at least 3.6kW of output at that temperature and at the required flow temp. 

Your combined SCOP of 2.3, which aligns with the manufacturer’s performance curve, shows it’s running inefficiently and that makes sense if it’s frequently cycling or needing a volumiser just to run stably. A correctly sized system should modulate down without short cycling, and should not need a volumiser just to mask design oversights.

Document exactly what the 6kW unit actually delivers at design conditions vs. your heat loss: this is your smoking gun.

I'm going to give this more thought as to how we can drive this to a resolution.

@heacol, if you're around, can you please provide some insight into this?

Posted by: @bart

↑

By way of an update: NAPIT's initial overview is more than a little disconcerting advising that PAS2035/PAS2030 only applies to RIWI/ventilation and concluding that ASHP oversizing and inappropriate use of volumiser is not evidenced by design calculations.