An Excel spread sheet has been used to calculate the heating and cooling.

One thing also calculated is that at peak energy the bungalow requires as much for the heating in the winter as for the cooling in the summer! So the hottest part of the hottest day and the coldest part of the coldest night need about the same amount of power! This is mainly because of the solar gain through the windows.

The calculations were compared with what has been used for heating and it is reasonable good. The calculated heat requirement is down to -8C although I cannot find records here of colder than -5C. It is assumed that the ASHP will supply all heating and hot water when required. (hopefully mainly hot water heating from excess PV power) There is a back up immersion in the buffer tank but I hope to never use it. 

The cooling is another thing all together.

The ASHP manufacturer informed us that if you run the cooling through the radiators below about 19C  this will cause condensation - They were right. When running the ASHP in the summer in cooling mode at 18C the radiators began to get wet because the warm air was being cooled and the cooler air could not hold as much moisture.( the buffer tank also started to do the same and this has to be sorted as well) This is what is know as the dew point!

Water coils are now designed to be added to the MVHR ducts to overcome this problem by cooling the air entering the building.  

The MVHR will be boosted with cooling to the east end of the building, or the west end of the building ( no south windows) when there is strong solar gain. 

As a trial the ASHP will be left on 24 hrs to see the effects.

The temperature of the heating water produced by the ASHP will be turned down to the lowest setting which keeps the desired room temperature.

Anything else let me know