Hi @AndyOrchard2, I would definitely challenge your surveyor on that one. Chances are whatever software they are using only has an open flue/open fire option. A log burner with stove plate draws very little combustion air, hence why units of less than 5 or 6kW (I forget which) don't require fixed ventilation inlet openings. Compared to stoves, open fires draw much more air up a chimney, several times more, due to the induction effect of warm air rising from the fire drawing in colder room air. That's why log burners are so much better space heaters, they are more efficient at combustion, and also much less cold air is drawn into the room the stove is in from cooler areas, because they only ingest air required for combustion with almost no induction of air.

We have an ASHP + 5kW log burner in the sitting room, the log burner was not factored into our heat loss estimate, as it's room sealed (or as good as) when not lit and the vents closed. Our sitting room came out at 1.5kW heat loss, but has two large glazed windows + two external walls. We changed two existing 1992 double panel rads for longer, lower, and higher capacity Stelrad K2s. They do the job nicely without the log burner on with CH flow temperature set to a maximum of 45degC.

One of the concerns of oversizing ASHPs is minimum turn down capacity. An ASHP will spend the vast majority of time in operation below maximum nameplate kW rating on part load. ASHPs modulate down to about 25%. Too large a unit with a minimum output higher than the house heating demand will then cycle on and off as the only way it can reduce output below minimum turn down. This isn't that efficient as the compressor is not chugging quietly away under inverter control, it is switching on and off, and each time it starts it's playing catch up and working a little harder, due to pre-programmed on-off cycle times that limit the number of compressor starts per hour.

If you have a log burner you have cost-effective secondary source of heating, which can be used to boost temperature quickly if need be, or to supplement your ASHP during particularly cold spells. Remind your installer of that and ensure they 'right size, but don't oversize' 

Thank you @allyfish for the really useful reply.  

It does seem a case of 'computer says no' which could be an MCS thing as the calcs work fine without the flue.

The driver appears to be the air changes as the calculator assumes 5 air changes an hour when including an ‘open flue’ compared to 1.5ac/hr without.

Your insight is great proof that an effective ASHP set up can be achieved with a log burner in situ given the ability to close vents etc., so hopefully some pragmatism can be applied to the model – although I’m not holding my breath…

Overall, it increases the space heating demand for the entire house by 27% (from 3.65kw to 4.76kw) so pretty fundamental to the overall design, but maybe not a show stopper based on your comment re ASHPs being able to modulate down c.25% which would be about right for real world comfort!

The original design was set at 50C flow temp too (as a result of this fandango around the flue) so your caution of oversizing is even more of a reason to tweak the model.

Your response was exactly what I was hoping to read so thanks again – much appreciated.