I'm not sure why the boiler is so big, I think the one specified was quite cheap to buy so that's what they went with. I didn't really get involved in that level of detail on the actual kit specified. The filters on our PAUL are quite easy to change, new design apparently.

I am a bit worried about overheating in the summer but really can't remember why he changed his mind on the ground loop.

Sounds like you're much more involved in the tech than I was, do you have a professional interest?