Ben1984

Interesting. Based on your experience it would seem that if the unit can meet building regs requirements it should be perfectly fine with day-to-day ventilation. So the single units should in fact cope just fine on my build (assuming they have been designed to meet building regs)

This is an interesting point, I remember reading something about this before. If this is true, why does everyone seem to use the size/volume of the house to specify the MVHR unit? Is this just to ensure it meets building regs?

Whilst I know very little on the subject, my concern is that Paul heat recovery and the Airshop have manupilated the figures a bit to try and make one unit 'work' when in reality it could struggle in certain situations. I have plenty of space in my plant room, so I'm currently inclined to go with 2 x Vent Axia units. But I won't be placing the order for a few months so I'm keeping my options open.

BPC's quote for 2 x Vent Axia units is by far the cheapest @ £4700. They also quoted for 2 x Zehnder units (Q350 and Q450) @ £8000 this is also cheaper than both quotes i've received for a single unit. I prefer the idea of one single unit, to simplfy the design and reduce maintenace costs but that is currently more expensive and I am concerned it would struggle when the house is busy.

CVC and BPC are both saying I need 2 units (or a large commerical unit that is not cost effective) The Air shop and Paul heat recovery have both specified the Zehender 600. (It's worth noting that Paul Heat Recovery missed some rooms on the initial design, but even after amending it they are sticking with the Zehender 600).

Following with interest. Currently experiencing something very similar - had 4 quotes for MVHR install (~400m2). 2 are adamant I need two units and the other 2 are saying the Zehender 600 will be sufficient.

Thanks (again). I thought i'd got my head around the ASHP/heat calculations but I clearly have more to learn. I worked out my heat loss to be in the region of 6.5KW (internal floor area of 390m2) allowing another ~2KW for hot water and adding on a bit extra I thought I would need an 8-10KW ASHP. But the quotes I'm getting from installers are for 12-18KW ASHP's. I thought this was because they were assuming a flow temperature of 45 degrees, but your post suggests that isnt the case. I'm still as confused as I was when I strated researching this! I will re-visit this tomorrow.

Thanks, it makes perfect sense. I am slightly surprised that you can achieve ~20 degrees internal temperatures when it's -9 outside with a flow temp of just 32.8 degs. I've had a few quotes for ASHP ,they have all been working off 45 degree flow - If I run it on a WC I assume I would need more pipework in my UFH and a smaller ASHP?

Interesting, I thought that was quite cool! This is obviously something else I need to look into! I have no idea what a "weather compensation curve" is.

Thanks Joth, some useful insight. I was very sceptical about putting water through the FCU below dew point and it sounds like I was probably right to be. I think I will go for ceiling or concealed wall (mounted high up) units in the 5 bedrooms and rely on the UFH downstairs for cooling. Aiming to run everything at the same temperature (~15 degrees for cooling and ~40 degrees for heating). Still not entirely sure how to size the units in the bedrooms if they are primarily there for cooling, i'll speak to some of the companies selling them.

Why would I need to run the ASHP at a colder temperature for a larger unit? I was under the impression that I could achieve more cooling with a larger unit without reducing the flow temperature.

Has anyone on here run water below dew point (~7 degrees) through their fan coils? - The reading/research I've done suggest most people run it around 14/15 degrees. I am happy to insulate the pipework to the fan units, but I think I'd still be a bit concerened about dew forming in voids if I ran it at 7 degrees

Thanks, I'm currently looking at concealed units - I can get a 5KW unit that has a grill measuring ~900mm x 200mm that should (in theory) meet the heating requirements of the room or I can spend an extra ~£250 and get a 11KW unit with a grill that measures 1600mm x 200mm. Trying to decide if it's worth paying more and getting a larger unit with noise and cooling in mind.

I am currently trying to work out a spec for our fan coils, wondering if you can help with a couple of questions please: I can get away with relatively small units if i size them for heating - is it worth oversizing them for cooling and/or quieter operation? Is there any meaningful difference between a ceiling and wall mounted unit in terms of performance (assuming everything else is equal)? I am currently planning to have 1 unit in each bedroom - should I consider adding 1 or 2 units downstairs in the main living space? EDIT: For clarification I will have UFH throughout the ground floor, the fan coils would simple add some extra cooling in the summer.

I had a similar experience with SSE - they originally said that their small connections team weren't able to quote without an accurate breakdown of the required load. After explaining I wasn't able to proivde one, they passed me to their large connection team (who can quote without that the load requirments but apparently it takes longer). The large connection team rang me two days later to say they looked at my enquiry and they'd made the decision to pass it back to the small connection team to quote! After going around the houses, the small connections team were really helpful and quick to produce a quote for a 3 phase supply - for the first time ever is was less than I was expecting to pay! Moral of the story - speak to them, there may well be a way you can proceed without that information.