Tarrington

That's true, a lot of the people we spoke to (including the main building contractor we're using) can only do air-to-water. Even the place we eventually found who can do both spent about an hour trying to talk us out of it (which is weird when that's their main USP).

It's really mysterious to me why reversible multi-split systems aren't more common in the UK. They're reasonably common in the US and some of Europe, but every supplier I've spoken to about them in the UK has answered as if they've never even considered the idea. The worst thing is, nobody has been able to clearly articulate why. Some people say they're not "powerful enough" to heat a whole house (they are) or that they're less efficient or more expensive to run than an air-to-water system (when on paper they're more efficient, because they don't need as big a temperature difference). Of course, aircon is becoming more of a need as the years pass, and that's a nice side benefit of reversible multi-splits; and we've found from our current place with direct air heating that the low thermal mass you get by cutting water out of the equation is really nice. I've seen some posts on here with better information from people who have actually used them, saying that underfloor heating feels cosier and an air blower creates a draught, making the room feel colder, so you can run to a lower thermostat set-point with underfloor heating: this makes more sense. But since it's quite a big decision I did a little more research into what systems are available - especially as heating suppliers are very vague about numbers. After that, it makes a lot more sense why people would shy away from air-to-air. Our house is going to be about 175 sq m and the room layout is such that we'll need 6-8 indoor units. For a start, most multi-split systems only go up to 5-6 units, so we'd probably need to split it into two systems. Second, the smallest indoor blowers I've found are rated for 2 kW of heating, even though most of our rooms would need less than 1. The vendors don't recommend using much more total capacity indoors than the capacity of the outdoor unit (the heat pump itself), so we'd end up with a system with about 15 kW total capacity, even though the house itself is well-insulated and won't need more than 6 kW (and almost certainly less, as we won't need to heat upstairs and downstairs all day). That seems hugely overspecified, while an air-to-water system would be able to do a single 6 kW water loop with no problems, and (with bypass valves) still allow some per-room temperature control. In addition, while there are some air-to-water units that can also do hot water for tops, that's not something you'll get from a multi-split, so there needs to be another solution there for hot water. Given all of this, it's slightly surprising that there aren't systems that can do both: air-to-water for underfloor heating and hot water, with the capability to send refrigerant to a small number of indoor blowers for heating or cooling extra rooms. What do you think about this analysis? Is the need to overspec the system as a whole the downfall of air-to-air systems? Why do you think they're not more common in the UK?

Hi, I signed up for the site as my husband and I are starting our first self-build house this year. We've found the plot and the legal process is running. We already have drawings, a mortgage offer, and a builder to do the work (if the start date lines up), so it's pretty exciting.