SomethingSensible

Thanks for all the replies. @JohnMo "Vastly overthinking" things is a fair assessment. Still, I'd much rather consider the possibilities and reject the overcomplex/unworkable/daft than blindly install a system which can't do something which is important to me. So, the electric boiler idea is 100% dead. Good riddance! Running pure electric towel rails through the winter feels like I'd be missing out on all that ASHP COP goodness. But if they are only on for a short amount of time compared to the more-or-less continuous heating zone, then maybe it's better? @-rick- I've looked a bit more at dual-fuel towel rail options and realised that the add-on heating elements aren't as expensive as I (mis)remembered. Is there a reason why this should be avoided? So whether I go for pure electric or dual-fuel, my towel drying concerns look like they are behind me. My remaining worry is the floors during the season when space heating is not required. The comfort of the bathroom floors isn't so much a concern for me - I can handle a wee bit of chill! - rather, if the floor does not dry properly I'm worried we'll soon be dealing with mould growth.

I’m doing a whole house renovation/extension including new roof, new slab and external wrap insulation. So everything should be well insulated. Downstairs will be UFH (‘slab UFH’). Upstairs the bedrooms will be rads with TRVs and the two bathrooms will have water-based heated towel rails and UFH (‘bathroom UFH’). (We had electric UFH in a bathroom before and it broke. Not doing that again!) The plan is to install an ASHP (and accompanying DHW tank). I’d like the ASHP to provide ‘comfort cooling’ during periods of extreme heat. I plan to install PV and battery at some point in the future, hopefully within 2 years, but do not have the budget for that right now. The ASHP company I've been talking to are suggesting a Samsung Gen 7 EHS Mono R290 unit. (12 kW, subject to heat loss calculations, to be done once I commit to them). From what I can tell, this supports 2 heating zones. They can have different temperatures if a mixing valve is used. Hopefully, most of the setup is as simple as it can be. However, I’d appreciate your collective wisdom on the best way to configure the bathrooms… “Best” in this case means most efficient (energy and cost) way of delivering the following behaviour… Winter Slab UFH and bedroom rads operate ‘normally’ (I’m not really sure what normal is yet) Bathroom UFH is on schedule to ensure bathrooms are comfortable bathroom UFH and towel rails - can be ‘boosted’ if necessary to dry towels and floor Rest of year No space heating required. bathroom UFH and towel rails are boosted manually when someone has a shower - to dry towels and floor. Heat wave Comfort cooling through slab UFH and bedroom rads Bathrooms can be left alone - everything will dry naturally! By boosting manually, I’m thinking something like this https://www.heatingcontrolsonline.co.uk/horstmann-secure-15-30-60-boost-timer.html but would also consider a smarthome solution using smart buttons My desire to have heat in the bathrooms when everything else is off seems to be a bit awkward. As far as I've been able to work out from my research, options for bathroom heat source seem to be... 1. ASHP - heating circuit Due to small load, I believe a buffer tank would be required for times when rest of house heating is off. This avoids short cycling but is bad for efficiency. If there was no buffer tank there would likely be short cycling which would also hurt efficiency. But which hit to efficiency is worse? 2. ASHP - DHW circuit Run bathrooms in parallel with tank on DHW circuit. Use return temperature limiters to avoid overheating the bathrooms. (https://www.heatingcontrolsonline.co.uk/return-temperature-limiters-c-44_49_61.html). This requires DHW to be on at times when it wouldn't otherwise need to be and could be more expensive. It also feels like it could be complicated to control to also ensure the bathrooms aren't being heated overnight (which, until we get PV, will presumably be the cheapest time to heat the DHW tank). 3. Electric boiler The heat source for bathrooms could be completely separate from ASHP and rest of house heating. Maximum/simple control but poor efficiency compared to ASHP running normally. But is it worse that ASHP running sub-optimally, as in option 1? I hope this all makes sense. It seems all the options involve taking an efficiency hit in some way, but I'm not sure how to make a fair (numerical) comparison. Thanks in advance for any pointers.