ap534

I've reflected on all the comments here - thank you. A2AHP is sorted with a local installer who will do a commission only price and I'll order that in the next day or so. Floor level will be brought up with PIR then T&G chipboard then vinyl click. I'll not bother with UFH. Telford do a 90l indirect UVC with a 21kW coil which looks good to me. Instant Electric shower and water heating isn't going to be practical from a loading point of view. The comments about poor insulation gave me a reality check and made me ponder whether to add anything to the walls now? The plasterboard will be screwed to 2" battens around the perimeter walls which gives me fixing points for shelves and also a a service void. In the large room, OSB will be screwed to the battens 1st and the plasterboard on top of that again to give fixings for cabinets, speakers, TVs etc. I was wondering whether to infill the space in between the battens with PIR and foam the edges, or is it not worth it because the battens themselves will be a cold bridge to the wall? Any other things to worry about while I'm at 1st fix?

Yes, I started off with that plan, however the electrical load became too high. I have a 16mm2 cable to the annexe - good for 50A which is approx 12kW. So I'd need to fit an 8kW shower max, which are not great for flow rate. I then have 2kw for a Quooker Combi style (we have one in the house) for the kitchen sink, which then leaves the bathroom basin HW to solve for. Then I have the A2AHP load plus any normal appliance load in the building. Alternatively I could fit a 8kW Redring instant heater which could also do the bathroom basin, and use another hot water heater tap for the kitchen unit. I will talk to my electrician again about whether we can go all electrical.

Ah ok, what I was thinking was putting insulation above the slab, moves that slab mass outside the internal thermal envelope, reducing the mass inside the envelope. With only 25 or 50mm extra insulation I guess it doesn't really move it outside, just slows down the heat transfer to the slab which is the point you made if I understand correctly?

I sense I've made a faux pas! Is there anything I should read? I don't really want tiles as they are cold and hard. Current plan is to get someone to fit glue down vinyl plank of a type that is not as slippy as the Karndean we have in the house. Or fit the waterproof vinyl click type. I was looking at sheet vinyl, but width is just too wide. Thinking about the floor if I don't put UFH in :- The slab is out of level by up to 20mm. Is it worth adding self levelling compound 1st to get a proper level to build up from or just put PIR straight down, then board, then self levelling compound on top of that? When we did the extension in the house they just added a few mm of latex compound on top of the slab - That was already level though. I need to fix the level for the floor so that the plasterboard/doors etc. can be fitted.

ok thank you, so forget the UFH then? Even the Wundatherm "rapid warm up system"? What would be a good way of raising the floor level 50 to 75mm given that I also want to reduce the thermal mass and not have the flooring cold to the touch? I need to self level anyway as there is 20mm difference in level in some places, but was wondering whether I can incorporate more insulation in the build up?

There's a 18kW ASHP (cheapish generic model) + 15m2 of solar matting. The solar matting adds about 9kW in Summer sunshine. The pool has a telescopic cover over it and a solar cover which helps to retain the heat. The PV provides most of the power for the pool during heating season. Yes, that was my starting point. The 2 aircon installers I spoke to said that 2 individual units work out around the same cost as a multi-split, and also modulate down better (If I was just using the office, then I'd only need a low level of heating/cooling to maintain the temp). Is the wall, ceiling, or floor the problem, or all of it?

Thank you for replying SteamyTea and Tonyshouse. The building is supposed to be to building regs (as of 2020). Insulation Insulation in the floor is actually 150mm PIR boards. There are 75mm PIR boards in the wall cavity, and the roof has 130mm PIR boards from memory - I'll need to check. I'm not sure if that gives you any more information or answers your question though!? Windows The building has 2 main rooms + a wetroom. The rooms are 5mx5m and 2.5m x 2.5m. The main room has a 4m double glazed sliding door, and a 2m double glazed sliding door, and a 1.5m wide double glazed window. Here's a video of what it looks like. It has had a tidy up since then! https://youtu.be/JifnW4bNmRw Usage Summer:- It is a multipurpose building, next to a small outdoor swimming pool. We'll use it as a place to sit and supervise the kids in the pool in the Summer, and somewhere for them to change and have showers. Doors open in this usage, DHW requirement is for multiple consecutive showers. We'd also use it as a home cinema. In this usage, we'd have the doors and windows closed - which is where the Summer cooling requirement comes from. Winter:- In the winter we'd use it at the weekend to go "on holiday" for the day as a lounge area or to go and watch a film. We'd want to decide with 30 mins or so notice and for it to be able to warm up quickly. In this instance the UFH would be nice, but not if it takes ages to heat up. Other usage year round - Home office (Cooling and heating)/general entertaining/somewhere for the kids to build a Scalextrix track without cluttering the house. In summary, it may not be used for a few weeks at a time, but when it is used, the demands on it are quite high. Heating The A2AHP seemed ideal, as if the building is not being used for a few weeks, then we could set it to keep the building dehumidified and at a base temperature at minimal cost. The Wundertherm system seemed to deliver rapid warm up by not heating the slab. It has the pipes within insulation boards, then a 5mm renovation screed then the floor covering. Either way I need to bring the floor level between 50 and 75mm. The UFH seems to be adding a lot of complexity and cost, so maybe I should forget it and just stick to the A2AHPs, and a simple UVC for the HW.

Hi, 1st post ? We have a new 50m2 Annexe with 1st fix happening now. It is block and render with 75mm cavity insulation, 130mm floor insulation covered by 100mm slab, 130mm insulated warm flat roof. We would like to be able to use it all year round as a living area/home office, but it won't normally be occupied or heated. So rapid warm up is essential. It will have a kitchenette and wetroom with shower and toilet. I have a 20m run of 25mm insulated flow and return pipes (https://www.flexenergy.co.uk/flexalen-600/) coming underground from the house which has a 37kW British Gas condensing gas boiler. Minimum output around 10kW. Stated heat loss through the piping is 15W/m at 80 deg temp. I generally run the house heating at around 50 deg C, then boost to a max of around 80 while heating the DHW. I also have 10kW of PV, some of which I can divert to the cylinder in the Summer months. My plan so far:- DHW 150l unvented cylinder with high area coil, heat from the house boiler. High area coil for rapid heat up, and future use with ashp if we replace the gas boiler. Heating/Cooling 2 x Midea wall mounted air to air units with external compressors https://mideauk.co.uk/products/blanc-series/. 5kW for main room, 3kW for office. UFH - Wunda 20mm EPS400 overlaid by renovation screed + vinyl plank. Heat load 2.2kW. UFH fed from house boiler. Questions Air to Air appears to be the lowest cost to heat the building, and the installers say that most people don't bother using UFH once they have air to air. Does this ring true? I like the thought of being able to sit on the warm floor in the winter. I'm being quoted by 2 different places roughly £1500 + £1400 + vat for the 5kW and 3kW Midea units installed. The units are readily advertised at £680 and £460 +vat. Is over £2k for install the going rate? If I put UFH in then I need it to warm up quickly, which led me to the Wunda system which seems to fit the bill. I don't want to be heating a slab for hours. Am I wasting my time with UFH? If I put the UFH in, then I think I need a buffer tank otherwise the boiler will be short cycling all the time. Then I read threads on here about also being able to switch in the buffer tank to pre-heat the DHW which then let's me reduce the UVC size perhaps to 80l? By the time I've added all that plus the controls, is there a lower cost/size alternative? What about a small external ASHP for the UFH.? Here is a floor plan, Here's my piping sketch so far, but not sure how to do the DHW pre-heat. I've omitted the various controls and valves at the house. Any pointers would be much appreciated please, as I'm finding it difficult to get advice I can trust!