MrsDeS

Thank you people. Radiators haven't been purchased. I thought it would be better if they were running at a lower temperature as well, but I'm more than happy without massive radiators if I should just run them normally. I'm hopeful we won't need them too much. Maybe we just go for a 'normal' sized combi and settle with the status quo. It's been no big deal and if the oversizing will still only result in a fairly rubbish shower, there's little point. As pointed out, the water thieves will be leaving at some point. DS is already saving for a deposit on a house and DD is determined to go to Uni in London and is in love with it, so I suspect we won't see her for dust. I'll end up following her ☺️

Morning! We've built a our new chalet-style bungalow of 190 square metres from SIPs. Having finally decided on a gas boiler with UFH downstairs (about 110 square metres) and oversized rads upstairs, my next challenge is the boiler. I want to make sure I get this right. Our heating demand should be quite low (SAP says 31kWh per square metre) and MVHR will recover some of it for us too. We have two bathrooms with showers. Hot water is the issue. We have two teenagers who can while away hours in the shower and I'm not much better! We have a combi boiler in our current house and I appreciate that hot water is constant and readily available. I cannot bear the idea of running out of hot water, so waiting to use my shower on occasion is far preferable to having the water run out on me in the middle of one. I understand that we should only need a small boiler for the heating to run efficiently and to have it kick into condensing mode as much as possible, but that if we did want to have two showers running and not have a tank, that we should oversize the boiler to accommodate a high flow rate (assuming we have that from the mains once the water is connected!). Is it okay to do that? I've also just read about using a smaller combi boiler with a small tank that could cope with feeding two showers and allow the tank to be topped up with a half decent flow rate that should prevent us running out of hot water. Have I got that right? Presumably that would help the heating run more efficiently, but how much of a concern should that be to me? Is it going to keep the bills down massively or will the heat loss from the tank end up cancelling out any savings on the heating side? What size boiler/tank should we fit in each scenario? Thanks in advance!

Thank you, people. You have been most helpful!

Could you help me with the maths, please? Would I run the radiators at the same temp as the UFH? What would that be, about 40 degrees? If I needed a 'normal' radiator of 1500 BTUs, for example.

The SAP has given it 1.7. I'll have a look again, but I'm minded now to go with gas and UFH with the view to changing over to ASHP when the boiler dies. ?

I did a bit of maths. Assuming the big number on the SAP is what I'd need to heat the house without the ASHP cutting the energy required, based on just heating and my current energy tariffs. Electricity £453 heating, £256 service. Gas £117 heating, £91 standing charge, £50 service. SAP says 'other fuel'. That looks like £241 in electric vs £37 (or maybe £63 if the ASHP is 170% as efficient) in gas in their estimates, not accounting for teenage addictions to endless showers. Is hot water really that cheap? Cooking will be induction/fan oven, so no difference. I'm adding this up as the ASHP costing me £659 a year more. Plus the additional cost of buying and fitting it. With that fact that most electricity is still produced with fossil fuels, I'm struggling to justify the additional cost. The payback on PV is over 20 years for us too. We've paid a lot on SIPs, on the MVHR and this looks like saving us nothing in bills and we've already spent a fortune insulating it and making it airtight. What would happen if I oversized the radiators now to accommodate a future ASHP when the boiler dies? Or maybe I do spend the extra now on UFH (we have 150mm of insulation under there) so that when it genuinely makes sense to go to ASHP, we are ready...

Thanks for the reply! I hadn't fathomed that it was less efficient that way as well. I'm doing a bit more reading now, thank you.

Hi, this is my first post! I've done a lot of reading here over the last few weeks and there's clearly some very knowledgeable people here, so I'd really appreciate a bit of advice. My husband and I have built our own house from SIPs. We first-fixed a Vent Axia MVHR but haven't moved on for a while. Even without heating, the house is more temperate than the leaky 1930s box we live in now. We're back, with funds to continue, but have procrastinated over the heating system. We were initially thinking of ASHP, then changed our minds to gas and didn't put UFH in with the slab. Doing it now as a retrofit looks pricey so we're stuck with radiators definitely if we go with the ASHP, because of the initial outlay. The house is a 190sqm 1.5 storey chalet bungalow. The 'as designed' SAP gives us a space heating energy requirement of 31KwH per square metre or 5766KwH, reduced to 3392kwH by an ASHP. Putting the numbers in for RHI, it looks like we'd get £3900 back if the requirement was calculated as 5767. We have one quote at the moment of £10k (plus VAT to reclaim) for a 11.5kW Mitsubishi with a 250 litre Kingspan tank. We will buy our own radiators. Connecting the gas is about £600 and a powerful combi boiler (to hopefully run the water for 2 showers at the same time) less than £1500 plus fitting costs, plus radiators. I think then, that if the ASHP and tank is about £6,000 then the RHI will (eventually) give us back the difference in the inital outlay, minus the additional cost of larger radiators. I've used an online calculator to calculate BTUs for the radiators if we used gas and they look happily small. I don't know how to calculate those upwards for the low running temperatures associated with the ASHP. The questions are: Is it really that level a playing field IF we are prepared to wait for the RHI to pay back? How much more expensive will an ASHP be in running costs and at what point in time will it actually start to pay back in real terms on heating bills? Part of me thinks that we've done the hard work in paying for the SIPs and the MVHR. Should we just stick with what we know and go with gas? Oh, and if we go with ASHP, how do I calculate how much bigger my radiators need to be? Sorry for the waffle. It's about as clear as my head can think about this. This bit is really not my bag!