SBMS

Yes agreed. I think MCS lets you discount rooms heated by other sources in heat pump design (MCs 3005). But I think you’re right that BUS requires 100% heat from heat pump. There’s a weird cross over I think that is unclear as to whether MCS when designed for BUS requires it and that’s probably an installer interpretation. Still on the fence about cooling with ASHP and sticking fancoils upstairs and ditching the AC.

It’s actually relevant for us as some rooms we may put AC in so wouldn’t be heated by heat pump.

I guess as long as it is sized to heat it doesn’t actually have to deliver heat to that room to be eligible for the grant though?

How come? Are you not allowed this?

We are using 22mm steico universal boards with pro clima solitex plus.

What size outdoor unit do you have @Thorfun?

Good point. It made a mistake and I’ve time shifted it now. But it does still make me think… it’s pretty hard to model correctly the cooling load.

Thanks @JohnMo I didn’t realise it was that automated. That’s interesting. I got chat gpt to model my cooling requirements during summer and it spat out this graph taking in my heat loss and cooling data along with this description of how it generated it. It still has a peak of around 13.4kW. 🔍 The whole-house 24-hour cooling load profile I plotted is not based on full peak (worst-case) loads Instead, it is based on: A synthetic sinusoidal profile interpolated between average and peak cooling loads – using column W (Average Load) as the baseline – and column AD (Peak Load) as the afternoon high 📈 What does that mean? At 4:00 AM, each room is assumed to demand only its average load (e.g., from internal gains or mild solar). By 15:00, each room is assumed to hit its peak load (from full solar and ventilation gains). The curve is room-by-room, then summed.

So if you switch the pump into cool mode but then the DHW falls for heat it reverts back to heat mode (and then back again to cool mode when water is heated)?

Do the units natively support switching from heat to cooling or is it work to ‘put the heat pump in cooling mode’? by my estimation it’d be around 4k for the fan coil units needed for cooling with ASHP versus significantly more (10k+) for a2a system.

I think the challenge on here is that there are those in the reverse ASHP camp and those in the separate AC camp. I don’t think there’s anyone that’s had both and can say which is more optimal, comfortable etc. I think the neatest idea is running heat pump in reverse (it’s there anyway) but the added complexity of dew points, lagging etc makes this more risky than AC. That’s where I am at moment. AC = guaranteed cooling, no risk. But more expense.

So when designing the UFH loops do we need to target a w/m2 To account for cooling demand? Ie we need an output of 63w/m2 in this instance?

How would one go about calculating the cooling output of the slab running in reverse? Say I have a room that is 11sqm with a cooling requirement of 700 watts... How could I calculate if the slab could cool that space?

What did you put in?

I don’t disagree with your logic @JohnMo and probably the average is even lower. I think the AC units have the ability to modulate pretty low so oversizing isn’t a massive deal and the price difference between say a 15kW or 25kW unit is pretty marginal. So does it hugely matter if I have a system larger than is needed most of the time? Appreciate it’s different with ASHP sizing and that’s important to get right. It might be different in the world of AC but all suppliers I’ve spoken to size to peak cooling load. fyi lots of south facing glass

Just to be clear that is Peak Cooling Load (the maximum instantaneous cooling demand). As I understand it, most HVAC designers will size equipment based off the peak load. I used these figures when calculating the peak vs the average for solar gain, temperature delta etc. Component Peak Value Average Value Ratio Solar Gain 600–800 W/m² 300–400 W/m² ~2× ΔT (cooling) 9°C ~5–6°C ~1.5× Internal Gains Max occupancy 50–75% occupancy ~0.5–0.75× Total Load 100% 50–70% ~0.6× typical My Average Cooling Load is much lower, around 9kW in the summer.

Continuously second guess as to whether to go fancoils and ASHP in reverse or true AC. Would be much more cost effective with FCUs and ASHP in reverse, but have concerns that it won't cool house sufficiently (how to calculate the ground floor cooling capacity??) and how hot water generation can happen at same time? I hear lots of people talking about manually setting jumpers on heat pumps to run in reverse, lagging of pipes for condensate etc. Sounds difficult to do as a whole house system. That, and the peak cooling load for our house (worst worst case) is circa 24kW. I don't even know what the 'cooling capacity' of a 12kW heat pump actually is (is it the same?? 12kW??)

What units you got there @Dave Jones?

We've decided on UFH downstairs (ASHP) and a VRF AC system to upstairs bedrooms and downstairs. This has removed any wet heating costs upstairs - no rads etc, and will allow us to use AC for top up heat in very cold weather (if needed - unlikely) and benefit from cooling in summer.

Thanks Nick good to hear! I’ve gone round the houses on this. Currently wondering whether the double height south facing atrium glazing justifies solar control glazing (extra £1200)!

We looked at norrsken etc much higher price than our current supplier. We are going with senior architectural pure line of windows triple glazed about 0.7 u value all aluminium. Sliders about 1.0 and bifolds about 0.9. Much better price than Alu clad.

Did you do ducted AC upstairs or wall units? Think we are going this direction..

It doesn’t you’re right I was reading someone else’s article on a blog and I don’t think the regs say that. I initially thought it was the greater of 43l/s for 5 bedrooms or 0.3 * square meters floor space. Think i went down a rabbit hole reading someone else’s blog. I think it’s as simple as 370*0.3=111 as I put originally for BR??

Sorry typo before. I read the above and worked out that the BR ventilation rate is 124l/s. 43l/s as per your table plus the adjustment as the floor area is greater than 100sqm. (370 - 100) = 270 m² × 0.3 l/s/m² = 81 l/s 81+43 from the table is 124l/s. Which I think requires a unit size of circa 450 cubic meters per hour. Seems close to the ideal 60% most recommend running a unit at although I wonder if the BR calculation is on the high side. Paul heat recovery calcs just seem a bit on the low side but I don’t know.

To be fair the AI calculated the BR value which I checked and it was correct (111l/s). The occupancy figure was around 120-130l/s. So fairly similar.