SBMS

Yes @JohnMo you're 100% right. So easy to get lost in the detail though. It's a fair point and perhaps I'm being a bit critical on them. I suppose last thing they want is to install something undersized or risk their MCS. Can they design the underfloor heating (e.g. the number of loops and energy output of the UFH) without knowing the 'as built' heat requirements though? I'd be happy for them to quote at the stage of wind and watertight if they needed to know the actual as built insulation and airtightness figures...

Fair enough!! Do you mind me asking what was the cost for your A2A ducted? Is it zoned?

Should have insisted on using fancoils you mean instead of ducted A2A? You wouldn't recommend A2A then?

Got it. What would a good permeability (AP50) value be? I am conscious that that it's not part of Jeremy's spreadsheet.. Those who aren't targeting good levels of airtightness could have their assumptions thrown. For example, if I target an AP50 of 1.0, which for my dimensions equates to 0.58ACH50 this is 574 watts of additional heat required from the ASHP. If I changed the AP50 to 5, then the heat loss is nearer to 2.7kW. Be good if the calculator allowed this as an input? I assume that MCS when doing their sizing calculations should take into account the AP50 (or ACH50) targets to generate an accurate figure? FYI - I found this diagram useful for other readers to calculate between ACH50 and AP50: From https://thefifthestate.com.au/columns/spinifex/why-we-should-change-our-building-air-tightness-metrics/#:~:text=It's really a very similar,m3) of a house..

I think I get it can I check? Given an air permeability AP50 of 1.0, the ACH is 1/20 of this so 0.05. Fed into the calculation above this would mean at delta t of 29 degrees for a 1200 cubic vol house the heat loss would be 574 watts (so this would be the added load on the ASHP?) Also - when passivhaus talks about the ‘maximum air permeability is 0.6ACH 50’ - am I right in thinking this isn’t the same as the air permeability figure seen on the SAP or done on the blower test (AP50) - although they frequently seem to be very similar numbers?

Just trying to work out the correlation between the air permeability (AP50) and what n should be? Im Assuming that ‘n’ is not 1.0 but then I don’t really understand how to derive the infiltration rate from the air permeability AP50 figure? the SAP does have a figure for infiltration rate of 0.05 but I don’t know how this is derived?

Interesting. What made you go split ducted air instead of fan coils?

Thanks @JohnMo this figure can’t be entered into Jeremy’s calculator though?

I think I see now that the AP50 designed as per my sap is 1.0. Does jeremys calculator capture the air tightness of the building? I understand the MVHR ventilation rate would assume a perfectly sealed controlled house. But if say, there’s additional ventilation loss (say another 0.5ACH) how does it account for that and where would it be entered? Im worried I’ve misunderstood this part of the heat loss calculator. If my score for AP50 was 1.0 And my surface area of the house is 450m2. Then if the volume of the house is 1200m3 then the ACH50 would be circa 0.375. If my MVHR was ventilating at a rate of 0.5ACH then does this mean my total air change figure becomes 0.875?? How does the heat loss calculator account for uncontrolled ventilation loss based on airtightness in addition to controlled ventilation from MVHR?

I think I've worked out probably what the supplier is specifying. If I change the Air changes figure from 0.5 to, say 5, the energy requirement jumps to 12kW which is probably what they are basing it off. Can I ask - the figure that is on the SAP which is the "Measured/design AP50" (air permeability) - is this the same as the 'Air changes per hour' figure on Jeremy's spreadsheet? If not, what is the difference? We are aiming for 0.5 to 1.0 on the air pressure test - is this the same as air changes or are they different? If different, what's the link between the air pressure test figure and the 'air changes per hour' figure in the heat loss calculator?

This is actually a great point. All our rooms upstairs except wet rooms will be carpeted and with thick wool as my wife likes that... I think that knocks UFH out (although might price up wet UFH in the bathrooms). Would you consider an AC system (split or ducted) upstairs - or fan coils, or just standard rads and hope it doesn't overheat?

North West - Lancashire. SAP guys have said there's a small risk of overheating in some of the bedrooms as we are using a 0.6Ug triple glazed pane (they've said a 0.4Ug would solve this but that's very expensive). I am definitely erring on the side of fan coil units - if they don't need to provide heat then it's just the capital install cost, but at least they are there for cooling if needed (they say it'll only get hotter). I don't know how familiar installers are with fan coil units though... The one part of the house I'm not DIYing is the plumbing (I hate it, and always get it wrong )

Thanks @JohnMo - found that just before your reply 👍 Think the current supplier is just going off a stock standard insulated house. They've calculated: I get 4.5kW - although what would be a sensible amount to include for DHW in worst case?

@ProDave correct - its rooms in the attic so technically 3 floors. I could do UFH upstairs (would rather not fit myself as I simply don't have the time for that - will be doing as much as I can such as MVHR but need to be realistic).. but it doesn't provide any cooling hence looking at either ducted AC/split AC or fan coils? House section for context:

@joe90 I'm not sure - the thing I'm trying to work out is the following... So on Jeremy's heat loss calculator, plumbing everything in its looking like a worst case -9 outside and 20 inside, its around: Total heat loss = 5885 W So if I didn't want any radiators upstairs, I would need to ensure that my UFH provided, in this worst case scenario, about 5.8kW (call it 6) output. Is that correct logic? Presumably if my house was a box with nothing on the floors like kitchen units, islands etc, and all heat rose vertically this would work. But my house isn't shaped like that, it's like this: I think this means that the 134m2 double story part, once I take away the unuseable floorspace (staircases, kitchen units etc) there's about 107m2 of UFH space providing the space heating requirements for approximately 134 m2 (ground floor bit), 117m2 (first floor) and 88m2 (attic rooms): Total power req (kW) 6 475 total sqm SQM UFH Power Delivery SQM kW Required Single Storey 1 44 44 0.56 kW Single Storey 2 92 92 1.16 kW Ground Floor 134 339 4.28 kW delivered in 107m2 First Floor 117 0 0 Second Floor 88 0 0 I suppose the question is - can 107m2 comfortably provide 4.28kW of heating output, and/or would it be preferable to have a lower output UFH with some upstairs radiators/fan coils upstairs (that can also provide cooling). What would be the disadvantage to this (bearing in mind we do want some cooling in the bedrooms as well)?

Nod - agreed and we are pretty well insulated to be honest. 200mm cavity, blown beads etc. I know your last build you put radiators and never used them but I can’t bring myself to put no heating upstairs. Cold wife and cold children.

Planning our upstairs heating for our build. Downstairs is wet UFH, ASHP. We’ve got a first and second floor (attic rooms). Some small risk from overheating due to a fair bit of glazing so would like some active cooling upstairs. Trying to decide what to put upstairs. UFH upstairs on two floors is coming out expensive. Mainly labour costs (we are not going to fit it ourselves). Seems that it’s overkill if heat requirement is low? And we can’t cool. I thought about putting split AC units in each of the 5 bedrooms and using them for heat as well. But not sure about having to remember to turn them on if it gets nippy, fans running etc? But cooling would be fine. I therefore thought about a ducted AC system upstairs (maybe a couple of zones) that could do heat and cooling? Other option is fan coils. I don’t quite understand the condensation risk - does all pipe work need to be lagged? Do I need condensate drain off at each unit? Are they as effective at cooling as an AC system? Would appreciate peoples throughts: Upstairs options option 1 - UFH + split AC option 2 - radiators + split AC option 3 - split AC (heat and cool) option 4 - ducted AC (heat and cool) option 5 - fan coil units (heat and cool)

Thanks @JohnMo - I have done the heat loss calculations. I might be missing it but where in the spreadsheet does it show a ‘peak energy input’ requirement that would help me size the heat pump? I can only see total monthly losses.

I think that’s what I’m trying to work out - what size do I need? Is the SAP calculation accurate enough to size the heating or does an MCS installer (which they are) produce a more accurate figure?

Sorry @JohnMo you’re suggesting I need a 6kw unit not a 16kw? am I right in thinking that January heat loss rate figure of around 4kw is a peak figure based on -9 outside? So a 6kw unit would comfortably cope? sorry @SteamyTea for miscapitalisation of the Ks and the Ws on my phone typing

is 35w/sq m good? Average? It’s masonry 200mm cavity with eco bead. 304mm posi rafters with blown in cellulose and external 22mm wood fibre sarking. 175mm PIR in the floor. Glazing averaging around 0.85 but a fair amount of it to be fair. aiming for 1.0 on airtightness. House footprint is 190 sqm PA ratio of 0.35.

One thing with this.. I might be misunderstanding but am wondering if we need UFH upstairs (we have a first floor and rooms in roof as well - So UFH on both floors). This adds quite a bit of cost. But if I wanted to minimise radiators upstairs I would need to work on a certain value for sizing the wet UFH loops in the slab to ensure it provided the right amount of output?

Thanks @JohnMo. Yes MVHR. Have found the heat loss in my sap for January: That figure is what’s confusing me as it seems the supplier has come up with a much higher figure. We’ve got a fairly standard form factor…

We have had our SAP rating done. It’s come out with a primary energy rate of around 23kWh per sqm. One of the ASHP suppliers has come back and intimated we might need a 1 x 12kW & 1 x 7kW system. They’ve calculated a 35 W/m2 requirement - so 16kW (over 450sqm - this includes a potential future extension). This has been done based on the worst case -2 outside +20 inside. We are trying really hard for good u values - 0.14 walls, 0.1 Roof and floor etc. Does this calculation sound correct? I appreciate this is the ‘worst case’ so the average will be better but can anyone comment on this W/m2 calculation? Is it our floor area that’s pushing us to two units?

I know someone whose husband died and because he didn’t put his wife’s name on the application they’re now chasing for CIL because he hasn’t lived in the house for 3 years. Outrageous.