SBMS

Thanks @JamesPa this is useful. May I ask what values your installer agreed to regarding your ACH? Did they use a higher ACH but allow you to factor in MVHR? Or did they just allow you to adjust the ACH value to accommodate the heat recovery and if so, what value did they use?

Presumably this allows me to prove my natural/uncontrolled ventilation - but what about taking into account the MvHR controlled ventilation rate - and its heat recovery capability? Can MCS factor this in?

Thanks @SteamyTea. So is it fair to say the heat loss calculation done by the MSC supplier is horribly going to oversize the heatpump? I imagine that I can adjust the ACH figures for natural ventilation once I have a blower test done pre ordering. We’ll target 1ACH50 - Obviously at 0.05 ACH I would die, hence I will use MVHR to make sure we don’t asphyxiate. I’ll probably run the MVHR at, say 0.3 to 0.5 which will push the ventilation rate back up to the MCS’ ventilation rate. My question then is whether MCS heat loss allows them to accommodate the heat recovery capability of the MVHR?

Surely only a tiny amount? If my house’s ACH50 is around 0.5, then the normalised infiltration rate I’ve seen used is 1/20 of this so 1/20 of the 698W is lost to ventilation. So the actual ACH is nowhere near 0.5 and the uncontrolled ventilation loss is around 35W. I then add my MVHR which, let’s say I run at ACH 0.5 but it’s recovering 96% of that heat loss. So yes it adds to the ventilation losses but only about 4% of that 698W figure (28W). so the actual losses to ventilation would be nearer 63W? Does this sound right?

I asked ChatGPT and it stated 695W so sounds like that part of the heat loss is correct? Am I right in assuming this drops to something like 28W with MVHR?

It is a small section in my example because two of the walls are internal (it’s a corner room). The maths still doesn’t make sense. I’ve asked the question but think the supplier has finished for Xmas so won’t get an answer before. I wonder if there is some sort of magic number for thermal bridging that increases the heat loss? Also - Does the ventilation heat loss look correct? The room is 181m3 and they’ve said the ACH to 0.5. I did mention that we are having MVHR but I don’t think the ventilation heat loss of 698w takes any heat recovery into account? How would I calculate the ventilation heat loss at 0.5ACH (without MVHR and then with MVHR at 96% efficiency - zehnder q600)?

Completely agree - this is what I'm trying to find out from the supplier as it doesn't seem to work.

I’ve done this already. The MCS calcs are miles out because they can’t account for MVHR. However I’m unclear what formula they are applying. I can make the heat loss work for the window but for the wall or floor I don’t arrive at the same value for heat loss…

We’ve had some heat loss calcs back from the ASHP supplier. I just wanted to check the maths. Given the following for the heat loss through a 1.0 Uw window: the window area is 32.9m2 and the resultant heat loss is 881w. Am I right in thinking therefore that this is calculating for an outside temperature of about -5.8 degrees (delta t of 26.7 degrees)? Not sure I understand the maths of the above regarding the heat loss through the wall/window and ceiling…

How come all MVHr designers size off the volume of the house, not the number of occupants?

Thanks. Never really thought about from perspective of number of people in house. Family of four with 2 dogs so my gut is that although it’s a large house, something like the q600 would suffice. @JohnMo why did you go for two units out of interest (and how on earth did you get that for 2k!)

What would you aim for? I did do my heat loss calcs targeting 0.3ACH as it was a ‘healthy minimum’. I suppose this reduced the need for a larger unit even more?

So if the house volume was, say, 900m3 and I was aiming for 0.5ACH the unit would need to do 450m3? (Obviously would want to run it around 70% so probably would need 645m3?) when they do their sizing do they account for target ACH?

Not sure what you mean by over ventilation? Re your comment on price over 20 years, it’s not really the motivation for the MVHR. More because we are aiming for high air tightness so need MVHR and the comfort element.

Also - can anyone help with a very simple way to size a unit based on the size of the house? I see figures on the units like 600m3/h airflow and 108l/s requirement for building regs - how can I calculate the requirements for the house?

Currently looking at MVHR options for our build. Around 390m2 Over three floors (room in roof) so we have been given two options from BPC for MVHR: 2x Zehnder Q450 (total price 7901) 1x Komfovent r700 (total price 6000) same ducting for both options. The zehnder is 95% heat recovery but the komfovent is 83%. However I would prefer to only have a single unit and there is a price differential of 2k which I probably wouldn’t make up in heat recovery costs vs the equipment cost difference over lifetime of the unit. I also don’t know if electricity running costs would be greater with 2 units. Anyone been in a similar situation or have any advice as to which route to proceed?

Is your ‘airtightness membrane’ a VCL? I think I read the differential of the vapour resistivity between the inside (membrane) and outer (OSB sarking) needs to be around 5:1 to ensure vapour migrates out. It’s worth just checking this because if it’s at risk of condensation at least you know it’s down to makeup and not poor workmanship (design not build). Knowing doesn’t fix your problem though. I’m surprised that even with MVHR you still have a problem as that should help. I would suggest your nuclear option is to fit a dehumidifier and keep that running. You can get ones that are wall mounted and the condensate outlet can be plumbed in.

Where is your roofing membrane?? Should be under your counter battens/on top of your sarking boards?

Thanks - it’s all contained in the system itself as it’s effectively a swim spa. So filtration etc is done within the gubbins - like a massive hot tub. No separate pump room required. Heating is done via a supplied ASHP. Ventilation is not supplied and I need to look at a dehumidifier and vent system separately. Is that a single block wall basement that your pool was dropped into? how did you waterproof the basement area?

Getting my SE to design it and can then get it quoted up. Will report back!

Absolutely amazing result and fantastic news. Fair play to the other person that dealt with this. I bet your build just become a lot brighter! After that you’ll be able To deal with any issues that come up and think at least it wasn’t as bad as your last 24 hours!

Do you work for an inflatable pool company??

I think 90cm would feel a bit more like walking in a bath - I was thinking 140cm was pretty shallow. On site today and the groundworker has dug our foundations down to about 1100mm from DPC. The hole was dry as a bone at least…

Did you receive a full liability notice and was it in your name?

Unfortunately reading the above I think they did. They’re usually the first to respond on an application to let you know CIL is liable…