HighlandHopeful

Thanks everyone for all your help today. I will go away and think about the floor.

Yes I am perplexed about why floor U value is so high, I wonder if I've calculated it wrong. I've used this for advice: https://www.gov.scot/binaries/content/documents/govscot/publications/advice-and-guidance/2020/02/tables-of-u-values-and-thermal-conductivity/documents/6-c---u-values-of-ground-floors-and-basements/6-c---u-values-of-ground-floors-and-basements/govscot%3Adocument/6.C%2B-%2BU-values%2Bof%2Bground%2Bfloors%2Band%2Bbasements%2B%2B.pdf The build up the floor is: On top of the soil, 50mm of sand, 50mm of concrete. Then 150mm air gap - as part of the calcs I had to imagine this sucking heat out through the floor and transmitting to the uninsulated parts of the foundations around the perimeter of the building. Then 150mm joists with 100mm rockwool in between (and a 50mm airgap) Then vapour barrier Then 2 layers OSB at 11mm each Then the wooden flooring but I haven't included the thickness of resistance of this yet, so U value might go down slightly, but not significantly. I used Resistance of the floor, Rf = 1/Uf - 0.17 - 0.17 And U-value of the floor, Uf = 1 / [(1/U0 ) – 0.2 + Rf] U0 was derived from the table on page 6.c.3 at the above link - our area to perimeter ration is 0.5 and the "Ventilation opening area per unit perimeter of underfloor space (m²/m)" is 0.003ish assuming that I calculate this by doing depth of air gap between soil and joists divided by perimeter.

Interesting, love the sound of the aluminium skirtings with UFH (although BFs dad very allergic to aluminium and will be helping with the build - does any other metal work?). Another reason against UFH for us, which I should have said, was that I thought you need a concrete slab for UFH, but we can't get a concrete tanker to our plot. We are having strip foundations so that we don't need a tanker.

Thank you for all of that, really helpful, I've updated my calcs to assume a much lower temperature, I'm not sure why but had it stuck in my head that you design for average lowest, not actual likely lowest. Re: how we are doing ventilation. I assumed 0.6 air changes per hour for the heat loss calcs as I read that passivhaus should aim for that. We are using heat exchange MVHR. Haven't got as far on that yet, but let me know if you think 0.6 is a stupid number to have put into my calcs! Thanks again, I'm chuffed you replied.

Thank you for helping! I've calculated our U value of walls 0.131, roof 0.105, floor 0.21. We have a lot of windows, which are 1.1 ish. For heat loss I used: Qp = ((F1 x Sigma AU)+(0.33F2 x NV)) x (tc - tao). F1 and F2 were taken from the textbook for panel radiator systems and were 1.00 and 1.10 respectively. Sigma AU was the sum of all the areas of elements multiplied by their U values N was the air change rate - I assumed 0.6 V volume of the house tc 21 degrees and tao WAS -5 but I've edited to -15 as I get now that I need to design for worst case as we won't have supplemental heating, not for an average case. I thought about it for a long time but I really want wooden floors and lots of rugs and carpet in a couple of rooms... Thanks for the other info, incredibly helpful!!!

Hi all, this is a long one, but I’ve been storing this all up since June 😊 My profile name is 'Highland Hopeful' as I joined here when I was still gathering info for my planning application of a new house in the Highlands. I'm super pleased that we got planning approval and now crawling our way through the Building Warrant application. It is equal parts breakthrough and breakdown, as we are doing everything ourselves. We are 37 and 44 and very enthusiastic, both have engineering (sound, electrics, civil, transport) backgrounds and are hands on people. I have found these forum threads so incredibly helpful, so thank you for existing, and haven't needed to post anything until today, but I would love to sense check a couple of things that have led to my ASHP sizing calculations as I’ve hit a wall. I have been using info from the forum, as well as a very useful textbook from the library called 'Building Services Engineering' by David V. Chadderton. The house is 19.2m x 4.8m, it has a 45degree pitched roof, it is one storey, so the ‘roof void’ is just open so that one can look up and behold it. It has one bedroom, and only my boyfriend and I will live in it, full time. The radiators and DHW will be heated by an air-to-water air source heat pump. We are not having underfloor heating. It will have triple glazed windows (I have already bought them – it was cheaper to design the house around what was available on seconds, than to spec them around a design). The questions I would love your thoughts on 😊 1. How to include DHW in ASHP Sizing Calcs I have calculated the U-values of walls, roof, floors, windows and doors and used this to calculate the heat requirement. I’ve assumed 0.6 air changes per hour. Outside temperature of minus5C. As a result, I got a heat requirement of 4kW. So I though woo great, I’ll get a 5kW heat pump. But then I read in the textbook that this heat requirement is only part of the ‘boiler’ power calcs (I’m calling it a boiler as that is what the textbook from the library is based on) as I need to include for losses (10%, so 0.4kW, fine) and the power required to heat the water in the cylinder. This is where I got confused. To calculate the ‘boiler’ power you need to know the time to heat up the water in the cylinder, and for that you need to know the power of the ‘boiler’ (circular reference warning?!), so I used 5kW as that’s what I wanted to buy, but then obviously that tells you how long it takes to heat the cylinder if the power of the system is 5kW, which then would need adding to my heat requirement of 4kW and losses of 0.4kW to give a ASHP size of 9.4kW... I didn’t like how the equation was self referencing, i.e. if I’d said I was having a 12kW system, that’s what would go into he heating time calc, which would then go back into the ASHP size, and could just go up forever... Is it that I need to specify what kind of reheat time I’m happy with? Does 5kW based on the info above seem reasonable (U value of walls 0.131, roof 0.105, floor 0.21)? How do I incorporate the need for the DHW. Ha even this question is going around in circles! A problem may be that I’m using ‘Boiler power’ calcs from the building services engineering textbook to try to work out ASHP size… 2. U-value of Pitched Roof I think there's a high chance I have done the U-value for the roof wrong. We are having a 45degree pitch, which is completely open i.e. doesn’t have a flat ceiling with a roof void above it. I have worked out the U-value of the roof as if it is just two flat pieces. In the textbook I am using it says for pitched roofs I should use the formula: U = 1/(RA cosβ + RR + RB) where RR is the resistance of the roof void (we don’t have one) and RB is the combined resistance of the materials in the flat part of the ceiling including the inside (we don’t have a flat bit of the ceiling). I have tried to find online a way of calculating U value for pitched roofs, but not coming up with anything useful. Any ideas? 3. Average Coldest Temperature I’ve assumed an average coldest temperature for the heat requirement calcs of minus5C, but I’m really not sure about that. We are right on the north coast looking at the sea, but still seemed very low – any thoughts? I couldn’t find a good source for that, so just used what I found. So sorry this is long and thanks for your time if you got this far! I’ve been working through this for months and have everything in a spreadsheet that will update if I change any of these numbers. I feel like I’m nearing the end so would love advice on these finer points.