Great_scot_selfbuild

Which is best and why, or is there nothing in it between 11mm OSB and 12mm Medite vent panels for the roof sarking. Our roof will have 350mm of warmcel between the rafters (U=0.11), with 50mm air gap above the sarking by means of battens & purlins supporting a corrugated steel roof sheeting. The original scope of the timber frame had 11mm OSB listed but the designer has specified 12mm Medite vent and so I’m being asked which I would like. The price difference is £10/sheet, and we need 164 sheets, so £1,640 uplift for Medite vent. Have any of you used both and what would you go for and why? We’re going for quite a high spec performance build, and have prioritised the fabric of the build. If there’s a material benefit/value to be gained then we would consider this extra cost, but we can’t afford to do it just on a whim. Keen to get the experienced view from this group. Thanks in advance…

Any recommended tools / method for the UFH layout/circuit designs?

@BartW Thanks for creating this thread - having read it now, I think I’ve saved myself a lot of work being led down the comfopost path by some M&E suppliers. I’m becoming less trusting of the technical competence of some of the companies I’ve reached out to and am finding myself doing so much double-checking and calculations myself, I’m very interested in how you’ve gone about designing the UFH / ASHP / DHW system. I’ll be starting my search for tutorials and advice on how to go about this, but would welcome any advice you can offer?

@JohnMo amazingly helpful and clear - thank you!

@JohnMo sorry, not sure I follow - you’re saying the 7.5kw isn’t the best fit? They’ve used this SAP figure to give a 4.35kw requirement, and then (presumably) scaled it up to a 7.5kw HP to cover the DHW. Not that I disagree, just trying to understand - which bit of this shows that 6kW isn’t big enough at -3? And is 7.5kw big enough at -3, plus covering DHW?

They used a statement in my SAP assessor’s report that stated heating requirement as 16.7627w/m2; 260m2. (Screenshot attached)

@JohnMo @SBMS @MikeSharp01 @jack @Marvin Thanks for all your input - most helpful. After challenging/questioning the suppliers' quotes they have revised their proposed solution and Cool Energy currently is looking most favourable (MCS umbrella etc.) and has reduced the ASHP proposed down from 14kW(!) to the following: Proposed Solution: We recommend the following setup based on your requirements: Heat Pump Output: We recommend a heat pump with an output of 4.354kW. Design Temperature: -3°C ambient temperature with a 35°C water flow temperature, suitable for underfloor heating (UFH). Heat Pump Model: We recommend the Cool Energy CE-ET10, which can modulate between 3.89 kW to 7.48 kW at the above design temperature. Hot Water Cylinder Size: 250L, calculated at 50L per person plus an additional 50L. Design Temperature Explanation: The design temperature of -3°C ambient and 35°C water flow is chosen to ensure your heat pump system provides reliable heating even on colder winter days while keeping the water temperature low enough for underfloor heating efficiency. This balance helps maximise energy efficiency and comfort throughout your home. My interpretation of this is that whilst the say 'recommend an output of 4.354kW' which sounds ok to me based on my estimated heat losses of just over 3kW plus an allowance for the DHW - I'm not sure how to calculate/estimate this? and haven't searched around yet; however their model can modulate between 3.89 - 7.48kW; am I correct in assuming this would be referred to as a '7.5kW ASHP'? Just wanted to share an update and see if anyone had an opinion on this proposed ASHP solution?

@SBMS Thanks. Does that heat loss mean 3kW heat pump?! (not clear what duration it is over - I saw another post that referred to multiplying by 24, assuming it to imply it is an hourly heat loss figure, but basically I'm still unclear. Volume - I hadn't clocked that was a manual entry and I hadn't changed it. You're correct, volume is 879m3; floor area is 131m2 but tall ceilings and upstairs within the roof space. Thanks again

@JohnMo sorry - can you explain what you mean here? (Assuming DHW = Domestic Hot Water), but not sure what you mean about allowing room for it?

Ok, so please excuse the basic questions... 1. Internal wall area = is this just the external wall dimensions? could I just as easily calculate the external wall area (noting that this ignores the depth of the first floor and the thickness of walls that are perpendicular to the external wall, but as it's just a simplified calculation I assume this doesn't really matter much) 2. Internal roof area = could I just measure the roof? Ours is a warm roof and the upstairs is almost all inside the roof fault (small top collar) 3. Internal floor area = assume this is the sum of both floors? And the most basic question till last... 4. Which cell contains the figure I'm looking for as to size of my ASHP? I've read some comments about multiplying the total heat loss power by 24(!?), but couldn't see anything on the spreadsheet that mentions or suggests this. Grateful for your help in spelling out the obvious to me! (screenshot attached but sure I've not got some of these bits right; temperatures not changed from the default)

Any advice on how you identify companies offering this, and then the tricky bit, assessing whether they are any good or not?

I clearly need to bump this up my priority to read up on this in detail.

So we’re going for a very airtight performance timber frame, and visited a nearby build who built to passivehaus spec etc. This was their second self-build and one of the comments they wish they had done differently was route the hob extractor directly outside rather than use a recirculating one, as when it’s operating the noise is noticeable and an annoyance. This may be because they’re comparing it to their previous self-build which went straight outside and wasn’t noticeable inside when operating; it could be that the recirculating extractor isn’t actually that loud. However, we’re in the position where it’s not too late to decide to send the extract air directly outside. We like the idea of it running almost silently, but how do we maintain the airtightness when it’s not operating? Anyone done this? Thoughts?

@Mr Blobby this is a useful thread - did your install and setup go well? Can you share any photos of the end result (pipes through the wall for intake and exhaust in particular? Thanks

@JamesPa thanks - very helpful and clear. I’ve had the same BPC sales rep come back now saying they have checked our SAP report and they have calculated it to be 8.5kW heat loss. I need to do my own check using the spreadsheet tool from Buildhub, but I suspect it should still be lower. TBH they’ve already lost my confidence for a few reasons, but not least defending their method, then running the calcs and suddenly it’s dropped from 14kW to 8.5kW with next to no effort. Then there’s the radio-silence when I asked for clarification about which part of Approved document F he was referencing (yes, F is ventilation - not heating…). As for finding a reliable installer, I’m now taking a different approach by looking at sourcing the parts separate to finding the installer. Everyone I’ve approached so far simply seems to increase their quote/cost by £7500 so they pocket all the grant (combined with the list of tell-tale warning signs you’ve listed).

Any recommendations for suppliers of ASHP/ hot water cylinders / components?

@JamesPa I noted this input on a previous post/response of yours and I've been on the lookout for those aspects in the quotes. There are very few that have come across as honest and accurate (disappointingly). @JohnMo@JamesPa Thank you both! Great input - very much appreciated.

I've been searching around (a lot) for ASHP and who to go with - this is one of the strangest responses. I've attached their quote; I then queried how they came up with the 14kW size. Our timber frame house will be almost passivehaus level of performance and at least 0.6ACH on air tightness. We're expecting (based on a similar house we've visited) to need a 6kW ASHP, UFH downstairs only and bathrooms with electric UFH pads and towel rails. This was their response to me asking why they came up with 14kW. What has Part F (ventilation) got to do with it? Grateful for your thoughts / experience of them (I've not ruled them out yet). "In line with part F regulations for new builds, the minimum heating loads & property floor area have been placed in the manufacturer's selection software. This is the result. If your property is not a new build, please provide your SAP report to allow us to verify & calculate a new unit."

Just starting to compare quotes I'm getting in - welcome thoughts on this (seems steep to me): MVHR Design: £675 + VAT MVHR Supply (est): £6950 Excl VAT (but can reclaim) MVHR Commissioning: £748 +VAT Certainly helping me calculate valuing my time! I haven't yet discussed with our warranty inspector - am I likely to encounter a problem with them if I design and commission it myself? (if I did, I'd have it all fully documented and evidenced, so hoping that this may be an option, if I can free up the time to do it).

@Kelvin useful detail - thanks

@Nick Laslett thanks so much for taking the time to put this collection of links and info together! I started on some volumetric calculations over the weekend and am coming round to the idea that this is something I could do myself (need to somehow free up the time!). I’ll spend some evenings this week going over this detail - TVM 😁

Which is better in terms of MVHR performance - rigid or semi-rigid ducting? Anyone have experience of both types? We have ample space to accommodate rigid ducting in our new-build and have visited a build nearby which has used rigid MVHR ducting throughout, but a lot of the quotes I'm receiving are proposing semi-rigid ducting (our plans were supplied, so they can see we have 425mm posi joists and can run pipes up alongside the 350mm timber I-joists to the loft space). My assumption is that the rigid ducting will provide a smoother airflow and therefore better performance of the system. I'm busy reading up on some of the existing MVHR threads - some excellent resources. I'm weighing up whether I design my own system; some of the design costs (alone) are near £1k; is this typical?!

Our design is for a very well-insulated timber frame house (walls & roof both U=0.11, triple glazed windows, 0.6ACH etc.). We’ve always been working on the principal that the UFH downstairs will provide enough heat for the house and upstairs we’ll just need towel rails and MVHR. I’ve read/been told that MVHR isn’t to be assumed as a system for redistributing heat around the house, but I’m just going by what we’ve seen on a passive haus locally and it certainly works. When it comes to the bathroom we realised that we probably do want some UFH of some kind, but perhaps this could just be an electric system and used on requirement. For those who have taken a similar approach, what are your experiences? I’m worried about the cost of an electric system as I’ve no idea on its efficiency/power consumption and as a result I’m wondering if we should just look at factoring in a wet UFH system for the bathrooms before we finalise the floor levels of the other rooms (don’t want a step between the bathroom and adjoining rooms). One of the responses I’ve had back when going out for quotes for ASHP simply stated that “MCS requires rooms to be under 10watts per m2 for no emitters to be allowed upstairs”. I’m going through the SAP heat loss calcs at the moment, has anyone encountered this being an issue if it’s slightly over 10w/m2? Does building control care? Copy of the upstairs floor plans and section for context / interest - the ceilings are very high, so when it comes to heating and sizing the MVHR I’m trying to work out how much the volume affects the sized of ASHP and MVHR.

@nod my thoughts exactly!

@LnP exactly this. I’ve lost track of the hours I’ve spent reading up on this. ChatGPT was fairly good at trying to explain the difference, but can never match getting feedback from those who have ‘walked the walk’ and done it. My Principal Designer has (only since this year) started to be asked to sign at the completion stage. I’ll go back and clarify what exactly this was and will update this thread.