Great_scot_selfbuild

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.

@nod thanks - our principal designer said they were now being asked to sign-off by building control at completion stage and think I was assuming there was an equivalent for PC too.

We’re part way through our build and we have a builder currently nominated with BC as principal contractor but anticipate this may change later on. Background: The timber frame will be supplied and installed by a separate timber frame company (chosen during the design process and prior to selecting the builder as our PC), and the windows and external doors supplied and installed by another company. This will take us up to being wind and watertight. Beyond this point we hope to be in a position to do a lot of the internal building work ourselves. I’m trying to understand how we get the BC completion certificate sign-off done and understand there is a principal contractor signature sign-off at the end? Is this correct, and what will the BC require to be signed at the end / by who? I’m asking here before I go to discuss it with our approved inspector as I don’t want him potentially mentioning it to our builder before we know what we want to do. I’ve read the CDM rules inside out (plus ChatGPT queries), but I’d rather hear from people who’ve done similar. It feels like we may need to have our PC (potentially) sign-off for the phase of work he was responsible for (lets say up to wind & watertight), and then take on responsibility as PC ourselves for the later activity. I equate this to builds where people start with one builder and for one reason or another they have to change builder part way through. The second one can’t (surely) be asked to sign-off work they weren’t responsible for. I know this group will have an answer from those who have walked this path before. (Our builder is excellent and if money were no object I would love to have him do it all - but unfortunately money is, actually a fairly big object) Thanks in advance…

@dpmiller @saveasteading I'm a little surprised but my searches are coming up with Velux being cheaper than Fakro (for the spec we're looking for), and I simply can't find Rooflite in the size we need. I had searched Fakro a while back, but it still seems to be surprisingly more expensive - I'd assumed that the more well known Velux name would have had bumped their price up. If anyone's found a good/different Fakro or Rooflite supplier then let me know. Spec: 940 x 1600mm Triple-glazed Centre-pivot Manual

@Tom this is probably a totally different thread, but RE: lead flashing - did you do this yourself? The lead flashing labour costs for our garage were ££££!

@saveasteading can you share a link? We do want them opening as a preference (certainly for the bathrooms), and hence having them matching. But that said, I like researching and making sure we’ve considered everything. That and money is getting tight… (predictably)

@JamesPa very useful advice - TVM 😁

Useful nudge RE: alternative approach to the ASHP grant. It certainly comes across as less of a grant, more of a 'we'll charge you £7500 more and try to convince you you're getting a saving'.

@ProDave thanks - good to know it's a valid approach (we've seen one, but encouraging to hear directly from others. Not seen Jeremey's heat loss spreadsheet - will do a search for it; the SAP output is certainly not signed up to the 'plain English campaign'!

@Gus Potter thanks. We certainly had that approach for the garage but would rather not spend huge amounts on tailored lead flashing, though we know it would work. There were no off-the-shelf flashing kits for the garage rooflights, whereas at least the velux does offer some. Good point about considering the corrosion aspect - the corrugated sheeting is plastic coated on both sides but depending on how the fixing and overlap is done, this certainly needs considering - appreciate it.

@Bramco I should have been clearer - the cladding question was about the timber cladding - I searched for Greencoat, but that seems to be the roof sheeting, unless my searching is off?

@JamesP lovely looking build. We gave our architectural technician the size of velux window we wanted and they have applied the same tolerance gap around the velux window size as the window company wanted for the windows elsewhere - how did your structural opening compare to the velux size on yours?

Well it's been a long journey getting here! We've finally got the SAP calculations done in detail and although we had the assessor use an assumed 6kW MCS listed ASHP, we haven't committed to any brand and are now looking at them all. Some time back we visited a local self-build with a similar performance and sqm specification and have used that as a handrail for what size HP we should expect to need (they had a 5.5kW). Additionally we need to choose the MVHR system, and confirm what overall system / combination will be right for keeping the house warm in the right places. We were advised that UFH downstairs and MVHR around the rest of the house (for a house with passive-levels of insulation) would suffice. Now we're getting close to finalising this though, we're questioning the detail - MVHR seems to extract heat (& humidity) from bathrooms, but if we only have a towel radiator in there will it be enough? Timber frame build summary: UFH downstairs MVHR to move heat around the rest of the house Electric towel rails in bathrooms (1 x family, 1 x en-suite upstairs) Walls and roof U=0.11 Floor U = 0.14 Target ACH = 0.6 Downstairs ceiling height = 2.9m Upstairs is into the roof eaves, with a flattened top at c.4m Area = 260 sqm total As you'll notice form the summary, the design has a large volume, hence my focus on understanding how an MVHR and heating system is sized. Welcome your thoughts & advice. Very early on we got some quotes from companies specialising in renewables and 'whole system' design - and we quickly scared by the VERY large quotes. Felt like they just saw us as a self-builder money pot, which is a long way from the reality!

@Bramco that looks like a very tidy finish. Our garage roof lights were actually from here - https://saris-extensions.co.uk/shop/skylights/pitched-roof/ but we’re looking for opening roof windows and part of the velux draw was the ability to plug in electric powered blinds and u-value. Not that we’ve committed yet so I’m continually reviewing options. What cladding is that you’ve used? Looks great.

Anyone willing to share an example of a construction drawing showing the installation of velux in a roof with corrugated profile sheet covering, plus advice on the flashing kit? I'm wanting to install the 7 x velux roof windows (940x1600) ourselves and looking for advice on which flashing kit we would need and how it will fit into the structural opening. Grateful for any experience / advice. We have all the other windows being installed by the supplier and although our builder could no doubt do these, I'm looking at which tasks we could manage ourselves and try and grapple some control of labour costs where it is more manageable for us to (perhaps) do. The installation would be timed to take place whilst scaffolding is still in place and I'm expecting the windows to be lifted in place from inside the house. Our fixed rooflights on the garage had huge up stands the windows simply sat on, whereas the rooflights openings for vellum have a structural opening (windows = 940x1600, structural opening = 960x1620) and we haven't yet received the construction drawings fro the architectural technician yet (on holiday until early Sep) - looking to understand what we should expect in advance of receiving the drawings. We'd like the velux to be flush with the roof rather than on upstands, and our roofing material will be plasticol coated corrugated (here - https://www.panelsandprofiles.co.uk/wp-content/uploads/2014/03/TPP-14-3-CORRUGATED-Roof-Load-Span.pdf) - photo of the finish attached as used on our garage. Cross-section of a building regs drawing attached to show the section view with the velux window positions. Our searches of Velux flashings for sheet profile roofing has led us here: https://www.roofingsuperstore.co.uk/browse/roof-windows/pitched-roof-windows/velux-flashings/filterby/code/edw.html TIA...

Yes - not keen on this, hence my focus on understanding this requirement and how it works on site. Thanks.

@Nickfromwales do you mind spelling out TBS?

Here's a site plan view