Babak

Hot water might become an issue on colder days and nights if I were to use the heat pump for it. But, on the other hand, even with today’s prices I reckon using immersion heater for a week or so every year to supplement the pump will be cheaper and faster than getting a bigger heat pump that will cost around £1,000 upfront and likely will fail earlier due to more frequent cycling. in theory at leat I should also be able to use subsidised immersion heating for HW on coldest days as I have PVs. Just a thought though.

so in the end I have just signed the paperwork for the smaller unit. The larger unit modulates down to 4kw and the one I went for down to 2kw. demand at -1 is 6kw with the unit maxing out at 6.8kw. I may still get a low loss header for defrost purposes though. HW is a potential issue. The unit I am getting is a Hitachi Yukati S Combi with a pre plumbed tank all within the same enclosure as the heat exchanger for the internal unit. The original decision was to go for a 260 litre version. However, as of June they have improved the model by changing the capacity down to 220litres. This is probably still OK for my family of 4 plus the occasional guests. The new version’s tank is a heat reserve, so hot water is heated on demand and cyclical heating to 55c is not necessary. I guess there is also slightly less available hot water due to exchange efficiency losses. On the other hand the unit is ultra super insulated and so if I time the water heating well, I could actually save money. I have about 4kw of solar PV’s (inverter size, actual PV’s are higher rated), so paradoxically enough may be able to use direct (not pumped) electric heating on the coldest days as they tend to be clear and cloudy days slightly warmer. With some tinkering I could potentially use immersion heating only when it is coldest so the heat pump just warms the house, assumption being the water stays hot until the next morning, ignoring intermediate use. thank you for your note about your 5kw ASHP. I was beginning to have my doubts about the heating specs given the entire planet and their dog wanted to upsize the heat pump and the UFH. It is good to read persistence does pay and that low energy designs actually work.

I would possibly have gone for the bigger output, but the outside unit is double the size as it jumps to two fans. But is also a point of principle. There is a design with certain calculations and also a largish built-in margin for errors, inefficiencies, etc. which was being ignored. unfortunately it became a point of principle when he decided to ignore a big part of the calcs for a very weak reason. He might have thought something in the lines of “what if the calcs are not right? What if the MVHR ends up recovering only half the heat? Etc etc”. I would have resisted those less on the basis of an SME going for gut feeling. But that was not the case….

Agreed, TH should be more open and upfront about their roofing system so the work that needs to be done is planned in advance. We have used aluminium tape over most of the panel joins internally. To top that off we are using double foiled bubble wrap as an extra layer of insulation and barrier before plaster boarding. I think if we do not do this there would be a very high risk of condensation through small gaps between panels where glue may not have been applied. To illustrate, we had a LOT of water coming in after the panels were set up. Huge amounts were due to dormers edges, which were formed by cutting the panels and glueing them together, not being fully water tight (and hence not air tight). But a couple of places we could actually see light coming in from the sky where there were no dormers. Just panels not being put up properly. We wised up to this only because we read the technical manual and had water ingress early on. The installation was done by TH, so we could easily have missed it had we relied on them fully. But also bear in mind you can lose air through door and window reveals not being properly sealed…. See the videos below for fun! We only managed to stop the leaks once the roof membrane was fully on, no amount of PU foaming inside or out was working. Fortunately there was enough rain in May and and June to realise we had an issue!! IMG_1421.MOV

To be fair to all installers, common sense is only common amongst those who have been through the problem and solved it already. It is surprisingly uncommon otherwise. to illustrate, I had similar issues with UFH designers and kept moving on after the first conversation. They mostly could not understand the low heat losses and wanted to stick with statistical heat loss factors for UK new build UK housing… I actually registered as an installer with one of the larger suppliers and got access to their online tool for working out heat requirements and pipe spacing based on floor and fabric types. Also had issues when looking for SE (found a really good one though - no prior experience of my ICF and roof, but with open mind). And ground works guys .… Majority just wanted to stick with how they done things, and only a few willing to listen and do some thinking /research (also found a really good groundwork’s guy!)

…. But there is a positive to all this….when looking through the forum for similar issues I came across a discussion of switching the heat pump to chilling mode. Looks like it is a little more complicated that I first thought, so can plan for it better…. 🙂

He has changed the numbers for anonymity. He is also saying it is for ‘him’, even though he is the installer. The actual numbers are just over 6 and just over 8kw, which are just below the two heat pump models output at -1. When you go from the smaller model to the more powerful model, the external unit gets an additional fan and becomes almost twice as big. Also a no no for us! Fyi this is the gentleman’s comment as he forwarded the MCS reply to me. I suspect his other installers have not actually installed in a house with MVHR. Dear Xxxxxx Please find forwarded the reply that I received from MCS regarding my question about using heat pumps with MVHR. I followed their recommendation and spoke to three accredited contractors, including Joe from Xxxxxx, who all said that when working out heat losses of buildings to size a heat pump they do not include the affect of MVHR. I personally agree with this approach particularly when using a modern intelligent modulating heat pump model like we are proposing for you. Best wish

So taking the above approach, when the MVHR fails, a couple of windows open. If it gets too cold, a couple of electric heaters kick in. Now try convincing my installer of it. TBH timelines are a bit tight (as with any new self build) and I have too much invested in the make and the installer, otherwise would just move on….

It is medium sized company and he uses a software for heat loss calculations. The issue is that he wants to exclude the impact of heat exchanged in the ventilation system, yet include the heat lost through outgoing air. I actually think he is genuinely interested in protecting me, but just does not quite get it. This is what he wrote to the MCS and the MCS reply (he changed numbers as I have for anonymity). He says he has consulted with three other installers as suggested by MCS below, but I reckon t(e installers are also unaware of the difference between MVHR and central ventilation. Query to MCS: I am thinking of buying an Air Source Heat Pump (ASHP) and having Mechanical Ventilation and Heat Recovery (MVHR) installed. The heat loss calculation for the house confirms use of a 16 kW heat pump without MVHR but reduces this to 12kW with MVHR. My concern is that if I have the smaller 12 kW ASHP installed and the MVHR breaks down or stops working for some reason then the heat pump will not be powerful enough to satisfy the heat demand for the house. Is there any MCS guidance about this to help me choose the correct size of heat pump when using MVHR ? MCS reply: What you are asking is to do with design guidance, and unfortunately our technical team are unable to provide this. So we would recommend that you speak to at least three contractors and see what their response is. I can refer you to our best practice guide, which I have attached. There is a very small section on MVHRs on page 13. But the best route would be to speak to some of our installers. If you have any other questions please don’t hesitate to ask. Kind Regards

it’s the ASHP installer who is specifying the heat demand, not the MVHR supplier. SAP efficiency 95%. I need to get hold of the calculations, yes. They are not room by room though, if I remember correctly.

LOL, yes certainly looks like he is not comfortable with this additional factor. We did talk about the failure scenario, when we need to open the windows. But yes even then it is unlikely the ventilation losses would be as much as he would have as default in his calcs, which probably assumes trickle vent as well as through the fabric losses.

I wish I knew. I think his line of thought is that should the MVHR unit fail, I should still have all of my heat demand met through the primary heating system. But thank you, I will also argue with him that the ASHP is more likely to fail.

Hi, have a question about incorporating the impact of MVHR when calculating heat loss. Background: New ICF build near completion. My Heat pump installer has calculated I need 8kw of heat at -1/21. However, after incorporating the 90% efficiency of MVHR into heat loss calculations, this drops to 6.5kw. This was done by reducing heat loss through ventilation to 10%. This also synchs with my UFH room by room heat demand calcs. Theoretical efficiency of the MVHR unit is over 95%, and my u-values are conservative, eg I have double bubble-wrap foil under roof but have ignored its additional R value. Same with impact of plasterboards, render etc. So fairly confident about the stated heat losses not being more. Issue: My heat pump installer INSISTS that I should ignore the MVHR because it could fail and if that happens in the middle of -1 degree winter I need to have additional capacity for heat. He wants to install a bigger system to suit the 8kw demand. The change to the bigger unit means my outside unit will double in size, which I absolutely do not want unless I really have to. My arguments has covered the following, none of which has convinced him: - extra cost of bigger system (£1,000) will take years to be recovered through multiple failures of MVHR just when outside is really cold - even ignoring the upfront costs, having to supplement heating with electric radiators/fans for a couple of weeks every few years (IF the MVHR fails), is not actually that huge. - MCS has been non-comittal on this and says it is a design issue for the installer to decide - What happens if the heat pump fails? What happens if there is no power for 10 days? Why do we need to assume the MVHR fails? But not anything else? What am I missing here? I realise I am a little on the borderline with the 6kw system which only has a couple of extra 100 watts of power at -1 outside, but I am quite prepared to supplement that with normal electric heating when I have to. MCS have not objected, and it is my risk to take. Any advice? Is there a rule about excluding the impact of MVHR when calculating heat losses? It is just so incredibly odd that my installer wants to disregard the heat recovery impact when working out ventilation heat losses, which sure.y goes against the spirit of MCS. Thank you!

Yes its odd how different installers (and manufacturers) have such differing ideas about what should be best practice. My Windows and doors (Rationel) are also designed to rest against check reveals which I guess is why they insist on having the render done first - so they can pressure seal against those extra lips on installation. But in my case I had to remove the check reveals that come with TH. At 2.5cm plus the 8mm of render they would have been too wide for the frame to sit against and still have enough frame left between the sash and the external opening. Plus my frames would sit a little further forward so the TH lips would have been in the wrong place. I am a little worried about the DPC. I think the order of things as they will happen in my case will be 1- Render all the way up to the opening maybe slightly lower, 2- put DPC/DPM on reveal base and use CT-1 or similar to seal against the render 3-Fix cill 4 - Install windows. Just keeping my fingers crossed that following Rationel requirements will at provide an installation assurance. The cill will be cut to go round the reveal (like normal installations) rather than into the polystyrene as suggested by TH. But if I am right, the cill was not the problem above, it was the lack of full DPC/DPM all the way below the cill? Not sure if it helps anyone else, but below is a hand drawn detail of how it should work in my case, except for the DPM and cill detail which will be slightly different (credit to my XXX for sharing properly drawn originals from elsewhere!). The cill will not turn back onto the render to close the air gap. What is Illbrook 295? I can find Illbrook, but not the 295. Is it tape or silicone type?

That’s a really good point. My renderers are supposed to commence in a couple of days so will ask the same. The build was finished in March so it has not been under the sun - certainly summer sun - for that long, but definitely a good point, thank you!