TerryE

If you use a warm slab (See MBC website profiles), though other specialist builders do these as well, then your main thermal store is the slab itself. Willis-only isn't cost effective for new build with current electricity prices. Better to go for energy efficient build and ASHP direct into slab circulating UFH water at ~35°C so CoP should be around 4. A few there have done this very effectively. The issue you will have is getting an MCSE installer to do this for you because their standard templates don't work for such energy efficient builds. However you don't actually need to use an MSCE installer as this can be signed off by your Binsp, esp is you have a private one. You just need to do all of the calcs to prove conformance to BRegs

IMO, a single correctly-sized system is always best and far easier to balance. Yes, there are circumstances where a split system might make sense for example a main house with a detached annexe, but not if you've got a single well connected living space of many rooms. If you are not careful and the two systems will just fight each other, and be really difficult to balance.

The main source of sound is the pump circulation, and this depends on the flow rate that you need to dump the heating to the slab. You can either work this out using math, or by trial and error. In general you want the flow to be as low as you can get away with, as this will generate the least noise. In my passive house I only have three loops covering the entire ground floor slab and I can get way with the pump on the lowest setting. Our manifold is in a cupboard in the ground floor toilet, and I can just about hear it when sitting on the can. 🤣

As Peter suggests this doesn't really make sense practically. The manifold might only be 50cm wide but you've got the pump return on the LH side so you are going to have to cut out at least one stud vertical which will then need cross bracing top and bottom and the bottom bracing will then impede the UFH out and return runs. Have you got 60 or 40cm stud centres? If 40 then getting it in is going to be problematical. OK if you really want to lose your UFH out and return runs within the studwork, then you may be able to notch back the CLS uprights to 40mm and brace/support the two cutouts sections lengths of CLS at 90°. That would allow you set back the manifold and pump 50mm or so and allow the UFH pipework to run cleanly within studwork, but then so you will still need to box out the manifold and pump area with a shallow cover maybe 80-100mm deep. Even so, this is all very tight and for very little gain IMO. Also heaven help you if you need to do any maintenance !!

I never found any problems with internal corners at all: I don't mitre them but cut one with a negative profile, with the straight section using a (japanese) hand saw, and the profile bit done with a fine tooth jigsaw with a cut-on-pull blade on the reverse side (easier to draw the profile). No probs with tearing or breakout. We didn't have that many external corners so it was fairly easy to plan the laying sequence so that I could pre-glue and pin the externals and let them dry before fitting, with the far end being the square cut pair of any internal corner, so the piece can be overcut 5mm or so; offered up and cut exactly to fit. It's 4 years since we moved in and we've got no noticeable wear and tear yet.

We went with HDF skirting in our new build. I did all of the carpentry and fitting; Jan the painting. In previous houses we used pine. I found HDF easier to work, fit and paint.

Two comments on this What really matters isn't whether they've designed or even built passive houses, but that you visit examples personally and get the occupants to confirm that they perform to spec as built. Our house is the more boring variety but at least it performs within a 10% tramline of as designed. I've come across examples where the as-built is off by factors not percentages. IIRC, the self-builders bible has a rule of thumb: the cheapest house has 4 corners and a simple tented roof. Add 10% for each extra corner. Add 50% if you applying to Grand Designs; add 100% if you are selected to appear. 🤣 I repeat: get an independent QS to recost for as-built before to commit to anything!!

Your bedrooms seem awfully constrained given the total wallspace. Curves and none right-angles might look graceful on paper but are a total PITA when it comes to adding furniture / wardrobes / hanging space, etc. Heating must be UFH (which is the best choice anyway) as you will have no available wallspace for rads. Why not space out BR1 & 2 with a separating double wall (only one side will need to be load bearing) and split this into 2 walk-in wardrobes for the two bedrooms? OK, I see that you have a decent walk-in for the MBR, but nothing for BR 1-3 and this is especially the case given that you've got opposing doors creating round-bed access corridors. This type of layout could work if you are only two occupants with three guestrooms for short stay visitors. I see in another post that you are future proofing for a family. However no modern child, teenager or adult would be happy with this type of bedroom layout. Where could they fit a desk / home office / hang a TV / bookcase? What about privacy if their private space is accessible from both sides? Yes, the BRs could be used as offices, but dual use (that is also usable for overnighting occasional guests) wouldn't work with this layout. I strongly suggest that you get your architect to do a decent 3D realisation, dressed complete with a sensible selection of wardrobes, dressing tables etc, and you and your partner do a virtual in-room walkaround to see it it works in practice. Also the thermal design is going to be bad; really bad. I take it from the drawings that pretty much the entire south facing aspect is glass. (I say this because these clearly aren't solid walls.) Even with modern triple glazed glass the U-values are going to be 0.8 W/m²K or worse in practice if you are going for bi-folds, so the house if going to be expensive to heat in the winter and you need to double-check UFH design limits. In the the other seasons the house will turn into a hot-house. Airtightness is also going to be a real issue. So I feel that you are going to find it difficult to achieve current SAP requirements, let alone anything like passive class. I also can't understand the rationale for the S facing hub layout. It looks more like a circular bargraph than a clean arc. What's with the glass rebates? The rooms and the outside space and connexion to inside would work a lot better if the polygon formed at the meet of the room was more like a fit to a graceful circular arch. The extra floor-space in the formed in these reentrants isn't really usable anyway, and will add significant to the construction costs. What is the roof design and pitching? What are your ridge lines going to look like? I know this might seem harsh, but this plan looks to me like a concept that an inexperienced architect might come up without decent senior partner review. I think that you are going to be horrified at the build cost. If you want to go with this design, then I would suggest that you pay the extra $K or 2 to get a decent independent QS to give you an estimated low/medium/high total build cost before you go any further. You will also need to be really careful selection your builder and main suppliers. This is an unusual design and will require truly experienced builders.

I live in the new build opposite the Rose and Crown. 😊 PM me you want to hook up / trade experiences. Terry

Oops, I am the Wally: my answer was to the MVHR post that I was working on. Sorry. 🤣 The only running costs for the Harvey is the salt blocks as you say. Clearly usage depends on your water consumption and water hardness which will vary dramatically depending on geography and hence from household to household. We buy the 12 packs for £80 odd and these last around 4-6 months, IIRC. I can check with Jan what we use, as she does the reordering.

Yes, everything apart from cold drinking water, as tea and coffee make better with the soft water. Electricity on our current tariff is about 10p / day and filters are maybe £15-20 per 3 months so around £100 p.a. PS. Mike, it looks like I'm not the only one to cross post on topics. 🤣🤣

My experience is of a similar ballpark to @joth's analysis. Yes, the MVHR uses less than 1 kWh / day which is 1 kWh more than trickle vents. But we have a 3 storey detached house built to roughly passive-class standards. We are all electric and haven't installed an ASHP yet because I can't make a payback case. Instead we use a Willis to heat our UFH mainly on E7 cheap tariff over the winter months. Last year we used just over 11,000 kWh electricity in total (over 70% cheap tariff), so yes the MVHR accounts for maybe 3% of our total energy use, but the heat recovery is around 95% and without this our heating requirement would be around 50% higher, so this one is a no brainer for us. Not to mention fantastic air quality throughout the house and free drying room. @JohnMo We do the annual service ourselves and bulk buy the filters, so the maintenance cost is small beer.

We have a 2½ storey house with external stone skin, using an MBC provided and built Larson strut twinwall TF that is racked with a single 15mm inner OSB3 skin with 2 × transerve internal load bearing walls also racked. We had the full SE load calcs as part of our BCont docs. This has proved extremely solid with not even a single hairline crack in any plasterwork.

We have a heating strategy of keeping our house at the same temperature 24 × 7 (accepting the ±½°C ripple that I mentioned above), though one side effect of having a super-insulated house is that even in winter the house only cools by around 1°C / day if the heating is off. But the same thermal design process needs to be applied even if you live in a conventional low-insulation leaking house: If you are using UFH as your primary heating source then if your house looses X kWh / per day heat externally, then your UFH heating strategy must supply the same X kWh to keep overall heat balance. That (~7 × Δtemp × Area) W is a good rough estimate of how much heat the slab can throw into the room environment, so the more heat you need to input then the higher the Δtemp needs to be, though once this gets up to 10°C then the floor starts to get pretty uncomfortable, and you really need to worry about potential hot spots under in-contact furniture, etc. Higher rates of temp rise can cause heat stress in the slab which isn't a good thing. This is why IMO you want to insulate where practical and cost-effective and try to remove external air leaks and use MVHR as this will dramatically reduce your overall heat requirements. A more standard higher O/P slab will benefit from a TMV mixer and buffer tank design, though because the water circulating in the slab is a lot cooler than that in a radiator installation, UFH installations are a far better fit to ASHP installations. Last thing to remember if you are going for a non-constant Time-of-day profile, is the issue of time lag. With a 100mm+ slab (especially if the UFH runs are placed at the base of this) then the heat has to propagate through the say 80mm of concrete before the surface is at temperature and the floor starts to radiate heat into the room. This means for example if you want the house to be warm when you come home at 6pm, say, then you might need to turn the UFH on at 2 or 3pm and likewise turn it off at 9pm, say.

@SuperPav, Mesh selection is really a choice for your SE or whoever did your slab design and it is driven by that. Using the mesh as a placement framework for the UFH piping is really a secondary bonus and shouldn't impact on mesh selection. As to slab heating, there are broadly two strategies at the extreme: Dump heat into the slab at a steady rate so that the slab remains at a slight Δt above room temp. Here it will radiate ~ 7 ×Δt × A W into the interior. If this matches the net heat loss of the house, then your house will stay at the set temperature. Calculate your total kWh house losses for the coming day, and for a given input heating rate this give a total heating time. Just dump this into the slab in one or more "chunks", and accept that this will result in a slight ripple (say under 1°C) on your internal temperature. We have a passive house and have adopted the latter as this was easier to implement, in terms of kit required, ease of control, and simplicity leading to maintenance risks and reduction. The thermal dynamics here is a different Q entirely and one where I feel more qualified to answer on. I did my slab design back in 2015, and I was one of the first ones on this forum to go with a UFH solution which went against the prevailing wisdom of including a buffer tank. I decided to use a 3 kW Willis heater to heat the slab directly and use it as the thermal store for the heat. I needed to do some modelling of the slab for me to be comfortable with this solution and to ensure that it's thermal performance during the heating cycle was well within sensible limits, before finalising on this implementation. Luckily my professional background give me some experience of doing this type of modelling, but the main challenge was one of producing a model of the correct simplicity yet enough detail for the exercise to be meaningful. I have previously documented this in my blog posts and and specifically in my thread, Modelling the "Chunk" Heating of a Passive Slab written in late 2016. For the modelling, I didn't want to get into the complexities of CUDA compute engines and CFD libraries, so to keep the math and computation simple enough to be computable in Fortran on a single core (which is what I used back in the 1980s when I did this sort modelling for a living), I approximated the slab heated by one UFH loop as a (radially symmetric) concrete pipe some 0.120m in diameter and 100m long with a 15mm heating pipe running down its centre and with water circulating through it and heated by a 1 kW element before return. This allowed me to use a simple radial approximation for the Fourier heat equation and to solve over time for the (r, l, t) coordinates over time and the length of the pipe. (See this post for more details.) OK, it's a model and an approximate one at that because the actual heat flow through the pipe is not radially symmetric as the area around the heating pipe is insulated below, radiating to the air above, and adjacent to another. Even so, this did allow me to investigate the overall varying heating characteristics over time and both along and across the concrete. This was good enough to confirm that a "Willis direct into slab" approach would work fine for me. I subsequently instrumented the actual implementation with lots of DS18B20 digital thermometers which have been continuously logging now for ~5 years, and the slab behaves as predicted. This can be summarised by more simple and intuitive approximation. The slab surrounding a single pipe heating loop can be thought of a (folded) long box of concrete that is insulated below, radiating to the air above and with similar boxes on either side (since these are being heated by a return run at a similar flow temperature). The UFH pipe flow is dumps ~1kW (in my case) heat along the centre of this long box, and so the water cools as it flows along the pipe. The overall Δt is largely dictated by the water flow rate which in turn depends on the pump head, flow resistance, etc. In my case this was the biggest difference between the model and actual implementation, as I went with a slower flow rate than my modelled 1 m/s, in order to keep the circulation noise to a minimum. (The UFH is in a services cupboard off or G/F toilet and I don't like to hear the pump noise when I am taking a dump). So as the long "box" heats you get a standard radial heat flow curve where the circulating water in the middle is maybe 5°C hotter than the surface during heating period, with maybe a 2-3°C drop along the 100m run length. As the heat is dumped into the slab, it slowly but steadily heats up over the heating period, say by 5°C or so over a 7hr heating window. Certainly if you walk over the slab at the end of a heating cycle in bare feet (as we do), you can notice the 1-2 °C variation across the floor between the flow and return legs of the UFH runs and any gaps in UFH coverage. Pretty much as soon as the heating stops, the heat from the warm spots spreads so the feel becomes more uniform (as the heat only needs to flow ~50mm or so through the concrete). We heat our slab overnight, so by midnight the slab might be ½-1 °C below room temperature. In the morning after heating it might be 4°C above room temp, and it is now radiating ~28 W/m2 into the environment. This rate will fall during the day as the slab cools, leading (in our case) to a ~1°C ripple on the overall air temperature. The integrated heat dump from the slab must match the overall house losses, so the colder it is the more we need to heat the slab; and the warmer, the less. I hope this makes sense.

Your profile is similar to those of us that have MBC warm slabs. The rebar mesh was laid on ~40mm soldiers and the UFH pipe tied directly to the mesh. Have a look at my blog; there are pics of this. And others have similar. Absolutely no probs. Remember to tape the UFH pipe ends before the pour, to prevent dirt ingress. Our slab was laid in late Oct, so we didn't fill and pressure test until after the frame and windows were up and the structure airtight. BTW, I note that you have a cold bridge between the slap and the inner wall leaf. You typically put a small EPS upstand here to prevent this. Plastering out + skirting will hide this.

We just bulk-buy the salt block and have to remember to check the unit once a week or so and add another couple of blocks when needed.

How often do you need to descale your kettle etc.? We considered our water "slightly hard". Your local water authority should publish a water hardness by geo location report, and ours was harder that I thought. We have a Harvey Salt Ion exchange softener on all water as Nick suggests. Absolutely no regrets, as we next get scale in our appliances, taps, shower screens, etc. Remember that if you have any thermostatic mixers on any of your potable lines then these can fur up and fail if your water is at all hard. As Nick says, anything other than ion-exchange is just snake-eyes.

On this one, a few of us have gone down a dissenting route and have been successful at it. I did the overall design for my house myself and spent a lot of time on the 3D visualisation to make sure that the internals worked as a whole. I then used a decent local architect technician to draw up the set of plans for planning submission, and put the money that I would have spent of architects fees into our TF and passive foundation by subcontracting a decent custom build TF passive house specialist firm (in our case MBC, though there are a few to choose from). I did all of the design validation myself. To be fair, I had recently retired when we started the build, so I could invest the time in doing the research and getting on top of all the issues. I do disagree with Mike on "PH experience doesn't matter". If you are going to invest in an architect's services then he or she should be competent to do what you require: although passive house design is any more difficult than classic UK build, IMO, it is just very different. For example, it is really hard achieving passive class performance with conventional foundations + beam and block floor + block and brick leaf walls. We used a warm slab (that is a floating reinforce slab with UFH in-slab on a structural EPS bed + twinwall TF with blown cellulosic filler: this technique achieves passive-class almost by design. Anyway have a browse of some of the blogs, and do your research. 😊

You need an electrian, though TBH is you own a multimeter, then this is probably something that you could buzz out yourself -- so lonf as you understand and are confortable with all of the 240V safe working pactices.

I would say one benefit. I think that the other major plus is that the plumbing is 1-1 point-to-point, so no hidden joins or branches. The "Clapham Junction" of runs in the ceiling running into manifolds is also worth planning out and routing properly to avoid getting into a rats' nest of pipework. Certainly in a new build pulling the Hep20 was a doddle: you just had to be careful to keep the pipe clear of any sharp edges such as EcoJoist webs. We used 15mm throughout for simplicity, but in retrospect we should done what Nick suggests and that is to plum the hot runs in 10mm. Oh dear, we loose an extra ½l of water and wait an extra 5-10 sec for the hot to run hot. 🤣 Picking up @Russdl's point, I went round carefully trimming all the flow rates before the final inspection, but our inspector never even bothered checking. However the only two that we changed back were kitchen and utility sink mixers. TBH, we doc't find the low flow rates on the ensuite basins an issue. If anything it's an advantage: before I fitted the regulators, if you turns the tap on full, then the flow was so strong, that it would shoot out of the sink and soak evertything.

Broadly the situation on non-heating days is that the indirect heat sources (solar gain, warm bodies, waste heat from misc electricals) generally exceeds the fabric heat losses, so the house will steadily heat above your desired setpoint. Having your MVHR bypass its heat exchanger so the outgoing house air is swapped for unheated external air is one mechanism for dumping excess heat. However, you don't want to do this if the external air is warmer than the internal setpoint as you would want the heat exchanger running to keep heat out -- hence the min / max setpoints on the MVHR. You can do the sums on flow volumes + temp delta for your system to work out the kWh equivalent effects. Or you can set them by experimentation, but there is no standard "right" values -- these will depend on the characteristics of your house. My Ventaxia has a set of zero-volt contacts which can be used by a home automation system to fine tune all of this strategy, and I did initially plan to do this, but in the end we adopted my wife's strategy: we just open the odd window when needed. (We live in the country so the air quality is good, so long as the farmers aren't muck spreading. 🤣) However, we do exploit the fact that our 3 storey house is a sun trap on the principle SE aspect and the rear NW stays cool until mid-afternoon, so we crack a rear window in the morning and (if needed) a front one in late afternoon; this plus the roof light at the top of the hallway gives a very effective heat pipe to dump excess heat. I know that some members would regard opening windows in a house with MVHR as heresy, but as we don't have special needs (e.g. asthma suffers), we don't see why not when we want to dump heat.

According to regs, compression joints in slab have to be accessible. No need for soldered joints once tested to be accessible. They fail because of cold spots / contamination preventing solder wetting / flow and so (typically) the flux residue is acting as a temporary seal that will ultimately fail. Ring fill have already got a 360° wet on the joint. So long as the pipe is clean and fluxed, then this will also wet forming a permanent seal. Same applies for elbows bringing pipe up from slab level. As Nick says, you can always pressure test before you pour concrete.

I know the professionals like Nick make it look easy, but just about anyone who is modestly handy can do this So if you aren't familiar with soldering, then just buy 28mm ring-fills. You can dry fit and cut the pipe to length, then joint the pieces conveniently positioned out-of-place. Look at some YouTube demos of soldering ring fill joints. So long as everything has been cleaned to virgin bright copper and fluxed, and the area is clear of contacts / cold spots then you can heat the pipe uniformly and let the heat flow into the joint. Once the ring of solder wicks out under heat completely around the ring, then you know that the joint is sound. Clean. Ditto any end 90 elbows that are in-slab And remember that any piping in-slab must be decently lagged both for thermal reasons and to ensure that the concrete does not come into contact with the copper.

I use a Sonoff Power-usage plug that I can control with my Home Automation system, but you can also get Android timer apps and they are also programmable with Google Home if you are happy letting Uncle Google control your devices.