TerryE

@Bitpipe @daiking thanks for the links; the more suggestions, the better for anyone readings this. The main benefits of these pre-tapped systems is that wiring them up is simple and if you have flexibility in layout (e.g. laying them out in a bed) then you can space them out or lose the excess cable by snaking and can thus working within the fixed spacing. The disadvantage can also be this fixed spacing. In our case, I am fixing thee spots to fence posts at 2.4m centres so I have a fixed layout and I don't want take-up-slack loops visible. I also plan to drill a hole through each post and run the main backbone straight along the Arras rail line. Hence for me, the wiring system which uses a bare SPT1 backbone with clamp-on taps is preferable, with the less visible joints the better. I am using a Zigbee controlled socket as I can control this directly from my Home Assistant system without and dependency on an external cloud-based service. I have a couple of Zigbee motion sensors, so my HA system can set a dusk to midnight + motion triggered on schedule.

Just a quick update on this. Jan and I have been bouncing this one around for the last few months. What we really want is a reasonable cover of "full moon" light levels or thereabouts along the boarder of our driveway and in our side passage -- enough to see the way to the bins or down the side of the house without tripping over obstacles en route. We live in a village and even through we live on the main street through the village, the general street lighting is poor and so there is almost no indirect light into our garden -- effectively zero if you are entering the garden with eyes adapted to indoor light levels. I've decided on 6×12V LED wall lights, 4 on posts along our front and a couple in the side passageway. 3W lights should give this full moon or better visibility and avoid dazzling our neighbours. It seems to me that many of these 12V systems are overpriced and over-engineered. For example one system that I looked at had fixed T taps at 2m spacing and each lighting spur required 2 more connectors and a min 1m cable to the device -- not much use when are fence posts are spaced at 2.4m and I want to run the cable across the fence under the top Arras rail with the light about 20cm from the cable. We've decided on a 12V garden system from Luxform. What I like is that this uses easily replaceable GR3.5 LED bulbs and simple close-on connector system driven by a simple 240V-12V transformer- based PSU. I'll use a Home Automation-controlled switch to control time-of-day and motion switching. I will post back an update after the system is installed and operationing.

Re UFH, you need to distinguish loops vs zones. Keep your individual loops as close to the same length as possible and < 100m. So we have 3 × ~100m loops (IIRC the shortest is 92m) all drawn off a 300m coil. 4 × 75m could have worked as well. Don't have any joins in your UF pipework below slab. The reason for the suggested 100m max is the head needed to get the necessary flow. I used the delta T across each zone to trim the flow rates and balance them, but once balanced, like @joe90 and @Pete, I run my 3 loops as a single zone. There's little point in doing anything else in a passive-class house, IMO. Ditto no point is trying to dick around with time-of-day temperature strategies. In terms of JSH's spreadsheet, the obvious Google search gave it as the 1st hit: Fabric and Ventilation Heat Loss Calculator, though IMO, you could dump most of the complexity about the under-slab soil temperature as it is going to be a pretty constant 12-14 °C for most UK insulated slabs that are in direct contact with the soil. Ventilated are another issue. Yup KISS - Keep it Simple Stupid.

We much +1 to @Bitpipe's comments. MVHR gives you lots of fresh air in the colder months without having to throw heat out of open windows or trickle vents. Brilliant -- but that doesn't stop us opening windows when it is warm enough. IMO, the only good argument for not doing this is if one or more of the occupants is allergic to pollens etc. You really need to think of the house as a system and understand where your relative thermal gains and losses are. IMO, the PH and SAP spreadsheets are just too complex to do these sorts of trade-offs. The sort of simple spreadsheet that Jeremy Harris or I developed is plenty detailed enough. The whole build process is one of trust(-ish) but validate and verify at every step. You need to understand potential thermal bridges and make sure to avoid them when practical. You need to be sympathetic to the fact that UK building trades in general have little or no understanding of the importance of the detailing needed to get good thermal performance, but this doesn't excuse sloppy workmanship. You either (i) need to pay the premium for a subcontractor that does have this understanding; (ii) validate and specify at every step, or (iii) accept that you will be disappointed with the as-built performance of your house.

As @Iceverge says, I have a reasonably large stone clad "cottage style" build to passive standards. As I said recently to @shuff27 when showing him the build, I can't think of any things that I'd tweak if we were doing this over let alone fundamentally change. Have a look at my blog.

Or a Jacuzzi, or more specifically not for DHW or conventional CH. It also has a flow inverter so it can cool as well as heat. Given that I only need <30°C for my slab, and I can't make the payback case for a premium ASHP, I must admit that I am seriously considering one of the rebadged Chinese imports for my UFH application.

That's quite a subjective view really. The hall way can act as the focal point unifying the house. We've got a fairly tight footprint and space usage was quite an important design factor, but we still have an open hall way going up all three storeys in our house. My wife and I think that this is one of its most effective design features. It pleases us and surely that's what matters.

@shuff27 has ridge height limits and this has resulted in the imposition of dormer-style windows. However I don't think this would preclude small Velux-style windows in the bathrooms. The LPA also has a think about large picture-style windows on the principle elevation, and overlooking neighbours, so increasing window areas might prove problematic.

Re the issue of cycling boilers, let's say you need 2 kW and you have a 12 kW boiler, this the boiler will be on:off on a roughly 1:5 cycle. Boilers need to be on for a decent block at a go, say 20 mins, this means that you will need to be off for 1h40 between each on and you will need a buffer tank capable of storing ~ 5kWh heat.

The issue here is gaps in the mortar. Let's say behind one board you have a gap near the bottom and another at the top, so you get cold air coming in at the bottom, being nicely heated to say 20°C by that nice board radiator and cycling back out at the top crack. The net result is that the void behind the board is cycling heat into the void between the brick and blockwork courses. I've seen horror story FIR walk-arounds where external walls are 10° colder than they should be because of this. You can't tell it is happening until the house is built and fully heated and by that stage the cost of remediation is prohibitive. I've also seen cases where it is clear from the FIR camera that whole areas of cavity insulation have been omitted. If this happens, by this stage what realistic remediation or recompense do you have from your builder? As @joe90 says, one option is to parge the wall with a cement slurry to seal the blockwork properly. Another is to go over it with a "fine tooth comb" before it is plaster boarded out, but this level of inspection is skilled and tedious.

Consider a double wall between the office and the master bedroom with the void split say 70:30 as built-in wardrobe for the MBR and shelving for the office. Such built-in features are part of the fix of the house and are VAT zero-rated. They also work much better than free standing equivalents, IMO.

+1 on the main issue with dot & dab being air-tightness. Builders tend to botch this by using the plasterboard as the main air-tightness membrane, and if you've got any cracking in or voids in the blockwork mortar lines (which you nearly always do) then you get convection cycling behind the plasterboard which just pumps heat out of the house. There are some good FIR camera walk-arounds of which show this. Far better to directly plaster or alternatively seal the block work and then batten out to carry the board. We keep our house at ~ 23 °C and I accept that we may be paying a 15-20% premium for this but that's 15% of a small bill anyway. The problem of 0.18 vs 0.12 is a more subtle one of tipping points. We loose minimal heat through our walls so we don't have or need any installed form of CH on the upper floors: no radiators, boilers or CH plumbing and this saved a lot of installation complexity and cost. You should use a Jeremy-style heating spreadsheet and have a play with the parameters and sensitivities and see what the correct mix / trade-offs is for you. Having a truly energy efficient house (if done properly) doesn't add materially the overall costs because of the consequential savings that you can make. Architects will design crappy performance houses, builders will build sloppily because that is what they are used to, and building to an energy efficient standard is just an inconvenience for them: this is the culture of the UK construction industry. But you will need to live with the consequences every day after you move in.

Turning this on it's head, why has your architect spec'ed such a crappy profile? Another layer of Dritherm would drop the U-value to nearer 0.11 W/m2K for minimal extra build cost albeit with a 470mm wall profile. It seems that architect's are just so f***ing conservative to be unbelievable. See @tonyshouse blog. Even so, you can't make any as-built thermal performance predictions until you have an air-tightness and MVHR design.

We've got some bad creaking spots in our 1st floor. Nothing to do with the anchoring of the flooring to the EcoJoists, and everything to do with the cross-joist brace only being nailed across the joists and not properly screwed in. Missed this until after we moved in and by then post second fit, so screwing the cross-brace to the EcoJoists would have been a total PITA. Workaround: avoid stepping on the small ~ 1m2 area which creeks . 20-20 hindsight, and I wish that I'd fixed that cross joist properly ?.

You need to do the thermal calcs. On a new build, you can get the wall / roof / slab U-values 0.15 or better quite easily IMO, and for minimal cost. The downside is wall depth needed, but if you plan this in, then there isn't a material cost -- except that it make makes it a little more inconvenient for your building crew -- hence you need to focus on quality assurance and avoid construction flaws (like sloppy or omitted insulation installation) which cause thermal bridges and will compromise the build. Next up if you have a thermal shell of this spec, are the air-related losses which will dominate your heat budgets, so you also need a level of air-tightness and MVHR. My personal experience of building a house of this class is that the internal heat flows are 10× greater than interior to exterior losses. The bottom line if you achieve this is that you will need to top up heat -- e.g. through a in-slab UFH installation -- but that you don't need any other CH installation or rads as room to room temperatures will only vary by a degree or so. (In the coldest 3 months, we also boost the heat for the top 2 floors by using a small oil-filled electric heater on a timer switch in my office on the 1st floor for a few hours overnight.) Overall, the whole house maintains a 22½-23°C temperature 24×7. We use a Willis for my slab heating, and I can't make the cost-benefit case to install an ASHP as I won't recover the installation costs over a 10-year payback. However, that is because we have a high-spec passive-class house. IMO, an initial install of an ASHP is a safe option for most reasonably energy-efficient new builds.

I agree that fitting two is good idea, but if you have a properly insulated slab then it will quickly get far too hot if pump 6kW into it.

Air tightness and MVHR? Thermal bridges? You have the sums using a basic heat balance spreadsheet as per Jeremy's linked above. What is the average outside temp for your winter quarter? Ditto daily heat loss? Plug in the numbers and per kWh electricity costs. Can you live with this run rate?

Something like this: 12000 BTU AC. IMO, using something like a Willis is part of a package: Wall, roof, floor U-values 0.15 or better. No major thermal bridges. Triple glazed windows and doors. Airtightness 1.0 ACHP or better MVHR with ~90% heat recovery. If you use a Willis solution, there are two separate Qs Will it provide enough heat to do the job? Will you be able to afford the monthly bills?

@bobh there is always a trade-off between upfront costs and run-rate ones. The Willis approach works for us because: My house is perhaps 10× more energy efficient than our previous one, so the total energy input we need to keep the house at a comfortable temperature is small. (e.g. the Willis ran for about 2½ hrs last night at E7 rate). I use an RPi to control how much heat and when I inject it into the slab, and so I can make most use of cheaper tariff rates. I am about to switch to an Octopus agile tariff to drop these costs further. However the weakness of this approach is that a Willis works at a CoP of 1, so you have to pay the full electricity unit price for every kWh that you use. If you can't afford an ASHP and you only have ~100m², then have you considered an air-to-air wall mounted unit? You can get these for around £500 and up for a 12000 BTU (~3 kW) unit that will run at a CoP of around 3.0. @Jeremy Harris did a topic on the one he installed. A bit noisy, and one room only but it is a cheap way to pump heat into a house.

@bobh, the Achilles heel of this approach is that at is core it is using resistive electric heating at say 9p / kWh (off peak) or 15p+ / kWh peak / single tariff to pump those kWh into the environment just like those 1970s storage heaters. Resistive = a CoP of 1.0 unlike an ASHP which might be 3-4x as efficient or gas which runs at maybe a third to a quarter of the unit price per kWh. The reason it makes sense in my case is that my house is maybe 10× more efficient in terms of heat conservation than a typical build. Yes, is is very cheap to install, but the run costs can be prohibitive is you don't have an energy-efficient build. Do the sums before you commit to this path.

Russ, I discussed the math in the above post. The Willis dumps its 2.88 kW into the water and how much this heats the water depends on the flow rate through the 15mm feed. If this is 1 m/s then 2.88 kW will heat the water by 4°C so if the return from the slab is at 26°C, say, then the output from the Willis at this flow rate would be 30°C and (assuming both are working and a flow rate of 1m/s) then water would be returning to the slab at 30°C. The slab is dumping heat into your living space at roughly 7W/m²/°C, so if you have an air temp of 22°C and a slab temp of 26°C and a slab area of 100m² then your slab will radiate 7 × 4 × 100 = 2.8 kW into your living space. With a single Willis, this will be in rough equilibrium. With 2, the extra heat would slowly heat up the slab raising the delta T from 4 degrees until at 30 °C (assuming that you didn't stop the heating) you would be dumping just under 6 Kw into your space and the slab would again be in equilibrium -- event though the living space might be getting very warm at this stage. This assumes of course that there isn't any major construction flaw in your slab design so you are not getting material heat losses through thermal bridging. For a given outside temp, your house will lose heat at a rate largely proportional to the difference between the internal and external temps. Double that difference and you double the heat top up needed. So using 100 m² slab assumption, if you are loosing 2.8 kW then your slab needs to be 4°C warmer than room temperature. Double that internal to external difference and your slab needs to be 8°C warmer than room temperature. If you are putting less into the slab it will steadily cool down; more and it will steadily heat up. With my computerised control I calculate an estimate of how many kWh I need to put into the slab and dump up to 7 hrs of that in over night at E7 rates. If this was manual then a simple paper table of external temp / heating time next to my timer would do just as well. But the net result of this block of heat is that I have too much heat going into the slab for steady-state and so it slowly warms up, in effect acting as a thermal store. At 7am, the floor is nice and warm and the room temp is slowly rising. This room temp typically peaks around 11 am and then it starts to cool. By maybe 4 pm, the it falls below my control setpoint, and my system starts to top up the slab. Again there is nothing in principle stopping you implementing such a control logic with a couple of timer switches and a UFH thermostat in series with one of them so that in off-peak you do a block of heat based on a timer setting, and in peak you use the UFH thermostat. Note that these thermostats implement a dead band so a 22°C setting might turn on at 21°C and off at 23°C. This dead-band must match the overall responsiveness of the slab. One caveat to note. Most timer switches have a power rating and in general UFH thermostats are designed to switch a boiler contact circuit and NOT a 12 or 24A power circuit. If you are going to use timers and UFH thermostats to drive a couple of 3kW heaters then it might be wise to drive them through a properly rates 240V/240V contactor. @ProDave, @Nickfromwales comments?

It is just under a year since this last post. The main change that I have made to my system is to add an additional RPi 4 running a standard Home Assistant install with the InfluxDB, Grafana, NodeRED, Mosquitto broker, and Zigbee2mqtt add-ons. This is a pretty standard Home Automation use-case. The RPi which runs the CH + DHW control has been stripped right back to a base Rasbian + MySQL + NodeRED. This is integrated into the HA system using the main MQTT broker and a bunch of topics, so the user interface is done entirely through the HA. Quite honestly my current RPi3 + 2 × ESPs is overkill as the entire CH + DHW system would fit happily on a single PiZero, though I would still need some custom glue to drive the SSRs. If I were proposing this setup to anyone else, I would also suggest dropping the SSRs and switch to a standard passive distribution box (the sort that my electrician would understand and comfortable wire), but with separate SonOff power relays reflashed with the Tasmota firmware, which would enable the HA system to control these directly. The SonOff power relays use a HF152F-T relay which is rated to 16A and can switch the 2.88 kW Willis resistive load for long durations. IMO, this circuit is a more robust design than the corresponding SumAmp control circuit, for example. The great advantage of this sort of approach is its cheapness and robustness. The major downside is that the homeowner needs some level of IT literacy and minimal computing skills to configure and to use it.

I am one of the early adopters and advocates of this system. I only have one Willis heating a 3 storey passive house. On the 13th at the height of the cold snap where the system predicted a 930 min heating requirement; this was implemented as 420 mins during the E7 window, with the rest split over hourly top-ups once the hall temp had fallen below the boost set-point (22.3°C ). Three days later, this requirement was down to 460 mins and yesterday it was at 199 mins. Did the system cope? -- well at one point the hall temperature did drop just below 22.2°C --- Brrrrrhhhh. This isn't going to be the case for every house. IMO, you really need one with as-built passive class performance, but if you have achieved this then it is fit for purpose.

?? ? You obviously don't know Jan. We share one car between us, and I am not allowed to collect waifs and strays of any sort. ?

By way of context our LPA is South Northants; they are notorious for their pickiness compared to the adjacent MK and Northants LPAs. The village is classed as what they call "modestly sustainable", which is a mid-grade classification that is a nod to the fact that whilst there are some estate developments done in the 70s and later, the core of the village dates back to the 1700s and includes quite a few nice cottages, etc. such the two listed properties adjacent to this plot. To be fair to the LPA, this policy has seen the general look and characteristic of the village improve considerably over the last 35 years, as (with one notable exception) the more eyesore pre-80s developments have been replaced by more sympathetic ones. I suspect that the existing garage stonework isn't constructed to the quality that they would like given its adjacency to the listed cottage, hence the demolition condition. To give an idea of the low bar they (at least used to) set for NMA, I applied for one to change my front door material, and this got a peremptory rejection as not a non-material change, (the planner didn't even bother to contact me). I immediately contacted him to ask why this was viewed as an MMA, given that the door isn't even visible from the public highway; his response was: we treat all changes to the principle elevation as material, and the fact that it isn't visible from the road isn't relevant as callers to the house will see that it is different from the approved plan. I can see a path to approval of the sort of changes @shuff27 wants, but in my mind the challenge would be to keep then to MMA rather than a full material application. If you don't plan to use the garage then one obvious area of cost saving would be to remove it entirely. We like many on the road (including the listed cottage opposite) don't have a garage. The house will need adequate off-road parking -- say for three cars -- but I can't see the need to mandate a garage, as a defensible planning decision, IMO Modern cars will happily survive unsheltered parking for their design life.