TerryE

Nick, thanks for the reminder to other readers that my whole discussion is only applicable to passive-class houses. I should have emphasised this earlier. I hadn't forgotten that, which is why I added my predicate. I've discussed this use before: my concern is that for ~8 months a year if I used a buffer tank then I would only be cycling the ASHP to keep it at 35°C say (with the associated heat losses) to save some E7 units. Averaged over the last 4 years, we use ~240 ltr /person / per day and maybe (on average) 30-40% of that is DHW, say 240 ltr raised from 10°C to 40°C or 7.2 MJ or 2 kWh at E7 rate = ~13p/day. (We run our dishwasher and waching machine coldfill overnight to use E7 units). Using a buffer tank would save maybe half of this, but the electricity would be mostly at peak rate, so when you factor COPs vs E7:Peak and heat loss from the buffer tank, then I might be looking at a saving of £10 p.a. I accept that the other major advantage is that doing this effectively doubles his SunAmp capacity. But I've also been thinking of taking this "use the slab as a buffer tank" paradigm one step further and adding a PHE into the UFH circuit. This way we could use the slab to preheat the main CW feed going into our SunAmp to ~20°C year-round, which will increase its effective capacity in winter by roughly a half -- for the cost of a PHE. So yes, having a bath in winter will suck a bit of heat out of the slab. Big deal. I plan to do the major top up using E7 electricity anyway. The rest of the year, taking a little bit of heat from the slab is no bad thing as we are in heat excess. @ProDave, we have an MBC frame and the thermal capacity of the cellulosic filler is pretty high -- roughly half of that of the slab. But yes, there is no point in circulating water when you don't need to. What I am planning to do is to monitor the out and returns sides of each UFH circuit with a DS18B20 digital thermometer. This have an absolute accuracy of ~½°C but an an adjusted relative accuracy of far better than 0.1°C. I am monitoring all loops to help me balance my loop flows. But yes once the slab is in steady state (and the temperature monitoring will tell my system this, then there is little point in having the circulating pump on.

Note to self: need to add a Strikeout icon to the editor bar!

@ProDave Dave, this is the point that would like to challenge: the need for a regulating manifold (and therefore a buffer tank to prime it). Let's just say that you want to circulate maintain a steady state by circulating water at 22°C which results in a heat transfer rate of ½kW into the slab. One method of doing this is to your ASHP continuously at an O/P of 22°C, say; another is use to buffer tank and pump 3kW into it at 35°C for 30mins or whatever your hysteresis on your buffer controls dictate, and then have a "space" of 3 hrs or so whilst this is slowly bled into the slab at this constant 22°C. A third strategy could be to take your ASHP down to whatever is can run at at its lowest output temperature and run it for 30mins say heating up the slab then turn it off, but leave the recirculation going. You don't need any temperature sensors in the slab, just a temperature sensor on the return flow from the slab. Circulate the water and wait whilst the slab (return flow) temperature falls. When it reaches 22°C, give it another 30 mins of ASHP top up and repeat as necessary. You don't need any fancy control of the ASHP, just a simple on off demand, as all you are controlling is the mark-space ratio. The numbers might need tweaked (and trimmed down to average out the sawtooth effect) and you might need a compensating adjustment for typical ambient/trended air temperature which modifies the turn on temperature, but in principle this could all be done with a few sensors and some control logic. No buffer tanks, no precision manifolds. OK I am a little different from most of you guys in that I am comfortable with this sort of instrumentation and IoT devices, and I've done quite a bit of embedded development over the years, so I am a lot more comfortable in doing this in S/W than COTS control H/W, but once I've gone around the buoy a few times and (if I've) got a workable system, then the approach once documented should be transferable for others to an off-the-shelf implementation.

I've been thinking about this issue of the buffer tank. Jeremy uses one with a precision mix-down as Nick and Neil have described above. Jason and Dave don't; Jack has one but doesn't use it at the moment. OK, Jeremy's approach clearly works, but I've really got to ask myself: If you are using the buffer tank for heating only, then why run one at 35°C, say, when you have a humongous concrete buffer at 20°C ? OK, the buffer tank approach is essential if you are using a Gas boiler or equivalent with can't modulate below 10kW, say, but if your heat source is an ASHP which can modulate down to ~ 1kW, then what does it does this complexity really add apart from another tank in your equipment room and a source of heat loss into a small room? The discussion might get a bit nerdy so maybe this is better addressed as a Boffin's corner topic. Give me a few days and I'll put together a decent strawman to explain my thinking.

I never had a problem with the plans provided by the TA, MBC and my staircase company all came out in the right metric units. So I don't know what the issue is. Sorry. When I have this problem I just set up a simple 2 column scaling spreadsheet to enter the data values and get the conversion automatically.

@Vijay On the ribbon at top there is a 2-D distance Icon (A double arrow with a ? over it. Click on this and this exposes a pop-up ribon with all sorts of variants, but the simplest approach is to us the default and click and click, then the distance dialogue appears. Note that Clt-scroll-whee does zoom and clicking down the scroll wheel allows you to pan. There's bunch of VARIcad youtube videos. OK, only a subset apply to the free viewer, but the viewing functions are the same in both. PS remember to use the @TerryE if you want to attract my attention to a post

@PeterWThe decrement delay for my wall profile is a couple of days so I don't need to be predictive; I can just use the last days average as measured or even the average of the last few days, or even stick a thermal probe into my cavity and take the measurement there (and let the stone skin do the integration for me). Other than that roughly yes as you say. I have pretty much the whole slab covered in three loops some at 10cm and some at 15cm centres, but these are run as a single zone so the 9kWh would raise the slab by just over 2°C, though this heat would itself take another hour or more to propagate from the UFH loops throughout the slab and be "seen" at floor level. So by about 09:00, say, the slab would be pumping an extra 1kW/hr into the living space and this would decay over the day as the room temperature slowly rose, and hence the ripple on the temperature that I mentioned. I don't control the release of the heat; the whole house is a single zone which has huge inertia. Clearly if the room temperature is trending down then I will need to trim up the overnight boost. The approach is going to be one of suck it and see. I don't have any upstairs heating at all, so the 1st floor rooms might be a few degrees cooler than the living room and kitchen diner, but that's no bad thing. I think that the main problem might be my son's room in the loft but that is because he has his gaming PC on most of the evening and it and the rest of his electronics chuff out about 300W plus his personal 200+W of course, plus additional boosts if he has mates around, so it might get a bit warm up there !! @StonesJason, my approach is a KISS baselining for the first year of operation. Our two approaches share many drivers, but with a different flavour of implementation. There isn't a single correct approach, and I might well follow your lead after I've got my baseline data. I've lead some embedded control based developments in the early days of my IT career and understand the maths of both linear control. And yes, I am planning to use an RPi for my main embedded system, running openHAB or Pimatic, with ESP8266s for point collection and control, but that's getting too off-topic.

Thanks guys, I'll pass this by Janet, and one of us will post back

That's the point I was making above. The time constant of both the slab and the thermal balance of the house (less solar gain) are very large. Any room stat no matter how accurate used in on-off mode could be unstable from a control perspective, IMO. If you are going to use the room stat then you need to smooth it to a similar time constant to the slab's -- well within a factor of 2 or so. As I've said in other posts I want to run our house for the first year or so with a very simplified but instrumented control regime. My initial approach will be simple, that is to use an electric inline heater to N kWhr into the slab each night during the economy 7 window (shifted to the end of the window) during periods of net heating, where N is my calculated average daily heat deficit for the house, given the daily average outside temp. I estimate that this will result in a ripple of less than 2°C in the room temp in the colder months. (I will use the MVHR bypass to dump excess heat in net cooling months.) This might not be perfect but its going to a huge step improvement over our current house, and this will allow me to collect enough data to design and select my final solution. We are still fitting out the house at the moment so it's unheated, but we've got a small domestic dehumidifier (~300W max) running in the loft to dry out the plaster and painting and this is chuffing out enough heat to keep the whole loft at a comfortable temperature.

On reflection I've decided to use a thin wood profile say 25×10mm with slightly rounded corners as an architrave/ tiling stop. Simple, not too fussy. @Nickfromwales, you must have a standard approach for this

What concerns me with a conventional controller is that the time constants for slab and the master stat (if based on a room temp) are way out of whack. It will take about 4kWhr of heat to raise the temperature of my slab by 1°C. So if you put 4kWhr into the slab when at equilibrium, maybe an hour later the slab will be 1°C warmer and dumping about ½kW additional heat into the room. The time constants and gains of the two systems are far too separated for a simple on-off control to be stable. I have some ideas on how to do this, but that's the problem: they are just ideas, because I haven't got my system commissioned and collected enough hard data to characterise the system and enable me to establish a decent control policy.

Dave, but to be honest I was thinking of your house

How much concrete do you have in your warm slab? I can see that this approach could work for a typical house, but I personally would be a little more concerned as the house spec moves towards the passive / zero-energy type performance. If you have the water to 30°C, say going into the slab, the how does the ASHP know when the demand is off? Your house has quite a large footprint, hasn't it? So if your slab gets up to 10°C above room temp and you've got 100m² heated area, then the slab will be pumping roughly 7kW heat into the house. Can you cope with that rate of heating without cooking? In our house if our slab gets much more the 3°C above room temp even in December, then we will start to overheat.

We are using matching HDF skirting and architrave throughout the house. Clearly we aren't using skirting in our wet rooms, but this does beg the question about the best treatment around the door frames into the wet rooms. Architrave is one option, but I am interested in alternative suggestions / approaches that other have tried and fond preferable

We have glass panels on our landings. According to the guy at Complete Stair Systems they are 2x 4mm annealed panels, laminated together 0.76mm PVB interlayer. I know because I got one of the dimensions -- the one on the loft floor -- wrong due to a mindfart on my part. I can cut down the oak pretty much invisibly to remove the excess 100mm, but the glass panel is another issue, so I decided to source it in the UK rather than pay a huge uplift to get a Swedish replacement.

He'll only make a mistake on this approach once.

The MBC lads said the the secret of digging clay is to dip your spade in a bucket of water between each shovel load. Maybe you need a pond somewhere where you can leave the bucket i for 30 mins at the end of the days or at breaks, and a old sleeper or equiv to knock the bucket out one. Just a thought.

@Sensus this discussion is about a house as build not some anonymous worst case. In our case (TF erected this time last year), the floorboards went direct from the pallet to being fixed in place on the joists. We had about 2 days of rain in the period before the sarking went on, so the top sealed surfaces did get wet and as the flooring wasn't the waterproof variety, if you now put a steel rule across the floor board joints, then for a few of these, you can see that there is about 0.5mm blooming / expansion at these joints. Nothing noticeable from a standing position. The impact in Leaway's case is for him to assess, based on his actual time-line and the weather exposure / damage, surely? However, if I recall correctly then he also has a similarly constructed MBC frame, so will be in a similar situation to us, and he probably also has a warm slab which involves a lot of concrete being cast on the inside the DPC. As to the humidity point, yes in general for this time of year, but where we live the temperature was ~14°C at midday and the sky was clear just a few days ago, so my advice to use natural ventilation when appropriate still applies. @Leaway, can I suggest that you leave a square of polythene, say 1m square weighted down on your ground floor for a 4-7 days and if the floor gets visibly damp under this, then most of the moisture is still coming from the slab. As I said, ours took over 6 months to dry out.

As Peter and I both said above, a good old fashion through-draft is better if the R.H. is less than 80% say. To be honest we just use a standard domestic dehumidifier. There are plenty to choose from in the £100-150 range, and they cost under a £ / day to run. IMO, this is fine to take out the moisture from a board skim or paint. Also handy later if you want to dry clothes indoors. This would not be man enough, in the case a wet plastered walls, and you'd need the heavyweight sort of dehumidifier that Ferdinand suggests. Sensus comments re the timber frame don't really apply in the case of a modern passive-style TF, as the VCL is usually on the inner layer and the only inner timber is the joists, floorings and service cavity battening, and these are kiln dried and only expose to the elements for a few days during erection. Also most self-builds don't work on the accelerated timescales that commercial large-scale developers work to, so the timber, slab, etc have more time to come to equilibrium. What is more of an issue is the moisture content in the floor / slab above the DPC / Radon Barrier. Our house took about six months for the slab to dry out properly. You might only have 50-100ltr of water in your plaster skims, but 7m³ of concrete will contain maybe 1+m³ of water or 100× as much!

We've discussed this here from time to time and on eBuild. IMO The easiest way to calculate the incident radiation is to use the PVGIS online calculator. You can either do this by window or wall face. Just put in the area of glass and the elevation as 90° (or the roof angle if a roof window). One of the columns in the calculated table gives the average incident radiation in kWhr/ day by month for your location and orientation. The % that gets into the room varies according to reflective coating and type of glass but is typically over 50% unless you've got a specialist coating on your windows. The numbers can be frightening if you've got a lot of glass. In our case we have smallish cottage-style on SE / NW facing walls and we get an extra ~2.5 kWhr / day for about 5 months a year dropping to 0.5 2.5 kWhr / day in Dec/Jan.

We are in a similar position. Turning on the MVHR at this stage is a bad idea. You'll just get it and all your piping choked with plaster and other building dust. IMO, just keep the place well ventilated by opening windows and allowing through drafts, so long as the humility isn't fully saturated. Also try a bog standard dehumidifier when it is too damp with windows closed. This will take out maybe 10ltr per day.

+1. I also miss his perspective. @SteamyTea, Nick maybe you could pass along the sentiments/

Those of us who are old enough remember stuff like this: A great way to polish the s**t on your arse!

As per my discussion above, the portal issue goes both ways. When I had my NMA turned down, one complicating factor was that they hadn't uploaded the contact details for the allocated planning officer and given that he didn't contact me, I didn't know who to contact -- until I was notified of the refusal, and by then it was too late.

John, if you aren't having a UVC then you'd probably want a TS. If you are driving your buffer tank to 40°C then as far as I can see there are too possible scenarios for using it: Passing the cold feed into the UVC though a heat exchanger driver by the buffer tank. In this scenario if the buffer tank is charged then the cold fill will be at 35+°C rather than 10°C say, and so if you are running your hot tank normally at 50°C, say, then you will only need to heat the water 15° rather than 40°. Using the buffer tank to preheat the UVC with a bottom coil if fitted, but this will only be useful if the tank around the coil is already below 35°C, say. But this would heat the entire tank in this scenario. I am planning to use a SunStack, so only the first option is available for me. The issue that have with this is that I am only looking at 90 or so heating days a year and the bookend 30 days or so are pretty marginal. So for ¾ of the year, I won't be using the buffer tank unless I keep it charged just for priming the H/W and if I do then I've got all of the heat waste associated with keeping the buffer tank hot all of the time just in case I want to run some hot water. When I do the sums I doubt whether I will get the payback to justify the complexity. The problem that I have is that I haven't got the data to optimise our system, and I don't want to over-engineer it unnecessarily. After having talked the pros and cons through with Jan, what we hare planning to do is to have as simple an electric only system for the first year with a double SunAmp and no buffer tank. I'll use a simple electric heater for the slab and an inline boost for the SunAmp -- and instrument it properly to work our true usage patterns, demands temperatures, etc. I estimate something like 90% of my draw will be on the E7 reduced tariff. Once I have a full years data, I will then be able to design an optimised system and work out the expected payback periods based on hard data.