TerryE

I use my electricity consumption actuals the help optimize my usage. I currently have an OVO account with a Smart Meter, and OVO provides decent online reporting of usage by 30min reporting period. A little bit of retro-engineering of their undocumented AJAX API gave me a daily batch script that run downloads these data into my HA MySQL DB (see this OVO Post for more details if you are interested). Unfortunately my fixed rate contract is coming to an end and I am now looking at OVO's over 30% price hike -- ouch -- so it's time to move supplier again, and this time I plan to switch to Bulb which is about 20% cheaper than OVO. The downside is that they are slow in rolling out smart metering, so I am going to lose my ½-hourly rate feed for the foreseeable future. Damn! What to do? So I am planning to implement a meter pulse counter. All modern electrical meters use a LED to flash at either 0.01 or 0.001 kWh unit intervals. (Actually most smart meters measure kVAh not kWh, though in our case nearly all of our load is resistive so the two are pretty close, but that's another story). So how? The key component here is an LM393 Light Sensor Module, which cost £2-5 on eBay. the LM393 is an analogue comparator and this little module has a trim-pot on it so you can set the analogue output threshold on the diode output to convert it into a clean digital signal for input into a GPIO input. The next thing is a WiFi enabled micro to do the pulse collection and reporting, such as an ESP8266, ESP32 or RPi0B. in my case this will be an ESP8266. The micro can either count the pulses directly or you can stick a counter in the way. I good choice here is the PCF8593P. This is primarily used as an RTC, but it also has an event counter mode which accumulates pulse falling edges in an internal BCD counter that is accessible to the micro via I2C. Hence the micro can deep sleep for 30 mins; wake up and read the event count, connect to the Wifi and report the count to my HA RPi3 and go back to sleep again. Don't use a Maxim DS1682 as some articles suggest as this powers up and load the count from EEPROM on the rising edge, then stores the new count back in EEPROM on the falling edge, and this read/store is done every event. This make the device ultra low power and really good for applications such as measuring uptime and up events for devices such as vending machine, but the EEPROM only has a nominal 50K write cycles. Do the math: for this application, the DS1682 will only last a few days before the EEPROM fails. The ½hr cycle for the ESP8266 makes it very low power, but the kicker is that the PCF8593P has a nominal power rating of 30mA, so the circuit has just too much base draw for battery power -- a couple of NiMH batteries with a capacity of 5000mAh will last about a week. ? So I will need to feed power into my external meter box to run the data collector. Bugger. It looks as if I am going to have to bring forward my plan to have a 12 or 24V 5A power bus for all of my garden and yard projects. 60W peak should be quite enough for most if not all of of my planned projects, and 12V is nice to work with thanks to availability of very low price and efficient Buck step-down DC-DC convertors; it is also safe enough to run around the garden with minimal or no safety protection. Comments and advice gratefully accepted.

IIRC we just rang up http://www.kytun.com/page/contact-us/18. We used one of their std slate edge profiles but we got the soffits, etc custom folded. They do std widths which they can fold as you specify. It was only about 10% more if I recall.

I used Kytun soffits and slate edges as well. Zero maintenance. Looks good. My slater thought it was a brilliant system. So it has my recommendation as well.

Where? Hopefully you unpacked it and stored it properly to acclimatise with the house environment. As to assembly, the most important thing is to make sure that you understand the assembly process start to finish before you begin, both in terms of the staircase flights and how you will fix them into position. There are lots of useful step-by-step guides and YouTube videos. so watch them through and dry rehearse it all before you start -- especially if you are doing upper storey fittings. I just googled tk stairs installation instructions and got a lot of hits. Make sure that you have the fixings and tools to hand and in position before you start. Validate dimensions carefully before you assemble anything: for example if your staircase is 5mm too wide for the plaster to plaster surface then it is a lot easier to use a table saw to remove the excess off the stairs and risers before you assemble than to start pulling plasterboard off the walls. You also need to think about how you will safely lift and position the flights whilst you fit them, because the last thing you want is an "oh shit" moment with a bloody heavy staircase over your head that is dangling unsafely. Also if you are painting the stairs or the stringers you might think of applying the base and primer coats before you fit the staircases and this is a lot easier to do when the stairs are sat on a couple of saw horses.

There was nothing in the installation instructions for ours, and in fact the unit is in a sheet steel carrier. We've never had any issues with this configuration and our electrician (who wired it up) didn't raise any issues at the time. IIRC, in terms of the physics, you need fero-magnetic medium which will generate local eddy currents and the local resistance then draws a lot of localised power (= local heating) within the medium. You really need something like cast iron or a steel+copper/aluminium laminate structure to produce decent eddying. BTW, steel is fero-magnetic. What I will do is to take the temperature of the cutlery (about 15cm) under a heating pan to see if there is any noticeable warming and report back

The other thing that I really like about the induction hob is that the energy is converted into heat in the pan itself, and hence the thermal spill below is minimal and managed by a small fan internal to the low profile unit. This low profile means that we have a 900mm draw unit directly under the 4+2 hob set up, with a large 900 cutlery draw and two pretty deep utensil and pan drawers directly under the unit. Ergonomically, this is brilliant.

Well I do have a couple of rings fed from an external propane cylinder as backup. (Complete with the GS cert for the installation). We did use them a coupe of times before Jan decided that she far prefers the immediacy and cleanliness of the 4 ring induction hob. Ditto me. We have turned it off at the bottle as we never use them. IMO, there's little point in installing a gas supply in a passive house because you can't really avoid installing an electric supply and the marginal installation costs / daily connection charge / annual maintenance are far higher than any delta in using electricity, even without PV but especially with.

Jason, I feel that both you and @pdf27 have similar concerns, and ones that I can only partially answer. This is almost as a consequence of policy / strategy on my part, and of "walking on the shoulders of giants". I saw that Jeremy's thoughts evolved in the course of his experience of controlling his own house system and he's rejigged his system 3 times and may do so again when he and his wife are in continuous occupation. So I decided to adopt a lazy and evidence-based approach, that is rather than develop and implement a complex fully featured system only to have to rework it later. Jan and I decide to do this in a number of phases so that each was "good enough for now", and then only implement subsequent phases once we had the experience, data and understanding to do so. IMO, there isn't a good rule book that we can use for this class of house design. Phase zero was to do appropriate design validation and modelling to ensure that the house as system had suitable macro characteristics for use to control and keep it within a comfortable living range. This convinced me that a simple Willis-heated UFH system would be capable of maintaining the house over the winter period, but that there was a question over whether we could cool the house enough over peak summer. Ditto using the 2×SunAmp for DHW. Phase 1 was using the Willis heater, circulation pump and 2×SunAmps controlled by a NodeRED instance on an RPi with a simple ON1/OFF1 ON2/OFF2 scheduler and a dozen DS18B20s collecting sensor data every 2 minutes (for 10 months now). I adjusted the ON and OFF times manually based on average outside temperature as required which was maybe a couple of times a week. We can't have PV so optimising this around a cheap E7 tariff just seemed economic sense. This experience and the data showed us that our house worked and performed within an acceptable tolerance of our design predictions. The summer heat wave has underlined that we do need an ASHP -- something that wouldn't really have been cost justified based on our data purely for the winter heating problem. However this is still desirable rather than essential. Unfortunately, I want to place it against the side of a new summer house style shed that we will be building maybe next summer as part of landscaping our rear garden. I don't want to have to move it, so we can't install it until after these rear ground works are completed. If we have to delay it another year then this isn't really a big concern for us. Also unlike a conventional central heating boiler, the ASHP isn't a critical system (as we can heat the house happily albeit more expensively with the Willis) so I can take a more cost aggressive view on its purchase and maintenance. So Phase 2 is to add autonomous control, and I will do this in the next month or so. This is a software only exercise adding a some extra nodeRED / Javascript modules; the interface is tablet / phone friendly browser-based, so just like my chromecast, Jan or I can configure the system through one of the various Android devices that we carry or have to hand. From our experience last winter, I am confident that a two period heating regime will give us a good level of control; that is where the OFF1 and OFF2 times are fixed and the ON1 and ON2 are set based on a daily adaptive calculation of T1 and T2 from three input: the daily average internal temp, the maximum internal ripple, and the average external temperature. We will always have a T1 and T2 to calculate, but sometimes their value will be 0. Incidentally some have expressed concerns about the "what if I drop down dead" scenario doing this sort of approach, but I have a son and son-in-law who both work in IT and are already dabbling with this technology themselves. I am sure that they can take over maintenance if needed. Phase 3 will be the addition of the ASHP and its integration into the heating system. My current thinking is that I will treat this very much the same as the Willis in terms of house control. I will set it to a specific fairly low output by setting the output temperature from the ASHP so that it gives say ~2kW and have a fixed number (say 3×) ON/OFF scheduling cycle, with the T1,T2,T3 set to produce a good thermal balance and acceptable ripple. The reason that I can do this is that the ASHP circuit is coupled to the slab UFH circuit through a plate heat exchanger (PHE). On the UFH side I know what the circuit flow rates are (as I calibrate these from the ΔT from the 3kW Willis element); I can measure / integrate the corresponding ΔT in and out of the PHE to compute the actual heat input from the ASHP into the slab; that is I am treating the ASHP as a black box heat source / sink. These total heat figures will plug into essentially the same control algo as for phase 2. At a tuning level, I can experiment with numbers of heating cycles / ASHP settings, on a day-to-days to find a good set to use, but I won't attempt to use these actively in house control. As I said previously we minor solar gains in winter in our house, so unlike Jason I don't have to adjust for this in my control system.

I heat from 00:00 to 07:00 (or bring the start time back if this is too much). This means that the slab is nice and warm under foot when we are pottering around at breakfast. The GFL room temperatures peak at around 11am and then fail by about ½°C by about 3pm, though we don't really notice this as we tend to be quite active in the house. If we need a top-up we do this from around 2:30 to 4:30 typically so the temperatures again peaks around 7pm and the house is at a comfortable temperature whilst we are sitting around in the evening. So this cycle is 21 kWh at cheap rate and 6kWh at peak rate, and the overall ripple is a little over ½°C. If the low point is uncomfortable then the easiest thing to do is to raise the overall temp another °C or so and keep the same profile. Sorry but this is wrong in my experience -- the main area of our slab is 10cm with the UFH at 5cm deep and pumping in 3kW on a top-hat profile still has quite a delay before enough heat reaches the slab surface to start raising a material amount -- probably about 3-4 hours, and that only starts the heating in the room which is why the peak room temp occurs some hours after the heating has stopped. OK, if you have a 5cm screed with the UFH embedded then the slab will be more responsive, but MBC-style construction were the UFH is fixed to the rebar mesh before poor has long time constants, so you either need to adopt Jeremy's very low (but largely constant) temperature with a simple area stat or my daily computed total and a fixed heating plan.

You are really losing me in this argument. If you mean design freedom then perhaps I can understand more. There is only a control (as in control system) issue if this type of arrangement is acts to constrain the control regime. In our case at the dead of winter we only need to maintain the floor at an average ~2°C warmer than the target environment temperature to maintain overall heat balance. We could easily heat the floor to 4 or even 6°C above room temp with our pretty standard UFH layout in terms of heat output (if we wanted to live in a dry sauna), so this design constrains nothing. All of the control parameters are still tunable: the heat input into each circuit, the flow speed, the min/max ontime, the on spacing. The control algo i about 100 lines of javascript. This is the easy bit. The main PITA in nodeRED is implementing a simple control panel. And BTW, you don't need to have any weather forecast terms in the control. We have a stone skin; this plus the cellulose-filled Larson strut frame give one averall decrement delay of well over a day, so we can just plug in actuals.

Not true. The whole point is that you reformulate the heatflow equation in radial form so that you work in (r,x,t) space instead of (x,y,z,t) space. The package will still form a mess over (r,x), but because dropped one of the curved dimensions, the effect is to drop the computation load a couple of orders of magnitude. Not really; just stick with the normal design guidelines which will be either 100mm or 150mm centres. What you do need to get right is the number of loops (you don't control them separate so they aren't really zones). But again here most people stick to just under 100mm since the UFH pipe comes on rolls which are a multiple of 100m and doing anything else creates wastage as you really don't want to have any joins in the concrete. You do get a pretty strong temperature gradient along the pipe which is why IMO you should always do a loopback layout so that the hot and cooler runs alternate but still give an overall uniform heating effect. One for @JSHarris to comment on really, but this will vary from heat pump to heat pump. AFAIK, what they control is the output water temperature which nothing to do with the environmental control. Perhaps the main difference between Jeremy and me is that he has gone the off-the-shelf approach and I do all my control in nodeRED on an RPi, so he is constrained by different implementation strategies. The time constants are on different timescales and the mechanisms sufficiently decoupled that the systems don't "fight". It's all pretty stable.

Yes, we had one in our old farmhouse. They are cheap and value for money. Good for a teenager's room, rental accom, or anywhere if you are really constrained on budget, IMO. We used the sofa in our old living room and it served its purpose, so no complaints, but to be honest we found that it just wasn't that comfortable to sit on -- which is partially why we went to the other extreme in the new house.

We split our plot to build our new property and since we bought it 15 yrs before registry was required in South Northants, we had to register it at the same time. This was when we discovered that one of our neighbours (or at least her nephew who arranged the sale of her property) had included a path that belonged to our property in her cottage's LRO registration (but never told us) 15 years previously. It didn't matter that the path was explicitly detailed in the original 1913 deed of sale as part of its hereditaments, the LRO took the position that the original registration took precedence unless varied by agreement or court order. What a costly and time-consuming PITA that was to sort, though in the end we decided that bribing the neighbour with an "inconvenience payment" to cooperate and sign the TP1, also having "no registered boundary disputes" was going to be cheaper and easier than the adversarial route. Also one side of our property was shared with another three neighbours and when we planted a laurel hedge 30 years ago, the shared fence posts were on a taught-line. 26 years later we looked at the recorded boundary and it was all over the place (and a couple of neighbours have moved they fenceline behind the cover of the hedge). Again in practice, he who registers first defines the boundary. So my advice is do register your property as soon as practical, and be prepared to discover issues when you do so -- ones that might take time and effort to resolve. You don't want to do this when you are working to a deadline. Because of all of the complications, this cost us about £2K in legal fees in the end and IMO our solicitor was incompetent. (E.g. they handn't bothered to read the 1913 deed of sale and it took me 5 mins to find the relevant paragraph -- after it was all settled and the handed over the old deeds).

Well maybe not conceal, but a fairly mute in wall plate comparable to an MHVR venturi. How about something like http://www.justfans.co.uk/catalogues/silent.pdf. I am sure if you shop around, then you should be able to find something with a better acoustic spec.

How about an in-wall 200mm vent between the snug and the hall? You can get "silent" fans (e.g. ~25-30dB) which can shift maybe 2m³/min which will be enough to stop the room overheating.

In my old house, you closed doors to keep the "coziness" in and the cold drafts out. In my new house, we rarely close doors because the rooms can overheat when containing a high body count, and start to feel a little stuffy. (You get so used to the "fresh air" feeling of an MVHR house that it is noticeable if the warm breathing body count exceeds the planned airflow and so the CO2 count starts to rise even slightly.) So we often compromise and leave them ajar. The one mistake that we did make was the positioning of our bedroom MVHR inlet, which is diametrically opposite the door into the hall. The issue here is that we have an ensuite which has its own extract effectively on the oppsite side of the same wall as the inlet, so the flowpath between outlet and inlet bypasses the body of the room. I leave the bedroom door about 1" ajar which cures the issue, but this does mean that the hall nightlights do shine through the crack into the bedroom.

@pdf27, Assuming that you have a conventional "double-back" layout for your zones and also have access to the Ansys heat flow modelling, then you can easily set up the heat flow model that I did, which is to approximate the slab as a concrete tube the length of a zone run and the radius set by the slab thickness and pipe spacing. This radial symmetry makes give a 2 spacial and one time dimension model which is computationally solvable over 10s of hours. OK, the radial symmetry assumption breaks down in reality because the UFH pipes are not in a cylindrical medium but set in a slab concrete with a insulating surface below and a radiant one above. Even so the model still gave an excellent prediction of the time response of the slab and the heat-off impulse response in the both the model and the actual slab shows that the radial component dominates the heat flow during heating: it's an extremely useful model. To me what this all underlines is that the slab itself is the biggest heat capacitor in the system, so there is little point in adding complexity of external smoothing using TMVs and buffer tanks. So long as you are pumping enough heat per day into your slab then a passive class house with warm slab + UFH + cellulosic filler will stay comfortable. In my experience your divide the year into three broad zones: No active heat management is required (roughly 6 months / year) because the intrinsic heat excess is enough to keep the house at a comfortable equilibrium and MVHR exchange / bypass gives adequate trim. One per day heat adjustment is sufficient (roughly 3-4 months / year). This is my overnight top-up / cool-down. Again so long as this adjustment gives enough bulk heat-balance, the MVHR exchange / bypass gives adequate trim. One per day heat adjustment is insufficient (roughly 2-3 months / year). Here for the mid winter months a single heating period starts to give a daily heat ripple that is noticeable, so you need multiple heating periods per day. For the UK climate range IMO you will never need more than 3, but at 4 the ripple will be less than 0.1°C. At the moment I use the day-to-day average temperature as a control feedback to compute the total amount of daily heat (and in future cooling) needed to apply. Because I use a fixed input heater this maps directly to heating time. If you are on E7 it does make sense to have an asymmetric cycle with a bulk heat overnight, but the tops can be spread through the day. It sounds like you are going to adopt similar ASHP heating scheme to the one that I plan to and @jack has done, which is to set the ASHP output temperature at a low setpoint (in my case around 27-28 °C). You then need to control the mark/space ratio to maintain the overall daily thermal balance. And the CoP at this set point is excellent. (Though if you have kids and use lots of HW and want to use your ASHP to preheat this then this would greatly complicate this approach.) There are a number of strategies here, eg. use a fixed cycle (say 6 hours) and then control the on time or use a fixed on-time and control the off-time to give a variable cycle, but IMO these are all based on macro thermal balance. This will work well for my house, but if you have "acres of S facing glass" then day-to-day highly variable solar gain will become an issue that you need to factor in. Jeremy and I started out at very different design conclusions from a very similar problem analysis and our solutions are conditioned by historic investment decisions. Even so we have significantly converged in our approaches. Jeremy uses a single internal datum, I daily average a couple of DS18B20s measuring room temperature. To be honest, if mid-winter heating was my only concern then I'd stick with the largely E7 Willis approach. Yes, the running cost is maybe £300 p.a. more than using an ASHP, but I have no complex mechanical systems to maintain and to replace every 10 years or so, so there is no cost benefit case here. The real issue that makes me plan to introduce an ASHP is the summer cooling one: for about a month a year, I need to dump heat from the house actively to keep a comfortable internal environment, and I can't do this by an additive heat solution. One last comment. I've mention the impact of solar gain which can throw a "big spanner in the works". You can get very sunny days in December and if you have a large south-facing area of glass, this can be a big pulse of kWh into the house. The other one that causes us fun is visitors. When our kids+family or others come to visit, then just the heat and activities around hosting these guests adds environmental control challenges. For example, 6 people sitting in a room will cause it to start to warm up noticeably!

Sorry if there is any confusion: floor temperature. The relationship between the flow temperature and the floor temperature is highly non-linear both in terms of space and time. It took me ages to get my head around this and it wasn't really until I did the modeling that I got a good feel for it. You just can't use steady state or linear approximations validly. I explained this in a little more detail in: Instead of tightly controlling the flow temperature and therefore putting in a very variable power input, I limit my power input. The slab heats steadily over about 4 hrs radially from the pipe centres before it even starts to approach an equilibrium where the floor temperature has risen enough for the heating to start to tail off. On a 7 hr @ 3kW heat, the final inflow temp is around 29°C and the outflow temp at 26°C, but the inflow temp collapses within a couple of mins of turning off the heat -- as the radial gradient collapses. The floor reaches max temp around 11:00 - 12:00. Conventional Gas and other boiler systems are simply not designed to output a steady power of < 1 kW. You have to add a lot of additional hysteresis to avoid constant start / stop and knackering mechanical components. As to Jeremy's formula IIRC, it is a curve fit from BRE for conventional UFH systems which typically need to output maybe 3× or more than a passive house UFH system. The exponential component is a fit to the convection effects that you will see if your floor is 5° or more than room temperature, but my floor never gets more than a couple of degrees warmer. I ain't going to argue 7 vs 8. They're the same ballbark as far as I am concerned. Maybe J still has the BRE doc ref.

We had a long thread about this a couple of years back. @JSHarris and @SteamyTea were two of the other main actors. The figure drops out of Stefan–Boltzmann as the radiant component dominates heat losses at the small delta T. There are minimal conductive losses on an solid / air interface and a few °C is too small to generate enough instability to create any convective flow in a passive house. In our case we have a mat slate floor which is rough at a micro level and about the perfect radiant surface. The figure would be lower for a carpeted floor, but not as much as you'd think because whilst carpet is a good insulator, the surface is almost fractile and so the effective radiant surface is a lot larger. Certainly looking at the slab temps and our rate of heat loss, this is in the right ballpark.

The microSD standard does not support TRIM; only SSD -- but the Linux kernel only supports TRIM over eSATA and USB3 interfaced controllers. (eMMC does which is why the new RPi clones which support eMMC and USB3 have a big I/O performance hike over RPi.) I believe that we have the same potential issue with cheap SSD clones, which is why provenance of SSD and microSD devices is of material importance.

I've just been doing a bit of research and it turns out that this isn't the case. Reputable manufacturers do have an on-card flash controller in front of the flash chip(s) ( and this is often an ARM chip! ) and this controller does implement wear leveling. See Bunnie:studios - 2Gbyte microSD card and Bunnie:studios - On MicroSD Problems which describes an epidemic issue of knock-off clones where the counterfeiters save cost by leaving out this controller and use a very simple interface which does no wear leveling. If you sudo cat /sys/block/mmcblk0/device/cid this will output a 32 hex digit output. Use https://goughlui.com/static/multicid.htm or layout below to decode. It turns out one of my SD cards is a bit suspicious, and has a questionable life, but luckily I only use them as /boot and effectively never write to them, so this isn't an issue for me.

Jeremy the easiest way to connect this is through a simple gateway. If you have a spare serial port off your PIC hub then you can hook it up to a Wifi-enabled processor. You might find the RPi zero W as the best choice because you can run Linux and script this in Python. Alternatively you can use an ESP processor and script this in Lua which is pretty Fortran-like. If you do want to go this second route them you do have access to one of the ESP/Lua gurus, but this is a case of Matthew 7:7 (an exercise for Google)

Yes, the ESP32 is a very nice device and a very good successor to the ESP8266. However the challenge with all of these devices is that the developer platforms are all community driven. There just isn't the commercial model to pay for funded environment development, and hence you pretty much have to work in C. I am one of the NodeMCU Lua developers. Espressif sent me some pre-release ESP32 samples about two years ago, but they've only become available to the general buyer at a reasonable price and delivery in the last 6 months or so. I and the other collaborators haven't time yet to port the firmware and all of the supported device libraries to the ESP32 I feel that is a bit of an uninformed view. You can now happily run a 5,000 line Lua application on one and control all of the I/O for a Home heating system (in my case 4 SSRs, 20 thermometers, 2 flow meters) on one. That's a little more than being a "Wifi chip". As to using SD cards, as you say they fail and regularly. And that is a total PITA. The nub of the issue is that the EXT4 file system really assumes that there is a decent wear-leveling flash controller between the processor and the flash memory chips themselves. Yes the Linux kernel does have some flash support but this is really to talk to the flash controller with garbage collection hints. This just isn't the case and like all file systems or pretty bare flash devices like your typical microSD card, you will hit the erase limit on hot sectors and the device will fail. That's why you need an SSD or a board that supports eMMC. ?? "create database ed character set utf8;" and most DB installs nowadays have utf8 as the default character set, so you don't need the last three words. Not that hard. I also agree that security can be a PITA, but it is less of a pain in the arse than the alternative.

Thanks for the clarification. That would be around 1.4kW sustained in our case. We would need the outside temp averaged over something like a 4-day period to be < -5 °C to hit that sort of number, and we have a 3 floor house. Have a look at my blog posts on the 3D modelling of the slab-heat flow. Last winter was particularly cold and we only went over the 1kW figure over two months. Because we have a slate floor we get a good 7W / K / m² off the UFH, and as I said using 1 or 2 strategically placed Dyson's can double that if there is the odd week when you need it. IMO, in terms of heat output and achieving overall heat balance we have a very comfortable margin in the GFL slab UFH. I don't know if you (two) have lived in a house of this class, but what we and all of our guests note is that there is no material variation anywhere in the house; no snug feet away from the cold utility. The only noticeable variation is that ~1°C drop going upstairs in winter. Because we haven't fitted our ASHP yet we do our bulk heat input using E7 overnight into the slab and the overall thermal inertia is high enough that we still only get about a 1°C ripple over the day, that's < 0.1°C/hr so you don't really notice it changing. Jan and I have decided that we will install an ASHP probably late next spring but I can only make a cost-benefit case if I do this myself. And yes paying for pumped kW heat at a third of the cost will impact our time of day heating profiles and the use of active cooling in the winter. Upstairs UFH seems a complex and costly solution to a tiny issue. Like my ensuite towel rail, if you do have a passive class house, then you[ll only use it because you've got it, and not because you need it. As @mike2016 suggests we are also considering an inline heater in the MVHR as an option. I suspect that the main reason for the imbalance upstairs is that if our exhaust is at 21°C and the outside temperature is ~0°C then the inlets into the bedrooms is at ~18-19°C and this is enough to bring the temperature down. We don't want to use this to provide bulk heating so I don't see @JSHarris concerns as being material in this usecase; this is more a trim control, and even a 1kW heater would be more than enough for this. However, I will do the maths and the cost benefits before doing this.

My control system runs on an RPi directly connected to a couple of ESP8266s that drive my pumps and heaters using SSRs. I've got DD18B20s all over my pipework. I use an SSRs to turn the pump. I don't bother with a flow switch. Why bother? I just take the temperature at each zone just before and after it enters and leaves the slab. If the pump is running and the water into the slab isn't heating as expected then something is wrong. PS. Ed, I suggest that you stick an SSD on your RPi (or buy one of the clones such as the Odroid C2 which as 2Gb RAM and an eMMC slot) and stick to MySQL/MarianDB ?