TerryE

I think that the main lesson from this is to ensure that your plasters use the bloody template in the first place!

It's only Dec /Jan where this might be a problem. The figures are all modelling and estimating, but I think that we're running a tad less than 20 kWh worst case -- call that £1.40 / day or £55 per month. It we spill into having to do peak rate top-up of another couple of hours, its another £1.20p. But until I've collected live data, then I don't know. I could more into the slab, kW-wise but I'd rather not plan on this. As I implied in the preamble, the fall-back is to put in a smallish monoblock ASHP, and if I am finding that I need to do peak rate top-up for more than 30 or so days a year, then we'll cross that threshold, I think. And picking up @joe90's point the unknown is the summer cooling issue. If I can't tune the MVHR bypass mode to dump enough heat then we'll need an ASHP anyway and this heating issue is no longer a problem, as the ~4× CoP makes a big difference to the electricity bills. As to the water preheat for the DHW, that's an issue for another blog post, but my main reason here isn't saving £s as I pay for the kW either way -- at least in net heating days. It's more an issue of guaranteeing flow rates and DHW capacity. Again models take you only so far. We need real operating data, but I don't have a Tardis.

We have a passive-class house where the net heating requirement to keep the house warm in the coldest winter months is approximately 1kW. The only heating system for doing this an underfloor heating (UFH) system base on 3 ~100m UFH loops buried in our passive slab. That's it; no upper floor systems; no towel rails; nothing. The reason for this is that our timber framed house is super insulated and air tight so there is very little temperature variation throughout the house, but that's all been covered in earlier posts. What I want to do in this post is to provide a simple explanation of how I am going to heat my house and how this works so that John (@joe90) and other forum members understand my approach. This basic heating strategy was first evangelised by Jeremy Harris (@JSHarris), but variants have been adopted by other forum members and their consistent experience is that it works and works effectively for this class of passive house. However, what I am doing is a slight variation on Jeremy's approach: I am using the slab itself as my main heat store, so no buffer tank. I will be heating it by circulating warm water through the UFH loops and this water will be heated by a simple small inline 3kW electrical heater element. The heating charge will normally be done as a "chunk" once per day during the E7 cheap rate period to take advantage of low tariff rates. However, I am also including in the design provision for the later addition of an ASHP, should the heating data collected over the first year show that there is a 10-year payback in doing this. As I said, Jeremy's approach has been well documented by him in his blog and by others. He has recently described that his system settles down into a repeating pattern over the colder winter months winter where his heating comes on for a few hours once a day in the early morning, and the heat in the slab is topped up during this period. This is broadly what I call "chunk heating": unlike a traditional house central heating system which is turning on and off pretty continually, the heat losses in our type of house are so small and the house has such a high thermal inertia that you can heat the it practically with a single daily top-up to the slab; this heat then "trickle feeds" into the house over the day. Yes, there is a slight residual ripple on the temperature in the house, but this is less than a 1°C undulation over the entire day and so this isn't really perceptible to the occupants. I am adopting this same approach, but shifting my heating period earlier so that it ends at the same time as the E7 low rate tariff ends. The main difference in my implementation is that I am heating the slab directly without a buffer tank. I wanted to get my head around this before committing to this decision, so I modelled this in some detail and covered all of this physics and modelling stuff in my Boffin's corner thread. This modelling has persuaded me that the mechanisms and dynamics of heating are pretty simple, and so in this post I want to cut out all of the equations and stuff (with one exception) and focus on describing what happens in plain terms. First, I am using a small 3 kW electric element to heat the water circulating in the UFH loops (the same type is used as a hot tank immersion heating element). Just like an electric shower this heats the water stream a step in temperature. Sorry I am a boffin, so I will call this temperature change ∆T. (BTW, the triangle is just the Greek letter D and is short for difference; blame Isaac Newton for that one.) Just like an electric shower, double the power and ∆T doubles; double the flow rate and ∆T halves, and if I do the sums for a typical flow throw my UFH loops, and for a 3 kW heater then ∆T works out at about 1.6°C for my system -- a lot less than a typical gas-boiler fed UFH installation, but my heater is puny in comparison. So if I start pumping 3kW of heat into my slab, then the system settles down after about 10mins and the heat output is pretty much the same along the entire 3 × 100m runs of UFH pipe, pipe work, that is each 1m of pipe dumps about 10W of heat into the concrete. This lifts the temperature of the concrete, and at the same time cools the water in the pipe pretty steadily along its length so it comes out at 1.6°C cooler than it went in. But cooler or hotter than what? The heat flows radially away from the UFH pipe creating a thermal gradient. [Boffin bit warning, and the only one] this gradient is pretty close to what is known as the steady state radial solution to the 1-D heat equation, which has a formula Tr = Tp - A.log(r/rp). where T is the temperature and r is the distance from the pipe centre, with the p subscript relating to the pipe/concrete interface. The A term is a function of the amount of heat flow. The main thing to note here is the general shape of this gradient: the temperature of the water ends up roughly 4-5°C hotter than the slab average for this sort of 10W/m value, and the temperature in the concrete falls away rapidly as you moving away from the pipe towards the average slab temperature. Since the volume of concrete goes as r2, the actual proportion of the concrete more than 1°C hotter than slab average temperature is small. So the overall effect of the heating is to slowly lift the average slab temperature. There is also a general heat gradient along the water in the pipe but once you get more than a few cm from the pipe centre the concrete is all within 1°C or so of the slab average. There are also local hot regions around the UFH pipes up to 5°C or so hotter than the overall average slab temperature. However, this is factors less than you will get with a conventional UFH system. A key difference of Jeremy's approach is that the water continues to recirculate after the heating is turned off, and now the water flow acts to redistribute the heat rapidly along the pipe levelling the previous 1.6°C gradient; at the same time (without the heat being pumped from the UFH pipe) this central warmer region rapidly flattens out as the heat flows outward, and within an hour or so hardly any heat variation remains and the entire slab is within ½°C of the slab average temperature. A good analogy here is pouring water into a bucket: the surface level steadily rises as you pour it in and the surface itself is a bit churned up by the act of pouring, but as soon as you stop pouring, it rapidly levels out to flat surface. OK in a real slab this is also complicated by the deep elements (the unheated ring beams in my slab are over a third of the total volume) and the heat does flow into these largely thanks to the high thermal conductivity of the rebar. But overall, the slab is acting as a heat battery soaking up the power that you pump in. The trick is not to put a somewhat arbitrary limit of the maximum input water temperature (say 25°C) as this will limit the amount of power that you can apply. This heat gets quickly spread uniformly throughout the slab. By the end of the heating period, the slab is 2°C (or whatever) warmer than the room temperature, and is starting to transfer heat into the room fabric at ~15W/m² whilst itself slowly cooling. This is more than the external heat losses in the house, so this heat both warms the air and the rest of the wall fabric. This creates a very slow rise and fall in the room temperature over the course of the day -- of roughly 1°C. But so long as you put in enough heat each night, the overall house temperature remains stable. So how much is "enough" heat? In my case I use a very simple strategy. I am using the UFH circulation temperature at midnight as my test. If it is less than the previous night, then I add a bit more heat than last nigh and v.v. Simple really.

Yup, the Wundatrade people have a series of excellent YouTube videos which explain how to intall their UFH manifolds and I only picked up about this by watching them.

The proper tool is only a fiver, so compared to everything else I am buying, its pissing in the wind not to.

@jack @JSHarris Thanks guys, it's nice to have this confirmed. I'll be ordering al this kit in the next 24 hrs.

High flows are the ones where you'd get pissed if they dropped in pressure / went of of mixing balance when someone turns on a shower or started running a bath: the main sinks, the baths, the showers. The low flows are what they say and flow rate isn't that important: the toilet cisterns, the hand basins, bidets, etc.

@Calvinmiddle David, I should have asked the guys when they laid the slab, but looking at the pipe it is 16 mm PexAlPex or PertAlPert, so you go for the 16mm Pert-Al-Pert adaptors on the manifold?

@JSHarris Jeremy how did you deal with the overflow (rather than the drain)? The Installation instructions state "the hose should be run downhill all of the way and terminate at the exterior of the building if possible" (as a warning pipe). My thinking is that I've now got 4 pressure relief / overflows I was think putting them all through tundishes to common waste into our stack but with a moisture / wetness detector so that I get an alarm to check them if any starts dripping.

@pocster. Seek and ye shall find. Search the forum for hep20 and you'll see some active threads on all this -- including mine.

AFAIK the discharge mode is essentially passive, so you can get H/W even if the power is out. But as to charge mode, have you got a standby generator for your current gas central heating system? If not, then you are in exactly the same situation today. I have two SunAmps in parallel so both have to fail to lose H/W. And then I have a Propane 2-ring gas hob in my kitchen so tea and coffee are still available. And the power would have to be off for more than 3 days before the house starts to feel a little cool. 5hWh / 2.8kW = just under 2 hrs if you've exhausted the store totally. In practice squeezing the last 0.1kWh of heat out of the device probably isn't worth it so the trick is to size your system so you rarely run out, but all of this is exactly the same as if you had a conventional UVC.

No, and in fact MDPE is approved for any use where it could be exposed to direct sunlight. Just search the forum for Hep2O and there's loads of background material.

Neil,as per Mr Punter, but it is also important to keep the rise and going exactly the same all of the way up the stair, and this includes turns, top and bottom stair. So you need to allow for your floor coverings at the top and bottom. The bottom tread rise is slightly less critical. The reason is that your brain is very good at placing your feet, but if the rise and going aren't exactly the same then it becomes quite easy to trip or stumble and this is very dangerous on a stair. So if you do need a turn in your stair then you will find it more comfortable if this is an evenly swept turn, though having this type of design costs more. A 45° split on the turn is the worst.

We do have a few hidden joins -- some of the runs travel vertically down the walls but need to come out at 90° to the wall (e.g. for the shower fittings) so these have elbows just behind the plasterboard. Other than that, none. If there is a leak at one of these, then at least it's just a matter of cutting a hole in the board fixing and repairing. But you want to buy a load of Hep2O end-stops, and then you can at least pressure test before boarding out -- if you don't want to take the risk.

As well as the ease of fitting, cost and significant risk of leeks, the Hep2O / manifold approach (or equiv from other vendors) is a lot easier to balance and has significantly less heat losses. If it's not too late in your design process, then centralise you heating / DHW tank / thermal store, manifolds, etc. in a single service area. That way your copper runs are only a few metres long at most and the hot works can be properly lagged to minimise thermal loses. You will still end up dumping heat from the hot radial runs, but this can be minimised if you split your hots into low-flow and high-flow and plum the low flow hots in 10mm.

All commissioned and working IMO, the main design issue with this is that we have standard 2×2 induction hob, a 120mm separator bar in the worktop and then the 2×1 gas hob. This makes for quite a wide worktop footprint: you need roughly a 98cm gap in your top cupboards above the hobs (where the extractor fits.)

My logic is this: the SAPV is a thermal store that enable you to charge it with E7 or PV electricity at times when electricity is cheap, and because f its low heat loses, you can then discharge it into heating your DHW when you need it. It is optimised for heat delivery in the 40-60°C range. In an MBC house with a passive slab (or other equivalent vendor offerings) you already have equivalent thermal store but one that is optimised for storing heat at 20-25°C and that is the slab. I had to get my head around using it this way, which was the reason behind my topic Modelling the "Chunk" Heating of a Passive Slab, but my current plain is to use a small electric water heater to pump a chunk of heat into the slab each night when I am in net heating days.

I don't know. I suspect that it's not, but it one of those things that will bite you badly if you don't and something goes wrong. Will you be insured? Will you be criminally liable?

Typo. It's amazingly -158°C. The entropy of a crystalline structure is a lot lower than the liquid phase which is why converting from one to t'other soaks so much heat. And this is also why the specific heat of ice is a pretty much half that of water. This effect is also why steam is so dangerous. If it condenses into water on your skin it chucks a lot of heat and causes serious scalding. If you run out of heat, then you run out of heat. It's store not a generator. But there's nothing to stop you topping it up during the day if you need to and one of the standard uses is to dump your excess PV into it during the day. My quote wasn't cheap John, this one is a bit off-topic but l will answer this. In essence a UVC is classified as a potentially dangerous appliance, which is not surprising really since if one burst at 3 bar and blasted a few hundred litres of scalding water around your house, then people could get seriously hurt. This is covered in Part G paras 3.17 to 3.21. Installation of a UVC is notifiable and must be installed and certified by a qualified installer. Your B Insp won't sign off your completion certificate without this if you have a UVC.

Yup the main difference is the max flow rates. I don't understand why they plumb up the cells in series rather than parallel.

@jack @JSHarris Have either of you got a data sheet on your water softener? I am looking for the pressure drop to flow curves. I did see a couple of figures quoted on one site, and assuming the typical power curve this gives 0.0276×f2.41 kPa for f in ltr/min which is 0.2 bar at 15 ltr/min, 1 bar at 30 and 2 bar at 40, but the 2.41 seems suspiciously steep to me; the power term is normally in the 1.8 - 1.85 range.

@JSHarris @jack this issue of when to use flexible hoses vs copper is an interesting one. The SunAmps are supplied with 2×½" (or 15mm) flexible hoses which seems daft to as the internal diameter of flexible fittings is slightly less that the equivalent copper pipe. The SunAmps give flow:PD curves up to 14l/min which a flow speed of ~1.7 m/s which is getting rather high, IMO. Surely it would be better to come out of the SA in 15 copper and jump to 22mm flexible if we really want flexible connectors. The best price so far that I've found for the Harveys Crown Water Softener is £885 inc VAT from Fountain Softeners complete with 22mm high flow install kit. BTW. Have you seem my latest blog posts giving an intro to SunAmps?

I just wanted to include a brief post explaining from a self-builder perspective why we have decided not to use an Unvented Cylinder (UVC), Thermal Store (TS) or combi-boiler for our domestic hot water (DHW) in our new build. Instead we are using 2 × SunAmp PV heat batteries heated by E7 tariff. So why? We decided that we don't need gas to be installed avoiding the Gas connection charges, per day supply charge and the maintenance costs on gas appliances. Big saving here. We don't have the room for a TS and we've heard too many horror stories about the problems of heat losses in a passive-house class new-build like ours, so no TS. We didn't want to get into all of the regulatory crap around installing and annual maintenance contracts for an UVC. So strike this one as well. So what is the alternative? The SunAmp is a thermal battery with an in-built heat exchanger (a bit like a combi boiler) which can store ~5kWh of heat for delivery in water typically at 50-65°C. Here is a simplified schematic of the store. (Note that I've left off all of the essential safety features such as the expansion vessel pressure relief and overflows to simplify this down to the functional essentials.) The guts of the device are a couple of Phase Change Material (PCM) cells which act as the thermal store. It in essence it works in one of two modes: Discharge Heating, where the CW supply flows through the two PCM cells and is heated to between 55-65°C and then blended with a CW mix in a TMV down to a preset output temperature. Recharge. When fed with an external electricity supply (typically PV or E7 off-peak tariff power), water is circulated internally through the cells and a 2.8kW heater to bring them up to an internally preset maximum temperature. So the SunAmps can only be charged by electricity, and there is no alternative form of heat supply. The form-factor is very small – two SunAmps side-by-side take up (d × w × h) 530 × 600 × 740 mm. The rectangular packaging also facilitates the use of internal vacuum pack insulation panels so the total standing heat loss is ~ 1kWh / day which is a lot less than a typical TS. The exact choice of PCM is specific to SunAmp, but the linked Wikipedia article lists the common ones with a phase change at around this 55-65°C range. However in terms of the physics of how this all works, it is easier to describe another common PCM that we are all familiar with and which has its phase change at 0°C: water. There are three material properties that you need to consider when looking at how a PCM works: the specific heats of the solid and liquid phases, that is how much heat you need to supply to heat 1 kg of water by 1°C and the latent heat of fusion that is how much you need to convert 1 kg of water at 0°C to ice at 0°C. I could give you the figures but a good way to think about is that you need the same amount of heat: To heat ice at -158°C to ice at 0°C To melt ice at 0°C to water at 0°C To heat water at 0°C to water at 80°C. OK these ratios and the fusion temperature differ for different PCMs (as well other properties which reflect the long term stability of the using it in cells, etc.), but that is all the proprietary stuff (discussed in the detailed below from Andrew Bissell). Even so, the bottom line is simple: the systemic heat losses are far less than alternative solutions, and Weight-for weight you can store roughly four times as much heat in a SunAmp PV store as a conventional DHW cylinder. As to why we have chosen the 2 × SunAmp PV approach, there were 2 main drivers for us: 5kWh isn't enough to meet our typical daily use, and 10kWh is so we will be able to charge our stores overnight at E7 rate and only need daytime top-up in exceptional circumstances. The pressure drop across the store in Bar is roughly 0.0142×f1.81 where f is the flow rate in ltr/min, and if you crank the numbers one store doesn't give us enough flow rate. Even so if we look at our planned use (I'll go into the figures in a later blog post), our household of 3 people has had an average use of 280 ltr/day averaged over the last 6 years. Most of this is hot water -- say 80% or at an average lift of 25°C, this amounts to 5,500kgK = 6.4kWh/day or 7.4 kWh/day allowing for heat loses. This will cost us £194 p.a. at my current electricity tariff for my household's DHW. Will I really realise the payback from additionally investing in gas or ASHP based DHW systems? I think not. PS. Slightly amended wording to reflect the earlier comment of Andrew Bissell quoted below.

Nah, I won't keep simple for you, but for normal mortals, that's a different matter.

@jack Jeff it was really useful for me :-) Thanks