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How an MBC WarmSlab Has Actually Performed based on 6 Years Data


TerryE

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In my topic Modelling the "Chunk" Heating of a Passive Slab, I discussed how I used a heat flow model to predict how my MBC WarmSlab heated by UFH + Willis heater would perform.  What I wanted to do in this post is to provide a “6 years on” retrospective of how the house and slab have performed as built based on actual data that I’ve logged during this period, and to provide some general conclusions.

In this, I assumed 15 mm UFH pipework, but we actually used 16mm PEX-Al-PEX pipework with an internal diameter of ~13mm.  At a nominal flow rate of 1 m/s, say, my three pipe loops in parallel have an aggregate flow rate of 0.4l/s or 1.4 m³/hr.  At this flow, a 3kW (2.88 kW measured) heater will raise this stream temperature by 1.7 °C.  However, when I commissioned the system, I found setting the Gunfoss manifold pump at a high setting (roughly equivalent to this flow rate) gave a very noticeable circulation noise in the adjacent toilet, so I tried the pump on its lower settings and found that the flow was almost inaudible on lowest one with in to return delta at the manifold still only about 5°C, so I stayed with this.  The actual as measured delta for two loops of 4.9°C and the third slightly shorter loop of 4.1°C (close enough not to bother balancing the flows out).  This corresponds to an actual flow nearer to 0.4 m/s or 0.56 m³/hr by volume.  When scaled to adjust for this lower flow rate, the actual measured temperature profiles are pretty close to those modelled.


I measured the actual Willis heater’s heat input as 2.88kW.  In analysing the actual slab heating rates, I found that this raises the overall slab temperature by some 0.45 °C / hour after the initial start up.  Plugging typical specific heat and density figures for the concrete, this is empirically equivalent to heating 25 tonne of concrete (Cmass = Ewillis/ΔT/SIconcrete = 2.88*3600/0.45/0.9 kg), or 10.6 m³ concrete by volume (23000/2400 m³). 


In the case where the Willis provides heating for the full 7 hour off-peak window (just over 20 kWh), at the end of this heating period the flow input to the slab is +9 °C above the initial slab temperature and the flow return is +4.4 °C.  The temperature of the concrete immediately in contact with the pipe will follow this same gradient, with this temperature excess decaying radially away from the pipe centres.  By the end of this heating window at the slab surface, there is barely a noticeable difference in the measured temperature of the floor above the out and return UFH pipe runs (perhaps 1°C).  These temperatures and gradients are also comfortably within the reinforced concrete’s design parameters.  As soon as the Willis is turned off, the internal temperature gradients start to flatten and any unevenness redistributed across the slab; the rebar reinforcing has a thermal conductivity 60 × that of concrete and this accelerates this, so that within an hour or so of the heating turning off, the overall slab is left about 3.1 °C warmer than at the heating start time (actually about 10% less than this, as the slab has already started to dump heat into airspace). 


In my original modelling topic, I mentioned that my passive slab has ~73m² of concrete 0.1m thick (~ 17½ tonne of concrete with another ~10 tonne of perimeter beams, cross bracing and steel rebar, with the UFH runs laid in 3 × ~100m long standard “doubled back” spirals (common to most UFH designs) on ~150mm centres and roughly 50 mm below the slab surface.  (Actually only 75% of the slab is covered by the UFH runs, because of the need to avoid proximity to ring beams, partition walls, areas under fitted cupboard areas, etc..) Nonetheless, this empirical 25 tonne figure is still consistent with the total volumetric 27½ total estimate if we assume that the rebar is effective at spreading heat through the wider slab over this multiple hour timescale.


In conclusion, based on this modelling and observation:

  • First recall our context: our house is near passive in class with a lot of internal specific heat capacity.  We only need about 1kW overall heater input in the coldest winter months to maintain overall heat balance, e.g. either by a resistive heater such as a Willis or an ASHP.
     
  • IMO, there are two extreme approaches to house heating: (i) “agile” tracking of occupancy patterns so the living spaces are only heated when and where occupied; (ii) a 24×7 constant comfortable temperature everywhere within the living space.  Our warm slab design is very much optimised for this second case, and our slab supplier did a good job in designing an UFH layout to match the slab characteristics to this 
    • The slab is covered in “doubled back” spirals with each loop using up a full 100m roll spaced on roughly 150 - 200 centres (and avoiding partition walls and cupboarded areas) so that each heats roughly 15 - 20 m² slab.  In our case three loops were enough, and there was no advantage in trying to squeeze in a fourth. 
    • Our 3 loops will happily take up 3 kW heat input.  Circulation speeds between ⅓ - 1 m/s seem to work well, with the only real difference being the slower the flow speed, the higher the delta between in and return temperatures.
    • The slab does just as its trade name suggests: it can be treated as a huge low temperature thermal store, but because of its extremely  high thermal inertia, one that is not rapidly responsive to heat input.  In our case, a heat input of 3 kW input will only raise the slab temperature by 1°C over a couple of hours, and radiating 1kW will drop the slab by only 1°C over roughly six hours.
       
  • In a true passive class house, one key to heating economy is the high level of thermal insulation coupled with a substantial internal heat capacity.  Trying to drive such a house in an agile manner is a fruitless exercise, so forget the traditional having room-specific thermostat control; forget having traditional time-of-day heat profiles.  It is far easier to treat all ground-floor rooms as a single thermal zone to be kept at a roughly constant temperature.
     
  • In my view, using a resistive heating approach (such as a Wills heater) as well as an ASHP can both work well.  In this second case something like the 5kW Panasonic Aquarea ASHP would be a good fit as it uses a modulated inverter compressor so it can heat the slab directly without needing a buffer tank.  The choice is a trade-off between running costs vs. installation costs.  In our case, switching from a Willis to this type of ASHP would save me about £600 p.a, in electricity cost, so I would really need to do the install for a net £ 3-4K to make the investment case feasible.  However I would like to defer this discussion to a separate thread because there are other issues that such an approach would need to address.

Edited by TerryE

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I should really pull my storage heaters apart, find out the mass of the bricks, and the thermal properties, rig up some individual monitoring, and see what they are actually doing.

Trouble is, they keep the house warm, and that is what I care about.

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@SteamyTea, I agree with what you say, however storage heaters are an old technology that is implicitly accepted as "doing what the name says".  This isn't the case with the warm slab construction technique. We've got all exterior walls, floor, roof with a ≤ 0.12 U-value; MVHR and airtight to ~0.5 ACH. Inside this insulated skin; we have a 20+ tonne slab that acts as storage heater, and it works just as you say storage heaters work.  The internal fabric of the building also adds to the thermal capacity.  We can keep our house at a comfortable temperature 24×4, 365 days a year, using a very simple control system.

 

We still get members here with builds that cry out for this approach, but they end up going with base slab, 200 mm EPS or equiv and UFH loops over a membrane clipped to this, then a thin say 40-50mm unreinforced self-leveling skim because that's what their architect has specified.  The single integrated reinforced slab with embedded UFH and an external insulation jacket is cheaper and simpler to build if you have a trained and competent construction crew.

 

"I want per room zones".  "I want to be able to heat up my house rapidly when I come home so I need a thin UFH layer."  All a mistake, IMO.  Our build has far less running cost than a typical build where each room's temperature yo-yos around, and there is all sorts of control complexity trying keep decent thermal control.

   

Edited by TerryE
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@TerryEYour example should be a case study for doing it right. 

 

I've bought an A2A ASHP to keep costs down. Currently supply half our 3100kWh via expensive day rate electricity with a simple resistive heater that has zero thermal inertia. 

 

Had we used your approach the case for the A2A would have been much slimmer.

However in 50 years time we will have probably used 4 A2A HPs with all the hassle of replacement and your slab will be still working perfectly. 

 

 

 

 

 

 

 

 

 

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@SteamyTea Nick, the more I think about your comment, the more I like it. 🤩 Just think of a warm slab as a 20 tonne (or whatever yours is) storage heater under your feet.  Don't worry out the internals; it just works.  You can stick 1,2,3, ... kW into it, and it will soak up the energy, and then slowly radiate the heat back into the internal airspace.  I feel another modelling exercise and post coming on.

Edited by TerryE
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I do like simple solutions, a former member here who had a very well insulated and draught proof house postulated that controlling the slab input temp to 1 or 2 degrees above what you want in the house is a self regulating and simple solution that should work with a slab with UFH.

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The question I have, is that a whilst a huge passive slab maybe the perfect solution to heating the house... 

 

.. Can I still fit a manual thermostat (not even connected to the heating, but with a satisfying click and red light) so that the Mrs can turn it on max temperature when she has cold tootsies, and turn it off completely when she has a hot moment. 🙄

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2 hours ago, FuerteStu said:

Can I still fit a manual thermostat (not even connected to the heating, but with a satisfying click and red light) so that....

Seems like a plan - I decided the kids were the bigger problem so I moved the dial on its spindle so it reads 5deg above the real value - not sure how much that has saved us over the years.

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@FuerteStu, with this type of house, yes you can put a manual thermostat on the wall, but don't connect it to anything!  Let's say your better half is feeling a bit chilly and cranks the stat up by 5° just before going to bed.  Well at ½°C / hr, it will be hours before you even notice any difference; by the time you do, the slab will be too hot and the rooms will carry getting warmer for hours, and there will be nothing that you can do about it short of throwing windows and doors open. 

 

An analogy: think of a canal boat vs. a little skiff with outboard.  The first is far more fuel efficient per tonne of payload carried, whilst the second is "agile"; however, canal boat can't respond to the tiller the same way that the skiff does.

 

We just set our house a to 22.4 °C average setpoint.  Because our heating is done overnight, there is a time-of-day ripple of ±½°C on this: never too hot and never too cold, so always comfortable. 

 

Last winter I dropped this to 21 °C to do my bit on fuel economy, but I also bought a cheap free-standing oil-filled radiator and put it in our living room.  20 mins on a low setting when we felt cold was enough to turn the room toasty.  Having something like this would give your wife the control that she likes over her sitting space.

Edited by TerryE
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I live alone, so put up with the ripples.

I suspect if I had my own wife, I would be warm with rage most of the time.

Someone else's wife, warm with passion maybe.

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On 08/10/2023 at 15:04, SteamyTea said:

I suspect if I had my own wife, I would be warm with rage most of the time.

Someone else's wife, warm with passion maybe.

I've had the same one for 45 years.  Still think that I am a lucky chappy. 🤣

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I have been logging temperature data from a dozen probes across the system: hall temp, the outs and returns from the slab, the willis, etc. ever since we started using the CH system after we moved in in late 2017.  I have started an exercise to mine this data in order to calibrate a simple heating model which gives a reasonable fit to actual house performance.

 

Take an example, the current heating algo computes the predicted heating time, and when the external temperature is low, this is invariably more than 7 hours, so this first 7 hours is dumped in during the E7 off-peak window , with any remaining heat only added if the internal hall temperature fall below a preset (~22°C).  This sometimes doesn't happen if the hall was slightly warmer than average at the midnight rollover.  So I have a bunch of days where the external temp was ~5 °C and the house was only heated from 0-7 UTC.  I can average these out to get a typical house response curve for this initial condition. Ditto when the external temp was ~10°C, say, though in this case I need to group by actual heat input. as the CH system is on for less than 7 hrs.  Also in warmer periods, the unheated slab still typically hovers at about half a degree cooler than the hall; this is because the ground is at ~10 °C below the slab, so there are still heat losses to ground, this set of reading can give me an estimate of these.

 

Anyway, I'll crank the numbers over this next week or so, and the next post here will be on what I've found.

 

One quick spoiler: my actual overall heat losses are about 50% more than what the simple JSH approach predicts.  So the as-built house is only low energy rather than true passive-class: we need ~20-25 kWh daily top-up in the peak winter months instead of 10 kWh or so, but this is still many factors less than a typical 2018 house of our size.

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7 minutes ago, TerryE said:

I have been logging temperature data from a dozen probes across the system: hall temp, the outs and returns from the slab, the willis, etc. ever since we started using the CH system after we moved in in late 2017.  I have started an exercise to mine this data in order to calibrate a simple heating model which gives a reasonable fit to actual house performance.

 

Take an example, the current heating algo computes the predicted heating time, and when the external temperature is low, this is invariably more than 7 hours, so this first 7 hours is dumped in during the E7 off-peak window , with any remaining heat only added if the internal hall temperature fall below a preset (~22°C).  This sometimes doesn't happen if the hall was slightly warmer than average at the midnight rollover.  So I have a bunch of days where the external temp was ~5 °C and the house was only heated from 0-7 UTC.  I can average these out to get a typical house response curve for this initial condition. Ditto when the external temp was ~10°C, say, though in this case I need to group by actual heat input. as the CH system is on for less than 7 hrs.  Also in warmer periods, the unheated slab still typically hovers at about half a degree cooler than the hall; this is because the ground is at ~10 °C below the slab, so there are still heat losses to ground, this set of reading can give me an estimate of these.

 

Anyway, I'll crank the numbers over this next week or so, and the next post here will be on what I've found.

 

Following with interest.  Do you use a recirculation pump for a bit of time (~30 mins) after a heat cycle?

 

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4 hours ago, Adrian Walker said:

Following with interest.  Do you use a recirculation pump for a bit of time (~30 mins) after a heat cycle?

 

Yup, certainly for dwell time -- I'd need to check the code to see how long -- really just enough to allow the heat in the 50mm or so of the concrete near to the UFH pipe to disperse and along the pipe.  By 8AM onwards the rate of slab cooling is pretty proportional to the Δt  between the slab and room temps.  The decay is a lot faster 7-8AM, but I suspect that is because heat is being drawn down mostly through the rebar from the 150mm slab area where the UFH loops run into the deeper cross and ring beams.

 

I also circulate the UFH for 8 mins before the hour every hour just to help even out any solar gain warming, etc., and I use the average return temp at the end of the 8 min period as a precise measure of the average slab temp.

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4 hours ago, TerryE said:

 

Yup, certainly for dwell time -- I'd need to check the code to see how long -- really just enough to allow the heat in the 50mm or so of the concrete near to the UFH pipe to disperse and along the pipe.  By 8AM onwards the rate of slab cooling is pretty proportional to the Δt  between the slab and room temps.  The decay is a lot faster 7-8AM, but I suspect that is because heat is being drawn down mostly through the rebar from the 150mm slab area where the UFH loops run into the deeper cross and ring beams.

 

I also circulate the UFH for 8 mins before the hour every hour just to help even out any solar gain warming, etc., and I use the average return temp at the end of the 8 min period as a precise measure of the average slab temp.

Thanks for your reply. Hopefully, your temperature data log will have periods with and without the recirculation pump running.

 

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I followed your @TerryE story a number of years ago and designed my system based on your posts. Thanks for all your detailed posts all those years ago.


My system:

  • Installed and running 4 years instead of 6 yours like yours
  • Heated by the same 3kw wills heater during the night
  • 100 sqm of polished concrete, 100mm deep and reinforced with fibres not a steel mesh.
  • Self installed Wunda 16mm PERT-AL-PERT PIPE, manifold and pumpset - 5 heating loops with similar double back loop design.
  • The pump is on a low nice quiet setting. I’ve no idea how to calculate how much water it pumps or the flow like you did nor do I feel I need to. It's working so I’ll leave it.
  • I’ve a temp difference of circa 5 degrees after the system is up and running. It’s the basic temperature dials on the manifold so this isn’t digital or recorded like yours. 
  • Portable oil heater for the misses for when she thinks she’s cold.
  • We’ve our temp between 20-21 degrees so lower than yours
  • It was a major refurb, not a new build but we did a PHPP for the house which I can compare to.

Main differences:

  • I don’t have all the fancy temperature probes or data logging you have. We have temperature sensors but they aren’t recorded. I could fix this by purchasing a few but don’t really feel the need.
  • I have an electrical meter on the wills so know exactly the energy going into the slab.
  • I don’t run the pump after the wills is off to spread the heat like you nor do I do it for a few minutes on the hour. I did play around with this for a while at the start but it doesn’t make a massive difference. This slab is all one large open plan kitchen / dining / living / entrance hall area. You do notice the floor warmer in the hallway nearer the manifold but this is fine as the heat rises in this double height area to the upper unheated rooms.

 

Things I’d change:
I don’t have the fancy controls you have nor do I have the coding skills to develop it. It’s therefore a much cruder timed system.
Note I’m based in Ireland, not the UK. For the first two years it was definitely cheaper to use a cheap wills than invest in an ASHP but the massive increase of the electrical unit rate has changed this. I've had the ducts fitted for years from outside to the wills heater so it’s an easy swap we’ll have to make soon. 
All loops are circa 90-100 meters long so I’d like to connect the ASHP directly to the slab avoiding a buffer too.

 

Question:
If I were to data log temperatures does anyone have any advice on what products to use that don’t require coding, are relatively cheap and what number would you advise getting and what to record? Do I go all out and record the flow and return temperatures for example?

 

My biggest achievement:
For the winter period 2021-2022 when everything was turned off in Spring the total units used was 3,347.1 when PHPP has a number of 3,349.0. Other years were higher or lower but that year was bang on!
 

 

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Just looking at this again from the point of view of controlling with a heat pump. Because everything is so slow (~0.5°C/hr temperature increase, and the increase across the 3kW heater smaller than the ΔT between flow and return, would the return flow temperature make a good leading indicator of room temperature? I'm wondering if that's a better way of controlling a very un-responsive slab than a room stat.

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