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The Thermal Design – Using An Active Slab



I am writing these posts for two main reasons. The first is for my benefit, in that I find that if I have got to the point where I can explain my thinking to others, then I've got to grips with the problem myself. The second is that I might just help others going down this same path, by documenting my thought processes.

It's three months since I wrote the Part I of these three Thermal Design posts, and I concluded this by saying that I intended to adopt Jeremy Harris's UFH For A Low Temperature Slab concept. Since then I have done quite a lot of modelling to understand how the house has a system will react to changes in input and output (as discussed in my last post). Basically, the house fabric has a huge thermal inertia, so I can use average weather conditions and ignore diurnal temperature changes, and even the odd day or so of unseasonal weather. The only wild-card in the heating equation is the issue of solar gain.

Part 2 was a bit of a mixed bag which summarised these issues, and to be honest in the process of getting to grips with some of these, I ended up pretty much rewriting chunks of this and cutting out a lot. It's take me some time to get my head around some of the secondary issues, to get to the point where I could do this post. I realised that once I approached this issue with the right mindset, then achieving this control is going to be quite straightforward. Perhaps the best place to start this is by a showing a graph of the net heat balance for my planned new house:

This is just a summary of the heat balance output from my spreadsheet of overall heat balance by calendar month. It shows three curves (note that this these curves now reflect poorer wall U values, a shift from 0.12 to 0.16):


  • Gross Heat Balance excluding solar gains. This is similar to the one that Jeremy calculated for his house and roughly similar to one that you might calculate for you own house if it is a near-PassivHaus specification. Note that daily household electricity usage ultimately ends up as waste heat within the fabric of the house and acts as a heating source, so this is included in the overall heat equation -- which is why this curve goes positive (up to 7kWhr/day in July/Aug) as well as negative (down to -12 Whr/day in Jan/Feb).
  • Ditto but less slab losses. I explain why I break these loses out below.
  • Ditto but adding in expected solar gains. These gains are just the season estimates based on the PVGIS data as I describe in my last post. My house is aligned on an S/E axis with no large south facing windows, yet even for my window configuration solar gain is quite an issue to be addressed.


Using an Active Slab

So the essence of Jeremy's and my approach is to have an actively controlled slab. By this I mean that I can to define a target temperature set point for the slab itself, as measured by a sensor in the slab, and there is an automated mechanism for adding heat and dumping heat so that it's temperature can be automatically maintained 24x7 within a defined dead-band about that set point.

The slab temperature is coupled to the room temperature: if the temperature difference between these is ΔT and the area of the slab is A, then the heat transfer between the slab and the area is linearly dependent on these, say h.A.ΔT, where h is a constant. A good ballpark for h is 7 W/m²K. This will could vary from house to house because it is dependent on floor coverings and treatments, but it is unlikely to be outside the 5-10 W/m²K range. If I crank in the numbers for my heat balance curves and slab area, a ΔT of less than 1°C will create this range of heat transfer even in the depths of winter.

For example, if I set the slab temperature at 22°C in January, (ignoring solar gain for now), then maintaining the slab at this temperature will pump enough heat into the house to keep the room temperature at roughly 21°C. Likewise if I maintain the slab at 20° in the summer by dumping heat, then this will dump enough heat out of the house to keep the room temperature at roughly 21°C. I am using the heat flow between the slab and the house's internal air to keep the overall house temperature near a desired temperature. The whole house is treated here as a single zone for temperature control purposes. I am not trying to separate out individual rooms or even floors.

Yes we will lose heat directly into the ground through the slab, but this doesn't directly factor into the temperature at which I need to set my slab.

This type of slab control is two sided: I need to be able to dump heat as well as add it. However, the average rates at which I need to add or dump heat are pretty low -- never more than ± 0.5kW. Because the slab itself has a significant thermal capacity (~ 2kWhr per degree), I have a lot of freedom in how this heat adjustment is timed, if I am willing to tolerate a degree or so temperature variation during the day.

I am planning to use a monoblock inverter ASHP for heating broadly the same as Jeremy discusses in his topic that I referenced above. One variation to Jeremy's solution that I am still assessing is to add an external buried ground loop as an extra controlled zone on my UFH. As the ground is varies in the range 8-12°C (depending on season) at 2m depth, dumping 400W or so from a slab at ~20°C doesn't take a large loop size or flow rate. (I will cover these details in a separate thread.)

Dealing with Solar Gain

This is a difficult issue to get to grips with. As tried to discuss in an eBuild topic, Rfc -- U F Controlled Slab + Control For A zero energy home (ZEH) on this, I can get a pretty good estimate of the average season gain for my window configuration and this is not trivial. Some (less than half) of the sunshine will end up on a ground floor and directly heat the slab. Having the slab circulation pump on (e.g. even 20 mins every hour if not otherwise heating) will effectively redistribute this heat across the entire slab, cooling the sun-facing rooms and heating the sun-shaded rooms. Once in the slab, these heat deltas will simple get factored into the overall slab control.

In terms of overall temperature control, it is the major proportion -- that where the sunshine lands on other than the slab -- and the heat from which then gets transferred into the room air is more problematical. However, in my case I have a simple remedy which will work for most of the year -- which is to leave my MVHR set in summer bypass mode. With this, I can define a trip threshold for exhaust air temperature (say 23°C) above which, the MVHR automatically starts to bypass the heat exchanger dumping hot air and drawing in cold air. This is an automated heat dump which in my case can effectively dump up to 1.5kW in winter though a lot less in summer.

The TBD issue is whether I also factor predicted solar gains into my slab temperature offsets, and this is one that I can't really model effectively, and (to be honest) not one where I think that I need to. I am not directly controlling the heat input into the slab, only its temperature set-point. If the air temperature does rise above the slab set-point then heat will flow into the slab from the air. My system won't demand extra heat input (and heat that I pay for).

I want to avoid the scenario where my system is actively heating the slab at the same time as the the MVHR is in active bypass mode, but accepting a 3°C offset should effectively eliminate this situation.

High summer is going to be the period that I need to consider more. This is when I will almost certainly need to dump quite a lot of heat, but the MVHR heat dump is going to be the least effective and this is where I might need to factor solar gain into the slab setpoint, but this is a case of refinement on my real house.

I am lucky in that my house plan and orientation means that solar gain isn't going to be a major problem for me.

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Comments from the original eBuild post:


joe90, 16 Dec 2014 01:26 PM

I too am following this thread with enthusiasm and have been to Jeremy's and was very very impressed with the whole build. My planned build may have a bigger overheat problem than most because of a South facing sun space across the whole back of the house but my thoughts are that this overheat will occur when the PV is producing the most so the cost of running a Heat pump to cool should not be a problem!!!. Also I am planning shades to reduce summer heat but can enjoy the sun through the rest of the year. I also am convinced that the sunspace could be a source of pre heated air for the MVHR during the shoulder or heating months. Jeremy's brilliant find was the electric in line water heater to raise water already warmed to the required usable temp for DHW, Well done JSH.

notnickclegg, 16 Dec 2014 06:06 PM

"I also am convinced that the sunspace could be a source of pre heated air for the MVHR during the shoulder or heating months."

I looked into this a year or so ago. I seem to recall that preheating air going in the MVHR unit has surprisingly little impact on its efficiency, because you reduce the temperature difference that drives the heat transfer.  Jack

TerryE, 16 Dec 2014 06:40 PM

Joe90, if you are talking about putting your inlet to your MVHR on within your sunspace, you have to think about the cons as well as the pros. The whole idea of the MVHR is to recover the exhaust heat into the input stream. As Nick says, additional preheating will have little benefit, but what it might mean is that you will nearly always have heated input air and this will in practice mean that you will lose the ability to use the "summer" bypass mode to dump heat. The amounts of heat that you need to add to a near PassivHaus are trivial to achieve with an active slab. IMO, mitigating excess solar gain is a more difficult issue to master.

jsharris, 16 Dec 2014 08:44 PM

One thing I can report very positively on is that an exhaust air driven air-to-air heat pump is a LOT more efficient thanan outside ASHP. The give away is that the EAHP in our MVHR never needs to use power by reverse cycling to defrost. I can hear when it defrosts, as the condensate pipe is a vertical drop length of 32mm waste pipe at the back of our airing cupboard, so the drops of water when it stops and defrosts are easily heard. Looking at the temperatures within the unit when this happens shows that it just switches the heat pump off and allows the slightly warmer air from the MVHR heat exchanger to defrost the evaporator. This seems very effective and has no impact on the overall COP, which seems to be well over 4 all the time (and up to 5 or 6 at times).

Clearly such a system is only able to supply a small amount of heat, and in our case I'm using it to provide fine trim adjustments, so it rarely has to deliver more than a few hundred watts around the whole house. It does this very quietly and efficiently, more efficiently than the ASHP that's mounted outside, that struggles to deliver more than about 40 deg C at the output. I regularly see temperatures of around 45 deg C at the MVHR internal output (lower at the duct outlets) when it's trimming the house temperature in very cold weather.

My inclination is to rely less on the UFH for the main heating system, and just use that to provide the background, very stable, low level heating, as required, and use the MVHR EAHP to do the fine trimming of house temperature, as it has a faster response time and seems to be more efficient.

I think the secret to efficiency is keeping the heat pumps very lightly loaded, so each is only required to produce a modest temperature increase.

joe90, 16 Dec 2014 09:23 PM

Point taken chaps!, my plans are a long way off but this is all good food for thought. When the plans get nearer to completion I will have actual figures to play with.

TerryE, 17 Dec 2014 01:05 AM

Jeremy, I find your last comment a little ironic. The thoughts that you lay out in it were pretty much my initial conclusions at the time when I first joined the forum and why I was going for a Genvex with no UFH in the slab. We've almost swapped positions!! You have persuaded me that your active slab system is the way to go, and at the same time you have decided that the Genvex should take point. :)

Even so at 20,000 ft, I think that we are saying the same thing: the τ of the active slab is just far too long for it to be usable for fine control. The tactical difference between the two schemes is that the Genvex embeds an ASHP and can be used for +/- fine control, but that the MVHR "summer bypass" function can only be used for - fine control so the slab needs to be trimmed slightly on the high side so that a - control can work over the required range.

I can always adopt the fallback that Neil suggested in one post: keep a little fan heater in the living room just in case we do something daft like leave the front door open and so need a quick heat boost.

jsharris, 17 Dec 2014 09:12 AM

My position has gradually shifted in the light of experience, which has revealed subtle differences between theory and reality!

The biggest difference is that I didn't bother to model either time constant of the house or the full impact of solar gain initially. This was an error, because if I'd done this then I would have been better able to understand the way the house can heat up quickly with even a modest amount of solar gain and that it takes a relatively long time to cool down.

The slab can heat up pretty quickly if fed with water at around 25 to 28 deg C, but takes a long time to cool down, plus the slab sensor takes an hour or two before it sees the true slab temperature, as it's deliberately placed around 150mm to 200mm away from the nearest UFH pipe on the "cold" side of the house (the bit that never sees solar gain).

If I get the slab to a temperature just a bit below that needed to heat the house to the target temperature, I can hold it to within about +/- 0.2 deg C, but if I want to increase the slab temperature it does overshoot by up to half a degree or more initially, and can get the house a bit too warm, especially if there's a bit of solar gain that comes along unexpectedly. The defining characteristic of the UFH in the slab is that it responds fairly quickly (half an hour or so) to increased input and heats the house quickly, but responds very much more slowly when the house is warm enough and doesn't need any more heat.

This is what I expected, as there is a high delta T between the flow water and the slab, so the heat transfer rate in is pretty rapid, but there is a very small delta T between the room temperature and the slab (often less than 1 deg C) so the heat transfer rate out is pretty slow. I hadn't anticipated the impact of solar gain on this though, which can make the slab temperature overshoot before the heat has soaked through to the sensor and got the heating system to shut down.

Running the slab a little cooler (20.6 deg C seems to be about the sweet spot for outside temperatures between +8 deg C and down to just below freezing) seems to work well. There is a small heat input from the slab, enough to keep the house between around 19 and 20 deg C with no other heating and the Genvex can quickly trim the temperature by one or two degrees up or down as needed. A fan heater, or better still a thermostaically controlled duct heater, would do much the same job, at little or no overall additional cost. as the capital cost difference would pay for many years worth of additional energy use I think.

I'll admit to being surprised at how effective the Genvex is at doing this, until I worked out that it was only being asked to deliver very small amounts of heat in reality, just a few hundred watts throughout the whole house at the most. I am pretty sure that the Genvex would not be up to the job of heating the whole house efficiently if it wasn't for the background heating from the slab, so I've sort of accidentally evolved our heating into a two stage system that can both heat and cool the house to within pretty close limits, yet without making either system work particularly hard.

TerryE, 17 Dec 2014 11:48 AM

Probably the topic for another blog post, but we've had an enforced delay in our plans. That delay plus your input and the experiences of the other forum members have given me the time and data to do theoretically what you have been discovering by a mix of theory and practice. I had a look at how you could use the slab to track / mitigate these short term largely solar-driven variations, and I discovered that you can't: the time constant means that I get horrendous overruns. The only thing that works is to have the set-point for the slab pretty much fixed on a seasonal basis, and if I do that all I am left with is a trim control issue. So my modelling and your evolution have converged to similar conclusions.

Your suggestion about a thermostatically controlled duct heater is a good option to consider. I am still mulling over the pros and cons of this external earth loop, but that's another topic. Thanks for your support.

stones, 18 Dec 2014 12:10 PM

In my own house, we have a mix of low temp radiators providing most of the background heating, and a wet duct heater in the ventilation system which warms the supply air. The combination of these two things gives us a comfortable and very responsive system.

Whilst we can deliver the actual radiator system flow temp to the wet duct heater, we have throttled this back to a maximum of 26C, as we have found this the optimum level for our house. In effect therefore we have two variable heating delivery methods (albeit the supply air temp only really operates in a 5 or 6 C band). If we so wished we could deliver less through the radiator system and increase the supply air temp or vice versa. The point, as Jeremy has found, is that the combination of heat delivery methods gives you options and a high degree of control over your internal environment.

Wet duct heaters are not especially expensive and can be sized according to your heating requirements. They also have the advantage (over direct electric duct heaters) that they can use the low temp water as provided by an ASHP as part of a low temp UFH slab or low temp radiator system.

bitpipe, 18 Dec 2014 12:41 PM

> I assume that you aren't going to lag the ground floor (bad idea in this case) so you are going to see that 7-10 W/m²K transfer between floors which is enough to couple the basement to the rest of the house.

This is the dilemma - we want to minimise sound transference between the basement and ground floor as the former will be used for teenager space, music etc.. however doing so reduces the ability to transfer heat/cooth upwards. We are considering have a UFH loop on the ground floor also. We're trying to avoid a concrete lid on the basement as it's more expensive than timber and complicates services.

> Whether a straight MVHR bypass will work you need or more active solution like Jeremy's Genvex approach really depends on your window configurations and whether you have alternative active shading on the S facing larger windows. Doing the trick with PVGIS will give you a good handle on the order of magnitude on this issue.

Luckily we have very little south facing glazing as our house has an W/E aspect. Most of the glazing is on the west side which is slightly north facing.

TerryE, on 16 December 2014 - 12:18 AM, said:

If you are using Gas for your active slab heating then you'll probably need a buffer tank, because you'll want to have decent burn runs on the boiler. You also need to thin about how you chill the slab down in the summer. A heat / cool ASHP is excellent for both heat input and dumping, but you will still need a dump approach with gas heating. I am trying to get to grips with how well Seamus' idea of a ground loop will work. In your case if you haven't built the basement one possibility might be to run a loop around the outside of the basement wall EPS before you back fill. Opening this zone would effectively allow you to bypass the basement wall insulation and dump heat that way.

I am doing a bit more research and modelling on this ground loop approach (I'll post a new topic on this in a week or so). I'd be a lot happier if Seamus had some reference examples of it working in practice.

This is exactly what we are thinking of - running a loop of UFH external to the EPS - should be easy to do and I hope the pipes would be sufficiently robust?

Given we're on mains gas (even though I've just paid nat grid £1500 for a service disconnection to facilitate demolition, reconnection is subsidised) we've never given consideration to ASHP - still debating on how much solar PV we can afford on our W facing roof.

TerryE, 18 Dec 2014 03:49 PM

You want to talk to Seamus about the dump loop placement. You've also got a long back garden so another option would be to add it there. You can compare my floor plans and elevations that I gave on an earlier blog post with yours. You need to do the internal heat balance calcs, but with a house split over four floors you might need active heating.

bitpipe, 18 Dec 2014 05:41 PM
Seamus recommended a capillary mat product which, to quote:  "buried 2m deep will be in close proximity to 27m3 of soil and can avail of its coolth. 1 degree x 27m3 = 27kWh cooling sufficient cooling for 5 days.'

If taking this approach, I'd likely place it against the north basement wall no point digging up more of the garden when we have such a large hole already :)

We've also done PHPP which showed a total annual heating requirement of 13kWh(m2a), but not figured any internal heat balancing. What so you mean by active heating?

wmacleod, 18 Dec 2014 10:21 PM

Nothing here about how big this mat is or how he is working out the 27m3 soil contact, I would be very careful to double check exactly what is being calculated. You are talking about installing into a backfilled area, it isn't likely to be soil going in, it will most likely be stone which will have very different conductivity than damp soil. Also you may see heat from the basement walls. This is likely to give very different results than just burying it deep in the garden. Also remember that backfill should be compacted against the basement walls in layers, this may cause grief if you puncture the mat.

TerryE, 19 Dec 2014 12:18 AM

If Seamus had some hard examples installed, working and with performance data, then I would be a lot happier. For me, I see that going this route is going to be at my own risk, so I want to do some modelling / simulations to get a better understanding of how these loops work in practice, much as I did in the earlier blog post, Modelling Thermal Lag. The difference is that I need to use the heat equation in 2D, rather than in 1D. (2D will be good enough). I don't want to go into more details here. Let me do the maths and run the simulations and post separately.

By active heating, I meant a space heating fall-back given that your house is on 4 flours, especially if you have sound proofed floors between your top-up heat source in the basement and your upper stories. This might simply be a inline air heating in your MVHR as Jeremy suggested. You need to do the heat calcs per floor just to make sure that the kids don't start turning the heating right down because they find the basement too hot and the first floor is a tad too cold. This shouldn't be a problem but if it is then remedying post completion might be a pain.

jsharris, 20 Dec 2014 02:56 PM

I've done a few quick and dirty calculations on using the MVHR with duct heating as a "trim heating" system, and it looks very promising.

As a base assumption I've assumed that the UFH puts a constant 400 W into the house whenever it's on. At the moment this is for 13 hours per day and that seems to be enough to keep the house at around 19 deg C at the lowest, and over 20 deg C at the warmest, over the current range of weather (which has varied from a few days at zero overnight rising to maybe 6 deg C during the day, to the last couple of days where it's been around 5 deg C overnight rising to maybe 11 deg C during the day).

Without the MVHR duct heating the house temperature has dipped to just over 19 deg C first thing in the morning. Using the MVHR duct heating it is back up to 20 deg C within around half an hour. So it looks as if the MVHR duct heating (which is about 1.5 kW maximum) can increase the house temperature by 1 deg C in half an hour. It's doing this with the MVHR fan speed still at the minimum setting though, so clearly isn't delivering full power (the MVHR adjusts the fan speed up if it needs to deliver a lot of heat).

The total flow rate through the MVHR in trickle mode is about 46 l/S, so at a typical heat capacity of around 1.21 J.lt.K and a heated air temperature of 40 deg C (typically the MVHR heats the air to between 40 and 45 deg C) then the MVHR duct heater can deliver nearly 1200 watts to the whole house at around 19 deg C. Given that this is nearly three times the output of the UFH it's no wonder it's so effective as a very powerful trim heating source. It's also very efficient, as the exhaust air heat pump is running with an inflow air temperature that is around 3 deg C warmer than the outside air temperature, keeping the COP of the heat pump up at well over 3.

In practice, the MVHR only goes into heating mode for a very short time on the very coldest mornings, and is set to get the house to 20 deg C.

The MVHR does have the advantage of being as programmable as a full, zoned, central heating system, too. The controller has the normal timer settings with a weekly calendar, plus has the option of controlling air valves to turn some parts of the air feed system on and off for zone control. For example, I could fit zone valves in the ducts feeding the bedrooms to cut the heating to them, allowing them to be a little cooler, and the controller would allow those valves to open and close at preset times and days,

As it stands, with the UFH only on the ground floor and the warm air from the MVHR being distributed fairly evenly between the ground floor and the first floor, the first floor tends to be a little bit cooler (maybe half to one degree) than the ground floor. I think that this is about right for us, as we prefer the bedrooms to be a bit cooler, but if others wanted the upstairs to be warmer than fitting duct heaters selectively to the upper floors would allow that (assuming they only need the same sort of modest heating that we do).

TerryE, 20 Dec 2014 04:06 PM

Thanks Jeremy for documenting the conclusion that I'd also come to. For almost the whole year the MVHR ,when in bypass mode, acts as an equally effective trim down because the same flow rate is dumping hot air and the heat it carries with it. This won't be the case in the height of summer daytimes and possibly early evenings but here we can just accept some slight internal heating, because the house will still be a lot cooler than outside especially is we are running the slab in cooldown mode.

The one area where we may need to consider a separate zone is my son's bedsit room -- mainly because his computer equipment and something 2-3 extra warm bodies etc. will result in it being a local hotspot.

jsharris, 20 Dec 2014 05:15 PM

You could look at fitting wet duct heaters/coolers and driving them from the heat source. If this is a reversible ASHP then you'd have the option to selectively cool the air to particular rooms, with just the penalty of some additional pipe work and control valves. If you opt for a radial duct system then all the duct heaters/coolers with their associated flow, return and condensate drains could be in the same location. A wet duct heater is a pretty simple device and fairly compact.

TerryE, 20 Dec 2014 08:02 PM

Hummmnnn. More research is needed :) Thanks
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