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The Thermal Design – Part I



Janet and I want an energy efficient house, but what does that mean in practice? The whole concept is still largely rejected by the UK building industry. In our initial research, we either found books like the House Builders Bible which are good but superficial introductions on the concepts but without serious detail or at the other extreme academic papers on micro details. There is precious little in between, and to be honest we have found far more gems of knowledge in this site. All my experience and intuition concerns living in a traditionally built house. An energy-efficient house is just a different beast entirely, so I discarded my intuition and put my trust in the physics, maths and engineering. Likewise, we only considered the views and recommendations of those who have actually lived in this type of house.

One of the first things that I did was to build up my own version of Jeremy Harris's Heat Loss Calculator.xls (which he first developed on this GBF topic). I plugged in the numbers for our own house design, but in reality there are only a few parameters derived directly from the house geometry that drive this calculation (my numbers are in brackets):

  • The internal footprint of the slab (71m²)
  • The internal surface area of the external walls, less windows (179m²)
  • The internal surface area of the roof, less roof windows (93m²)
  • The area of windows (23m²)
  • The total volume of the internal living spaces (419 m³).

Each of these is multiplied by a factor derived from the design to give a heat loss per °C: a U value in the case of the first four. The last is more complex in that I had to build up a composite heat-loss based on the rate of air exchange, its unit mass and specific heat, and importantly the recovery efficiency of the MVHR.

For a typical winter external temperature of 4°c and an internal 21°c, this gives a delta of 17°c for fabric heat losses. The slab delta is somewhat different in that the ground temperature under slab is far more constant – say 10°c at the centre of the slab and maybe 6°c at the edges in the winter raising to 15°c in the summer. Plugging in our current design values (from our frame supplier, MBC) gives the following heat loses for a typical January day:

  • Slab: 97 W (9%) – 71 m² x 0.105 W/m²/K x 13°
  • Walls: 364 W (32%) – 179 m² x 0.120 W/m²/K x 17°
  • Roof: 171 W (15%) – 93 m² x 0.105 W/m²/K x 17°
  • Windows: 313 W (28%) – 23 m² x 0.800 W/m²/K x 17°
  • Air change: 180 W (16%) – 419 m³ x 0.025 W/m³/K x 17°

That's 1.1 kW in total, or as I sometimes say to friends, the whole house could be heated by a single 1-bar fire. Clearly this heat loss varies according to season, so if I plug in an overall temperature profile for my location, I then get the following daily heat losses in kWhr :
The house is reasonably balanced as a system: no single component dominates the heat losses. However, this wouldn't be the case if we dropped the heat recovery element of the MVHR, for example, as this would increase the air change heat losses by roughly 5x, becoming the majority of total heat loss. This is why the inclusion of MVHR is such an important component of energy-efficient design.

If we look at the risks and sensitivities in this sort of calculation, then they broadly fall into two categories: failures in airtightness and thermal bridging at boundaries. (Googling these highlighted terms will give background explanations of what these are). All internal surfaces in a properly implemented energy-efficient house are within a few degrees of the internal temperature, which also means that there are no internal condensation surfaces. A serious consequence of thermal bridging is that surface temperatures can drop significantly at the bridges below the internal dew-point, causing surface condensation.

Failures can occur with sloppy design or poor attention to detail during construction, so I believe that it is a lot more important to find a frame manufacturer / assembler who gives us confidence that these issues will be effectively addressed (as failures here could cost kilowatts of heat loss or mouldy surfaces) rather than whether the walls have a nominal U value of 0.12 or 0.14 (which varies the total heat loss by roughly 60W at most). However, this heat calculation isn't the whole story:

  • We typically have three occupants in the house and being alive we radiate heat – roughly 300-400W between the three of us.
  • The 23m² of windows can let in up to 1 kW/m² incident energy in direct sunlight.
  • Our average daily electricity usage in our current house is 16 kWhr / day. We will cut this a bit in the new house, but this is a combination of lighting, DHW and running electrical equipment – fridge, washing machine and my live-in son's Gaming PC + Xbox.

Apart from some of the DHW used (which literally goes down the plug hole), all of this energy eventually ends up as waste heat within the living environment, and therefore adds to the general heat budget – that's roughly 1kW plus the solar gain. This 24 kWhr/day or more means that we will often have a heat excess within the house. On the other hand clearly these temperatures are profile averages and there will be periods where the temperatures will be lower, and below zero for extended periods. However, even a doubling of the temperature deltas only give a running heat loss of approximately 2 kW. My overall conclusions are:

  • Our overall heat budget will be in near equilibrium for large parts of the year.
  • At most the sustain heat input requirement will be of the order of 2kW peak.
  • We need to manage heat excess efficiently and automatically up to say 2kW.

I want to expand on this last "automatically" point: in our current farmhouse with its 2ft thick walls, the house environment is sufficiently stable for a large part of the summer and autumn that we turn off all heating and leave windows ajar all day, relying on natural ventilation. We feel that warmer weather should result in periods where we can do the same in our new energy-efficient house. However, we don't want to be forced into the situation where we have to dash around the house a few times a day opening and closing curtains, blinds and windows just to keep the house at a stable temperature: in general, the house should look after itself.

This imbalance (or a lot more on sunny days with the solar gain through windows) is a material issue and to put this in perspective consider:

  • The mass of the liveable airspace in the house is some 500Kg with a heat capacity of just over 0.5 mJ/K or 0.14 kWh/K.
  • The specific heat of the slab is less than that of air (0.75 kJ/kg K), but the mass of over 7m³ of slab concrete is significantly more (16.4 tonne) giving a heat capacity of 3.42 kWh/K and the plaster board, etc. within the walls adds perhaps another 10% to this capacity.
  • There are various equations for the heat transfer between the slab and the air above it but a good ballpark is 10 W/m²/K – that is roughly 0.7 kW/K for the entire slab.

The slab and the other fabric which sit within the thermal envelope of the house has over 20 times the heat capacity of the air inside the house. So if we were to heat the input air from the MVHR to 10°C above room temperature, this will transfer about 0.7 kWh heat into airspace of the house in one hour at 0.5 ACH – the same as running the slab at 1°C above room temperature. In extremes we can easily lift the slab temperature say 5°c to increase the heating slab effect five-fold, but at another 2x the air heating route will start to be problematic with noticeable effects on air quality and background noise if we increase the ACH to do so..

Our initial intention was to use an integrated MVHR + ASHP(Genvax), but there was always a concern that this would be inadequate to cope with extreme cold spells, so we planned to use a supplementary closed wood burner. However the problem with any stove is the minimum output (typically 2-3 kW) which is simply far too much for the living room to cope with. So we abandoned that idea: there's no point in installing a stove that you will never use in practice.

This analysis plus Jeremy's reasoned argument also convinced us that an active UFH slab was the way to go, for the following reasons:

  • It's a relatively low cost option.
  • It addresses a saleability risk: "I want to fit a gas boiler"
  • With a suitable ASHP it enables active cooling as well as active heating.
  • In circulation mode it is extremely effective at distributing the heat throughout the entire slab from any hot spots caused by direct sunshine through the windows.

With this reasoning the advantages of a combined ASHP+MVHR just seemed to dissolve, and we've now decided to abandon the Genvax in favour of a standard passive MVHR system. We have still to chose the ASHP, but we are looking at a low power (say ~5 kW) monoblock inverter-based design alone the lines of Jeremy's active slab approach, but more on this in later posts as I finalise details. We also need to think about the vertical temperature gradients in the house (our hallway / landing rises through all three floors into the loft space). Hence this is only part I of the thermal design. :)

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The original eBuild comments:


warby  21 Sep 2014 109:17 PM
Excellent Blog.  Have you considered using an inexpensive cooling/refrigeration unit, instead of using the ASHP in cooling mode, to cool the slab. The ASHP would be cheaper and could be replaced directly by a gas boiler if required in the future. The ASHP would only operate in heating mode and I would have thought made the overall heating and DHW design easier.
Looking forward to your next instalment.   Martin

TerryE  21 Sep 2014 11:24 PM
I am sufficiently confident in the figures that the only scenario where a gas boiler would come into pay is if we ever decide to sell the property and the potential new owners don't believe the data. They will have the option of fitting a gas boiler. In fact one of the decisions that we have taken is to dispense with gas altogether. Jan prefers an electric oven and modern induction hobs are as good as gas.

We want a monoblock inverter ASHP to avoid the need for an expensive certified installer and for acoustic reasons. Prices start at around £2K and most include cooling modes anyway. As we have seen in Jeremy's discussion, the main issue is the control system and getting one off-the-shelf is problematic, so I am currently planning to implement this myself, but that's for a later post. :)


TerryE  21 Sep 2014 11:28 PM
One other thing. I mentioned mining various topics in the site for background info. Here is a list of some of the ones that I found useful. I've added the originating poster, post date with the number of replies and views in parenthesis.


Hope this is useful.

joiner 22 Sep 2014 05:17 PM
I bet you have a hell of a job getting to sleep, and wake up at some ungodly hour to go looking for pen, paper and a calculator!

jsharris  22 Sep 2014 06:39 PM
I wish I'd been in your position when I was trying to get to grips with this!

Just like you, I found getting good information very difficult, and, as you know, I seriously under-estimated the solar gain and my pessimistic view that the house would need a fair bit of heat was just wrong (although the numbers were telling me this, I frankly didn't wholly believe them!).

Today was a good example for our house. I'm currently laying bonded bamboo flooring to the first floor bedrooms and landing. Today wasn't that warm (around 19 deg C max) but around 14:00 the Genvex went into active cooling mode, as the ground floor air temperature went up to 21.7 deg C. The slab temperature was around 19.1 - 19.2 deg C, and within a couple of hours the temperature was back down below 21.5 deg C, and then the Genvex switched back to normal ventilation.

I think that a cheaper air-to-air heat pump (such as the ~£500 split units that are readily available) would meet the quick air cooling requirement more effectively than the Genvex, and would also provide a quick air heating system if needed, but this does depend on the house layout (we have the advantage of a tall, central, hall).

If doing things again I'd probably fit a standard MVHR, keep the current ASHP and UFH heating/cooling system but fit a reversible air-to-air unit high in the hall space.

TerryE  22 Sep 2014 11:17 PM
Thanks. Just standing on the shoulders of giants. Need to think about this quick response issue though.

PS edit. Jeremy, Jan say this comment reads a little unctuously, but the fact is that the only reason that my and your starting positions are different is because your learning curve has been pretty well documented on this site and the GBF, as well as thought provoking input from Nick, David, Neil and so many others, so one of my main reasons for writing this up is to help others going down the same design path.

And not all of us have been on TV (it's amazing what Google can throw up): Jeremy Harris and the Beach Bums!

joiner  25 Sep 2014 03:25 PM
LOL. Tried to find that clip ages ago.  "Come on, Jezza!"

jsharris  26 Sep 2014 07:26 AM
I'm just grateful that there are only a few, relatively recent, clips of bits of TV with me in that are online, really just some BBC stuff and some of the Scrapheap stuff.

I know someone with a boxed set of the Onedin Line, who delights in telling me that I appear in every episode. I was a deckhand helping to sail the Charlotte Rose, the pretend square rigger used in the series, that was really a converted Baltic Trader (and which sailed like a pig with the square rig). Thankfully I was out of shot on pretty much all of the Poldark filming, as I was just bent down keeping horses still for any shots where Robin Ellis and Angharad Rees were chatting together on horseback, and the pilot science programme I did for a TV production company (for Channel four, I think) never made it to the screen.

TerryE 28 Sep 2014 07:57 PM

A footnote to this post: I've just tweaked the power figures after cross checking with Jeremy Harris's figures -- though it's not enough to change any of the other content. Out of interest the figures for Slab, Walls, Roof, Windows and Air change are:

    My house: 9%, 32%, 15%, 28%, 16%
    Jeremy's 13%, 26%, 18%, 31%, 12%

In otherwords, they are broadly similar because we are both using similar insulation technologies. The main reason for these slight shifhts is that Jeremy's house is 1½ stories and mine is 2½, so the ratios of roof:walls:volume are different since my house is more cube-like and I have a smaller percentage of window space.

NSS  05 Oct 2014 04:45 PM
Just made some swift calculations based upon our own design (for a 160m² 1.5-storey chalet) using a combination of known values for walls/roof/windows, a guestimate for the slab (of 0.2) and an assumed similar value to your own calculation for the air change figure, and it gives me the following...

  •     The internal footprint of the slab (117m²)
  •     The internal surface area of the external walls, less windows (155m²)
  •     The internal surface area of the roof, less roof windows (164m²)
  •     The area of windows (33m²)
  •     The total volume of the internal living spaces (355 m³).
  •     Slab: 304 W (21%) – 117 m² x 0.200 W/m²/K x 13°
  •     Walls: 290 W (20%) – 155 m² x 0.110 W/m²/K x 17°
  •     Roof: 306 W (21%) – 164 m² x 0.110 W/m²/K x 17°
  •     Windows: 387 W (27%) – 33 m² x 0.690 W/m²/K x 17°
  •     Air change: 151 W (11%) – 355 m³ x 0.025 W/m³/K x 17°

Total = 1.44 kW
Appreciate it is only part of the calculations needed, but does that seem a reasonable starting point?

TerryE  05 Oct 2014 05:10 PM
You can also download Jeremy's spreadsheet (you need to do a SaveAs and explicitly use an xls extension), but this sounds the correct ballpark for a near Passive design. The one important caveat is this issue of detailing and (i) linear heat-losses around windows, meets of panels, slabs, etc., and (ii) air losses at the same if the frame designer or constructor doesn't know their stuff, which is why supplier selection is important. Jeremy's spreadsheet also shows how to factor in seasonal average temperatures to get overall per month figures. Have a look through the topics that I linked to. One of these is probably a better place for discussion.

NSS  06 Oct 2014 09:52 AM
Have now downloaded Jeremy's spreadsheet. Suspect it will take me a while to get my head around it!

recoveringacademic 17 Oct 2014 06:26 AM
Thanks folks for a really interesting exchange of ideas. Remind me to get some sleep sometime soon.....

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What leaps out at me from this post is its clarity, which suggests you have a clear and thorough understanding of the subject. I think it would make a good primer for anyone wanting to learn about high performance houses, applied to a specific case.


Thanks Terry.

Edited by gravelld
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Just a brief codicil written just over a year after we moved into the house - gosh, how accurate this modelling turned out to be as built.   Our house is a 300mm twinwall TF on passive slab with a local natural stone skin, and it essentially performs as per this simple design calc.

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Reading your blog now as we get into the early stages of planning our build - this entry in particular has been most helpful. A big thank you!

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