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The Great "Thermal Mass" Myth


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#41 jsharris

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Posted 23 January 2015 - 06:21 PM

View Postwmacleod, on 23 January 2015 - 04:11 PM, said:

I don't want to take this further OT, so to bring it back on topic - Jeremy, can you tell us what your TMP figure is in your SAP calculation? I am interested to see whether SAP methodology tallies with what you are seeing.

SAP gives the TMP as 250, so in the "medium" category, but there isn't really enough data put into SAP to calculate the thermal response accurately (for example, SAP doesn't know the specific heat of all the internal structure, or the nature of the insulation in terms of decrement delay or specific heat).

#42 bassanclan

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Posted 23 January 2015 - 06:50 PM

Where would thermalite blocks come into the 'list' when they have a typical density of 660kg per cubic m?

#43 jsharris

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Posted 23 January 2015 - 07:01 PM

With a lack of data readily available, I'd take a stab at saying that the concrete content will dominate (as the specific heat of air is next to sod all) so very, very, roughly then they probably have a specific heat that's around 1/3rd of that of concrete, so maybe around 300J/kg.K. I could well be seriously in error in this though, as I've not been able to find any data from a quick web search.

#44 slidersx200

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Posted 23 January 2015 - 08:04 PM

By searching for autoclaved aerated concrete I managed to turn up this statement: "the thermal mass index, which is the product of thermal conductivity and volumetric heat capacity, is about 1/60 of concrete".

A second source in the same set of search results claims: Specific heat capacity 1.3 kJ/kgK (0.31 Btu/lbs°F , 15% better than EPS

Edited by slidersx200, 23 January 2015 - 08:05 PM.


#45 brickie

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Posted 24 January 2015 - 08:41 AM

Wikipedia says-
"Thermal Maas (b.2/5/64) was a central midfielder for Feyenoord 1982-87 before a persistent stray eyelash cut short his playing career.
He now works as a rep for a pencil company in Rotterdam. "
He does exist.

#46 SteamyTea

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Posted 24 January 2015 - 08:41 AM

:D

#47 sarahsouthwest

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Posted 24 January 2015 - 12:00 PM

The terms 'thermal mass' and 'thermal store' have frequently been used for our build in the sense of holding heat and slowly releasing it. I could understand this, and it seemed like a good idea.

Is it the misuse of the terms that is bothering people or is there a basic error in the assumption that block will outperform, say, wood, as a heat store?

View PostNeilW, on 23 January 2015 - 09:11 AM, said:

It's not the mass that matters really. It's the volume.

For example a kg of concrete occupies 4/10ths of the space of a kg of water.

So you have to have the heat density related to volume before you can order the materials. So you need to multiply the values in the OP together.

That gives the order as:

Water
Plaster
Granite
Concrete
Brick
Wood

Which is pretty much as you expect.

Surely Neil's list indicates that granite and concrete are "better" - more efficient? more effective?? - than brick and wood, so it makes sense to build with granite and concrete...?

(My little brain is struggling with this. Even from Cornwall I can hear everyone groaning that I'm a complete numpty and fair enough.)

#48 DamonHD

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Posted 24 January 2015 - 12:19 PM

You are not alone. It is not in the least intuitive for me either, and I second your "is wood really better than granite" (eg by volume or by £) question.

Rgds

Damon

#49 SteamyTea

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Posted 24 January 2015 - 12:28 PM

If you think of this as a drop of hot water hitting a sponge, say there is a 10°C temperature difference, as the drop disperses into the sponge it looses temperature. The leading edge of the dome will be the coldest, very close to the original sponge temperature.
So the diameter is the temperature gradient, and the volume is the energy stored.

Now imagine trying to get get that energy back out of the sponge a few minutes later (without squeezing it as then it becomes a heat pump). You are going to get virtually nothing out energy wise and it will be at a lower temperature because the energy will have been dispersed into the sponge. The drop that was tiny has expanded into to a greater volume but at lower density.

You also have to remember the small temperature difference that this is taking place at. Even if your room got to 40°C and the wall was at 10°C to start with, that is only a 30°C difference.
Compare that with a storage heater that has an electric element that is probably at 350 - 400°C, and it is going into just a few kg of bricks (they are probably about the same SHC of concrete), you can see that the energy, driven by the temperature difference is hundreds of times greater.

#50 declan52

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Posted 24 January 2015 - 12:36 PM

Don't think I have ever read a topic that contained so many different terms either correct or not that describe how effective a material holds an amount of heat.
My internal sponge has absorbed more than enough info and to be truthful am still none the wiser!!! Shouldn't have bunked school the day they where doing that bit in physics.

#51 sarahsouthwest

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Posted 24 January 2015 - 12:43 PM

I get the sponge analogy. I understand the storage heater comparison. But I'm lost when trying to extrapolate that into practice. (Damon, I'm glad it's not just me...)

I'm sort of getting, regardless of what anyone says re thermal mass/thermal store, it's all basically smoke and mirrors because the difference between say, block and wood, is not enough to make a serious impact on maintaining a comfortable temperature in your house.

Yes? No? It depends?

#52 DamonHD

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Posted 24 January 2015 - 12:52 PM

Well, we know that water is a very good store of heat when we want a thermal store, for example, and we can even pipe it to the places that need heat.

So my selfish question always is, compared to that baseline as an example, what materials are better or worse for energy or thermal storage by volume (because I'm space constrained), price, and weight (because my structure has load constraints).

Rgds

Damon

#53 SteamyTea

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Posted 24 January 2015 - 01:00 PM

The differences between wood and stone is in the number for each. You have to decide if you are using Heat Capacity (the volume) or Specific Heat Capacity (the mass).
So say you have a wall to build and it is 4 m by 3 m by 0.2m, that will have a volume of 2.4 m3, From that volume you can work out the mass (density times volume).
Alternatively, say you want a stone wall that has a mass of 500 kg with a surface area of 12m2. Then you have to work out the thickness (mass / (area times density).
It is all in the units, you just rearrange them (easy to make mistakes, I constantly do) to get what your looking for.

Here is a quick chart to show how how the water droplet works. May or may not help. Sadly a brain is not like a sponge, it discards a lot of knowledge, bit like opening a windows on a hot day, it gets rid of the excess heat but you cannot get it back.

Attached Files


Edited by SteamyTea, 24 January 2015 - 01:01 PM.


#54 SteamyTea

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Posted 24 January 2015 - 01:05 PM

Damon
Liquid water, will have to be over 4°C, which compromises the performance slightly.

The main reason we use water is that it is cheap, plentiful, pumpable, virtually harmless and has a fantastic SHC and VHC.
It also works in the temperature range that we want.

Edited by SteamyTea, 24 January 2015 - 01:05 PM.


#55 Nickfromwales

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Posted 24 January 2015 - 01:30 PM

OW!! :wacko:
Small brain hurting so, so badly but I can't stop trying to understand this 'myth'.
My two penneth is that for a solid construction house you have to heat it long and low for the ambient temp to acclimatise. With a timber framed house it would be the opposite.
So it's clear :huh: to me that a timber frame house would respond to heat changes quicker and be more comfortable to live in.
Just getting my cyanide pill ready for the replies :o
Regards, Nick.

#56 jsharris

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Posted 24 January 2015 - 01:37 PM

View Postsarahsouthwest, on 24 January 2015 - 12:00 PM, said:

The terms 'thermal mass' and 'thermal store' have frequently been used for our build in the sense of holding heat and slowly releasing it. I could understand this, and it seemed like a good idea.

Is it the misuse of the terms that is bothering people or is there a basic error in the assumption that block will outperform, say, wood, as a heat store?



Surely Neil's list indicates that granite and concrete are "better" - more efficient? more effective?? - than brick and wood, so it makes sense to build with granite and concrete...?

(My little brain is struggling with this. Even from Cornwall I can hear everyone groaning that I'm a complete numpty and fair enough.)

The problem is that the concept of heat capacity and the close linkage between heat capacity and a building's heating/cooling demand over time, so any given heating input (or heat loss rate) is really unrelated to the mass of the materials it's built from.

The amount of sensible heat energy that the effective interior part of the building can store is pretty much independent of mass, for a lot of reasons. Probably one of the hardest to understand is the effect of the thermal conductivity of the materials, as materials with a high specific heat (i.e those that can store a lot of heat for a given volume or mass) may not be able to allow heat stored from deep within the material to have any useful effect at controlling the house temperature response, over the small temperature range that we're looking for to build a comfortable house.

Here's a really simplistic example using our house (only because I know a fair bit about it). All the walls, internal and external, and all the ceilings (both floors) are the same structure, 12.5mm plasterboard with a ~3mm plaster skim, with a 50mm air void behind (with all the insulation the other side of that void) or some rockwool behind them in the case of the internal stud walls. If I was to just ignore the concrete ground floor and the timber first floor, and only look at the heat capacity of the plasterboard and skim in the walls and ceiling areas, and if I assume they are all 15mm thick gypsum (a pretty reasonable assumption for 12.5mm gypsum plasterboard, with two layers of paper either side, plus a 3mm gypsum plaster skim) then I get the following figures:

total area of internal and external walls (allowing for doorways and windows) = 385m²

Total volume of gypsum in walls and ceilings = 385 * 0.015 = 5.775m³

The volumetric heat capacity of gypsum is about 3 MJ per m³ per deg C, so the amount of sensible heat (the heat energy) stored in just the plasterboard walls and ceiling of our house will be around 17 MJ or so for a 1 deg C change in temperature. MJ aren't very friendly units when we're looking at heating bills etc, so converting this amount of sensible heat into kWh gives a figure of about 4.7 kWh per deg C change in temperature.

In winter, our average heating requirement is only around 7 or 8 kWh per 24 hours, so the above very simplistic calculation, ignoring the heat capacity of the concrete slab ground floor, the oak doors and stairs, the timber first floor and the internal air volume, shows that just the volume of the plasterboard and skim will store enough heat to cause the whole house to only drop by about 1 deg C in around 12 to 14 hours in winter, with no heating to the house at all.

Next, let's look at the concrete slab. That has a volume of about 7.5m³ and the volumetric heat capacity of concrete is around 2.1 MJ per m³ per deg C. Because of the relatively poor thermal conductivity of concrete, it isn't reasonable to assume that the whole slab can work to deliver heat to the house, so let's assume only the top 2m³ is useful in terms of stabilising the internal temperature. The effective heat capacity of the ground floor slab is then around 4.2 MJ for a 1 deg C change in temperature. Again converting this to kWh gives a figure of 1.16 kWh, so, added to the plasterboard walls we now have a total of around 5.8 kWh or so of heat stored in the walls and floor (ignoring heating and all the timber in the house, and ignoring the air in the house and the water in the UFH etc) and this alone will allow the house internal temperature to drop by 1 deg C over a period of around 20 hours or so, with no heating.

There is actually a heck of a lot more sensible heat (heat energy) stored in the house, though. The sensible heat stored deeper in the slab, that in the timber first floor and that in all the timber internal doors, frames, furniture etc will all make a significant improvement. What I've tried to do above is show that even a very thin, light skin on the inside of a low energy house can store enough heat to stabilise the temperature such that even if the heating goes off for a few hours the house will barely cool down at all, which is one of the features people usually find makes a house comfortable to live in.

#57 ProDave

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Posted 24 January 2015 - 03:33 PM

My BIL lives in a 300 year old Welsh Farmhouse with 3 foot thick solid stone walls.

He tells me the urban myth that once you get them warmed up, they keep their heat for a long time is a load of nonsense, and in fact it cools down as quick as any other house he has known.

Edited by joiner, 24 January 2015 - 03:45 PM.


#58 joiner

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Posted 24 January 2015 - 03:55 PM

This place was built in 1848. Solid brick. The south gable end faces due south. Go out there on the coldest day after the sun has been on it and you'll feel the heat coming off it long after sundown. That's what most people understand when you use the term "thermal mass".

As for solid-walled stone buildings? Our friends live in a SW-facing solid granite cottage in North Wales, just outside of Harlech. Even in mid-summer you need doors and windows open to get the interior even 'warm' and it costs an arm and a leg to heat it in the winter. It's why we don't like visiting with them! That's an example of "thermal mass" not working. ;)

#59 jsharris

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Posted 24 January 2015 - 03:55 PM

View PostProDave, on 24 January 2015 - 03:33 PM, said:

My BIL lives in a 300 year old Welsh Farmhouse with 3 foot thick solid stone walls.

He tells me the urban myth that once you get them warmed up, they keep their heat for a long time is a load of nonsense, and in fact it cools down as quick as any other house he has know.

Probably a good illustration of the thermal conductivity problem. If you take a 3ft thick stone wall, then the inside will be below room temperature slightly and the outside will be very slightly above the outside air temperature. There's therefore a temperature gradient across the wall, and this means that the layer of stone just a couple of mm below the surface will be colder than the room, so cannot transfer heat into it until the room temperature drops below that temperature.

In our case, all of the insulation is behind the plasterboard, so pretty much all of the sensible heat stored in the plasterboard is available to help stabilise any drop in room temperature.

As I've illustrated above, just the plasterboard on all the walls will provide enough heat input to allow the house temperature to drop by a degree in around 8 hours or thereabouts in winter. There really is no need for a lot of mass inside a well-insulated, thermally efficient building, just enough heat capacity to smooth out the peaks and troughs caused by external doors opening and closing, periods of high heat input (baths, showers, cooking) and periods between the heating or cooling system needing to operate.

I suspect there's little merit in aiming for more than about 1 deg C in 24 hours cooling with no heat input at all, in practice, as that should be more than enough to allow a small heating or cooling system to control the house temperature pretty well.

Edited by jsharris, 24 January 2015 - 03:55 PM.


#60 SteamyTea

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Posted 24 January 2015 - 04:11 PM

If you look at my hypothetical chart you can see how the energy drops of really fast and the temperature follows. Initially the energy drops at 3 times the rate of the temperature, but after 3 'time units' they match near enough and at about a 1/10th of the values.