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Nick

Thermal properties, insulation and R values.

I can find lots of technical information on the web about how well concrete insulates, but I don't know how to use it. I do not understand R values, and such.

I have a simple question. If I have a hot fire, say 700'C on a concrete slab, 5 inches thick, will it get hot below the slab. How hot? Regular concrete, nothing special.
sean

How long will your fire be there?
Nick

Dunno. 2 hours?
OtleyLad

Here's a pretty good explanation of heat transfer.

Here's the relevant formula:

Rate of transfer (Watts) = kA(T1 - T2)/d

k is different for high and low density concrete (0.2 - 1.5)
A is the area (sq metres)
T1, T2 Inside and outside temp
d thickness (metres)

This will tell you how many watts come through the concrete and then you can use that to heat up whatever is on the other side (air?).

Simples Confused
Nick

Thanks.

So, how hot is 2,500W ?

Smile
OtleyLad

Thanks.

So, how hot is 2,500W ?

Smile


Enough to raise the temp of 2.5 litres of water by one degree C in one second. So it could be boiling in less than 2 minutes.

Wood has a much lower specific heat than water so in theory it could reach burning temp (232C) in a similar time (if it were pressed up close to the concrete).
Nick

Thank you. A clear and helpful answer. Smile
onemanband

Building regs for a fireplace hearth are min 125mm thick concrete with a 50mm air gap(to combustable material), or 240mm with no air gap.

So yeh 5 inches of concrete slab probably will get hot on the underside.
Nick

Well, watch this space with interest. I hope all will be well. Smile

Hope, of course, is not a strategy.
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