Looking for a Glass Hearth?
We get asked about Glass hearths regularly as a way of seeing the floor underneath. The only issue is glass conducts heat!![](https://mrstoves.com.au/images/blog/fourierslaw.png)
Being the technical specialists that we are instead of just making an assumption we did the maths. See below why it is not a good idea for a glass hearth:
If we use Fourier's law and assuming glass can actually withstand the temperature of a burning log (say 700 degrees C) as most tempered glass have a shock value of 250 degrees C you would be looking at approximately 25 minutes for the sub floor to heat up 90 degrees above ambient:
q = (k / s) A dT
where
q = heat transfer (Watts - This is what we want to calculate)
k = material thermal conductivity rating (it is generally 1.05 for glass)
s = material thickness (9mm = 0.009m)
A = heat transfer area (30 x 30cm = 0.09m2)
dT = t1 - t2
= temperature gradient - difference - over the material (700 - 90 = 610)
Plugging the values into Fourier's Law we get:
= 1.05 / 0.009 * 0.09 * 610
= 6405 Watts
Assuming a 0.5kg log that has fallen out with a calorific energy value of 18MJ/kg (9MJ for 0.5kg) we can convert Watts (J/s) into how many seconds it takes to transfer through which looks like:
= 9000000 (Joules) / 6405 (Joules / Second) / 60 (seconds to minutes)
= 23 minutes
So it takes 23 minutes for a burning log to get the floor underneath to above 90 degrees above ambient when on a 9mm piece of glass assuming it doesn't crack under the temperature first.
Conversely a 10mm FC sheet works out at 1 hour 20 minutes for heat to transfer through.
If we use Fourier's law and assuming glass can actually withstand the temperature of a burning log (say 700 degrees C) as most tempered glass have a shock value of 250 degrees C you would be looking at approximately 25 minutes for the sub floor to heat up 90 degrees above ambient:
q = (k / s) A dT
where
q = heat transfer (Watts - This is what we want to calculate)
k = material thermal conductivity rating (it is generally 1.05 for glass)
s = material thickness (9mm = 0.009m)
A = heat transfer area (30 x 30cm = 0.09m2)
dT = t1 - t2
= temperature gradient - difference - over the material (700 - 90 = 610)
Plugging the values into Fourier's Law we get:
= 1.05 / 0.009 * 0.09 * 610
= 6405 Watts
Assuming a 0.5kg log that has fallen out with a calorific energy value of 18MJ/kg (9MJ for 0.5kg) we can convert Watts (J/s) into how many seconds it takes to transfer through which looks like:
= 9000000 (Joules) / 6405 (Joules / Second) / 60 (seconds to minutes)
= 23 minutes
So it takes 23 minutes for a burning log to get the floor underneath to above 90 degrees above ambient when on a 9mm piece of glass assuming it doesn't crack under the temperature first.
Conversely a 10mm FC sheet works out at 1 hour 20 minutes for heat to transfer through.