Can a sealer be used in a fireplace, firepit, barbecue, or other area that gets extremely hot?
That is not a good idea. Do not use a sealer, or other chemical product, where it can burn and release fumes.
How hot is too hot?
Do not use a sealer on any surface that at its hottest, you cannot maintain hand contact for 15 seconds.
In floor heating systems:
They could not get hot enough to effect the sealer as you would not be able to walk on a surface that hot.