Repairing Tile and Grout with Radiant Heat
How do you repair cracked grout when there is a radiant floor underneath? If water seeps through, will it damage the radiant floor?
You should do your best to repair the cracked grout. I don't know the extent of the cracked grout, but you can do a few different things.
Your could buy the same color grout at Home Depot or a tile store and follow the directions on how to mix it. Then if the crack is big you can just patch it with the mixed grout.
If the crack is very small, then you might have to make the crack bigger by removing the grout with a grout saw, which you can by at Home Depot when you buy the grout. It's cheap and small, used by hand.
If the crack is along the edge, where the horizontal meets the vertical, you could fill the crack with caulking of the same color. This will be more flexible and most likely won't crack again, like it would if you just replaced with grout.
These are a few things you could do and without seeing it, it's hard to say what is best. The radiant heating shouldn't be bothered by a little water coming in, as it heats up the water will evaporate. But having said that, you should repair the grout to minimize any water seeping in.
I hope this helps.