Could underfloor heating affect the grout on tiles above it? Specifically could it cause the grout to break up and fail?
Frank

was the original grout a flexible grout ? Everything expands and contracts with heat - even just a little bit might lead to problems so I would expect to see a flexible grout used where there is underfloor heating. Also was the underfloor heating on when the grout was setting ? if so then this may have accerlerated the setting time and weakened the grout

I think it was a fliexible grout, but can't be sure as i didn't lay the floor myself. I don't think the heating was on when the floor was being tiled. The grout doesn't come up in large lumps but is slowly wearing away. Would you suggest trying to seel it with something or scraping out the remaining grout and regrouting?
Thanks Frank