I don't know the answer but are the floors solid concrete or suspended?
Are you going for water based heating or electric?
AFAIK, most underfloor heating systems need/benefit from having some kind of insulation applied underneath them. Without sufficient insulation you may find that you have to run the heating at full whack to notice any gain. Why, for example, let your set up heat the foundations of your home?
I have no idea where you found your tiler and am therefore not willing to comment on him/her. I have however worked in a few homes recently where the tiler was found through one of those "recommendation/vetted sites".
Each of the tilers was 3 times quicker than me as a tiler (I am a decorator that occasionally tiles) but every single one cut corners. None of them bothered to use SBR or dedicated acrylic primers over the OSB flooring or on new plaster. I guess that they don't want to turn up do a couple hours of priming and then wait until the next day before they can start tiling.
Anecdotally, I would say that about 10% of tradespeople found through those kinds of sites that I have encountered are good quality, 50% provide a finish that looks ok from a distance but that is not up to spec and the rest are chancer muppets who take advantage of the fact that most customers don't know how to spot a quality job.
I would strongly recommend that you speak to the makers or suppliers of the underfloor heating rather than trusting a tiler who you may have found on the internet.
Sorry, I am not trying to rain on your parade but I am seeing more and more poor quality work from tradespeople found via the kind of websites that I mentioned earlier. Rubbish "builders" have always existed but now that punters are expected to leave online feedback, they feel obliged to leave 5 star reviews because "the guy was nice". By way of an example, my GF decided to use one of those sites to get a decorator to repaint our hallway (even though I am a decorator- she was unwilling to wait for me to have a free week). He charged her for 5 days labour. The day he finished, she paid him, left him a 5 star review and then asked me to sand back all of the newly painted woodwork and walls and then paint all of it again in the same colours.