If it runs off a relay, it's probably a 2-wire 24V setup. I had in-floor heating with a thermostat that looked like that and that's what it was. If that's what you have, you'll need a 24V thermostat that's battery powered (since 2-wire setups don't have constant power to the thermostat). I think I used this one to replace it and it worked fine:
https://www.amazon.ca/Honeywell-RTH2...RRAS01W8WH1GCN