There was a STANDATA information bulletin issued in 2009 by Alberta Safety Services that banned domestic water from being used in in-floor heating. It may have changed since then, I haven't double-checked, but if it you really, really, really wanted to use a domestic water heater for space heating you should push the water through a heat exchanger to keep the fluids separate.
Either way using a domestic heater is a cockamamie solution. A multi-floor condo building with in-floor heating would almost certainly use a dedicated central system with a proper boiler. It's substantially more efficient.
Not to mention he wouldn't have a
zone valve if the hydronic heating was run off the domestic hot water tank.
