I seem to recall the impetus being a couple people having died due to carbon monoxide poisoning due to tanks having been improperly installed and run all winter to maintain the in-floor heating setpoint.
There are some concerns about bacterial growth in the in-floor piping system. At the very least you should have a pump installed to flush the pipes every single day. Depending on how your system was designed and installed it may run either above or below the typical 140 F setpoint of a domestic hot water system. If it runs above that it should have a mixing valve to temper the water going to the fixtures in the house. If it runs below 140 it's... bad. Legionella will thrive in <120 F water.
(The fact that the domestic, potable system should always be at 140 F and the heating system should be... whatever it needs to be... is another reason it's a bad idea to combine the two systems using the lone tank heater.)
Also, from what I recall, the maximum length of pipe on a combined heating/domestic water system should have always been less than 50 ft on the heating side, which means your system shouldn't do more than about a room (otherwise it should have a heat exchanger and separate fluid running in the floor). I can't remember if that's in CSA B214 (the standard for installing hydronic heating systems) or if it's a local variance.
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