We're looking at getting a thermostat to run the forced air heat/ac for our entire house, and to run our HRV as well. There are some thermostats that offer the dehumidification controls built in, which is what we want. We have a radiant zone on the first floor where this new thermostat would be installed - the installer has said we could use the Aux control of the thermostat to trigger this zone. Has anybody heard of doing that? Also, do systems that combine forced air heat & hydronic ever stage these together, so the radiant is stage 1 & forced air the second stage? Thanks

you definitely want to stage the radiant 1st.

2 stage heat, yes. typically I'd use a separate dehumidistat.

Is there a 'typical' logic that two stage heat pump thermostats use when deciding to go from stage one to stage two?

I don't know what you'd call typical. Some have a fixed differential (i.e. 2 degrees below setpoint) and some use PID logic (oh no I'm gonna fall short!).

The new Honeywell prestige 2.0 stages the radiant first, and the differential before forced air kicks in is set to 4 degrees (max). There may be some logic that seeks to satisfy heat with stage 1 before stage 2 kicks in, beyond the temp differential, but it's not user configurable.