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Well, that sucks.

Sometimes you can cut a longer terminal block down to the five terminals you need. So if you have something longer? You "might" be able use that.

Or you can combine two smaller ones? I wish I could be more definite in my response. It is a matter of trial and error for sure and worth a try if you cant get exactly what you need.

It looks like a heat pump. Unfortunately any thermostat set up for heat pump, the auxiliary can't become the primary.

One option would be to program the thermostat for 2 Stage AC, 1 Stage Heat. If the reversing valve is energized in cooling, jump R and O, or Y and O at the condenser. Then of course the radiant is the only heat source, and that's probably not why they purchased a heat pump.

Seperate thermostats might be the best way. There's obviously no floor sensor, so if the radiant thermostat is at 70°, the heat pump thermostat could be set to 69° or lower.

Looking at the manual, it appears that this thermostat cannot operate the two systems independently. The "auxiliary" system will engage when the setpoint is 3 degrees above the current room air temperature. At that point, the FAU would engage. I do not see any possibility of operating the two systems independently.

Of course, if they don't push the recovery too far, the FAU would not engage.

I never did that work but I used it one time.

I remember my first commercial boiler job with a combination Gas/Oil burner.  Might have been a PowerFlame.  I did the commissioning myself and could not get it to run correctly on low oil.  So it never got to high oil.  I could coax it to high oil flame and get that set up properly but the low oil would not light.  I could get the low oil to light but the High oil was smokey.   Spent 2 days trying to get it right.  Finally I called Weil McLain tech support and they recommended that I purchase their start up service that came with one year of labor warranty.  

I bit the bullet and paid the fee.  When the tech from WM came, he was actually another contractor like me and I told him my problem. He did exactly what I already did and found the same problem as I did.  But he had experience that I did not have, and he went off book.  He replaced the nozzles with different specifications and got the firing rate on low and on high to be compatible with each other.  That nozzle change I was not prepared to do.  Live and learn. The book isn't always right. Once I had that knowledge I never needed to purchase the start up service after that.