I am not an expert for your setup, but I will offer some comments.

I have never heard the Trane warranty will be voided if you don't use their thermostat. The Honeywell Vision Pro thermostats are used frequently with Trane equipment. Call Trane if you want to get an answer on this.

I assume you have a thermostat for the boiler? Then my suggestions is to lock out the two heat pumps and the boiler at a set temperature. For example if you determine you want the set point to be 35 degrees, the two heat pumps would be locked below this temparature, and the boiler would be locked out above this temperature. You would need thermostats which have an outside temperature senser. I am not sure if the Trane TCONT803 has this feature. Others on this forum can comment on this.

Right now, my downstairs thermostat (Honeywell 7-day programmable) controls the down stairs a/c and the boiler. Sometime in the Fall, I switch over to heat and just use it for the boiler. Works fine.

I want to do something along the lines mike_home suggests -- depending on the outside temperature, the system chooses he heat pump or the boiler. When in boiler mode, I want the HP/air handler "off" -- presumably the newer Honeywell thermostats can do that?

The Honeywell Visionpro IQA thermostat with an outside temperature sensor should be able to accomplish want you want to do. It would lock out the heat pump and enable the boiler at a specific outdoor temperature. You should also lock out the upstairs heat pump at the same temperature.

It sounds like this contractor only wants to work with Trane thermostats. Trane may have an equivalent theromstat, but I am not aware of it. I also don't understand the comment that the air handler needs to run at 50% if you switch it manually. Do you have confidence in this contractor?

The contractor is highly rated (on Angies list). They were very professional; did a detailed Manual J; looked through my electric/fuel bills to see what sort of system fits, etc. I assumed Trane was imposing such constraints.

Trane has a t-stat the Comfort Link II which I believe can do what you suggest --but is pricey and charges $8.99/ month for Wifi enabling it. Something like the Honeywell Prestige series does everything it does so perhaps that is the route to take...

RaminVA,

You do not have to use a Trane t-stat. The use of any other t-stat in no way voids any warranty from Trane.

The 50% fan thing he is talking about however is correct. Trane xl16i requires a variable speed air handler that will only run at 50% with a call for continuous fan. There are easy ways to work around it though if you know what you are doing.

Thanks, harlemhvacguy. The systems are going to be installed next week. I might as well install a t-stat that integrates well. Any recommendations on the t-stat? Or is the workaround something I can do (via the operating system) after installation?

It can be done in the setup of the thermostat. They will have to set it up as a dual fuel with back up heat as gas with furnace( boiler in your case) controlling the fan. in this case no fan. The tcont803 and tcont402(non programmable ) will both do this as well as many other honeywell t-stats.