So, I researched this subject like crazy on this forum and across the web and never found an answer that I was satisfied with, so last night I tried what I am about to share and it appears to work to my satisfaction.
1) 2 stage thermostat. A heat pump thermostat is not a 2 stage thermostat. What I have is an Ecobee EB-STAT-02, so it is the onlly one I have tried this with, but I am sure other manufacturers have programmable digital thermostats that will work as well. The Honeywell IAQ and Prestige are examples.
2) 3-way zone valve
3) 120VAC to 24VAC 40VA step down transformer. This can be a Hard-Wired type, or a Plug-In type.
(i will upload a drawing later)
1) Install 3-way zone valve between Input and output of Water/Air heat exchanger as follows.
A) Hot from boiler (or DHWHX out) to 3-way zone valve COMMON.
B) 3-way Zone Valve NC (Normally Closed) to Water/Air Heat Exchanger IN.
C) 3-way Zone Valve NO (Normally Open) to one side of plumbing Tee.
D) Water/Air Heat Exchanger OUT to second side of plumbing Tee.
E) Third side of plumbing Tee RETURN to Boiler.
2) Electrical Hookup
A) 24VAC terminal 1 to 3-way Zone Valve terminal TH
B) 24VAC terminal 2 to Thermostat RH. (If there is a jumper on thermostat between RH and RC leave it in place)
C) 3-way Zone Valve terminal TR to Thermostat terminal W1 (may be labeled W O/B). If this is an existing installation your furnace W terminal is probably connected here. Move it to Thermostat terminal W2.
D) If you haven't already done so in (C) above, connect Furnace terminal W to Thermostat terminal W2.
That should do it. The way this is going to work is:
Thermostat will call for heat, energizing the solenoid in the 3-way zone valve thus allowing hot water to flow through heat exchanger and turning on the furnace fan. If after a preset amount of time (my thermostat seems to be set for about 10 minutes) if the room temp has not risen sufficiently the furnace burner will start. My thermostat also keeps the zone valve open so the boiler is still helping to heat. I haven't tried this in practice yet, but I have to believe that with the water temp being 165 or so, the likelyhood of the furnace burner lighting is small.
By the way, if your thermostat is already controlling and wired for one of the new 2 stage furnaces you are just going to have to try this or ask an HVAC professional.