? 2 different water sources to heat a baseboard ?

[000utback]

I have 3 adjoining rooms sharing a radiant floor system controlled by a Tekmar 521. The t'stat is in room 3, the largest room, NW exposure, lots of old glass windows. The t'stat is controlling to ambient T within floor limits, also have a floor T sensor. An antiscald valve limits the floor water T to 150F. Little Taco recirc pump supplies 4 floor radiant zones, each zone with his own thermostat and fed through a Taco zone valve. 4-zone Taco control board. That's a bit of info about the total house there, not just the 3 rooms.
The t'stat also controls a second stage loop for the same area, supplying fin tube baseboard directly from the boiler. A separate pump, controlled through an Argo 4 channel board. (each of my major 4 zones in the house has their own thermostat and pump). Diverter tees are used in a single pipe system.
So, room 2 is way too cold. If I turn up the t'stat in room 3, and reduce the mechanical thermostatic valves on room 3 baseboard, the boiler just runs a lot. I am looking at replacing the 7' basic fin tube with a high efficiency unit with 75% higher output. But that's only helpful when we're in stage 2 heating (meaning 1.5F below setpt in room 3).
Now we get to my question. What if I ran the fintube baseboard in room 2 on floor system water most of the time (since the floor system runs a lot), giving me a low operating temperature but almost continuous heat output; then linked control valves to the stage 2 command, so that it switched to boiler HOT water whenever stage 2 was activated? Would a pair of 3-way valves accomplish this, one in supply, one return? I have easy access under the floor for changing the PEX tubing.
Any assistance appreciated.

 

[Dana]

Pictures & diagrams are worth 10,000 words. I don't have a clear enough picture yet.

It sounds like room 2 and room 3 are on the same zone, but room 2 only gets heat during the thermostat's second stage (no radiant floor)?

And all of the second stage baseboards are running on a mono-flow tees, rather than a direct series-plumbed loop?

Even at 150F the fin-tube would be putting out ~350 BTU/hr per running foot. What is the design heat load of room #2 using Manual-J or IBR heat load calculations? It's possible (even likely) that room #2 could go from being under heated to overheated, even without swapping it out for higher-output baseboard if running stage-1 water temps continuously.

Getting to the right solution usually requires a room by room load calculation. Making room #2 it's own zone (with a separate zone valve) might work just fine at stage-1 water temps. But without knowing the load numbers it's hard to make that call.