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Thermostat Placement in zone with panel rads w/ thermo valves
...since it is a 5 Zone System, I now see why you have a Zone Valve on this loop.
Yes, you could leave this zone open all the time when running with an Outdoor Reset Control (or even without it for that matter), as the the TRVs will control the indivudial Zones on that loop.
Outdoor Reset is the most cost effective way to run your system and also results in inceased system effieciency and comfort level by minimizing temperature swings in your other zones operated with standard thermostats and zone valves. Without Outdoor Reset, you would be supplying the water temperatrue needed at the coldest day of the year even when it is milder outside and when the heatloss of the building is reduced because of the milder outdoor temperatures. With the same supply water temperature as needed at the coldest day of the year (Outdoor Design at 100% Heatloss of the Building), then the tehrmosatts would naturally call for a shorter period of time and then be off for a longer period of time, which does create the temperature swings and possible expansion noises (especially with Fun Tube Convector Baseboard) every time the building is not at aits maximum heat loss.
Based on your application, it sounds like the 260 tekmar Boiler Control would be best suited. -see attached D260 Manual or visit www.tekmarcontrols.com for more info, or call 250-545-7749.