I'm looking for professional opinions as I currently have a gas boiler / hydronic radiator heating in a 100+ year old home. Currently, I have the old round dial mercury thermostats. It's a 2 wire (Rh, W1) system. I have a Burnham series 2 gas boiler 15+ years since it was installed (Recently moved into the house and there was no paperwork with it, but the last service noted on the side of the boiler was 15 years ago).

My question is that I have been told by over a dozen different heating and cooling technicians in the area that a programmable thermostat should not be installed on a gas boiler / hydronic radiator heating system... End of story. However, every company I talk to that makes thermostats (Nest / Honeywell / EcoBee) have all told me that is either a lie or an old wives tale and their products will work fine. So which is it?

Little back story. 4 months ago I purchased 2 Nest thermostats as I have 2 zones (1 for each floor) and it was never really working properly because I kept getting power loss errors. I figured it is because I do not have the infamous C wire installed, but Nest kept saying that does not matter and their system will run fine on 2 wires only. Nest finally agreed to pay for a tech to come and look at the system, and the tech flat out said the Nest should not be installed. He explained that (1) due to the modulating needed for the boiler, the nest was preventing the boiler from running in a low mode, and instead always commanding the boiler to operate on high which was resulting in overheating the water, and causing the limiter to kick the system off. So the room temperature could never be achieved because the water temp was always stopping the boiler from radiating enough heat to bring up the room. It was also causing constant over cycling where the nest was just constantly trying to run to achieve that room temp. Nest of course said that was not the case, but eventually caved and allowed a refund.... So I'm back with the old Mercury thermostats. Afterwards I decided to call other technicians in my area and they all agreed without even looking at it that any programmable thermostat will do exactly that, and I can only use a non programmable thermostat.

Does everyone agree, or is that something that may have been true at one point, but newer thermostats that have come out have improved and found a way to work? I realize I will have to install a 3rd wire for C and I'm fine doing that if I can actually use a programmable thermostat.