Note: some thermostats are 120 volts be sure yours is 24 volts before you mess with it.
This is a simple Honeywell 24 volt heat only thermostat. the simplest way to eliminate the thermostat as the cause of the problem is to remove it from the wall and tie the 2 wires together.
This is only good if you only have 2 wires. If you have three or more wires tying them together can burn out your transformer, so don't do it. go to the test the end switch page instead.
Another way to see if your thermostat is working is to check if the zone valve it controls is open.
Most zone valves have some type of manual lever so that you can by pass the thermostat and open the zone valve. If the zone valve is opened by the thermostat the lever will move easy with little or no resistance. If the zone valve or zone valves are open, your thermostat or thermostats are working.
If it is hard to move the lever your zone valves are not open, but this does NOT mean the thermostat is bad.
Other problems may cause the same thing.
If your zone valves are open go back.
If your zone
valve or zone valves are not opened by the thermostat please go to the
transformer page to see instructions on
how to test the transformer.
Some programmable thermostats require a AA battery or other type battery to function if the battery dies you have no heat.
Try replacing the battery with a new one.
If this solves your problem and your boiler starts working, great! but if not please use my 24/7 help line.
Other programmable thermostats have a third wire that brings 24 volts as a power source for the thermostat to work, so that a battery is not needed to power the thermostat, it may have a backup battery to save the programming in case of a power outage, but the battery does not run the thermostat as the other type does.
This is a much better thermostat but you must not cross the wires to test.
This will fry the transformer and could fry the thermostat also.
I know I have not completely covered the thermostat at this point, but we need to eliminate other things before we can say for sure it is the thermostat. 90 percent of the time the thermostat is not the cause of the problem and it always is the first thing the home owner or the inexperienced plumber replaces.