My boiler flue damper is not opening and the unit will not start up.

I do not believe it is the thermostat because I have tried three different thermostats and none correct the problem. I also tried bypassing the thermostat by connecting the red and white wires. The boiler still won't light.

I noticed that when the thermostat calls for heat, the damper as not opening, so I figured a bad damper motor was the problem.

I bought a new damper motor. When I plugged the new motor in, the damper closed. When I turn up the thermostat to call for heat the new damper will still not open, so the unit will not light.

What can I try next?

Not to sound like a wise guy but honestly your best bet is to stop throwing parts (money) at the problem & call a local pro who should be able in short order to diagnose & fix the cause of your problem.

Well your (and most syatems) run on 24 volts.. sounds like you might have a bad transformer and if it is working look if your getting less than 17 volts it is bad and needs to be replaced . With a multimeter set it to low voltage and touch red led to red wire and black lead to white ( should read 26 to 28 volts ( that's normal) don't worrie that its putting out more than 24.without the damper opening it will not read open and unit will not fire.. that is a safety its supposed to work that way.. but sounds like you tried everything else.. that's the only other thing in the way.. good luck