I have forced air gas system. Honeywell thermometer T87N is on auto and heat temp is set to 65 but when home twemperature goes below 65 then also heat system doesn't go on. Also check honey well acqustat switch is current 140 degree. I try with dismiss tochange switch or relay to 120 130 140. But heat didn't start.I think my well mcclain cg3 boiler pressure gauge is not on. Is there any trial error I should do to figure. These is the most I figured out. Excepting some help please send me a reply.

If the thermostat is calling for heat and all the circuits are good to go you should have heat. You need to use a meter and find if all the circuits are working correctly.

First you say you have a forced air gas system then you say you have a boiler. Which do you have ?

I have a wellmclain cg3s boiler connect to ductwork. Its odd

Sounds like you have a boiler but also fan coils to blow the heat out like a regular system. Not to odd.

Does the boiler run at all. In other words do you have hot water?

Boiler gauge? It should register or be replaced. It is a very important tool to be able to read. Without this gauge the boiler could be out of water and you may not know it.

We need to find this out before going further. (water problem?)

How much experience do you have with this type of system?

I have an smith boiler it is about 5 years old. I will not start. I change the thermocouple, changed the power supply and relay, the gas is on and lite but it will not start. The pump, pumps, the low water cut off is on and working but no starting. Could it be the gas value?