Call for heat: circulator running, boiler not firing (pilot lit)

HomeownerDustin

I have a 40+ year old HydroTherm.
Energy efficiency folks were doing vent checks, which required the boiler to fire.

I turned both (2) thermostats up, I heard the clicks to call for heat on the thermostats. There are clicks at the boiler. I can feel the circulator pumps whirring but the boiler doesn't fire up beyond the pilot light (which is always lit). The pipes get mildly warm but nowhere close to the very high temps I'd normally feel.

Important info: A couple weeks ago I went to check if the thermostat for one of the zones was still active (in an unused part of the basement), and I had it call for heat while the zone valve was shut off. The boiler did fire up then, but it's not firing up now.

I'd like to avoid an expensive fix since we're intending to literally get rid of the boiler ASAP. All I need to do is to get the boiler to fire so that the energy guy can measure vent flow and we can proceed with energy efficiency work. Is this something I can try to trigger on my own, or does it sound like I need to call a plumber?

109A_5

Hello @HomeownerDustin,
Sounds like the solder joints have failed for the burner control at the Aquastat relay. Or maybe the Vent Damper end switch issue. Some other safety limit has tripped. A wire fell off somewhere. Gas valve failure. All just guesses without observation and making measurements.

EdTheHeaterMan

The manual gas valve may be closed, there is a manual gas valve on the gas control valve where the wires from the thermostat connect to the gas valve.
Can you take a close up picture of your gas valve. some of those old HydroTherms have a millivolt gas valve. if you have one of them there may be an easy temporary way to get the flame on for a test.

Post a picture of the gas valve so we can see where the wires connect.

HomeownerDustin

Yes I do have that component. I see that I can turn it to pilot and press it in. 

Also when I just called for heat again at the thermostat it smelled a bit like natural gas, if that suggests anything. 

HomeownerDustin

Also, I just hit the test button in the relay and nothing fired up when the light turned green. 

Gas valve is set to”On”

HVACNUT

If you raised the heat and then smelled gas, it could mean the valve is opening but for some reason(s) not igniting the burners.
If that's the case, then IMO, your job is done, and you should call a gas heat professional. 
You might have someone post, "clean this." Or, "Stick a paper clip in that hole." But your dealing with an older boiler that doesn't look to have the safeties (I don't see a rollout) a modern boiler has. I'm all for DIY when there's no chance of things going boom.
While making the appointment, ask if the tech will bring a fully functioning, fully charged combustion analyzer. 

Steamhead

That's a millivolt gas valve, which uses a powerpile rather than a thermocouple. The powerpile generates roughly 750 mv which operates the gas controls on the boiler. This miniscule voltage is not forgiving of bad connections- check these first.