Old Weil-McLain boiler not firing FIXED!

Nov1070

UPDATE! Zone valve fixed it. I understand it was probably an intermittent end switch failure.

Good morning. I have an old Weil-McLain boiler that I'm having issues with. Had two service guys out to look at it and neither one seemed to know much about it as they didn't really do any testing. Third guy was a no show no call. All 3 trying to sell me a new boiler which we cannot afford right now. The sticker on the side is blank where the model and serial numbers are and I was told that means its very old. This is a hot water baseboard system with a large holding tank and single zone.
The pilot is always on but randomly the boiler does not fire and we wake up to a cold house. We accidentally realized that turning off the emergency off switch and flipping it back on would start the boiler immediately. I was told to try a new thermostat last year but the problem came back. Tried another thermostat to no avail. Touching the thermostat wires together does not fire the boiler. When it does fire with the switch it heats the water just fine and performs normally until it doesn't. It seems like a pretty simple system with a transformer for the thermostat and two more inside the boiler housing. I diagnose automotive electrical and computer systems for a living so with a little guidance and some service information maybe I can find the issue myself.


Thank you in advance.
Chris

EBEBRATT-Ed

My first guess would be a bad thermocouple. The thermocouple sticks into the pilot flame and it's purpose is to prove the pilot flame is burning before opening the main gas valve. You should have e2 tubes running to the pilot burner. 1 is pilot gas(usually an aluminum tube) the outer one is the thermocouple

They get week with age. You can probably find one at a big box store a good hardware store or an HVAC/plumbing supply house would have one

Jon_blaney

There are a lot of Youtube gas furnace/boiler trouble shooting videos. Some with the same problem you are having.

STEAM DOCTOR

EBEBRATT-Ed said:

My first guess would be a bad thermocouple. The thermocouple sticks into the pilot flame and it's purpose is to prove the pilot flame is burning before opening the main gas valve. You should have e2 tubes running to the pilot burner. 1 is pilot gas(usually an aluminum tube) the outer one is the thermocouple They get week with age. You can probably find one at a big box store a good hardware store or an HVAC/plumbing supply house would have one

Pilot is always on. Probably not thermocouple. Turning electrical switch on and off, won't affect pilot. Would venture to guess that its a relay problem. But only a guess. Testing while it's down, is the only true way to know. 

Nov1070

Forgot to mention the thermocoupler is new. I agree that one of the relays is not energizing but need to figure out which one. will check YouTube

kcopp

make sure the relays are plugged in all the way. They have a tendency after a while to loosen up and not contact.

HVACNUT

If you do automotive electronics then set your meter to AC and follow the schematic.
The thermostat wires connect to R and G on the control relay so on a heat demand there should be 24v across G and C.
Is there a vent damper? Doesn't look like it.
You could turn off the power and check all connections and splices.
The relay plugs in. You could remove the clip and pull out the relay and reseat it.
Make sure everything that should be grounded is grounded. 

Nov1070

HVACNUT said:

If you do automotive electronics then set your meter to AC and follow the schematic.
The thermostat wires connect to R and G on the control relay so on a heat demand there should be 24v across G and C.
Is there a vent damper? Doesn't look like it.
You could turn off the power and check all connections and splices.
The relay plugs in. You could remove the clip and pull out the relay and reseat it.
Make sure everything that should be grounded is grounded. 

No there is no damper but I was doing some testing last night with my Fluke. I had 24v at the transformer and 115 at the gas valve, 115 at the other transformer, 24v at the low voltage terminals of the other transformer. With the switch on the boiler was still not kicking on. The zone valve didn't feel right but the boiler fired when I slid the lever all the way over. I then checked voltage at the gas valve with the boiler running it was 24v. I switched out the zone valve and woke up this morning to a warm house. Its been awhile since that happened. Not sure if that was the problem or I inadvertently fixed a loose connection somewhere. I'm still trying to understand the system and how it functions but its working right now.

Shane_2

Are you sure about 115 at the gas valve? If I was in a customers home and found line voltage on a gas valve, I would be VERY concerned.

Assuming the gas valve is safe and functioning, I think your most likely suspect is the zone valves. I didn't see them in pictures or mentioned before. What make and model? They have motors and end switches which don't last forever

Nov1070

Shane_2 said:

Are you sure about 115 at the gas valve? If I was in a customers home and found line voltage on a gas valve, I would be VERY concerned.

Assuming the gas valve is safe and functioning, I think your most likely suspect is the zone valves. I didn't see them in pictures or mentioned before. What make and model? They have motors and end switches which don't last forever

I will check again tonight but I'm fairly sure I saw 115 across the two terminals on the valve. It was late and I had a long day so I may be mistaken. It was def 24v with the boiler running. I'm still getting to know this setup and as I mentioned I did swap out the zone valve for the living space and the system was still working this morning. I had this problem with the hot water a couple years ago and the zone valve was the issue then also. Do you think that would explain why flipping the emergency switch got the boiler to fire?

HVACNUT

Hopefully you were in auto range and reading millivolts. 

Nov1070

HVACNUT said:

Hopefully you were in auto range and reading millivolts. 

Now that you mention it the meter was on auto and it very well may have been reading mV. Its dark in the utility and the LCD on my 25 year old Fluke is a little sketchy. Def going to look again. That would explain a lot. Thank you.

mattmia2

If the end switch on the zone valve is intermittent, cycling the power will cause the zone valve to close then open again and cycle the end switch. If there are multiple zones, the problem should only be with one zone unless all the end switches are bad and it only heats when the good zone valve is calling.

Nov1070

mattmia2 said:

If the end switch on the zone valve is intermittent, cycling the power will cause the zone valve to close then open again and cycle the end switch. If there are multiple zones, the problem should only be with one zone unless all the end switches are bad and it only heats when the good zone valve is calling.

Heat is still working. Thank you for your input!

mattmia2

You might want to turn up one zone's t-stat at a time and make sure the boiler fires for each zone. You may have simply found the last end switch that failed.