Old Weil-Mclain Boiler No response.

I don't think there is a pilot fire constantly on. When it's working and when heat is called, the unit will make clicking sound like a gas stove and after a while the fire will start.

Right now nothing happens when heat is called.

Vent damper is closed.

Do you mean the main power switch? Yes I turned it off and on a few times for testing. In the photo it's off.

The Red light on the waterlevel guard is off.

I haven't connected or tested any thing else except the voltage between R and C on the transformer which is 26 V, because the wiring is confusing to me.

The google nest thermostat was taken from next door neighbor. We both have the same thermostat, also the wiring on the two thermostats are same as shown here, R, C, W are wired.

However on the transformer the W terminal and the center terminal are not wired to anything.

If you may, please let me know what are Y and W? I'm pretty sure R is Hot for power and C is Common?

So I should :

1. test the voltage between R and Y without disconnecting anything.
2. Splicing together the blue wire nuts (or use a wire to jump between R and Y should do the same thing?)

I don't know how the pressuretrol is wired but my understanding is that it should be on the same safety device chain? Do you mean if it's wired at 120 Volts then it is not the issue here because the transformer is working?

Thanks!

I'm a little confused but I guess you two are talking about what voltage goes to the vent damper and the pressuretrol.

The vent damper is connected only to the gas valve which gets power from the transformer so it has to be 24 V, I think.

Not sure about the pressuretrol and waterlevel safety, I think I'll open the switch box and peek inside.

Thanks!

The wire "under the boiler" is a heavy orange wire and i think it is the spark wire.

Here are pictures of new voltage test results and the vent damper insides. There are 2 spring contacts that would move if the motor turned. I marked them A and B. I'm thinking to push these 2 contacts temporarily when heat is called and see if the boiler try to fire up.

Edit: a little googling suggest that the switch B is to stop Motor once the vent is fully open, at the same time switch B will be closed to signal Gas valve to fire up. Is this correct?

Thanks!

Not sure what you mean but this is the thermostat where R, C, W are wired, while on the transformer R,C, Y are wired.

I haven't jumped it yet. Will try later. I understand now that it bypass the thermostat and directly call for heat by connecting R (hot) and Y(on safety chain).

The Yellow wire connects to gas valve, but it's possible that it gets wired inside the gas valve unit to go through the vent damper first. The vent damper is wired only to the gas valve, nothing else, inside a coil pipe. As some people pointed out, the pressuretrol and water level guard are probably wired on the 120V side of the transformer to cut power to the transformer if safety measure is not satisfied. Since the transformer has 26V, it doesn't look like it's the issue of pressuretrol or waterlevel guard.

No wiring diagram anywhere, inside or out. There's a warning sign inside the cover saying "installation is not complete if wiring diagram is not attached here", and it's not attached there…

What still confuses me is how exactly are the gas valve/ignition/vent damper wired through R/Y/C, and why does R-Y measure 26V?

Update:

I jumped the R and Y on the transformer and the damper motors turned and boiler started to fire up!

I guess I'll first clean the contacts and redo the wire nuts and see if that works.

I'm still confused about a few things, like why is there a wire between R on transformer to the gas valve, and why does R-Y measure 26V with and without heat calling.

Thanks everybody for the help! Wish someone can answer my confusing too!

You are right. While cleaning i opened the junction box. The pressuretrol and water level guard are actually wired to the 24V ends. So I'm testing them now.

Must be one of the two.

I'll clean the pressuretrol pigtail and and LWCO probe tomorrow. They haven't been cleaned for at least 5 years i think.

I tried jumping the 2 terminals on the pressuretrol while calling heat and boiler didn't fire up so it's probably not the pressuretrol. I'll still clean it.

The LWCO has too many wires though, not sure what to jump. It's warning light is off though. Is it possible that a dirty probe would keep the circuit open even though low water light is off? I don't really know how it works.

Judging by the number of wires the pressuretrol doesn't have constant 24V but the LWCO does?

Yeah my brain refused to work when seeing those wires.

I also think the yellow and red is the safety circuit on the lwco because that's the color for the 2 wires on pressuretrol. I'll try to trace it tomorrow.

It does not have auto water feeder. I need to add some water manually about every 2 weeks if it's constantly running.

I jumped the red and yellow wires on the water level guard and the boiler fired up. Now just cleaning or replacement.

Thanks everyone for the help! I learned a lot and finally figured out how this unit works.

Still couldn't find a spill switch though. Got to keep that in mind.

I traced all the circuits and drew a diagram for future reference. Whoever wired this thing was trying to confuse people with wild color switching.

Forgot to test it but it must have power. The wires came from the same wire nuts that goes to the thermostat.

New issue is that the water probe is seized, or very tight. I don't have experience to tell so soaked the threads with PB blaster oil and waiting.