Pre-purge timer bypassed

JeremyG

I have a Weil McLain HE boiler, circa 1983. It's a power-vented unit and had a bunch of parts replaced (according to the service sticker) in the years leading up to buying the house in summer 2017. The manual was helpfully left by the previous owners, and it shows a pre-purge timer connected to the gas valve in the schematic. The 30-sec timer is still there, but no longer connected. (And untested, but cheaply replaced with an ICM102 or similar if it's not working.)

Incidentally, the gas valve runs a standing pilot, which wasn't the original configuration. Apparently it had a electronically-lit pilot from the factory. I'm guessing the tech who replaced the gas valve just did the simplest installation and disregarded the timer.

How important is doing a pre-purge? It's a shared flue with the natural-draft water heater. Both were checked for CO and correct drafting when I had an energy audit.

Thanks

Fred

Was this an oil fired boiler that has been converted to gas? That would explain the by passed pre-purge.

JeremyG

No, it's always been a gas boiler.

ratio

Pretty sure that swap is a call-the-lawyers no-no. I'd see about getting the correct parts reinstalled.

That said, it's my understanding that the pre-purge is to forcibly clear the combustion chamber and flue from any residual combustion products & ensure no unburnt fuel remains; with a standing pilot that shouldn't be able to happen.

HVACNUT

Can you post some pics of the boiler with the door off, and of the wiring diagram?

I know the more recent HE boilers use Hot Surface Ignition. I dont know what they used in '83. Cut me some slack, I was in 10th grade.

Newer revisions use a Honeywell control module and ALL controls go back to it with molex connectors, so theres no way to bypass any safety that will allow a normal cycle.
Its possible your boiler is OEM standing pilot and Electronic ignition was an option.

I'm curious to see the controls used on that boiler. There has to be a centrifugal or pressure switch that must close to prove clear flue passages from the draft inducer.
Dont know what the time delay is for.

JeremyG

My question is about the pre-purge timer and whether it's an important safety item to have in place.

There's a pressure switch that enables the gas valve. I can't put my hands on the manual right now (ugh, thought I'd just seen it). From memory, the delay-on-make timer is in series with the gas valve so that there's a 30-sec pre-purge before it lights.

Here's a photo of the gas valve.

ratio

It is required to have a pre purge long enough to give some certain number of air changes (I forget how many) in the combustion chamber & flue before the ignition point, but you're so far off the reservation that I wouldn't worry about it. You own the liability now, unless you can find the original installer; the lawyers'll eat him up.

Tim McElwain

Those boilers had a White Rodgers Cycle Pilot System (Mercury Pilot System) on them as original controls. There were a lot of problems with Cycle Pilot. Weil-McLain authorized a retrofit using a Hot Surface Igniter and they provided the kit to do the changeover. Your installation is now a code violation and is unsafe as the changes carried out were not authorized by the boiler manufacture. I would get in touch with Weil-McLain if I were you.

The prepurge would be required with the Hot Surface retrofit by the way.

JeremyG

@HVACNUT: FWIW, it looks like the boiler was originally a "cycle pilot" and W-M offers an HSI upgrade. Not cheap, but impressive that an old unit can be modernized.

https://www.supplyhouse.com/Weil-Mclain-382-200-449-Complete-Conversion-Kit-from-Cycle-Pilot-System-to-Gold-Control-HSI-Natural-Gas

JeremyG

Thanks @Tim McElwain. I stumbled across some similar info about the HSI upgrade right about the time you posted. I will contact W-M.

Learning lots of good stuff here. Unfortunately discovered that there have been condensation problems likely due to an oversized boiler (about 2x) and no bypass loop with a converted gravity system. The thing has to run for 15 min to get to 140º. Hopefully this boiler hangs on while I figure out my upgrade path for heating, DHW and finishing the basement.