Delayed Ignition

Old 10-17-18, 01:30 PM
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Delayed Ignition


I have an old Weil-Mclain chimney vent gas water boiler (it is a furnace, not a hot water heater).

Last winter, the furnace started making popping noises/explosions that was very worrisome (and unsafe).

I read a lot of the answers on the site, and removed the 4 cast iron registers, and thoroughly cleaned them. I also cleaned the pilot light and there is a good flame. The air setting to the registers seems ok and the colors of the flames coming out of them are normal like an oven burner.

I have just turned on the boiler, and sure enough it started doing it again.

It does this when:
  • 1. It shuts off i.e. when the desired temperature is reached
  • 2. When the thermostat is hovering around the set temperature. It then seems that it is trying to turn the ignition on and off, without waiting for a sufficient period (degree change etc.)
What could be causing this?

Could it be the thermostat? I don’t think it should be turning on and off, within one minute

Could it be the flue? I have never cleaned it but it is 6” wide

Old 10-17-18, 06:43 PM
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I'm completely lost. If it heats water it's a boiler..... not a furnace.

I can't offer much help as I have no idea what you have there.
A model number would be helpful.
Pictures are very helpful...... How-to-insert-pictures

Delayed ignition is usually caused by clogged holes in the burner.
It would need to be removed and thoroughly cleaned.

Have you considered calling your gas provider for service ?
Old 10-17-18, 08:57 PM
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Hi Pete,

Thanks for replying. It is a boiler. The model is a weil-mclain E-D-5

Upon further examination,

It ignites fine when cold or if it has not lit for a while (as mentioned , I thoroughly cleaned the cast-iron burners)


It gives a “poof” sound when shutting off. (it does not seem to be an "explosion" per se. and I have read that this could be "normal")

Furthermore, it turns on and off very quickly when it is reaches the set temperature. Then it starts these small “poofs” or explosions and I have to reduce the thermostat to shut it off.

I have read that this is called “short cycling”.

For example. Let’s say the therm is set to 70 deg.

If the room is at 68 it will ignite fine, without any explosion etc.

When the temp hits 70, then it starts this series of poofs and I have to turn down the therm to stop it.

However, if I let the temp fall back down to 68 and adjust the therm to 70 it will again ignite ok.

I am not sure what is causing this. As far as I can tell there is no filter to clean or replace

My question is what can be causing this?

I thought it might be the thermostat since I read that there are thermostats that prevent this by forcing a delay between start and stop

Any help is appreciated. Thanks
Old 10-17-18, 10:59 PM
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To me it sounds like the thermostat is a good first place to look.
Old 10-18-18, 03:22 AM
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I've had "delayed ignition" when:

1) a Transformer was weakening and about to quit; or

2) my Electrodes were out of adjustment.
Old 10-18-18, 03:40 AM
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Have you replaced the thermostat lately? How old is this boiler? Is it hot water or steam? Have you or someone tried to adjust the burner's primary air shutters? (this is usually only possible on older units). If it makes this whooshing sound only when it shuts down, it could be from worn out burner orfices, if this is an older boiler. You may need to have a "qualified" service person look at this unit.
Old 10-26-18, 12:35 PM
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I believe it was the thermostat.

I have a Lux 500 thermostat

The differential or "swing" was set to 4 degrees (too high)
Yet the therm kicked in when the temp was just 1 degree below the set temp,

The MINIMUM ON/OFF TIME is set to 5 minutes so when the set temp is reached is should not kick in for 5 minutes. It was not working since the "swing" was set to 4 degrees

I set the swing to 1 degree and it seems to be working. Thanks

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