Steam boiler works great when cold, won't re-ignite when hot

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Old 09-23-14, 12:05 PM
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Steam boiler works great when cold, won't re-ignite when hot

I purchased a home this summer and am trying to get the heat started for the season. Per the purchase inspector's advice, I drained the boiler until the low water mark on the sight glass and refilled prior to starting for the season. I then flipped on the burner switch and set the thermostat a couple degrees higher than ambient and I heard the pilot ignite (a couple of clicks then it was on), then the main burners came on. A few minutes later we had steam and the house warmed up nicely. I let it run for the night but it wasn't that cold out, so I don't know if it even had to cycle again all night.

Fast forward to a week later, it is now cold enough at night for the heat. When I set the thermostat to "heat", everything works at least once. The burners light, the water boils, and the home heats up. However, every time I wake up in the morning, the house is 3-4 degrees cooler than the thermostat set point. I flip the burner switch off and back on (thermostat still calling for heat), and I hear/see the flue close and re-open, then the pilot attempts to ignite but all I hear is clicking- no gas is flowing. This goes on for about a minute then it gives up trying to light. The water level is fine per the sight glass and the main gas valve is on. Interestingly, the boiler is still hot, although I can touch the steam pipe without injury so clearly it hasn't run in a while. If I shut everything off for a few hours and let it cool all the way down, everything works again.

Does this sound like something obvious? I have a thought that it might be the low water cutoff giving a false reading since it makes a loud click when I turn the burner switch off and on during this time but not when cool. I read an article on the internet about how to bypass the probe to test it but can't quite make heads or tails of how to do it. It is a Honeywell Guard Ring with a probe sensor, but no switch in the "test" knockout. Is it possible that the LWCO turns off the gas but the boiler still tries to fire?

My other thought is that perhaps it is related to the pressure. The cut-in is set at the lowest possible setting but I don't see an adjustment for the differential, and the gauge doesn't seem to work since it has always said 2.5psi (boiler off, cold, hot, no difference). But I'm not sure why it would be trying to ignite if this were the case.

Thanks in advance!
 
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Old 09-23-14, 04:12 PM
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Tell us the make/model of the boiler please.

then the pilot attempts to ignite but all I hear is clicking
So you've likely got a 'spark ignition' system and the pilot is failing to light.

The control ( what control model do you have? ) will not open the main gas valve unless the pilot lights.

This would seem to be the root cause of the problem, for some reason the pilot only wants to light when the system is stone cold... that's weird.

And of course, no pilot, no burners.

At this point I don't believe the problem is with the LWCO. If there is low water the control should prevent even the pilot from trying to light.

This assumes that the LWCO is wired correctly! So let's assume the opposite for purpose of discussion... WHAT IF the LWCO is wired incorrectly?

You MIGHT have what are called 'slow returns' where it takes forever and a day for the condensate to come back to the boiler, and this will cause low water on subsequent tries.

What do you see in the sight glass as far as water level when the boiler WILL fire, versus when it WON'T fire?

My other thought is that perhaps it is related to the pressure. The cut-in is set at the lowest possible setting but I don't see an adjustment for the differential, and the gauge doesn't seem to work since it has always said 2.5psi (boiler off, cold, hot, no difference). But I'm not sure why it would be trying to ignite if this were the case.
I doubt it's a pressure issue... but let's talk about it anyway.

When a steam boiler is cold, there will be ZERO pressure. When the boiler is HOT and making steam, you should see a MAXIMUM pressure of the "CUT IN" plus the "DIFF".

First, tell us about the pressure gauge. Is the gauge mounted on the top of the 'pigtail' siphon tube along with the PressurTrol control? If it's not an 'internal siphon' gauge (it will say this on the dial face) then it MUST be. Gauges without internal siphon must never be mounted where live steam can enter the gauge.

I suspect that the gauge is toast.

One common problem with steam boilers is that the siphon tube gets clogged. This can cause the boiler to run and run and run because the PressurTrol never senses the pressure... OR they can run up to a point and shut off, but if the pressure remains in the tube and the control doesn't see the pressure drop down again, they won't fire again until the pressure bleeds off... IF it ever does. It depends on just how clogged the siphon tube really is.

The minimum "CUT IN" pressure is fine. That's where it should be. The other dial, the DIFF is under the cover of that control. (I'm GUESSING that it's a Honeywell 404 variety, is it?)

The diff should not be set more than 2, and 1.5 is better.

A home should NEVER need more than 2 PSI of steam to heat properly.


So, here's what I would suggest:

1. Replace pressure gauge.

2. Remove and clean the siphon tube... even if it's NOT the problem, it probably needs to be done anyway, because it WILL be a problem in the future.

3. Get us the make and model of the boiler so we can review the wiring of the LWCO to make sure it's correct.

4. Tell us what you see on the sight glass gauge.

5. Post some pictures of the system.
 
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Old 09-23-14, 04:18 PM
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By the way, this book is absolute REQUIRED READING for homeowners with steam heat.

It is the best $25 you will ever spend on your system:

We Got Steam Heat! - A Homeowner's Guide to Peaceful Coexistence, by Dan Holohan
 
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