Low-water Cutoff Question

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CalyerBK

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I am a homeowner with a steam boiler that is probably 20+ years old. I seem to be having a problem with my low water cut-off, unless I am not understanding how it works.

The boiler has an automatic water feed, so whenever I release water through the blowdown pipe, the water feed fills the tank right back up. No problem. To test the low-water cutoff, I closed the valve that sits right before the water feed mechanism, choking off the water supply, then released the tank's water through the blow down valve. Even when the tank got down to practically empty, the boiler kept firing. (I reopened the valve so the tank filled back up before any accident could occur.)

Seems like in the past, the boiler would shut off until I restored the water supply. Perhaps I am remembering it wrong, but I don't think I am.

So is my low-water cutoff valve shot, or am I misunderstanding how it works?

Thanks
 

Dana

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The low water cutoff should indeed interrupt the burner operation when it's below the minimum level, with a set of low voltage switch contacts. The level is detected by a float (which could be stuck), which is mechanically connected to the switch contacts with a lever arm. The details vary by model & manufacturer. Do you have picture of the suspect cutoff switch?

Boiler8.gif


Also, it's not a tank- it's filling the volume of the boiler directly.
 

CalyerBK

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Helpful info, and nice diagram. Not home right now but will take pics when i get back.
 

CalyerBK

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Took some pics from various angles. The make and model is in the fourth pic. Thanks again...
 

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Dana

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According to the manual for the McDonnell #67 the contacts can either be low-voltage or line voltage. I believe you can open it up and test the swich contacts separately with only two screws on the end where the wiring goes in. Turn the power off, disconnect the wiring and open it up- the float and switch contact assembly looks like this, when you pull it out:

153825-5.jpg


With an ohm-meter on the contacts you should be able to tell if/when the contacts make & break as you flex the float bulb back & forth. I believe the switch itself can also be purchases as a separate assembly if the contacts are fused together or something. It comes in a couple of flavors depending on whether yours is a millivolt system (not likely) or line voltage (more likely.)
 

CalyerBK

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Called in a pro. He gave the cut-off device a number of whacks with his wrench and it started working. Said sometimes it gets stuck when you haven't done a blow-down in a while, and suggested i just test it again from time to time over the next few weeks. If it stops working again, get it replaced. ($600 w/labor, not too terrible)
 

Dana

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Sure you want to leave a critical piece of safety equipment to the occasional thwack-reset?

Keep an eagle eye on it, or you could end replacing your boiler (maybe even your house) sooner than you wanted to.
 
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