Get to know your steam boiler’s probe-type low-water cutoff.

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Last week I discussed how to check the mechanical low-water cutoff on your steam boiler. However, more and more new steam boilers are equipped with “probe-type” low-water cutoffs that use an entirely different mechanism to detect a low-water condition and shut down your burner before an unsafe condition occurs. (For more on that, see this post on the evolution of the low-water cutoff control.)

The typical probe-type low-water cutoff is threaded into a port on your steam boiler provided by the boiler manufacturer specifically for this purpose. It’s below the normal water level but just above the level of the bottom sight-glass port. This type of low-water cutoff operates by monitoring the electrical continuity of the boiler water. Simply put, when the probe is exposed to water, it allows the burner to operate. And when it’s exposed to air (as in a low-water condition) it does not.

McDonnell & Miller PSE-800

Some probe-type low-water cutoffs allow the burner to operate for a little bit after they sense a low-water condition — usually about 30 seconds. This is to prevent nuisance shutdowns due to temporary conditions, such as foaming or slow return of system condensate. A flashing light on the low-water cutoff control enclosure usually indicates this type of condition. If the probe senses water again within the 30-second window, the light will stop flashing and the burner continue firing without interruption.

Probe-type low-water cutoffs are either manually or automatically reset. Most residential steam boilers use automatic reset to let the boiler continue heating once the low-water condition is corrected — as when an automatic water feeder is used. A manual reset would require human intervention — a strategy that may not be in your best interest if the boiler were to shut down on low water when you’re away for a few days during the winter.

With either type, you can periodically check the device’s electronics by pressing and holding the test button while the burner is firing. A light will usually flash for a period of time and then the burner will shut down. Releasing the test button should start the burner and return the system to normal operation.

A probe-type low-water cutoff doesn’t require the same weekly maintenance as its mechanical cousin. But during the annual maintenance of your system, I’ll check your cutoff by lowering the water level to simulate an actual low-water condition. And every five years, I’ll remove the probe for cleaning and inspection. If your probe is older than 10 years, I should replace it for you.

With just a little attention, your probe-type low-water cutoff should provide years of reliable protection for your steam heating system.

Heidronically yours,

Wayne