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

How to Test Your Steam Boiler Mechanical Low-Water Cutoff

Probably one of the most common low-water cutoff controls used on residential steam boilers is the McDonnell & Miller Model 67. It’s a mechanical control that uses an internal float to monitor your boiler’s water level. A drop in water level lowers the float, activates a switch and shuts down the burner as a safety measure. Otherwise, without enough water, a boiler can dry-fire and create a dangerous condition. (See last week’s post for more on why we use low-water cutoffs.)

McDonnell & Miller # 67

All mechanical devices are subject to failure at some point. The low-water cutoff can accumulate rust and sludge that impedes the movement of the float. If too much debris accumulates and the float can’t drop during a low-water condition, the control can fail to operate when you need it most.

A simple weekly test you can do yourself can make a huge difference in the reliability of your low-water cutoff. Combined with a more thorough annual maintenance and scheduled replacement (every 10 years), you can be confident that your low-water cutoff will be ready when or if you need it.

Weekly maintenance involves “blowing down” or flushing your low-water cutoff by opening the lever-operated ball valve to flush out the sludge, rust or other debris.

Here’s how:

1.     You’ll want a 2 – 5 gallon bucket — preferably metal. The water you’ll be flushing is VERY HOT and can deform a plastic bucket.

2.     Verify the boiler water level is at its normal level. Adjust it if necessary by activating your water feeder or opening the manual fill valve.

3.     Turn up the thermostat so the boiler’s burner is firing during the test.

4.     Open the valve on the bottom of the low-water cutoff completely.

5.     Watch the water level as it drops in the gauge glass. (This is the glass tube on the side of your boiler that shows the water level.)

6.     As the water level drops to near the bottom of the gauge glass, the low-water cutoff should shut down the burner. (If it doesn’t, have it serviced immediately.)

7.     Close the valve and refill the boiler to its normal water level.

8.     The burner should relight.

9.     Reset the thermostat to its normal setting.

That’s all there is to it. Feel free to comment if you have any questions or would like to share your low-water cutoff experience. Good luck!

Hydronically yours,

Wayne

Your Low-Water Cutoff

Today’s boilers incorporate many controls to improve safety. And, arguably, the most important of those is the low-water cutoff.

In the late 1800s and early 1900s boiler explosions were not uncommon. Thousands died or were injured in the name of central heating. To be sure, there were many causes for these boiler incidents, but the most common was the low-water condition.

Boilers could lose their water through leaks or evaporation. When they did, the boiler metal would overheat. Then, either manually or through an automatic feeder, water would be added to the hot boiler. When this happened the water would immediately flash to steam. And as water turns to steam, its volume expands over 1600 times — which quickly over-pressurizes the boiler and results in a catastrophic failure.

In the early 1900s the issue of boilers losing water through leaks in their return piping was addressed by one of the largest insurance companies at the time. The Hartford Insurance Company had to pay many of the claims that resulted from these boiler explosions so they developed a piping scheme that kept water from escaping from the boiler in the event of a return-pipe leak. It became known within the industry as the Hartford Loop.

The Hartford Loop reduced boiler failures caused by return-pipe leaks, but it didn’t address other low-water situations such as a boiler crack, evaporation, or water-feeder failure. Boilers at the time needed human attention and intervention to maintain a safe water level. Even a short period of inattention could have disastrous consequences.

In 1926 McDonnell & Miller Co. introduced the first low-water cutoff. It was an automatic device that monitored the boiler’s water level and shut down the fuel supply before the water level dropped dangerously low. Combined with an automatic water feeder, the system keeps a boiler running safely with much less personal attention. It’s also saved countless lives.

Today, low-water cutoff controls are considered standard equipment on steam boilers and most hot-water boilers. They provide a level of safety and peace of mind that we’ve come to expect from our heating systems. But they still need some attention and a little routine maintenance to remain reliable safety devices.

In my next post, I’ll talk about things you can do to improve the safety and reliability of your low-water cutoff control.

Heidronically yours,

Wayne