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

Less is More

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Turn it down. Turn it way down.

I’m referring to the pressure setting on your steam system. The single easiest thing you can do to improve the performance, comfort and fuel economy of your steam heating system is to reduce the pressure. It should also be the first thing you do when things aren’t working right.

It’s tough to do. It seems counterintuitive. That back addition or attic bedroom isn’t getting enough heat, so it seems to make sense to turn up the pressure to force the steam into the far reaches of your system. But it seldom works. And what’s worse, it usually creates even more problems.

Most of the components in your steam system operate best within a range of pressures. Take main air vents, for instance. They come in lots of shapes and sizes AND pressure ranges. I was recently in a home with end-of-main vents that weren’t working properly. They seemed to vent OK at the beginning of the cycle, but appeared to stop working before the main was completely vented.

As it turns out, these particular main vents are designed to operate at less than 3 PSI. At higher pressures they close off tightly ­— rendering them essentially useless. A check of the pressure control had the system set at 7 PSI. So as the steam pressure rose inside the pipes, the vents worked until the pressure got to 3 PSI, then they’d stop venting. Since this was a one-pipe system, the radiator vents had to do double duty venting the rest of the main, all of the risers and the radiators.

A standard Honeywell Pressuretrol

This meant the radiators farthest from the boiler were not getting much heat. By the time the radiator vent in the attic allowed steam in, the radiator in the dining room (where the thermostat is located) had been full of steam and heating for some time.  It satisfied the thermostat and shut the boiler off just as the attic radiator was getting started.

When I turned the pressure down at the boiler, the main vents were able to completely vent the mains and allow the radiator vents to get back to work, venting only the risers and radiators. That’s when the attic radiator finally got some steam and heated the previously unusable attic space.

 

It can be worse on a two-pipe vapor system, as this usually has just one vent at the end of the dry return. If this single vent shuts off on pressure, everything stops heating. But the boiler keeps firing, trying to raise the pressure — wasting fuel.

A Honeywell Vaporstat

So what pressure is right for you? I haven’t met a residential space heating system yet that needed more than 2 PSI. And many need even less. In fact, on most of the replacement boilers I install, I routinely discard the factory pressure control that comes with the boiler and replace it with a Vaporstat. The factory-supplied device can only control the pressure down to 1PSI.  A Vaporstat regulates the pressure in OUNCES, which allows me to control the system at LESS than one pound of pressure — usually between 4 and 10 ounces. This can save a significant amount of fuel while improving comfort AND system performance.

Heidronically yours,

Wayne