Steam Boiler Low Water Cutoff Tripping: What to Do

Read the glass first

What normal looks like. Weil-McLain puts the normal water line halfway up the gauge glass. Pennco says the level may rise and fall as the boiler cycles on and off, which is normal, but water must always be visible in the glass.

What the light means. Burnham’s cast iron user manual says that with no automatic feeder fitted, a red or yellow Low Water light on the cutoff means the boiler needs more water. Hydrolevel says its amber lamp means the water is below the probe.

If the burner keeps firing with the glass empty, the cutoff is not stopping it. Shut the gas off at a point outside the boiler, as the first rule says, add no water, and call. McDonnell & Miller’s 69 manual gives two causes, a float shield loaded with mud or sediment, or switch contacts fused together.

Top up by hand, the maker's way

Burnham’s steps, with no automatic feeder. Burnham says steam boilers need water added now and then, to replace the small amount lost through radiator and system vents. With the Low Water light on, wait at least 15 minutes for all the water in the system to return to the boiler. Then open the manual feed valve, and stop when the glass reaches the boiler’s normal water level.

Each maker’s own rule. Burnham says ask a qualified installer or service agency if you are not sure where the manual feed valve is, or if you cannot see the water level in the glass. Weil-McLain says never add cold water to a hot boiler, and Peerless’s 63 and 64 manual says turn the gas off and call.

How often is too often. Burnham says that if water has to be added by hand or automatically more than once a month, a qualified installer or service agency should inspect the system for leaks or defective vents. That is check 6.

Water in the glass, light still on

The glass can be wrong. When the glass shows the right level and the amber lamp is lit, Hydrolevel’s first check is a plugged gauge glass. Weil-McLain’s user manual shows owners how to flush its glass through the pet cock, with the boiler pressure low.

A probe cutoff. Burnham says the probe sits in the boiler water and can foul, so it needs inspecting every year. Hydrolevel says only qualified personnel service its CycleGard, with the power off and the boiler thoroughly cooled. Its list goes on: the probe wire on its terminal, the ground between probe and boiler, which too much Teflon tape or sealing compound may cut off, and oily residue on the probe, cleaned off with the boiler skimmed. McDonnell & Miller says its PS-802 needs water that conducts, and water that is too pure may need boiler water treatment.

It stops for a short while, then fires again

Built-in waits. McDonnell & Miller’s PS-802 waits 15 seconds after water reaches the probe before the burner can run, and 10 seconds after it leaves before it stops it, to prevent nuisance cycling. Burnham says the Hydrolevel CG400 on its Independence keeps the burners off until 30 seconds after the water is back above the cut-off point.

A level test. Hydrolevel’s CycleGard takes power from the burner at set times so foam can settle and it can read the true water level, and its green light shows the test running. The last four digits of the model number set it, from every 10 minutes for 90 seconds to every 20 minutes for 60 or 90 seconds.

Water still out in the pipes. Weil-McLain says modern steam boilers steam for less time than older, larger ones, and one with only a low water cutoff can shut down and cycle while it waits for condensate to return. Its installer’s test times the return, and over 10 minutes it recommends a reservoir pipe or a boiler feed system. More in a boiler that keeps turning off.

The water bounces, surges or foams

Dirty water. Hydrolevel says foaming comes from machining oils, minerals from make-up water, grease and other dirt in the boiler water, and that cleaning the boiler will eliminate it. McDonnell & Miller’s PSE-800-M manual says some foaming is common in certain boilers, and points to the boiler maker’s cleaning steps.

After new work. Burnham says soldering or brazing pastes and fluxes left in the system can cause surging, an unsteady water line and wet steam if they are not removed when the boiler is boiled out.

The piping near the boiler. Burnham says wrong near-boiler piping sends wet steam into the system, and lists short cycling on low water, flooding, hammering and radiators that fail to heat.

Chemicals. Weil-McLain says some boiler water treatments can make the water level unstable, foaming agents cannot be tolerated in a steam boiler, and homemade cures or boiler patent medicines must never be used.

It keeps needing water, or the feeder overfills it

Where the water goes. Bell and Gossett’s Steam Team says a steam system will always need feed water. It escapes by evaporation through leaky air vents on the radiators and mains, made worse by pressure kept higher than needed, and even a few feet of buried return line has a good chance of leaking.

Weil-McLain’s list. Leaking return lines and underground pipes, faulty steam traps and air vents, and an automatic feeder installed or adjusted wrongly. It says continual fresh water shortens a boiler’s life: minerals build up, overheat the cast iron and cause section failure.

The feeder. Hydrolevel’s VXT counts the water it adds, to help find a leak, and waits for condensate to return before feeding. It locks out after two feed cycles with no burner restart, to prevent flooding. Weil-McLain says a feeder that feeds before the condensate returns overfills the boiler, and McDonnell & Miller says never use an electric automatic feeder with a manual reset cutoff.

A float feeder and cutoff. McDonnell & Miller’s Series 47 manual lists a float chamber clogged with sediment when the boiler gets too much or too little water. Little or no flow when its blowdown valve opens may show it, and the fix is cleaning the chamber, float and bellows.