Date: February 13, 2017
Cast iron low-pressure boilers can last for decades. 50 year old steam boilers, though not typical, are not unheard of particularly if they’re well cared for over the years. However, there is a tendency for owners to forget about an important regular steam boiler maintenance task that will help the unit run more efficiently and trouble-free.
The advent of automatic water feeders for convenience has been welcomed. These little devices free owners from the chore of venturing into the basement to add water to steam boilers. That is the good news. The bad news is because there’s no need to monitor or add water, many customers forget the importance of flushing low water cut-offs on steam boilers.
Before we go any further let me explain the function of a low water cut-off. The low water cut-offs monitor 
water level in steam boilers as well as large hot water boilers. For safety, they are wired into the burner. If the water content inside the boiler drops below a pre-determined level the low water cut-off will not allow the oil burner to fire. If a burner were to fire into a boiler with insufficient water the extreme heat would crack the cast iron section(s). At that point, the boiler would leak and require replacement
Pictured below are the two most common types of low water cut-offs used in residential steam boilers. On the right is a probe style low water cut-off while on the left is a #67 style. The probe style is maintenance free however, the #67 should be flushed weekly. Inside the housing of the #67 sits a float that travels with changes in water levels. Without regular flushing dirt and muck can accumulate inside the low water cut-off which can cause the float to stick in the up position. If your boiler is equipped with a #67 style low water cut-off, I am sure you have noticed the how dirty the water is when you flush it. Like the #67, a probe style low water cut-off controls power to the burner. However, rather than operating with a float it uses an internal probe that senses water level. Because it does not employ a float that could stick, it does not require regular flushing. Steam boilers with probe style low water cut-offs should, however, be flushed at least once per year. When you schedule your annual oil burner service or cleaning request the licensed oil burner technician flush your steam boiler. We do not recommend the homeowner perform this task for fear of scalding oneself.
Proper steam boiler maintenance is very important. If your boiler is equipped with a #67 low water cut-off be sure to flush it every 7-10 days during the heating season. If your domestic hot water is sourced from your boiler remember to flush once during the summer. Not sure how to perform this simple task? Contact your full-service oil heat dealer for instructions.