low water cutoff

The Importance of Low Water Cutoff Maintenance

What are Low Water Cutoffs?

Low water cutoff controls are safety devices on steam boilers that are designed to shut the boiler off in the event it runs low on water. When a steam boiler runs lows on water and does not shut down a “dry fire” could result. A “dry fire” is when there is no water in the boiler to regulate the temperature. As a result, this can cause the boiler to overheat, warp or even crack resulting in extremely costly repairs or even a complete boiler replacement.

One way to prevent low water cutoff device failures on your boiler is routine maintenance. According to McDonnell & Miller (a low water cutoff control manufacturer), depending on the low water cutoff device your boiler has, they should be blown down and tested on a daily basis. For example, on an MM #157S or #150S, if your boiler operating pressure exceeds 15psi this should be performed on a daily basis. If the operating pressure is below 15psi, then weekly testing should be done.

The process to blow down a MM #157S or MM #150S low water cutoff is as follows:

“When blowing down a control at pressure, the blow down valves should be opened slowly. The piping needs to be warmed up and stagnant water in the drain piping needs to be pushed out. Suddenly opening a blow down valve causes steam to condense, which can create water hammer. Damage to components can occur when water hammer occurs due to improper blow down piping. For these reasons, McDonnell & Miller recommends a dual valve blow-down system for each control. Blow down the control when the water in the boiler is at its normal level and the burner is on. Open upper valve (#1) • Slowly open the lower valve (#2) • Water in the sight glass should lower. • As the water in the sight glass lowers, the pump should turn on. • As the water continues to lower in the sight glass, the burner should turn off. • Slowly close the lower valve (#2). • Close the upper valve (#1) • The water level in the sight glass should rise, first turning on the burner and then turning off the pump. NOTE: On manual reset models, the reset button will need to be pressed after the water level has been restored before the burner will operate.” (“Series 150S/157S Low Water Cut-Off/Pump Controller”)

If the boiler does not shut off a service call to Controlled Combustion should be placed right away.

How often should low water cutoffs be serviced?

Aside from performing daily/weekly blow-down testing, the low water cutoffs should be opened and inspected annually to ensure safe and reliable operations of your boiler. Debris and sediment should be removed and the device should be replaced if it is worn or corroded. Having low water cutoff maintenance performed annually will help ensure that your boiler safety devices are functioning properly.


“Series 150S/157S Low Water Cut-Off/Pump Controller.” McDonnell & McMiller, 2008, mcdonnellmiller.com/boiler-controls/combination-low-water-cut-offspump-controllers/series-150s157s-low-water-cut-offpump-controler/#product-tab-literature.

When it comes to boiler compliance and boiler maintenance, it is crucial to work with a heating contractor that understands the entire heating system.  CC Controlled Combustion Co. Inc. specializes in all areas of your hydronic heating system and steam heating system.  Our range of services falls into five categories.  1) Compliance, 2) Maintenance, 3) Service, 4) Replacements & 5) Upgrades. Our 1) Compliance (aka Boiler Compliance or NYC Boiler Code Compliance) includes, but is not limited to NYC DOB Code Compliance (Annual Boiler Inspections LL4, LL87, LL95, LL97, Gas Pipe Inspections LL152), NYC DEP Certificate Renewals for Triennial Inspections, NYC DEP Registration Renewals, NYC DEP Annual Tune-Ups, NYC DEP Annual Backflow Preventer Inspections, NYS DEC Fuel Oil Tank Inspections (Monthly Visual Inspections for Above Ground Petroleum Bulk Storage Tanks and Annual Tank Tightness Testing for Underground Petroleum Bulk Storage Tanks).  NYS DEC Petroleum Bulk Storage (PBS) Application filing.  We also work with customers to cure the various DOB Boiler Violations, DEP Boiler Violations & DEC Violations. Our 2) Maintenance (aka heating system maintenance or preventative boiler maintenance) includes Burner Overhauls, Boiler Cleanings (Fireside Boiler Cleanings & Waterside Boiler Cleanings) on Cast Iron Sectional Boilers, Condensing Boilers, Firebox Boilers, Scotch Marine Boilers (both Firetube Boilers & Watertube Boilers), Boiler Water Chemical Treatment, Low Water Cut Off Control Maintenance, Condensate Neutralizer Kit Replacements, Domestic Hot Water Coil Flushing, Heat Exchanger Flushing & Valve Exercising.  Our 3) Service is available 24/7/365.  We offer trustworthy 24/7/365 Emergency Boiler Service.  We responded to service calls from our Service Contract customers during Super Storm Sandy, COVID19 Pandemic, on all holidays and in any weather.  Our Certified Welders, Electricians & Plumbers allow us to resolve a wide array of service requests during normal business hours. Through our 4) Replacements (aka Boiler Replacements or Boiler Installations), we perform like for like boiler replacement, bid on a spec from a professional engineer or suggest a different size or type of boiler.  We offer gas conversion services with utilities like Con Edison and National Grid, and chimney liner installations.  5) We can upgrade parts of your existing boiler or heating system for better efficiency, reliability, and compliance.  Some of our common upgrades are fuel conversions (#4 oil to #2 oil and or natural gas), linkage-less control (aka parallel positioning), variable frequency drive (VFD) installations on motors, and energy management systems with apartment temperature sensors.  We are an approved vendor for Consolidated Edison Rebates (Con Ed Rebates), National Grid Rebates (Nat Grid Rebates) and New York State Energy Research and Development Authority Real-Time Energy Monitoring (NYSERDA Rebates or NYSERDA RTEM Rebates).

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