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Why residential hot water boilers need LWCO’s?

Why residential hot water boilers need LWCO’s?

Volume 6/ Issue 2/ August 2019

There are both legal, and practical considerations that create the need to install mechanical or electronic low water cut-off (LWCO) control on a residential hot water boiler. From the legal side, jurisdictions have adopted codes which state when an LWCO must be installed, while the practical side is based upon system conditions.

ASME (American Society of Mechanical Engineers)
ASME Boiler and Pressure Vessel Code (BPVC) have been universally adopted as the minimum requirement for the manufacture, installation and maintenance of boilers. Section I, for Power Boilers, requires low water protection. Steam heating boilers of any size, regardless of where they are installed, must have a low water cut-off per ASME BPVC.IV. The same code only requires that hot water boilers with input greater than 400,000 btu must have a low water cut-off. In lieu of an LWCO, coil type boilers above 400,000 btu input, which require a flow of water to prevent overheating, shall have a safety device (typically a flow switch) to prevent burner operation when the flow of water is inadequate.

ASME CSD-1 (Control and Safety Devices)
CSD-1-2018 is an additional ASME standard for Controls and Safety Devices for Automatically Fired Boilers. As per Section 4, CSD-1 Part CW-120a requires at least one LWCO on all steam boilers, however the requirement for hot water boilers (Part CW-130a) have exception for residential boilers.

IMC (The Internal Mechanical Code)
This is a newer standard that is being adopted by jurisdictions. It is a consolidation of codes written in the past by BOCA, SBCC, and other independent code councils. Section 1007.1 of the IMC states “All steam
and hot water boilers shall be protected by a low water cut-off control.” If it’s a hot water boiler, it must have a low water cut-off.

2014 New York City Construction Code
All steam and hot water boilers shall be protected with dual low water cut-off control.
Hot water boilers located within a dwelling unit supplying only that unit and having a total heat input of less than 350,000 Btu/h (1025 kW) may be protected by only one low water cut-off control.

The Reducing Valve
There has always been a controversy about whether to keep the fill valve open or closed after initially filling a hot water heating system. Bell & Gossett recommends closing the fill valve. If the valve is closed and there is a leak in the system, no water is added to the system which may cause damage to the boiler and flooding. Also a fill valve has a strainer, debris (sand, silt, minerals, rust, etc.) that is present in the water can clog the strainer. If the strainer is clogged, an open valve is no guarantee that water will flow, but if it does, a flood could result. The best practice in a hot water system is to fill the system, close the valve and install a low water cut-off to protect the system.

Piping Elevation
Some systems have piping for radiators, snow melt, and tankless water heaters below the minimum safe water level of the boiler. Boiler manufacturers and organizations such as the National Fuel Gas have recognized this. Each has added a section in their literature or standards that indicates that if a hot water boiler is installed above level of radiation, then a low water cut-off should be installed

For many years, industry leaders have identified the need for low water cut-offs on hot water boilers. They agreed that the only way to detect a low water condition is with a low water cut-off device. No other safety device can determine if water is present. In 1997, McDonnell & Miller introduced the Series RB line of probe type low water cut-offs. Designed for use in residential hot water boiler applications, they feature a green “power on” LED, a “low water condition” red LED, and high sensitivity for use in a water, and water-glycol mixture. The series RB are equipped with a self-cleaning probe for years and years of worry free protection. The Series RB can be installed in either the boiler tapping or supply riser and are easy to wire. They are an excellent choice as the device to sense a low water condition in a hot water boiler. Remember, even with the many other safety devices (temperature limits, pressure relief valves, flow sensors, etc.) installed on a hot water boiler, the low water cut-off is a low cost component which will protect the boiler and system from damage if a low water condition occurs.

Give your customer and yourself peace of mind, and install low water cut-off. It is a low cost way to protect property, health, and even life.

Since 1924 McDonnell & Miller company is protecting boilers in the USA and oversees from dry fire and offers many excellent mechanical and electronic low water cut-offs for hot water boilers.

Our featured electronic controls provide two different versions, based upon the power supply.

McDonnell & Miller RB-122E low water cut-off for residential hot water boilers is an excellent choice for oil or gas hot water boilers with 120V burner circuit.


RB-122-E Low Water Cut-Offs

  • For residential and commercial applications
  • Electronic operation
  • Easy to install and wire
  • Red LED indicating low water condition
  • Green LED indicating power is on
  • Test button
  • Automatic reset
  • No blow down required
  • 20,000 ohms probe sensitivity
  • Maximum ambient temperature 120°F (49°C)
  • Maximum water temperature 250°F (121°C)
  • Maximum water pressure 160 psi (11.2 kg/cm2)

For more information regarding Series RB-122E MM-238REVO.pdf.


For more information regarding Series RB-24E please visit our website or download the pdf files MM-288C.pdf and MPF-009B_RB-24E.pdf.

If you have additional questions regarding McDonnell & Miller products please see our website or contact our factory representative for your area mcdonnellmiller.com.

Click here to download the August 2019 SteamTeam PDF file.

by Jarek Berezowski, Assistant Product Line Manager-Steam Products, McDonnell & Miller