McDonnell & Miller Frequently Asked Questions

Volume 4/ Issue 1/ May 2017

1. Q.  How do I maintain Low Water Cut-Offs?
Find detailed maintenance instructions for McDonnell & Miller low water cut-offs in the installation and operating instruction manual (IOM) that ships with the product. You can also get the IOMs online at

Here’s a brief summary of required maintenance. Please refer to the appropriate IOM for detailed instructions.

Probe-Style Low Water Cut-Offs 
• Test annually
• Inspect self-cleaning probe every 5 years (annually if not self-cleaning)
• Replace probe every 10 years
• Replace complete control every 15 years

Float-Operated Mechanical Controls
• Hot water boilers: blow down quarterly
• Steam boilers below 15 PSI: blow down weekly
• Steam boilers above 15 PSI: blow down daily
• Disassemble and inspect annually – including float chamber and equalizing piping
• Replace head mechanism every 5 years
• Replace control every 15 years

A Special Note on Commercial Steam Boilers
The National Board of Boiler and Pressure Vessel Inspectors (ASME) recommends daily blow-down and testing of all low water cut-offs on commercial steam boilers. Many local codes require this.

2. Q. I installed a new electronic water feeder – why is my boiler flooding?
A. Common causes of flooding include:
• Feeder pump not balanced to boiler requirements
• Faulty swing-check in return header
• Plugged equalizing pipe connection
• Leaking tankless hot water coil in boiler
• Attendant over-filling boiler through the hand bypass valve
• Dirty water resulting in priming and carryover
• Too small a difference between level of dry return and boiler water level
• City water pressure above 150 lbs. (May need pressure-reducing valve in feed line)

Faulty installation can cause flooding. Remember that the closing level of make-up feeders on heating boilers should be set 2″ to 2 1/2″ below the manufacturer’s recommended boiler water line. This lets the system return balance varying steaming rates without adding make-up.

B. Always troubleshoot the whole system, not just one control. Boilers often flood because of a malfunctioning low water cutoff or clogged return pipes. To determine if the feeder is shutting down completely, do a “broken union test.”

• Make sure the boiler water level is above the feeder’s closing level.
• Close the outlet valve in the feed pipe – running from the feeder to the boiler.
• Uncouple (break) the union between feeder and boiler.
– Lower the boiler water level to observe water flow through the feeder.
– Raise water back to the shutoff level to verify that no water flows out. If water does
come out of the union, the water feeder is leaking and needs to be repaired or replaced.

– Open the outlet valve to verify there is a steady flow of water coming back from the boiler.
If not, check the feed line for lime or scale, which can hold the feeder open due to back pressure.

• Check for leakage past the bypass valve by opening the union below the bypass valve.
– If there is a leak, repair or replace the bypass valve.

Broken Union Test

3. Q. Why won’t the valve shut off drip-tight?
Do the broken union test to be sure the valve really is leaking. Often the valve is not leaking, and the cause of the flooded boiler is in the system.

If the valve is leaking, your best option is to replace it—especially if it’s an old-style pilot action valve or metal-to-metal valve. Bring it up to date with a cartridge valve. Initially, you’ll have to replace the whole valve (models SA47-101-102 & SA51-101-102…pre-August 1995.) After that, you’ll need to replace only the cartridge if the valve fails to shut off drip-tight.


Click here to download the May 2017 SteamTeam pdf file