Common questions about M&M low water cutoffs, flow switches, and boiler controls. Contact our team or call (901) 458-2000 for selection or cross-reference help.
What does a McDonnell & Miller low water cutoff do?
A McDonnell & Miller low water cutoff (LWCO) monitors water level inside a steam or hot water boiler. If the level drops below a safe minimum - due to a leak, failed feedwater pump, or operator error - the LWCO cuts burner power to prevent dry firing. Dry firing a low-water boiler can cause cracked sections, boiler explosion, or fire. The LWCO is required by ASME code on virtually all steam boilers and most commercial hot water boilers.
What is the difference between the McDonnell Miller Series 150S and Series 93/94?
Both are float-type mechanical LWCOs for steam boilers rated to 150 PSI. The Series 150S installs directly on the boiler body in a compact housing, providing low water cutoff and optional pump control. The Series 93 and 94 are heavy-duty controls with an integral water column that includes tappings for gauge glass and tri-cock installations - required by many codes on commercial steam plants. The Series 94/194 adds an integrated pump controller. Series 93 uses a magnetic repulsion mechanism (no bellows) for reliable float operation. For older commercial steam boilers, the 93/94 series are the most commonly specified replacement controls.
What is the difference between the McDonnell Miller PSE-801 and PSE-802?
The PSE-801 and PSE-802 are both probe-type electronic LWCOs in the PS-800 Series for residential and light commercial steam boilers (max 15 PSI). The PSE-801 operates at 120 VAC; the PSE-802 at 24 VAC for compatibility with modern boiler controllers. Both use a conductivity-sensing probe with no moving parts. Manual reset (-M) variants are available for applications requiring operator acknowledgment before restart. For high-pressure or large commercial steam boilers, M&M recommends a redundant mechanical LWCO alongside the probe control.
How does a McDonnell & Miller flow switch work?
M&M paddle-type flow switches insert a paddle into the pipe through an NPT tapping. Liquid flow deflects the paddle and actuates an internal SPDT switch; when flow stops, a spring returns the paddle to the off position. The switch output can be wired normally-open (flow proven) or normally-closed (no flow alarm). M&M flow switches are commonly used as chiller flow proving switches, boiler circulator flow interlocks, and process flow switches. The FS8-W and FS-254 are the most common HVAC models; the FS7-4W covers 1Ā¼" pipe with NEMA 4X rating.
Can I replace just the head mechanism on a Series 93 or 94 control?
Yes. M&M designs their high-pressure controls so the head mechanism (switch and electrical components) can be replaced separately from the water column body. The 93-HD replaces the head on Series 93; 93-7B-HD adds the #7B switch; 94-7B-HD covers Series 94. This allows field repair without replacing the entire water column and its piping. Call (901) 458-2000 with the existing control model number and we will identify the correct head mechanism.
What is a McDonnell Miller RS Series remote sensor?
The RS-1 remote sensor series are probe holder assemblies for the Series 750 and PCH/PCL electronic level controls, allowing the probe to mount separately from the control module. Available in RS-1-LP (low pressure), RS-1-HP (high pressure, 250 PSIG rated), and RS-1-BR-1 (single level, 250 PSIG, 406Ā°F) variants. The EA-RS electrode assembly (354401) is the replacement probe element for RS Series holders.
