Boiler Safety Controls Since 1924

McDonnell & Miller Boiler Controls & Flow Switches

Low water cutoffs, high pressure LWCOs, electronic probes, flow switches, and liquid level controls for steam and hot water boiler systems -- stocked and shipped from Memphis, TN.

Authorized Distributor Family-Owned Since 1967 Same-Day Shipping Ships from Memphis, TN Technical Help Available
Shop All M&M Products (901) 458-2000
Authorized Distributor
Direct from Xylem / ITT -- not gray market. Genuine M&M parts with full manufacturer warranty.
Technical Support
Experienced staff can help you cross-reference series numbers, identify replacements, and spec the right control.
Same-Day Shipping
Orders placed by 2 PM CT ship same day from Memphis -- next-day to most of the US.
Parts Cross-Reference
Give us your M&M part number -- we identify the correct replacement including HD head mechanisms.

McDonnell & Miller Product Lines

M&M manufactures boiler safety controls and liquid management products for commercial and industrial steam and hot water systems. Memphis Control Center stocks the most commonly replaced series across all major product lines.

Float
Float-Type LWCOs
Series 61, 63, 64, 67, 69, 42S
Low water cutoff for steam and hot water boilers. McDonnell & Miller pioneered the float-type LWCO design in 1924.
High Pressure
High Pressure LWCOs
Series 93, 94, 150S, 157S
For steam boilers operating above standard pressure ratings -- commonly specified on large commercial and light industrial systems.
Electronic
Electronic Probe Controls
PSE-801, PSE-802
Solid-state probe-type low water cutoff -- no float mechanism. Used where space is limited or float controls are not practical.
Flow
Flow Switches
FS series
Paddle-type flow switches for liquid detection in pipe sizes from 3/4" through 10". SPDT output. Used in HVAC and boiler systems.
Level
Liquid Level Controls
Series 21, 25A, 51
Float-operated liquid level controls for pump, valve, and alarm applications in tanks, condensate receivers, and open vessels.
Parts
OEM Replacement Parts
Head mechanisms, floats, switches
McDonnell & Miller genuine replacement parts including HD (heavy duty) head mechanisms, float balls, and switch assemblies.

Top Selling M&M Controls

Most frequently ordered McDonnell & Miller controls at Memphis Control Center. Call (901) 458-2000 to confirm stock and same-day shipping availability.

Part Number Stock # Description
FS-251 120611 Flow switch -- 3/4" to 2" pipe -- SPDT paddle type
FS-254 120610 Flow switch -- 2" to 8" pipe -- SPDT paddle type
FS8-W 120601 Flow switch -- 3/4" to 1-1/2" pipe -- NEMA 4X wafer style
FS4-3 114400 Flow switch -- 1-1/4" to 2" pipe -- SPDT
93-7B 163000 Series 93 float-type LWCO -- side mount -- steam boilers to 50 PSI
93-HD 162400 Series 93 HD head mechanism -- switch replacement without pulling water column
94-7B-HD 166400 Series 94 LWCO with HD head -- top-mount high pressure steam
EA-RS 354401 Replacement electrode probe assembly for RS Series probe holders
150S 171702 Series 150S float-type LWCO -- side mount -- steam boilers to 15 PSI
FPC-1000 144704 Electronic LWCO -- 24V/120V -- auto or manual reset -- steam & hot water
750-MT-120 176207 Series 750 probe-type LWCO -- 120V -- manual reset -- obsolete, call for replacement
PSE-801 153827 Electronic probe LWCO -- 120V -- for steam boilers to 15 PSI
PSE-802 153927 Electronic probe LWCO -- 24V -- compatible with modern boiler controllers
67 149400 Series 67 combination LWCO & pump controller -- steam -- side mount
64 143600 Series 64 LWCO -- steam -- side mount -- low pressure residential
RS-1-HP 176199 High pressure remote sensor probe holder -- 3/4" NPT -- 250 PSIG rated

Most-Specified M&M Series

Technical overview of the M&M series most commonly specified on commercial and industrial boiler systems.

Series 150S -- Steam Boiler Low Water Cutoff
Float-type LWCO for steam boilers. The 150S is the standard replacement for older Series 150 installations. Probe-actuated version: 750/751P/752P for systems requiring electronic probe rather than float.
Float Type Steam Boilers SPDT Output Side-Mount
Series 750 / 751P / 752P -- Electronic LWCO
Probe-type electronic low water cutoff. No float mechanism -- uses conductance probe for water detection. 751P adds manual reset. 752P is the combination feed/cutoff version. Used where float controls are impractical or where space inside the boiler is restricted.
Probe Type Electronic Steam & Hot Water Manual Reset Option
Series 93 / 94 / 193 / 194 -- High Pressure LWCO
For steam boilers rated above standard residential pressures. Series 93 -- side-mount, steam. Series 94 -- top-mount, steam. Series 193/194 -- combination feed and cutoff versions. HD (heavy duty) head mechanisms available separately (93-HD, 94-7B-HD) for field replacement without removing the body from the boiler.
High Pressure Steam Side or Top Mount HD Head Available Feed & Cutoff Combo
PSE-801 / PSE-802 -- Electronic Probe Controls
Solid-state electronic LWCO with conductance probe. PSE-801 is a single-probe low water cutoff. PSE-802 adds a second probe for feedwater pump control. Used on hot water boilers and steam systems where a float-type control will not fit. EA-RS is the field-replaceable probe assembly (stock 354401).
Solid State Hot Water & Steam Pump Control Option Replaceable Probe

M&M Flow Switches -- FS Series

McDonnell & Miller paddle-type flow switches detect liquid flow in pipes. SPDT contact output. Stocked across the full FS series for 3/4" through 10" pipe. Browse all flow switches →

FS-251 3/4" -- 2" pipe · SPDT FS-254 2" -- 8" pipe · SPDT FS8-W 3/4" -- 1-1/2" · wafer FS4-3 1-1/4" -- 2" pipe · SPDT FS7-4 3/4" -- 4" pipe · SPDT FS6-3 1" -- 3" pipe · SPDT FS5-2 3/4" -- 2" pipe · SPDT All FS Series → Browse full flow switch catalog
McDonnell & Miller -- Chicago, 1924. M&M invented the float-type low water cutoff for steam boilers and has been the standard for boiler safety controls ever since. The company operated independently in Chicago for decades before being acquired by ITT Corporation. In 2011, ITT spun off its flow control businesses into a new company -- Xylem Inc. -- and M&M has operated as a Xylem brand since then. The controls are still engineered to the same specifications that made M&M the industry standard. Memphis Control Center has stocked M&M controls since 1967 and carries the full range of replacement parts and current production series.

McDonnell & Miller

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McDonnell & Miller specializes in manufacturing water level controls and flow switches for boilers and steam systems. They offer a wide range of products designed to ensure proper operation and safety in commercial and industrial applications. McDonnell & Miller products are known for their reliability, accuracy, and durability. Whether you need a low water cutoff switch for a boiler or a pump control for a cooling tower, McDonnell & Miller has the solution for you. Trust McDonnell & Miller to provide high-quality water level controls and flow switches that meet the needs of your system.

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Need help selecting the right M&M control?

Our team identifies series, voltage, and replacement parts daily - we'll find the right M&M for your boiler.

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McDonnell & Miller - Frequently Asked Questions

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 1¼" 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, 406°F) variants. The EA-RS electrode assembly (354401) is the replacement probe element for RS Series holders.

Related Categories

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For emergencies, outside of business hours, call #901-604-7069