McDonnell & Miller Low Water Cutoff: 67, 150S, 157S, PS-800 — In Stock, Same-Day Shipping | Memphis Control Center
Selection + Troubleshooting Guide

McDonnell & Miller Low Water Cutoff Guide:
Select the Right Control — and Fix It When It Fails

The low water cutoff is the last line of defense between your boiler and a catastrophic dry-fire. McDonnell & Miller has been building the industry standard for over 80 years. Here's how to select the right model for your boiler, maintain it correctly, and troubleshoot it when something goes wrong.

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McDonnell & Miller Distributor Same-day shipping by 2 PM CT In business since 1967
McDonnell & Miller
Industry standard since 1936 · a Xylem brand · Memphis Control Center Distributor
Float type · Probe type · Electronic · Mechanical · Combination feeder/cutoff · Steam & hot water boilers · ISO 9001 Certified · Xylem Water Solutions

Why the Low Water Cutoff Is Your Boiler's Most Important Safety Control

A steam boiler needs water to move heat away from its metal surfaces. When the water level drops below the minimum safe level, heat accumulates rapidly — and without the water to absorb it, metal temperatures climb fast enough to cause catastrophic damage, tube failure, or boiler rupture. The ASME Code for Low Pressure Heating Boilers requires that every automatically fired steam boiler have an automatic low water fuel cutoff. The low water cutoff's job is simple: when the water level drops to the critical point, it cuts power to the burner.

McDonnell & Miller has been engineering these controls for over 80 years. Their general catalog states it clearly: "Secondary (redundant) low water cutoff controls should be installed on all steam boilers with heat input greater than 400,000 BTU/hour or operating above 15 psi." On boilers that matter, two controls in series are the right answer — not one.

McDonnell & Miller Distributor

Memphis Control Center stocks common M&M LWCO models — same-day shipping on in-stock orders by 2 PM CT.

Same-Day Shipping

Boiler down for a LWCO? Most common models ship same day on orders placed by 2 PM CT. Don't lose a day of heat.

Application Support

Give us your boiler type, max pressure, and mounting configuration — we'll specify the right model fast. (901) 458-2000.

Two Types of Low Water Cutoffs

Every McDonnell & Miller LWCO falls into one of two sensing categories. The type you need depends on your boiler and application:

Float type — A float chamber mounted externally on the boiler tracks the water level. As the boiler level drops, the float drops with it. When it reaches the cutoff point, the float trips a snap-action switch that cuts power to the burner. Float types have been the standard since the 1920s and are proven, reliable, and field-serviceable. Models: 67, 150S, 157S, 63, 64, 764, 767, 61, 93/193, 94/194.

Probe type (electronic) — A conductive probe inserted into the boiler uses the water's electrical conductivity to complete a circuit. When water drops off the probe, the circuit breaks and the burner shuts down. Probe types require no float chamber and no blow-down valve, and include built-in delay features (delay on break = 10 seconds, delay on make = 15 seconds) that prevent nuisance shutdowns from a surging water line. Models: PS-800 (steam), PS-850 (hot water), 750, RB-24, RB-120, RB-122E, 150E/157E.

Which type is right for you? Float types are the traditional choice for most commercial and industrial steam boilers — proven, serviceable, and familiar to boiler techs. Probe types are ideal where the water line is unstable or surging, where a blow-down valve is not practical, and for residential hot water boilers. The McDonnell & Miller selection chart recommends probe types when water line fluctuation exceeds 2 inches.

Selection Matrix — Find Your Model by Boiler Type & Pressure

Match your boiler type and maximum operating pressure to the correct McDonnell & Miller series. This follows the official M&M system selection chart from their general catalog.

LWCO Selection by Boiler Type & Pressure

From official McDonnell & Miller MM-825(A) General Catalog selection chart. Always confirm with boiler manufacturer's required LWCO tapping location.

Steam · Up to 15 psi
PS-800 / 61 / 67
Low Pressure Steam — Primary LWCO
PS-800: Probe type, 15 psi max
61: Float, 20 psi max
67: Float, quick hook-up, 20 psi max
767: Float, 2.5" NPT side mount, 20 psi max
Steam · Up to 150 psi
150S / 157S / 93/193
High Pressure Steam — Primary or Secondary
150S: Float, 150 psi max, no water column
157S: Float, 150 psi, integral water column
93/193: Magnetic repulsion, 150 psi, stable water line
150E/157E: Electronic version, 150 psi
Steam · Up to 250 psi
94 / 194 / 750B
Very High Pressure Steam — Section I
94/194: Magnetic repulsion, 250 psi, 1.25" NPT
750B: Electronic probe, 250 psi max
Connection: 1.25" NPT, 10-bolt flange
Hot Water · Up to 50 psi
63 / 64 / RB-24
Residential/Commercial Hot Water
RB-24: Electronic probe, 50 psi, residential
63: Float, heavy duty, 50 psi
64: Float, quick hook-up fittings, 50 psi
764: Float, 2.5" NPT, 50 psi
Hot Water · Up to 160 psi
PS-850 / RB-120 / RB-122E
Commercial Hot Water Boilers
PS-850: Electronic probe, 160 psi max
RB-120: Electromechanical, 160 psi
RB-122E: Electronic, 160 psi
Probe sensitivity: 7,000 ohms
Hot Water · Up to 250 psi
750 / 750B
High Pressure Hot Water / Industrial
750: Electronic probe, 250 psi
750B: Electronic probe, 250 psi, relays included
Sensitivity: 20,000 ohms
Manual reset: Available (CSD-1 compliant)
Boiler down for a LWCO? Common models ship same day — order by 2 PM CT from Memphis.
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McDonnell & Miller Model Breakdown

Here is the full detail on every major McDonnell & Miller low water cutoff series — what each one is designed for, how it works, and when to specify it.

Float Type · Low Pressure Steam · Most Common Residential/Light Commercial
McDonnell & Miller Series 67
The Standard Low-Pressure Steam LWCO
Steam Only Most Common Up to 20 psi Quick Hook-Up

The Series 67 is the most widely installed low water cutoff on residential and light commercial low-pressure steam boilers. It's a float-operated control that mounts directly into the boiler's gauge glass tappings using the included quick hook-up fittings — making installation and replacement straightforward without additional piping. The float chamber tracks the boiler water level; when it drops to the cutoff point, dual precision snap-action switches cut power to the burner and can activate a water feeder or alarm.

The large float chamber provides stable operation even in systems with some water line movement. The lever-operated, full port ball valve makes blow-down easy and thorough. Available with optional low-voltage switches for millivolt (self-generating) gas valve circuits, and with manual reset for CSD-1 compliance.

Use the 67 when: You have a residential or light commercial low-pressure steam boiler (under 20 psi), direct gauge glass tapping installation is practical, and you need the most commonly stocked and serviced LWCO in the industry. For side-tapping installation on the boiler body rather than the gauge glass tappings, specify the 767 (2½" NPT side mount).

Steam only
20 psi (1.4 kg/cm²)
Float / mechanical
Quick hook-up (gauge glass tappings)
2 snap-action (burner + alarm/feeder)
Full port ball valve included
120V / 240V, 7.4A full load
Optional (67-M)
Need a Series 67 or 767? Most configurations in stock. Request a Quote →
Float Type · High Pressure Steam · Commercial & Industrial
McDonnell & Miller Series 150S
The High-Pressure Steam LWCO/Pump Controller
Steam — Up to 150 psi Pump Controller Monel Bellows Combination LWCO

The Series 150S is a combination float-operated low water cutoff and pump controller for commercial and industrial steam boilers rated up to 150 psi. Where the Series 67 handles low-pressure residential service, the 150S is designed for larger commercial boilers with a separate water column, operating at higher pressures. The Monel bellows provides corrosion resistance in demanding steam environments.

It includes two snap-action switches: one single pole, single throw for pump control, and one single pole, double throw for low water cutoff and alarm actuation. The 150S-MD (Maximum Differential) variant addresses a known characteristic of the 150 series: at lower operating pressures, the bellows is more pliable and the on/off differential narrows, causing nuisance burner shutdowns. The MD models have factory-adjusted setpoints that lower the burner-off point approximately 3/8" below the casting line, compensating for this narrowing at lower pressures. Maximum pressure is 50 psi for the MD models.

Use the 150S when: Your commercial or industrial steam boiler operates up to 150 psi, you need an LWCO with integral pump control, and the installation does not require an integral water column. If you need gauge glass and tri-cock tappings built into the water column assembly, specify the 157S.

Steam
150 psi (10.5 kg/cm²)
Float / Monel bellows
1 SPST (pump) + 1 SPDT (LWCO/alarm)
1" NPT equalizing pipes
7.4A full load, 345 VA pilot duty
Max differential — nuisance shutdown fix
Available (150S-M)
Need a 150S or 150S-M? Request a Quote →
Float Type · High Pressure Steam · Integral Water Column
McDonnell & Miller Series 157S
The 150S with Integral Water Column
Steam — Up to 150 psi Integral Water Column Gauge Glass Tappings Monel Bellows

The Series 157S is functionally identical to the 150S — same float mechanism, same Monel bellows, same switch configuration, same 150 psi maximum pressure — but it adds an integral water column. The water column assembly includes ½" or ¾" NPT tappings for gauge glass and tri-cock installations, which are required by code on larger commercial boilers where the gauge glass must be mounted on a water column rather than directly into the boiler.

This makes the 157S the right choice when codes require a water column, or when the boiler layout makes it more practical to have the gauge glass integral to the LWCO assembly rather than separate. The 157S-MD provides maximum differential operation for nuisance-shutdown prevention at lower operating pressures (same as 150S-MD, max 50 psi for MD models).

Use the 157S when: Your boiler requires an LWCO mounted on a water column with integral gauge glass tappings, your operating pressure is up to 150 psi, and you need combined low water cutoff and pump controller functions. The 157S is the specified standard for most larger commercial steam boilers.

Steam
150 psi (10.5 kg/cm²)
Integral — included
½" or ¾" NPT
Float / Monel bellows
1 SPST (pump) + 1 SPDT (LWCO/alarm)
Maximum differential — up to 50 psi
Available (157S-M)
Need a 157S or 157S-M? Request a Quote →
Electronic Probe Type · Low Pressure Steam · Residential & Commercial
McDonnell & Miller Series PS-800
The Electronic Probe LWCO for Steam
Steam — Up to 15 psi Electronic / Probe No Blow-Down Required Self-Cleaning Probe

The Series PS-800 is McDonnell & Miller's electronic probe-type low water cutoff for low-pressure steam boilers (15 psi maximum). Unlike float types that require a separate float chamber and regular blow-down, the PS-800 mounts directly into the boiler tapping with no blow-down valve required. The self-cleaning probe uses the boiler water's conductivity to sense the water level — when water drops off the probe for 10 continuous seconds (Delay on Break), the burner shuts down. When water returns to the probe, a 15-second Delay on Make allows additional feed water before the burner restarts, preventing rapid cycling.

Diagnostic LEDs make field troubleshooting straightforward: a red LED indicates low water or a shorted probe; a green LED indicates power and test status. Available in 120V and 24V models, with optional extended barrel, remote sensor, and manual reset configurations. The self-cleaning probe design minimizes scale and sediment accumulation.

Use the PS-800 when: Your application is a low-pressure steam boiler (15 psi maximum), you have an unstable or surging water line where the DOB/DOM delay features are needed, you want to eliminate the blow-down requirement of a float-type control, or you're installing on a boiler that doesn't have suitable gauge glass tappings for a float chamber. Not suitable above 15 psi steam — use the 750B or 150E/157E for higher pressure steam applications.

Steam only
15 psi (1.0 kg/cm²)
250°F (121°C)
Electronic probe, 7,000 ohm sensitivity
10 seconds
15 seconds
Not required
120V or 24V models available
Need a PS-800 or PS-801? Request a Quote →
Electronic Probe Type · Steam & Hot Water · Commercial & Industrial
McDonnell & Miller Series 750 / 750B
The Electronic LWCO for High Pressure and Hot Water
Steam or Hot Water Up to 250 psi Electronic Probe CSD-1 Compliant

The Series 750 and 750B are McDonnell & Miller's electronic probe-type LWCOs for commercial and industrial applications. The 750 uses a remote sensor (ordered separately) and is the primary LWCO for hot water boilers up to 250 psi. On steam boilers, the 750 with manual reset (MT models) functions as a secondary (redundant) LWCO only — it is not rated as a primary cutoff on steam per the official M&M catalog. For primary steam LWCO duty at high pressure, use the 150E/157E or 94/194. Green and red LED indicators provide immediate status at a glance.

The 750B adds integral relays, providing direct switching of loads without requiring an additional relay panel. The 750B-C3 and 750B-C4 configurations include a chamber assembly with sensing probes for side-mounted installations. Manual reset models (MT suffix) meet ASME Standard CSD-1 requirements — if the control is in a low water condition when power is interrupted, it remains in lockout when power is restored and requires manual reset after water is re-established.

Use the 750/750B when: You need electronic LWCO coverage above 15 psi, your boiler is hot water rather than steam, you need a CSD-1-compliant manual reset control for commercial or industrial service, or you're protecting a Section I boiler operating above 150 psi.

Hot Water (primary) · Steam (secondary/manual reset only)
250 psi (17.3 kg/cm²)
120°F (49°C)
Electronic probe, 20,000 ohm sensitivity
Adds integral relays
Manual reset models (MT suffix)
120V AC
Yes — new design
Need a Series 750 or 750B? Request a Quote →
Float Type · Steam & Hot Water · Heavy Duty
McDonnell & Miller Series 63 / 64 / 764
Heavy-Duty Float LWCOs for Steam and Hot Water
Steam or Hot Water Up to 50 psi Heavy Duty Two No. 2 Switches

The Series 63, 64, and 764 are heavy-duty float-type LWCOs suitable for both steam and hot water boilers up to 50 psi. The 63 is the workhorse heavy-duty model with a No. 2 switch included — providing the extra switch needed for alarm or water feeder actuation alongside the cutoff function. The 64 adds quick hook-up fittings for direct gauge glass tapping installation (similar to the 67 but for higher capacity). The 764 features a 2½" NPT tapping for side mounting directly on the boiler body.

All three are suited for commercial and industrial applications and are rated for any steaming capacity. Optional float block and manual reset configurations available on the 63. Use the TC-4 Test-N-Check valve on hot water system installations to prevent system water loss during blow-down testing.

Use the 63/64/764 when: Your boiler is steam or hot water up to 50 psi, you need a heavy-duty float-type control with included No. 2 switch, or your installation configuration calls for direct boiler tapping (64/764) rather than equalizing line mounting (63).

Steam or Hot Water
50 psi (3.5 kg/cm²)
Float / mechanical
No. 2 switch included
Quick hook-up fittings
2½" NPT side mount
120V, 10.2A full load (63); 7.4A (64/764)
Use with TC-4 valve
Need a 63, 64, or 764? Request a Quote →
Magnetic Repulsion · High Pressure Steam · Large Commercial & Industrial
McDonnell & Miller Series 93 / 193 / 94 / 194
The Magnetic Repulsion Pump Controller
Steam — 150 or 250 psi Pressure-Independent No Bellows Proportional Control Option

The Series 93/193 and 94/194 use magnetic repulsion instead of a bellows — eliminating the bellows as a potential failure point and delivering water level setpoints that remain constant regardless of operating pressure. This is the key advantage over the 150S/157S series: where bellows-based controls have pressure-dependent setpoints that shift as operating pressure changes, the 93/193/94/194 maintains the same cutoff point at 10 psi as at 150 or 250 psi.

The 93/193 are rated to 150 psi with 1" NPT connections; the 193 adds an integral water column with gauge glass tappings. The 94/194 are rated to 250 psi with 1¼" NPT connections and a ten-bolt flange for Section I high-pressure boiler service; the 194 adds the integral water column. The optional 7B switch provides a 0–135 ohm proportional control signal for boiler feed pump control, maintaining a constant water level rather than on/off cycling.

Use the 93/193 when: Your commercial or industrial steam boiler operates up to 150 psi and you need pressure-independent water level setpoints, especially if you have significant pressure swings during operation. Use the 94/194 for Section I boilers up to 250 psi.

Steam
150 psi (10.5 kg/cm²)
250 psi (17.6 kg/cm²)
Magnetic repulsion — no bellows
Pressure-independent setpoints
Adds integral water column
0–135 ohm proportional signal
1.25" NPT, 10-bolt flange
Large commercial steam boiler? 93/193/94/194 is your LWCO. Request a Quote →

Not Sure Which Model You Need?

Give us your boiler type (steam or hot water), maximum operating pressure, mounting configuration, and voltage — we'll specify the right McDonnell & Miller model and part number fast.

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Maintenance Schedule — Official McDonnell & Miller Guidelines

The following blow-down and inspection intervals are from the official McDonnell & Miller MM-825(A) General Catalog maintenance guidelines. Failure to follow these intervals is the most common reason LWCO controls fail to operate correctly when needed.

Series
Maintenance Interval
Max Replacement Interval
150, 157, 150S, 157S, 158S, 159S
Blow down and test daily. Inspect annually.
15 years
69, 169, 269, 369, 469 (Built-in float)
Inspect and test annually.
10 years
67, 767 (Low pressure float)
Blow down weekly. Inspect and test annually.
10 years
61, 63, 64, 764 (Heavy duty float)
Blow down weekly. Inspect and test annually.
10 years
42, 42S (Combination feeder/LWCO)
Blow down daily. Inspect and test annually.
10 years

Critical blow-down procedure: McDonnell & Miller recommends a dual valve blow-down system for commercial and industrial boilers. (1) Open the positive shut-off ball valve. (2) Open the throttling gate valve slowly until drain piping heats up, then open fully — observe that water level starts falling in the gauge glass. (3) Verify that pump contacts have closed and burner contacts have opened (burner shuts down). If the burner does NOT shut down during blow-down, immediately close all valves, shut off the burner, and service the control. (4) Close throttling valve, then ball valve. (5) Confirm water returns to normal level before leaving the site. Suddenly opening a blow-down valve without warming the drain piping first causes steam to condense, creating water hammer that can crush floats and damage linkages.

At each annual outage: The official M&M guidance calls for disassembling, inspecting, cleaning, and checking all float-type LWCOs for proper calibration and performance — not just testing. Scale, sediment, and corrosion inside the float chamber will affect control operation long before the float physically fails. Annual internal inspection is not optional on commercial boilers.

LWCO Troubleshooting Guide

These are the most common field complaints about McDonnell & Miller low water cutoffs, what causes them, and where to start.

Common LWCO Problems — Diagnosis & Direction

Field reference guide. Always verify water level and system safety before resetting any control. Have a qualified boiler technician service the system if the root cause is not clear.

Burner shuts off on low water — but the gauge glass shows water
Most likely causes: Float chamber filled with sediment preventing float from rising correctly · Float stuck or waterlogged · Float chamber not equalizing correctly with boiler (blocked equalizing lines) · Water in gauge glass is reading high due to foaming/priming in boiler
Start with: Blow down the control. If float doesn't drop during blow-down, the equalizing lines may be blocked. Check for boiler foaming — check pH (should be 7–9) and water condition.
Nuisance low water shutdowns — cuts out and resets repeatedly
Most likely causes: On 150/157S: pressure-dependent setpoints narrowing at lower operating pressures (specify 150S-MD or 157S-MD) · Water line surging due to boiler priming, foaming, or near-boiler piping issues · Condensate not returning fast enough · System water loss exceeding feeder capacity
If 150S/157S at low pressure: upgrade to MD model. Check near-boiler piping. Check condensate return rate. Consider PS-800 or probe-type control if water line is inherently unstable.
LWCO does NOT shut off burner during blow-down test
Most likely causes: Float corroded to the chamber body — not dropping freely · Float waterlogged and sinking even when water level is normal · Switch contacts welded or failed · Electrical bypass in the control circuit (someone may have jumped the control out)
This is a safety failure — shut down the boiler immediately. Do not return to service until the root cause is identified and corrected. Disassemble and inspect the float chamber. Check switch continuity.
Probe-type LWCO (PS-800/750) shuts down — "shorted probe" LED lit
Most likely causes: Scale buildup on probe bridging signal to ground · Probe touching the boiler body or tapping threads · Wrong probe length for tapping depth · High dissolved solids in boiler water creating a continuous conductive path
Remove probe and inspect for scale bridging from tip to body. Clean with descaling solution. Check probe insulator for damage. Check boiler water dissolved solids — may need blowdown or chemical treatment.
LWCO cuts out immediately every time — no water in boiler at all
Most likely causes: Actual low water condition — condensate not returning, leak in system, water feeder not working · Manual reset not pressed after a previous lockout · Jumper or wiring fault in the control circuit
Verify water level in gauge glass first — DO NOT reset without confirming water level. Find the source of water loss before returning to service. Check water feeder operation and condensate pump.
Water coming out of the blow-down valve won't stop / leaking
Most likely causes: Worn or damaged ball valve seat in the blow-down valve · Debris preventing valve from seating fully · Blow-down valve left partially open after testing
Attempt to fully close and re-open the ball valve to dislodge debris. If it continues to weep, replace the blow-down valve at next maintenance window. A continuously leaking blow-down valve will cause gradual water loss and false low water events.
150S/157S — pump runs but burner won't fire after low water event
Most likely causes: Manual reset model (150S-M/157S-M) — lockout requires manual reset button after water restored · Burner control also in lockout — check flame safeguard reset · Control wiring at switch incorrect (pump and burner circuits swapped)
Verify water level is restored above cutoff point. Press manual reset button firmly. If control does not reset, check water level again — may still be below cutoff. Verify wiring matches the control's terminal diagram.
How often should I replace my LWCO?
Per official M&M guidelines: 150S/157S series — maximum 15 years. 67, 63, 64, 764, 767 series — maximum 10 years. These are maximum intervals assuming regular blow-down and annual inspection. In hard water or harsh service conditions, replace sooner.
If the control is approaching or past its replacement interval, do not wait for a failure. A low water cutoff that fails to operate is not a nuisance — it's a safety hazard. Replace proactively on a scheduled maintenance window.

McDonnell & Miller LWCO Comparison at a Glance

McDonnell & Miller Low Water Cutoff Comparison
Feature 67 / 767 150S 157S PS-800 750 / 750B 63 / 64 93 / 193
Boiler Type Steam Steam Steam Steam HW primary
Steam 2° only		 Steam or HW Steam
Max Pressure 20 psi 150 psi 150 psi 15 psi 250 psi 50 psi 150 psi
Sensing Float Float/Bellows Float/Bellows Probe Probe Float/Bellows Mag. Repulsion
Water Column — — ✓ Integral — — — 193 only
Blow-Down Weekly Daily Daily ✗ None ✗ None Weekly Yes
Pressure-
Independent		 No No (MD helps) No (MD helps) Yes Yes No Yes
Pump Control — ✓ Yes ✓ Yes — — — ✓ Yes
Manual Reset Optional -M suffix -M suffix Optional -MT suffix Optional -M suffix
Replace By 10 yrs 15 yrs 15 yrs Per insp. Per insp. 10 yrs Per insp.
Best For Residential/ light commercial steam Commercial steam, no water column Commercial steam, water column req. Low-pressure steam, unstable water line High pressure or hot water, industrial Steam or HW up to 50 psi Commercial/ industrial, stable water line

Frequently Asked Questions

What is the difference between the McDonnell & Miller 150S and 157S?
The 150S and 157S are functionally identical — same float mechanism, same Monel bellows, same snap-action switches, same 150 psi maximum pressure, same pump and cutoff functions. The difference is the 157S includes an integral water column with tappings for a gauge glass and tri-cocks. Code and boiler manufacturer specifications often require the gauge glass to be mounted on a water column rather than directly into the boiler on larger commercial boilers — in those cases, the 157S is the correct choice. If no water column is required, the 150S is the more compact option. Both are available with manual reset (M suffix) and maximum differential (MD suffix) options.
What does the "MD" suffix mean on 150S-MD and 157S-MD?
MD stands for Maximum Differential. The bellows in the 150/157 series is pressure-sensitive — at higher operating pressures the bellows is stiffer, providing a wider differential between pump-on and burner-off setpoints. At lower operating pressures the bellows is more pliable, and the differential narrows. This narrowing causes nuisance burner shutdowns in low-pressure applications because the pump-on and burner-off setpoints get too close together. The MD models have factory-adjusted setpoints that lower the burner-off point approximately 3/8" below the casting line, compensating for the narrowing at lower pressures. If you're having nuisance shutdowns on a 150S or 157S at low pressure, the MD variant is the solution. Note: MD models have a maximum operating pressure of 50 psi.
How often should I blow down a McDonnell & Miller low water cutoff?
Per the official M&M maintenance guidelines: commercial and industrial boilers — blow down daily. Residential boilers — blow down weekly. The 150S/157S series should also be inspected annually and at each annual outage should be dismantled, inspected, cleaned, and checked for calibration. The 67, 63, 64, and 767 series should be blown down weekly and inspected annually. Use the dual-valve blow-down procedure: open the ball valve first, then slowly open the throttling gate valve until the drain piping heats up — do not open the blow-down valve suddenly, as this causes water hammer that can damage the float and linkage.
Why does my 150S/157S keep causing nuisance low water shutdowns?
The most common cause on a 150S or 157S is the pressure-dependent setpoint issue — the bellows becomes more pliable at lower operating pressures, narrowing the differential between the pump-on and burner-off points. The fix is to upgrade to the 150S-MD or 157S-MD which have factory-compensated setpoints. Other causes include boiler foaming/priming (check pH — should be 7–9, and check water condition), near-boiler piping issues throwing water around, condensate not returning fast enough, or a blocked equalizing line between the control and the boiler. If the nuisance shutdowns persist after checking all of the above, consider a probe-type control (PS-800) which is immune to pressure-dependent setpoint shifting.
What is the maximum replacement age for a McDonnell & Miller LWCO?
Per the official M&M general catalog maintenance guidelines: the 150/157 series (including 150S, 157S, 158S, 159S) should be replaced at a maximum of 15 years. The 67, 767, 63, 64, 764, 61, and 42 series should be replaced at a maximum of 10 years. These are maximum intervals assuming regular blow-down and annual internal inspection. In hard water areas or harsh service conditions, consider replacing earlier. An LWCO approaching its replacement interval should be replaced on a planned maintenance window — do not wait for a failure. A low water cutoff that fails to shut off the burner is a boiler explosion risk.
Does Memphis Control Center stock McDonnell & Miller low water cutoffs?
Yes — Memphis Control Center stocks common McDonnell & Miller LWCO models including the 67, 150S, 157S, PS-800 series, and others. Same-day shipping on in-stock orders placed by 2 PM CT. Call us at (901) 458-2000 with your boiler type (steam or hot water), maximum operating pressure, mounting configuration (direct tapping or equalizing line), and voltage — we'll identify the right model and part number fast. If you have the existing model number from the nameplate, that's all we need for a direct replacement.

Need a McDonnell & Miller LWCO — Fast?

Memphis Control Center stocks McDonnell & Miller low water cutoffs with same-day shipping by 2 PM CT. Boiler down? Call us — we'll get you the right model and get it moving.

Request a Quote Shop McDonnell & Miller Call (901) 458-2000
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