5D51-78 – FAN & LIMIT CONTROL (11″)

If your furnace blower won’t kick on, runs constantly, or the furnace overheats and shuts down, the fan and limit control may be the problem. This combo control turns the blower on/off by temperature and provides high-limit safety. The 11-inch element is for furnaces needing a longer plenum reach.

Specs: 11-inch element, adjustable fan differential, fixed high-limit cutoff, Honeywell L4064B series

Replaces: 5D51-78, L4064B2210

Fits: Older gas furnaces using a Honeywell L4064B fan and limit control with an 11-inch element.

Need help? See our Furnace Short Cycling Guide.

In stock at Open To Public HVAC Parts in Dallas. We test parts at the counter before you buy. Call or text (214) 340-9421.

Specs

• TypeCombo fan & limit control (mechanical)
• Disc Size11" disc
• OEM CompatibilityHoneywell

Cross-reference numbers

This part replaces the following OEM and aftermarket numbers:

• 5D51-78

Not sure if your number matches? Call (214) 340-9421 or bring the old part in — bench-testing is free.

Fits these models

• Honeywell L4064-style combo fan & limit control — used on older standing-pilot and oil-fired furnaces; controls both blower fan-on/fan-off AND high-limit safety
• Common applications: Coleman, Carrier, Rheem standing-pilot and oil furnaces from the 1960s-1990s
• Match the stem length (5" / 8" / 11") to your heat exchanger / plenum geometry

What does L150° mean on a limit switch?

The L-rating is the trip temperature in Fahrenheit. L150° opens (breaks the safety circuit) when surface temp hits 150°F. Common ratings on residential furnaces: L120°-L260°. Match your equipment's data-plate spec EXACTLY — wrong rating = nuisance trips OR no protection.

Auto-reset vs manual-reset — which do I need?

Match what your equipment was designed for. AUTO-reset switches close back when temp drops below the reset point (convenient, but can hide developing problems). MANUAL-reset switches require pressing the button after each trip (safer because it forces you to investigate). Most furnace primary limits are auto-reset; secondary / rollout safeties are manual-reset.

Why does my limit switch keep tripping?

The switch isn't usually the problem — it's doing its job. Common root causes: clogged air filter (most common), dirty blower wheel, undersized return duct, closed-off supply registers, failing blower motor, slipping blower belt (older units), cracked heat exchanger, oversized burner. Diagnose the airflow / heat-source issue before just swapping the switch.

What's the difference between micro and 3"/7" disc?

"Micro" is a snap-disc style limit switch (small, ~1" diameter housing, common on modern furnaces). 3" / 7" refers to the disc diameter on older bimetallic limit controls and combo fan&limit units. Verify your existing switch's style + mount geometry before swapping; they are NOT interchangeable.

Can I jumper the limit switch?

NO. The limit switch is a safety device protecting against fire and heat-exchanger damage. Bypassing it can cause carbon monoxide buildup, melted ductwork, cracked heat exchangers, or worse. If yours trips, fix the airflow / heat-source cause.

How long should a limit switch last?

15-25 years typical. Most failures are caused by the underlying root cause (repeated over-temp cycles fatigue the bimetal disc) rather than the switch itself. Replace the air filter on schedule and the limit lasts.