HH18HA449 – MANUAL LIMIT SWITCH (MICRO, LARGE BASE, L350°)

If your furnace is short-cycling (turning on and off repeatedly), a failed limit switch may be the cause. This Carrier OEM manual reset switch opens at 350°F — that rating makes it a flame rollout safety device. Push the button to restart. If it keeps tripping, get the combustion checked.

Specs: Opens at 350°F, manual reset, micro size with large base, SPST

Replaces: HH18HA449

Fits: Carrier, Bryant, and Payne furnaces calling for HH18HA449.

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

• TypeManual-reset micro limit switch
• Trip TemperatureL350°F
• Mount StyleLarge base
• ResetManual (push-button after trip)
• OEM CompatibilityUniversal

Cross-reference numbers

This part replaces the following OEM and aftermarket numbers:

• HH18HA449

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

Fits these models

• Universal manual-reset micro limit switch — secondary safety on residential gas furnaces and electric heat kits; requires push-button reset after each trip
• Common applications: most residential furnace rollout safeties and heat-kit secondary limits
• Match the L-rating, mount style (large base / teardrop / wide mount), and connector style to your existing switch

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.