When the Furnace Says “Nope” ❌
If you're working with a 3.5-ton R-32 AC and gas furnace system, and the high-limit switch keeps cutting off the burner, it’s more than just annoying—it’s a warning shot. That switch is a safety backup, not a daily player.
Instead of resetting it and crossing your fingers, let’s dig into what causes these trips, how to fix them, and how to prevent callbacks down the line. Whether you’re installing a system from The Furnace Outlet or troubleshooting one that’s gone sideways, this guide’s got your back. 🔧
What Does the High-Limit Switch Actually Do?
The high-limit switch is your furnace’s thermal watchdog. It monitors the heat exchanger temperature—and if that surface gets too hot (typically over 170–200°F depending on the model), the switch opens and shuts off the burners. No heat = no fire hazard.
It’s not a defect—it’s protection. But when it’s tripping constantly? That’s a sign something’s seriously off. And if you ignore it long enough, the switch itself will fail, or worse—you’ll damage the heat exchanger or blower motor. 💥
Top 6 Reasons Limit Switches Trip on 3.5-Ton R-32 Systems
1. Restricted Airflow (AKA Dirty Filter Syndrome)
This is the #1 cause—and also the easiest to fix.
✅ Check the return air filter.
✅ Look for collapsed filter media or cheap aftermarket junk.
✅ Confirm the blower motor is running at the correct speed.
Air can’t move = heat builds up = switch trips. Simple physics. If you're consistently undershooting airflow for a 3.5-ton system (you want around 1400–1600 CFM), you’re setting yourself up for trouble.
If you need help with airflow math, ACHR News has a solid breakdown.
2. Blower Speed Set Too Low
Factory blower settings don’t always match your duct layout. A 3.5-ton furnace typically requires high or med-high blower tap settings for proper air delivery.
✅ Check the DIP switch or speed tap settings.
✅ Adjust ECM profiles or PSC motor wiring as needed.
✅ Measure static pressure across the coil to verify airflow.
Use a manometer and verify you’re staying below 0.5” WC total static (return + supply). For static pressure tips, HVAC School's guide on airflow is a great reference.
3. Clogged or Undersized Ductwork
Sometimes the duct system is the real villain—especially in retrofits where the homeowner upgraded the equipment but left the old metal in place.
🔍 Check for crushed flex duct, leaky boots, or kinked runs.
📐 Measure return grille size and total cross-sectional area.
📏 Use a ductulator (or app) to verify compatibility with 1400–1600 CFM.
A common red flag? A 14” return for a 3.5-ton system. That’s almost always undersized.
Check out ACCA Manual D if you’re doing duct design the right way.
4. Dirty or Blocked Evaporator Coil
Even if airflow was good on install day, months (or years) of dust can create a wall of thermal resistance in the evaporator coil. That mess kills airflow and overheats the furnace.
🧽 Inspect the coil during your tune-up.
🔦 Shine a flashlight from the opposite side to check for blockage.
🛑 Don’t clean with acid—use a non-corrosive coil cleaner and low-pressure rinse.
If the blower can’t push air through the coil, your limit switch is going to earn overtime.
5. Malfunctioning Limit Switch or Loose Sensor Mount
Sometimes, the switch itself is the issue.
🛠️ Check the temperature rise with a thermometer at the supply and return plenums.
📏 Typical range is 35°F–65°F for gas furnaces.
🔌 Confirm the sensor is mounted tight to the heat exchanger—loose sensors will read hot even when they’re not.
If everything else looks good and your switch is tripping at normal temps, you might have a faulty sensor. Replace it.
Need guidance on measuring temp rise and diagnostics? Carrier’s service bulletin on limit switches (search their technical section) is worth bookmarking.
6. Flame Rollout or Burner Issues
This one’s less common but more dangerous.
🔥 Look for delayed ignition or flame rollout at the burner manifold.
📹 Use a mirror or phone camera to inspect ignition sequence.
🚨 If flames are escaping the combustion chamber, shut it down and check for:
-
Cracked heat exchanger
-
Obstructed flue
-
Over-firing
A rollout trip can sometimes “look” like a high-limit fault, especially on older boards with generic fault codes.
Tools You’ll Want for This Job
✅ Digital manometer (static pressure testing)
✅ Thermocouple or IR thermometer (temp rise)
✅ Multi-meter with continuity and temp modes
✅ Combustion analyzer
✅ Filter puller and coil brush
Some pros also use apps like MeasureQuick to log live system data. A solid read: MeasureQuick overview for HVAC techs.
How to Fix It (Step-by-Step)
Let’s say your switch is tripping. Here’s how I’d walk it down:
-
Kill power and remove the furnace doors.
-
Pull the filter and inspect for debris or collapse.
-
Check the blower motor speed setting.
-
Measure static pressure with a manometer.
-
Inspect evaporator coil visually or with flashlight.
-
Measure supply/return temperature rise.
-
Reset the limit switch and restart the system.
-
Watch ignition cycle and burner pattern.
If airflow checks out and temp rise is normal, you may have a bad sensor. Replace it, and test again.
How to Prevent Future Trips (And Save Yourself a Callback)
🏷️ Tag the unit with proper filter sizes and MERV rating
🧽 Recommend coil cleanings every 12–18 months
📐 Upgrade ductwork if return is undersized
⚙️ Confirm ECM settings on every service call
📄 Leave your service records near the furnace for the next tech
Homeowners don’t always realize these issues are preventable. Educate them. Give them a filter schedule and tell them why airflow matters.
Final Thoughts: Don’t Just Reset—Diagnose
Limit switches are like smoke alarms. You wouldn’t just yank the battery and walk away, right? Same goes here.
These 3.5-ton R-32 systems, especially ones like the high-efficiency Goodman and Daikin bundles on The Furnace Outlet, are built to run clean and efficient. But they depend on airflow. Skip that, and the system will protect itself—at your expense.
Curious about how to handle R-32 safely for these systems? Visit my guide: What Every Tech Needs to Know About 3.5-Ton Systems.
So next time that limit switch starts acting up, remember:
Don’t reset and run. Dig in, diagnose, and fix it the right way.
- Mark, your go-to tech
