High Limit Issues in Furnace Systems

If you've been in the HVAC industry for any amount of time, you've encountered a furnace displaying a high limit code. The high limit is a critical safety device that shuts down gas flow to a furnace when excessive temperature rise occurs. While changing a filter often resolves the issue, taking a few extra minutes to examine the entire system can prevent callbacks and ensure long-term reliability.

Understanding High Limit Operation

The high limit switch monitors the temperature of the heat exchanger. When the temperature exceeds safe operating limits (typically 160-200°F depending on the furnace model), the switch opens and interrupts the 24V signal to the gas valve. This prevents overheating and potential heat exchanger damage.

Symptoms of High Limit Tripping

When the high limit opens due to excessive temperature rise, several symptoms appear:

• The 24V signal feeding the gas valve is interrupted
• The main blower continues running to cool the heat exchanger until the switch closes again
• The house cools down and fails to maintain temperature, even though the furnace appears to be "running"
• The furnace may lock out for a period of time after multiple trips
• Short cycling may occur as the limit opens and closes repeatedly

Required Tools

Before beginning diagnosis, gather the following tools:

• Screwdriver and nut driver set
• Digital multimeter
• Temperature probes (with magnet attachments preferred)
• Manometer with static pressure tips
• TrueFlow grid or equivalent airflow measurement device (for precise CFM verification)
• Jumper wires
• Spare high limit switch (common ratings: check manufacturer specs)
• Flashlight for visual inspection

Diagnostic Procedure

Step 1: Airside System Verification

Excessive temperature rise is almost always caused by insufficient airflow. Start with these checks:

Filter and Air Restrictions

• Check and change the filter. A dirty filter is the most common cause. High MERV filters (13-16) can create excessive pressure drop even when relatively clean. Consider recommending a lower MERV rating (8-11) for residential applications.
• Verify all supply and return registers are open. Many homeowners close registers in unused rooms, not understanding this restricts airflow and can damage the system. Explain that the furnace is sized for the entire duct system and needs adequate airflow to operate safely.
• Inspect for blocked returns. Check for furniture, curtains, or other obstructions covering return grilles.

Internal Component Inspection

• Inspect the evaporator coil. A dirty AC coil significantly restricts airflow. On 90%+ efficiency furnaces, also check the secondary heat exchanger for blockage or debris.
• Remove and inspect the blower wheel. A dirty blower wheel can reduce airflow by 30-50% even when it doesn't look severely restricted. Look for dust buildup on the leading edges of the blades.
• Check blower motor capacitor. A weak capacitor can cause the blower to run at reduced speed, decreasing airflow.

Ductwork Evaluation

• Measure and evaluate duct sizing. Use proper duct sizing calculations to verify the system can move required CFM. General rule: 400 CFM per ton of cooling, or follow furnace manufacturer's airflow requirements (typically 1000-1400 CFM for residential units).
• Look for crushed or kinked flex duct, disconnected joints, or undersized trunk lines.

Static Pressure Testing

Static pressure testing is critical for identifying restriction locations:

1. Drill test ports if not already present (1/4" hole, 6-12" from furnace on supply and return sides)
2. Measure total external static pressure (TESP) with blower running
3. Compare to manufacturer specifications (typically 0.5" w.c. maximum, but check rating plate)
4. Measure supply and return separately to identify which side has the restriction:
   • High return pressure (negative): Filter, return duct, or evaporator coil issue
   • High supply pressure (positive): Supply duct restriction or closed registers
   • Both high: Overall undersized duct system

Acceptable static pressure ranges:

• Below 0.4" w.c.: Good
• 0.4-0.5" w.c.: Acceptable, but monitor
• 0.5-0.7" w.c.: Marginal, airflow likely reduced
• Above 0.7" w.c.: Excessive, will cause problems

Temperature Rise Verification

Temperature rise is the difference between return air temperature and supply air temperature:

1. Measure return air temperature 6-12" before the furnace
2. Measure supply air temperature 6-12" after the furnace (in supply plenum)
3. Calculate temperature rise: Supply temp - Return temp = Rise
4. Compare to rating plate specifications. Most furnaces list an acceptable range (example: 40-70°F rise)
5. Aim for the middle of the range (55°F in the above example) to allow for filter loading over time

If temperature rise is too high: Airflow is insufficient - continue troubleshooting airside issues.

If temperature rise is too low: Possible overfiring or excessive airflow - check gas pressure.

CRITICAL: Return Air Temperature Monitoring

This is frequently missed and causes callbacks. Temperature rise can be within specification, but the furnace can still trip on high limit due to elevated return air temperature.

Maximum return air temperature: Most furnace manufacturers specify a maximum return air temperature of 80°F. Always verify this in the installation manual, but 80°F is the industry standard.

The problem: If your return air temperature is elevated (85-90°F or higher), even a "normal" temperature rise will push you over the high limit rating.

Example scenario:

• Return air temperature: 90°F
• Temperature rise: 56°F (within spec of 35-65°F)
• Supply air temperature: 146°F
• High limit rating: 160°F
• Problem: You're only 14°F below the limit with no safety margin

As the system continues to run, heat buildup in the space or ductwork can push that return air temperature higher, causing the supply air to exceed the high limit setting and trip the safety.

How to diagnose elevated return air:

1. Monitor return air temperature over time - Don't just take one reading. Watch it for 10-15 minutes of continuous operation. If it's climbing steadily, you have a problem.

2. Look for supply-to-return short-circuiting:

   • Supply registers too close to return grilles (within 10 feet in same room)
   • Supply registers blowing directly at return grilles
   • Both supply and return in the same small space with inadequate mixing
   • Door undercuts too small, causing pressure imbalances that force air back through returns

3. Check for inadequate return air pathways:

   • Closed bedroom doors without transfer grilles or jump ducts
   • Single central return trying to serve entire house
   • Return grilles in wrong locations (example: return in hallway but supplies in bedrooms with closed doors)

4. Evaluate building load and insulation:

   • Poor insulation causing excessive heat gain
   • Undersized system running continuously in hot attic or mechanical room
   • Furnace located in unconditioned space with high ambient temperature

Solutions for elevated return air temperature:

• Relocate return grilles away from supply registers (minimum 10 feet separation recommended)
• Add transfer grilles or jump ducts to isolated rooms so conditioned air can return to the system
• Install additional return drops in key locations to improve air circulation
• Change register type or deflection to prevent direct supply-to-return airflow
• Improve door undercuts to allow airflow (minimum 1" gap, 2" preferred for bedrooms)
• Install door louvers in problematic rooms
• Insulate or relocate ductwork if ambient conditions are contributing

Key takeaway: Always monitor return air temperature for 10-15 minutes during operation, not just at startup. If return air exceeds 80°F or climbs continuously, you have a system design issue that must be addressed regardless of what the temperature rise calculation shows.

Gas Pressure Verification

Even on airside issues, always verify proper gas pressures:

• Manifold pressure should match rating plate (typically 3.5" w.c. for natural gas)
• Inlet pressure should be within manufacturer specs (typically 5-7" w.c. for natural gas)
• Incorrect gas pressure can cause incomplete combustion or excessive heat output

Step 2: Electrical System Diagnosis

If all airside components check out and the high limit continues tripping, investigate the electrical system:

Thermostat and Wiring Verification

1. Remove all thermostat wires from the furnace control board
2. Use a jumper wire to simulate a heat call (jump R to W terminals)
3. Verify normal operation with direct call for heat
4. If the system operates normally with a jumper but not with the thermostat connected, the issue may be thermostat-related or a wiring problem

Grounding and Power Quality

Poor grounding can cause erratic control board behavior:

• Verify dedicated circuit from the furnace all the way back to the electrical panel
• Check all junction boxes to ensure ground wire continuity (many electricians fail to continue the ground through junction boxes)
• Confirm all grounding points on the furnace are clean and secure
• Test for proper ground using a multimeter (should show very low resistance to ground)

High Limit Switch Testing

1. Allow the switch to cool completely (15-20 minutes after last heat cycle)
2. Disconnect wires from the limit switch terminals
3. Measure resistance across the switch:
   • Should read 0-2 ohms when closed (cool)
   • Should read "OL" (open/infinite) when hot/tripped
4. Test for grounding: Measure from each terminal to the metal cabinet. Should read "OL" (no continuity). If you have continuity to the cabinet, the switch is grounded and must be replaced.
5. If the switch tests good but you suspect it's faulty, monitor it with your meter while the furnace heats up. Watch for it to open prematurely.

Control Board Input Verification

If the limit switch tests good, verify the board is receiving the signal:

1. Locate the limit circuit on the wiring diagram (usually labeled "LIMIT" or "LS")

2. Identify where the 24V signal exits and returns to the board (typically through a Molex connector)

3. With the system calling for heat, backprobe the Molex plug to verify 24V is present at the input terminals

4. If 24V is absent, work backward through the circuit toward the transformer until you find voltage. The problem exists between where you have voltage and where you don't:

   • Broken or cut wire
   • Pinched wire with damaged insulation
   • Loose connection
   • Corroded terminals

5. If 24V is present at the input, remove the Molex connector and inspect it:

   • Verify female pins are fully seated in the connector housing
   • Look for signs of corrosion, heat damage, or loose fit
   • Reconnect and retest

6. If voltage is present but the board still doesn't respond, the control board is not recognizing the input signal and likely needs replacement

Step 3: Less Common Causes

If you've exhausted the above steps, consider these less common issues:

Flame Rollout

• High limit may be tripping due to flame rollout rather than temperature rise
• Inspect the rollout limit switch (usually mounted lower on the burner box)
• Check for blocked flue, cracked heat exchanger, or burner misalignment

Failed Heat Exchanger

• Cracks or holes in the heat exchanger can cause irregular airflow patterns and hot spots
• Perform visual inspection with flashlight and mirror
• Consider combustion analysis if suspected

Defective Gas Valve

• Rare, but a gas valve that doesn't close completely can cause residual heating
• Monitor with manometer to ensure manifold pressure drops to zero when the valve closes

Short Cycling from Other Causes

• Oversized furnace for the space
• Pressure switch issues (on high-efficiency models)
• Failed flame sensor causing ignition retries

Prevention and Customer Education

After resolving the issue, educate your customer:

• Filter maintenance schedule: Show them how to check and change filters monthly during heating season
• Register management: Explain why closing registers can damage the system
• Annual maintenance: Recommend yearly professional service before heating season
• Symptoms to watch for: Short cycling, unusual noises, or reduced comfort

Summary Checklist

Airside Issues (90% of cases):

• ☐ Filter dirty or too restrictive
• ☐ Closed or blocked registers
• ☐ Dirty evaporator coil
• ☐ Dirty blower wheel
• ☐ Undersized or restricted ductwork
• ☐ High static pressure
• ☐ Weak blower motor or capacitor

Electrical Issues:

• ☐ Failed high limit switch
• ☐ Poor grounding
• ☐ Damaged wiring
• ☐ Thermostat issues
• ☐ Failed control board

Other Issues:

• ☐ Incorrect gas pressure
• ☐ Flame rollout
• ☐ Cracked heat exchanger
• ☐ Defective gas valve

By following this systematic approach, you can efficiently diagnose and resolve high limit issues while building customer confidence and reducing callbacks.