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Page 3 - LIMIT SWITCHES

# Limit Switches - Furnace Safety and Fan Control Switches

Shop replacement limit switches for gas furnaces and air handlers. We stock high limit switches, fan limit controls, rollout switches, and auxiliary limit switches from all major HVAC brands at our Westland, Michigan warehouse.

## Why Buy Limit Switches From Technical Hot & Cold?

**Complete Limit Switch Selection**
From simple snap-disc switches to combo fan/limit controls, we stock the safety switches your furnace needs.

**OEM Quality**
Genuine manufacturer switches and premium aftermarket replacements from trusted brands like White-Rodgers and Honeywell.

**Same-Day Shipping**
Safety switches in stock ship today when ordered before 3PM EST from our Westland warehouse.

**Expert Switch Identification**
Limit switches vary by temperature rating. Our team helps identify the correct replacement for your furnace.

## What is a Limit Switch?

A limit switch is a safety device that:
- Monitors furnace temperature
- Shuts off burner if overheating occurs
- Controls blower fan operation (on combo switches)
- Prevents dangerous over-temperature conditions
- Required by safety codes

**Located:** Mounted on furnace plenum or heat exchanger
**Function:** Temperature-activated safety switch

## Types of Limit Switches

### High Limit Switches
Safety switch that shuts off burner at set temperature.

**Features:**
- Opens (breaks circuit) at specific temperature
- Manual or auto-reset types
- Mounted on furnace plenum
- Typically 160-200°F setting

**Common Settings:**
- 160°F, 170°F, 180°F, 190°F, 200°F
- Setting depends on furnace design
- Must match OEM specification

**Types:**
- **Manual Reset** - Must be manually reset after trip
- **Auto Reset** - Automatically resets when cooled

### Fan Limit Controls (Combination)
Combined fan control and high limit safety.

**Features:**
- Controls when blower turns on/off
- High limit safety protection
- Adjustable fan on/off settings
- Usually has dial with adjustable range

**Typical Settings:**
- Fan ON: 90-130°F adjustable
- Fan OFF: 70-110°F adjustable
- High Limit: 170-200°F fixed

**Advantages:**
- One device does two jobs
- Standard on many furnaces
- Adjustable for comfort

### Rollout Switches
Safety switch for flame rollout protection.

**Features:**
- Mounted near burners
- Detects flame outside combustion area
- Manual reset (usually)
- Indicates serious problem if tripped

**Settings:**
- Typically 250-350°F
- Lower than high limit
- Located at burner area

**Important:**
If rollout switch trips, DO NOT simply reset!
- Indicates dangerous condition
- Cracked heat exchanger possible
- Blocked vent possible
- Requires professional diagnosis

### Auxiliary Limit Switches
Additional safety switches for specific areas.

**Applications:**
- Heat exchanger protection
- Duct high temperature protection
- Multi-zone system safety
- Commercial equipment

### Spill Switch
Detects draft spillage near draft hood.

**Features:**
- Monitors for combustion gases
- Manual reset
- Indicates venting problems
- Safety shutoff

## Popular Limit Switch Brands

**Premium Brands:**
- **White-Rodgers** - Industry standard limit switches
- **Honeywell** - Quality safety controls
- **Therm-O-Disc** - Precision limit switches
- **Emerson** - Reliable safety switches

**OEM Brands:**
- **Carrier** - OEM limit switches
- **Trane** - OEM limit controls
- **Lennox** - OEM safety switches
- **Goodman** - OEM limit switches

## How to Select the Right Limit Switch

### Identify Your Switch Type:

1. **Read Old Switch**
   - Part number on switch
   - Temperature rating marked
   - Auto or manual reset
   - Voltage rating

2. **Check Furnace Manual**
   - Specifies correct switch
   - Temperature settings
   - Part number

3. **Common Information Needed:**
   - Temperature setting (°F)
   - Voltage (24V or 120V)
   - Auto or manual reset
   - Number of terminals
   - Mounting type

## Temperature Settings Guide

### High Limit Switch Settings:

**80% Furnaces (Standard Efficiency):**
- 170-200°F typical
- 180°F most common
- Higher for larger plenums

**90%+ Furnaces (High Efficiency):**
- 160-180°F typical
- 170°F most common
- Lower plenum temperatures

**Factors Affecting Setting:**
- Furnace size and BTU rating
- Plenum size
- Airflow (CFM)
- Return air temperature
- Duct design

### Fan Limit Control Settings:

**Fan ON Temperature:**
- 90-130°F adjustable
- Typically set 90-110°F
- Higher setting for short cycles

**Fan OFF Temperature:**
- 70-110°F adjustable
- Typically set 80-90°F
- Must be below Fan ON setting

**Adjustment for Comfort:**
- Higher Fan ON = fewer cold air blasts
- Lower Fan OFF = more heat extracted
- Balance comfort and efficiency

## Limit Switch Troubleshooting

### Symptoms of Bad Limit Switch:

**Furnace Short Cycles:**
- Burner shuts off too soon
- Limit switch opening prematurely
- May be faulty or wrong setting
- Check for airflow restrictions first

**Furnace Won't Start:**
- Limit switch stuck open
- Failed in open position
- Manual reset switch needs reset
- Check continuity with meter

**Blower Runs Continuously:**
- Fan limit stuck closed
- Switch welded contacts
- Blower won't turn off
- Temperature dial may be faulty

**Frequent Limit Trips:**
- Not necessarily bad switch!
- Indicates underlying problem:
  - Dirty filter (most common)
  - Dirty blower wheel
  - Closed/blocked registers
  - Undersized ductwork
  - Blower motor failure

### Diagnostic Steps:

1. **Check for Reset Button**
   - Manual reset switches have button
   - Reset and see if furnace operates
   - If trips again immediately = problem

2. **Test Continuity**
   - Power OFF!
   - Remove one wire
   - Use multimeter on Ohms
   - Should show continuity when cool
   - Should open when heated

3. **Check Installation**
   - Must have good thermal contact
   - Mounted securely to plenum
   - Sensing element in correct position

4. **Verify Temperature**
   - Use thermometer near switch
   - Compare to switch rating
   - Switch should open at rated temp

## Limit Switch Installation

**Safety Critical:**
1. Turn off all power (breaker and switch)
2. Turn off gas
3. Let furnace cool completely
4. Verify no voltage before touching wires

**Replacement Steps:**

1. **Photo Connections**
   - Document wire colors
   - Note terminal positions
   - Label if necessary

2. **Disconnect Wires**
   - Remove one at a time
   - Note which terminal
   - May need small screwdriver

3. **Remove Old Switch**
   - Usually mounted with 1-2 screws
   - May have mounting bracket
   - Note exact position

4. **Install New Switch**
   - Same position as old switch
   - Secure mounting screws
   - Ensure good contact with plenum
   - Proper orientation (sensing element down)

5. **Reconnect Wires**
   - Match wire colors to photos
   - Secure terminal screws
   - Ensure tight connections
   - No exposed wire

6. **For Fan Limit Controls:**
   - Set fan ON temperature (typically 110°F)
   - Set fan OFF temperature (typically 90°F)
   - High limit is usually fixed

7. **Test Operation**
   - Restore power and gas
   - Call for heat
   - Verify burner starts
   - Verify fan starts at set temperature
   - Monitor complete cycle

## Fan Limit Control Adjustment

**Setting Fan ON Temperature:**

1. Start with 110°F
2. Observe burner to fan delay
3. Too long delay? Lower setting
4. Cold air blast? Raise setting
5. Typical range: 90-130°F

**Setting Fan OFF Temperature:**

1. Start with 90°F
2. Observe fan run time after burner off
3. Want to extract residual heat
4. Too long run? Raise setting
5. Typical range: 70-110°F

**Rules:**
- Fan OFF must be below Fan ON
- Minimum 10-20°F difference
- Don't set too high (wastes energy)
- Don't set too low (cold air blasts)

## Common Installation Mistakes

❌ **Wrong Temperature Rating**
- Must match furnace requirements
- Too low = nuisance trips
- Too high = safety risk

❌ **Poor Thermal Contact**
- Switch not tight against plenum
- Senses incorrect temperature
- Causes erratic operation

❌ **Wrong Switch Type**
- Manual reset vs auto reset
- Voltage rating mismatch
- Wrong terminal configuration

❌ **Wiring Errors**
- Reversed wires
- Wrong terminals
- Loose connections

## When Limit Switch Trips

**DO THIS:**
1. Turn off furnace
2. Check and replace filter
3. Verify all registers open
4. Check blower wheel (clean if dirty)
5. Reset switch if manual reset
6. Try operation again

**If Trips Again:**
- Indicates serious problem
- Do not keep resetting
- Call HVAC professional
- Possible causes:
  - Cracked heat exchanger
  - Blower motor failure
  - Severe duct restriction
  - Undersized ductwork

**For Rollout Switch:**
- DO NOT simply reset
- Indicates dangerous condition
- Requires professional inspection
- Possible causes:
  - Cracked heat exchanger
  - Blocked vent
  - Burner problems
  - Carbon monoxide risk

## Limit Switch FAQs

**Q: Why does my limit switch keep tripping?**
A: Usually not a bad switch. Check:
1. Dirty filter (most common cause)
2. Dirty blower wheel
3. Closed or blocked vents
4. Ductwork too small
5. Blower motor not working properly

**Q: Can I adjust the high limit setting?**
A: On most switches, NO. High limit is factory set for safety. Some commercial switches allow adjustment - follow manufacturer specs.

**Q: What's the difference between manual and auto reset?**
A: 
- **Manual Reset**: Must physically push button to reset. Used where you want to know it tripped.
- **Auto Reset**: Automatically resets when cooled. More convenient but hides problems.

**Q: Can I use a different temperature rating?**
A: Not recommended. Use the temperature specified by furnace manufacturer. Wrong setting = safety risk.

**Q: Why does my blower run too long?**
A: Fan OFF setting may be too low, or fan limit switch may be failing. Adjust fan OFF temp higher.

**Q: Can I bypass the limit switch?**
A: NEVER! Limit switches are critical safety devices. Bypassing creates fire and safety hazards.

**Q: How long do limit switches last?**
A: 10-20 years typical. They can last the life of furnace if not frequently tripping.

## Preventive Maintenance

Reduce limit switch trips:
- Change filter monthly
- Keep blower wheel clean
- Don't close too many vents
- Annual furnace maintenance
- Monitor furnace operation
- Address issues promptly

## Need Help Selecting a Limit Switch?

Our HVAC parts specialists are here to help! Contact us:
- **Phone:** [Your phone number]
- **Email:** [Your email]
- **Hours:** [Your business hours]
- **Location:** Westland, MI - Local pickup available

**Have your furnace model number or old switch information ready. Browse our limit switch selection below or contact us for expert assistance!**

Can't find what you're looking for?

If you don't see your model or part, call us! Our friendly technicians are standing by. We can help you find the part you need with guaranteed fitment. Speak with us today at (734) 326-3900.

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