Make:  Any

Model or Series:  Any

Type:  80% Furnace

Problem Description:  Furnace locking-out on Code 4 - High Limit

Observed Symptoms:  

• Temperature rise higher than furnace's rated temperature rise.
  • The furnace nameplate should show the rated temperature rise.

Things to Check:  

• Check Return Static Pressure (between furnace and filter) - [should be less than .5" w.c.]
  • Higher than normal static pressure indicates restricted return airflow
    • Clogged air filter
    • Restrictive air filter
    • Return duct too small
    • Return registers covered
• Check Supply Static Pressure (between furnace and a/c coil) - [should be less than .5" w.c.]
  • Higher than normal static pressure indicates restricted supply airflow
    • Clogged A/C coil
    • Supply duct too small
    • Supply registers closed
• Check Heating Fan Speed
  • Most newer furnaces have separate fan outputs for heating and cooling
  • Try increasing the heating fan speed
• Check Manifold (outlet) Gas Pressure - [should be between 2.5 and 3.5" w.c. for Nat. gas, 9 to 11" w.c for LP]
  • If static pressures and airflow are correct, you can try lowering the gas pressure.
  • Try to get the actual temperature rise within the furnace's rated temperature rise range.
  • Stay between 2.5 and 3.5" w.c. for Nat. gas, 9 to 11" w.c. for LP