Home Furnace and Air Conditioning Thermostat

craftman

Repair a Home Furnace and Air Conditioning Thermostat

White-Rodgers Mechanical Thermostat

Home-Cost.com 2006

The furnace thermostat is the control device that determines when and how much heat will be produced by the furnace. This tutorial describes how to troubleshoot the low voltage thermostat often made by Honeywell, White-Rodgers, Lux, Robertshaw and others.  This type of thermostat runs off of a transformer reducing the incoming line voltage from 120 volts down to between 12 and 24 volts, depending on your furnace.When a furnace does not work it may be caused by problems with the furnace or the thermostat.
See How to Troubleshoot a Gas Furnace for information on possible furnace problems, causes and repairs.
But now let's take a look at troubleshooting problems that could be caused by a faulty thermostat and how to make the repairs.

PROBLEM: Furnace Produces No Heat

Furnace Produces No Heat Possible Causes

• Circuit breaker tripped or fuse is blown.
• Thermostat components dirty.
• Loose wires or open connection.
• Bad battery on digital electronic thermostats.
  

Possible Repairs

• Reset circuit breaker or replace fuse.
• Remove thermostat cover and gently clean components especially the bimetallic coil and the switch contact surfaces if the thermostat does not have a mercury switch.
• Set the thermostat to its lowest setting and clean the bimetallic coil with a soft brush or compressed air. Set the thermostat to its highest setting and clean the coil again. Reset the thermostat to its desired setting.
• Tighten any loose screw terminals. If a wire is loose from its terminal, place wire back on terminal clamp and tighten screw.
• Check for loose connection or disconnected wire on low-voltage transformer usually mounted on furnace
• On electronic models, check to see if the display is working indicating power.  If it is not working, replace the batteries in the electronic thermostat.
  

Heat Either Does Not Reach or Exceeds the Set TemperaturePossible Causes

• Thermostat mounted crooked.
• Heat anticipator not set correctly.
  

Possible Repairs

• Remove cover of thermostat and loosen screws holding unit to wall.  Level the thermostat. Re-tighten screws and replace cover.
• To adjust the heat anticipator, please see Heat Anticipator Adjustment.
  

Furnace Turns On and Off Frequently ("short cycling")Possible Causes

• Dirty thermostat components
• Heat anticipator not set correctly.
  

Possible Repairs

• Remove thermostat cover and gently clean components especially the bimetallic coil and the switch contact surfaces if the thermostat does not have a mercury switch.
• To adjust the heat anticipator, please see Heat Anticipator Adjustment.
  

By Bob Formisano
http://homerepair.about.com/od/h ... trblshoot_tstat.htm

altitude

在加拿大生活就是要成为全能手。昨天去RONA看了看，工具真全。

craftman

就是。至少知道一些，心里才踏实一点。