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Heating & Cooling Maintenance, Repair & Troubleshooting FAQ from NortherHeat
Check out these tips and explanations, below, from our Heating and Cooling Expert. Of course, you're also welcome to post questions to this board for more information: Just click the "Ask about Heating & Air Contidioning" button at the top of the page.
Thermostat wire color codes explained. Please note. Not all installers use the proper color codes when they install or replace equipment. It may be necessary go go to the furnace or outdoor unit to verify what wires are for what purpose. Also be sure there are no splices in the wire that could change the wire colors between components.
R (red) or Rh (Red heat) 24 volts from equipment.
Rc (Red cooling) 24 volts from transformer in cooling equipment.
Note: If only furnace has a transformer and cooling equipment does not jumper Rc and Rh.
W (White) sends 24 volts to furnace control to start the heating cycle.
W2 (no standardized wire color, usually whatever wire color is available) controls second stage heat.
Note: W2 is most often used for heat pumps to control what is called emegency heat or Auxillary heat, and most often will use the white wire.
Y (yellow) often (blue) is used, controls cooling unit (outdoor condensing unit) also is used for heat pump heat.
Y2 (no standardized wire color, usually whatever wire color is available) controls second stage cooling.
G (Green) controls the fan "on" operation of the furnace/air handler. Also often is used to start the blower for many electric furnaces.
C (common) most often brown but can also be black or other color available to installer.
O (orange) Energizes heat pump reversing valve for cooling (Trane and most other brands).
B (Blue) Energizes heat pump reversing valve for heating (Rheem, Ruud and Weatherking).
A heat pump is an air conditioner that can change refrigerant direction.
Just as an air conditioner absorbs heat inside the home in the summertime and expels it outside. In the winter a heatpump reverses refrigerant flow, absorbs heat outside and expels it inside the home for heat.
Heat pumps are very efficient when compaired to other heating sources. However when the outdoor temperature falls very low, lets say below 25 degrees F. there is very little heat outside to absorb and their efficiency decreases.
Temeratures from the register will typically be around 90 degrees F. Thus people often feel as though their furnace is blowing cool air. Because of this low vent temperature a heat pump will run a long time to keep a home warm.
The outdoor section of the heat pump becomes very cold and will form frost, and even some ice. To correct this problem the heatpump will go into a defrost cycle to clear itself from all this frost. Basically it is running like an air conditioner now. Often steam will be seen when it is defrosting, the outdoor fan stops turning and the back-up heat source will start to offset the the fact that the air conditioner is now on.
The heat pump should be installed up high on stands to keep it out of the snow, and give all the defrosted water a place to go.
All heat pumps with a back up heat source (this is all cold climates) need a 2 stage heat, pump, thermostat, or other electrical modifications must be made to the out door unit to work properly.
Standing pilot.
Pilot light that is constantly lit. Typical in older furnaces, uses this flame to ignite the main burners.
Intermittent pilot Pilot light ignites each time the furnace starts.
Two main types are
1. glow plug (called a hot surface ignitor) ignites the pilot light.
2. spark (a small electrical spark) ignites the pilot light.
Direct ignition
Similar to intermittent pilot in that a spark or hot surface ignitor is used to light the burners, most common method in newer furnaces.
How do I know if the problem is in the thermostat?
Disconnect power at the furnace or breaker box.
Remove thermostat from wall and jump R (Red) to W (White)
Restore power, did the furnace start to run? If yes the thermostat was bad. If no the problem is in the thermostat wire or furnace.
Disconnect power to furnace again. At the furnace find out how the thermostat is connected to the controls. Again jumper R and W. Retore power. Did the furnace start? If yes the problem was the wire if no the problem is in the furnace.
Using a volt ohm meter. Turn the thermostat to heat, turn it up above the room temperature, at the furnace check for 24 volts betweem W and common, chassis, ground, if 24 volts is present at W the thermostat is working fine.
Same method can be used for fan (G)
and cooling (Y)
Red wire should have 24 volts at all times.
Note Most furnaces have a safety door switch for the blower compartment, this button must be depressed to power on the furnace.
Age of equipment, best you can tell...
Brand (Manufacturer)...
Efficiency...see here
Natural gas, LP, forced air, boiler, heat pump, air conditioner...
Package unit (A/C and heat in one unit), split system, (furnace inside A/C outside)...
Natural draft furnace.
24 vollts is always present to the thermostat R terminal.
When the thermostat closes its switch to call for heat, 24 volts is sent out of the thermostat on the W terminal.
This 24 volts goes back into the furnace, then typically through 1 or 2 safety devices to the gas valve.
If the standing pilot is lit. The gas valve opens, and the gas to the burners is then ignited by the pilot light.
The heat exchanger is heated untill a device tells the blower to come on, usually a fan and limit control. Also see camstat fan and limit control.
When the thermostat is satisfied, the switch in the thermostat opens, the gas valve closes, stopping fuel and the fan continues to blow untill the fan and limit cools, turning off the blower.
High efficiency operation.
24 vollts is always present to the thermostat R terminal.
Thermostat sends 24 volts through white wire to furnace.
Furnace checks pressure switch, and safety switch's for proper positions (open/closed).
If vent and vacuum hoses are clear the pressure switch, closes. (only a few pressure switch's open to prove venting)
After inducer plower purges the heat exchanger, either a pilot lights, a hot surface ignitor (HSI), or a spark begins.
If the furnace is an intermittant pilot model, once the pilot lights, current is sent through the flame by a flame sensor, if current is present through the flame the main gas valve opens and the burners ignite.
If the furnace is direct ignition, The HSI or spark ignites the flame. A flame sensor passes current through the flame, if current is present the main gas valve stays open and heat exchanger begins to warm up.
Most high efficiency furnaces then use an electronic timer on the circuit board to start the blower after the burners are proved.
Thermostat opens when desired temperature is reached.
The gas valve closes, burners extinguish, and the timer starts again to turn off the fan.
Linear Mode
