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This article describes several tricks of the trade used by professionals when installing room thermostats and wiring them up. It describes the standard wire color conventions for HVAC thermostats.
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9 Wiring Tips for Room Thermostats: Helpful Pointers Regarding 24V Thermostat Wiring & Heat-Anticipators
Let's note first that the type of heating system installed determines what thermostat wires are necessary and how they must be connected. Our complete thermostat wiring guide for various types of heating or cooling systems and thermostat brands and models is found at THERMOSTAT WIRE CONNECTIONS. Additional wiring details are at THERMOSTAT WIRING DIAGRAMS if you prefer those.
Also see HVAC SYSTEM TYPES, and see for our complete guide to wiring different types of heating, cooling systems & various brands and types of room thermostats.
- Thanks to reader " Helpful Pointers" Regarding 24V T, 10/7/2012
Tips for figuring out which thermostat wires do what: how to identify (R) (W) (B) wires. These tips are from the 1949 Honey Heating Control Handbook [copy on file] for figuring out the function of thermostat wires whose color cannot be determined (for LOW VOLTAGE series 10 type circuits).
Watch out: DANGER OF DEATH BY ELECTROCUTION: don't try this experiment if your thermostat uses line voltage (120V). The procedure Honeywell described was ONLY for low voltage (nominally 25V) equipment.
Watch out also about shorting thermostat wires together on newer HVAC equipment using circuit boards. That advice is only for old, simple series 10 type control circuits. As we report in the article above, on some modern heating and air conditioning equipment shorting wires can blow a circuit board component or fuse. If you've already made this mistake, check for a blown fuse on a circuit board in your furnace or boiler controls.
Watch out: do not short any wires together. Turn off power & confirm it is off. Don't forget to turn off all electrical power involved with your heating system before working on thermostat wiring, and confirm that power is off where you are working by using an appropriate test instrument such as a VOM. Shorting thermostat wires to ground or together is likely to damage components in the thermostat itself or in other heating system components, as this reader's report below illustrates:
Note: your fuse marked with an "E" was probably being read upside down and was a 3-amp fuse. Using a fuse one amp greater incapacity is probably not too dangerous, but it's best not to overfuse as doing so risks equipment damage. Certainly a larger fuse is asking for wiring or device burn-up should a future short circuit occur. - Ed. & reader comments
Since shorting thermostat wires can damage a circuit but can also blow the low voltage transformer, also
Reader Question: possible shorted thermostat wires
Our furnace thermostat failed after an adjacent water heater installation. I noticed that wires were pinched between the water heater and the gas pipe. After I loosened the safety straps and freed the wire, the thermostat works. (I reset the furnace). Were squished wires the cause of a temporary short and will the wires keep working if they are undisturbed? - Larry 8/2/11
Reply: check for shorted wires and for thermostat transformer damage as well as for blown fuse
Yes it's surely possible that your thermostat wires were shorted.
Now a short in those wires sometimes lets you off the hook with no trouble, since the wires and thermostat are basically an "on-off" switch that calls for heat (or no heat).
Question: reader comments on shorting out thermostat wires & fusing requirements
Greetings, not sure if anyone saw this or not, but whoever S.R. is above in the description about shorting out wires seems to maybe have missed the fact that the "E" they refer to is actually a "3" amp fuse and not a 4 amp fuse.
My HVAC installer of 30 years who recently installed 2 furnaces for us and is installing my heat pump agrees that it is a 3 amp fuse. We purchased an extra one just in case. Putting a 4 amp in there may not be good for the board as it may allow too much current for a relay or something and you may fry a piece on the board which is most likely not cheap. I hope this helps - D.M. - 3/12/12
Thanks D.M., We posted your important fuse size comment right in the article
Reader Question: Where does the white wire go?
On a York roof mount heat and air system thermostat wire white go to the com terminal or the white terminal - Gary
Not sure for your system which wire goes to which terminal - take a look at the wiring instructions on the appliance or in the installation manual to be sure. If you give me the product name and model number we can help find the installation and wiring instructions for a certain answer. I didn't find a customer service telephone number for York but the company's website includes consumer help and is found at http://www.york.com.
Another reader offered: on a York roof mount heat and air system thermostat wire white goes to the com terminal or the white terminal
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