How do I check for shorted T-stat wires

Ken C.

I had a service call for a hot-air furnace (sorry, guys). The customer said she set the t-stat on 70, but the house temp would go above 90 because the furnace wouldn't shut off. The heat-off-cooling switch on the thermostat would not shut the furnace off, she said, so she had to use the service switch on the furnace. The furnace is a newer Rheem 80% (IFC, spark ignition) with A/C coil. She said the thermostat was replaced recently, but that didn't cure the problem. I pulled the t-stat and noticed that a couple of the wires (there were four) had the insulation nicked in a few spots. The wire had a lot of slack, so I cut the bad wiring off and re-wired the 'stat. Ran the unit through a couple of cycles, and it shut off properly. Turning the t-stat switch from "heat" to "off" also shut off the unit, so I'm hopeful I cured the problem. (My lack of confidence is due to my lack of experience on the heating side of P&H trade.) My questions are: 1) Although I didn't see any evidence of arcing in the t-stat wiring, could a short-circuit in the wires have caused the unit to keep running? 2) I know how to test two-conductor t-stat wire for continuity, but is there any test that will reveal a short-circuit?

Mark Hunt

Ohm meter


Disconnect the wires from the thermostat and the furnace and check for continuity.

With the wires disconnected and seperated, you should have no continuity.

Sometimes installers strip a bit more wire than they need and pack them into the thermostat. I'd look for overstipped wiring or perhaps a staple that was driven in too far.

It sounds as though there is definately a short somewhere, finding it could be the hardest part of the job.

Good luck.

Mark H

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flange

it sounds like your 24 volt hot wire was stripped to far, and when the stat was installed it shorted to the white wire giving you a constant call for heat. the easiest way to check that is to end the call for heat, and check down at w1 on the furnace to see if you have voltage present.

jim lockard

I have

gone on quite a few of those calls. #1 I hate 20 gauge wire too many problems. I always run 18 gauge with a couple of spares mostly 18/8 for everything. Oh disconnect your T-Stat spread your wire ends so nothing touches. Down at the furnace at the P.C. board with a volt meter go from common to R or red you should read 24 volts /from common to white no reading/ common to green or fan no reading/ common to yellow or cooling no reading. any of these read then that your short. Best Wishes J.Lockard