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Question DetailsAsked on 12/10/2016

My furnace won't blow air or heat,what's wrong? We just replaced the thermostat .

The furnace is a Trane and it's about 12 years old.

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2 Answers


If you changed the thermostat because the furnace was not working and you thought it was because it was a bad thermostat, maybe it was and the thermostat was not changed/connected right (see debugging procedure below) or maybe the thermostat was not the source of the problem at all - and the original problem is at the furnace and still in effect.

If when you say "we just changed the thermostat" if you mean a technician just did it in last day or two, then call him back to fix it. Most common error - not turning furnace power back on, not turning setting switch on thermostat to HEAT instead of OFF or A/C, and not stahying long enough to check for proper operation.

If furnace was working, then you changed the thermostat to get a better or programmable one or because it was not tracking temperature or programming correctly, then likely you did something in the course of changing the thermostat - common issues in that case:

0) turned the furnace power off to do the changeout, and forgot to turn it back on - if it has a status or power-on light, check that is on - and if flashing an LED code light, check in manual for what the flashing code means

1) replaced it without turning the thermostat off first, which can confuse the furnace controls at times

2) replaced it with the wires to the thermostat live (furnace power on), and may have touched the wires together or groundede the power wire (they are almost always "live, usually at 12V or 24V but occasionally 48V or 120V, coming from the furnace power) and may have confused the thermostat or (rarely) burned out a solenoid or transformer by keeping them in contact too long or grounding the power wire out solidly

3) you connected the wrong wires to the wrong terminals at the thermostat - maybe mixing up A/C and Furnace wires for instance, or if there was a neutral wire (usually white) mixing that up with a live (usually red but sometimes black) wire - which might or might not have confused the furnace controls, and may or may not have (rarely) fried the thermostat or furnace power supply or controls

4) may have wired the new thermostat in with it in the HEAT or A/C position rather than OFF and confused or damaged (rarely) the thermostat

5) forgot to put batteries in new thermostat (if digital), or put in dead or weak one(s), or put one or both in backwards, or battery contacts were not clean, or forgot to take out a protective plastic shipping strip over the battery contact in some units shipped with batteries in (some have a pull strip of plastic that has to come out to have the batteries actually make contact with the contacts in the thermostat)

6) did not set the anticipator to the correct setting for type of heating unit you have (usually on back of thermostat)

7) did not turn it to OFF before programming a digital thermostat, which can confuse the thermostat

8) incorrectly programmed or failed to program so it is not trying to hold the temparature you think you set (check by - on digital thermostats - manually setting a much higher temporary temperature like 80 or 90 degrees and see if that makes it kick on

9) failed to hit END or whatever the final button in the process is on the thermostat - so it thinks it is still in programming mode

10) if wireless/web controlled or compatible, did not complete the programming or it is still trying to accept wireless input and has not reverted to automatic operation to function per programming - so still in the PROGRAM mode

Different units vary of course in their details, but generally if something did not get seriously fried the following will correct a thermostsat installation problem:

a) setting thermostat control switch to OFF,

b) shutting off power switch to furnace (and A/C if you have A/C thermostat control wires connected to thermostat),

c) make sure anitcipator setting is right (will not cause failure to operate but makes it get too hot or not hot enough by a few degrees if set to wrong type of heating system),

d) make sure right wires are connected right places on the thermostat,

e) make sure that the batteries are good and making good contact and are facing right way,

f) then program the thermostat (if digital), including checking time and AM/PM is set right

g) turn furnace power back on,

i) turn thermostat switch to HEAT or FURNACE or similar named setting,

j) check if furnace turns on when thermostat says room temperature is below the desired heat set - or use temporary setting to much higher temperature to see if it comes on

k) check operation of furnace - that it is kicking off when thermostat room temp reading says it is at or above the programmed temp, and kicking back on again when the house cools down again. You need to wathc through 2 complete on-off cycles to be reasonably sure that it is working right. If in a hurry you can help cooling along by opening windows or doors

If your thermostat is a simple on-oiff mercury switch type, then the "programming" part involves just moving the temperature setting dial or arm to the desired temp. Will still have an anticipator setting on the back (occasionally on front behind the temperate setting lever) - usually a thin metal tab you swivel to the correct setting - see instruction with thermostat on selecting the anticipator setting. On digital one with numeric anticipation settings, manual for furnace should tell what number to use on most thermostats.

If still does not work, then unless you are a serious DIY'er, you need a Heating and A/C (your Search the List category) technician to do some electrical checking to figure out what is wrong.

Answered 3 years ago by LCD



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