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airheatman
airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 8051
Experience:  Furnace, A/C and heat pump specialist.
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Furnace heats in air conditioning mode, Was advised to

Customer Question

furnace heats in air conditioning mode
JA: Did you double-check the thermostat's settings? And what about the filters?
Customer: Was advised to replace thermostat, that's been done and it doesn't stop making heat ever.
JA: Do you plan on doing the work yourself?
Customer: Had two techs come out here, first one did fall check up and second one tried to fix the mess the first one left. So now my 88 and 90 yr old parents have all heat all the time. IF they use the fuse shut off switch located on the side of the unit IN the basement...
JA: Anything else we should know to help you best?
Customer: Last tech thought the thermostat was bad BUT new thermos acts the same way.. The furnace WAS running flawlessly UNTIL first tech serviced it.
Submitted: 5 months ago.
Category: HVAC
Expert:  airheatman replied 5 months ago.

Lets start with letting me get a few more details.

1.If you turn thermostat to OFF does it still heat

2.Model on thermostat

3.Model on furnace

Customer: replied 5 months ago.
Heats in all positions, Thermo #CT87N Same model that was running flawlessly before Furnace # G14Q36019 Lennox plus
Expert:  airheatman replied 5 months ago.

OK if heating in even the off position lets start by determining if issue is in thermostat or not.

Disconnect all thermostat wires from thermostat and see if it goes off. We will go from there.

Customer: replied 5 months ago.
after all wires removed unit stopped
Expert:  airheatman replied 5 months ago.

OK, So that tells us thermostat is the problem.

Tell me where each wire was going on stat (EX Red to R, Green to G, etc)

Customer: replied 5 months ago.
All wires were on proper spots red on red- green on green- white on white- and yellow on yellow.
Expert:  airheatman replied 5 months ago.

OK now put them back one at a time starting with red and tell me when furnace starts

Customer: replied 5 months ago.
its on off heat and nothing
Expert:  airheatman replied 5 months ago.

What is on heat?

Expert:  airheatman replied 5 months ago.

Have you put all wires back one at a time?

Customer: replied 5 months ago.
With thermos set to heat- all wires put on one at a time and nothing.
Expert:  airheatman replied 5 months ago.

Ok so that is good news, All wires are connected now and furnace is not on YES?

Customer: replied 5 months ago.
correct all on, set to heat and auto fan and all quiet temp is set low at 60
Expert:  airheatman replied 5 months ago.

OK now raise temp setting up about 4 degrees above room temp and see if heat comes on

Customer: replied 5 months ago.
running
Expert:  airheatman replied 5 months ago.

Now lower and see if it will cycle off

Customer: replied 5 months ago.
does not shut off
Expert:  airheatman replied 5 months ago.

Give it about 3 minutes and make sure numbers are well below room temp

Customer: replied 5 months ago.
room temp 72 thermos 60
Expert:  airheatman replied 5 months ago.

Burners and fan both still on?

Customer: replied 5 months ago.
Yup.
Expert:  airheatman replied 5 months ago.

Hate to say but you have proved issue is in tstat if furnace went off with tstat disconnected.

Customer: replied 5 months ago.
Wouldn't a relay stuck cause the same prob..?
Expert:  airheatman replied 5 months ago.

Just to be sure nothing else we can do, make sure the E/ F on thermostat is SET to F and cycle rate is set to Switch 1 and 2 both OFF

Customer: replied 5 months ago.
we did that before we contacted you.
Expert:  airheatman replied 5 months ago.

There is no such relay on furnace as it is controlled by a circuit board and even if relay was stuck, disconnecting wires at thermostat would not unstick it.

Customer: replied 5 months ago.
Okay. Thanks. Second tech returned to do what he can..
Expert:  airheatman replied 5 months ago.

Let me know hat you figure out.Curious to know as the Honeywell is usually pretty reliable.

Customer: replied 5 months ago.
According to his info there IS a relay under circuit board that dictates how units runs. Jury is still out...
Expert:  airheatman replied 5 months ago.

I still say if you disconnect thermostat wires form furnace and unit shuts off issue has to deal with thermostat unless it was coinidental

Expert:  airheatman replied 5 months ago.

Let him know it stops when disconnecting thermostat