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Tim H.
Tim H., HVAC Professional
Category: HVAC
Satisfied Customers: 3971
Experience:  Experienced in all aspects of HVAC, both residential and commercial.
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Furnace is not blowing hot air

Customer Question

Furnace is not blowing hot air
Submitted: 1 year ago.
Category: HVAC
Expert:  Tim H. replied 1 year ago.

Hi, I'm Tim and I'm an HVAC expert here to assist you with your questions. Please provide all information I request as it makes it more difficult to troubleshoot something when I'm not there onsite. Essentially, you become my eyes and ears. Thanks for the opportunity to help. At the end of our dialog, you will have a chance to rate my service, but please wait until we are finished and you're satisfied you have all the information you need.

I'm sorry to hear about your issues with your furnace. When the furnace shuts down, what LED flash code do you get?

Expert:  Tim H. replied 1 year ago.

Also, can I get full model number? Lennox made two furnaces beginning with G40, the G40UH and the G40 DF.

Customer: replied 1 year ago.
G40UH, I haven't seen any flashing lights on the circuit board
Customer: replied 1 year ago.
Posted by JustAnswer at customer's request) Hello. I would like to request the following Expert Service(s) from you: Live Phone Call. Let me know if you need more information, or send me the service offer(s) so we can proceed.
Customer: replied 1 year ago.
I didn't know it was an additional charge for a phone call.. I just need help trying to troubleshoot the furnace.. It's blowing cold air when the thermostat is set to heat
Expert:  Tim H. replied 1 year ago.

OK, if it's blowing cold air, there will be a flash code in the small window in bottom of the furnace. I need that code.

Customer: replied 1 year ago.
How will this code appear? With the furnace powered on? The furnace is on and circuit board cover is off and I don't see a light
Expert:  Tim H. replied 1 year ago.

It will not be seen in the top panel. It's in the bottom panel viewed through a small window. The code appears with furnace on, correct.

Customer: replied 1 year ago.
Green and red flashing rapidly
Expert:  Tim H. replied 1 year ago.

OK, now I need you to count the number of green flashes followed by a pause and the number of red flashes followed by a pause.

Customer: replied 1 year ago.
They are flashing at the same time at a rapid pace and I haven't seen a pause
Customer: replied 1 year ago.
Green light is just flashing now
Expert:  Tim H. replied 1 year ago.

If they are both flashing simultaneously at a fast rate, that's a call for heat. Is the inducer motor at the top of the furnace running?

Expert:  Tim H. replied 1 year ago.

How many green flashes followed by a pause?

Customer: replied 1 year ago.
The green light is flashing at a half second pace continuously
Customer: replied 1 year ago.
When the light was flashing I counted up to 60 flashes and no pause
Expert:  Tim H. replied 1 year ago.

That means that the prove switch (pressure switch) was not made. Check for blocked vent or inducer not running. Also, pinched pressure switch hose or bad pressure switch.

Customer: replied 1 year ago.
Green and red lights came back on flashing
Expert:  Tim H. replied 1 year ago.

First off: Is the inducer motor running?

Customer: replied 1 year ago.
I'm not too sure what the inducer motor is
Expert:  Tim H. replied 1 year ago.

The inducer motor can be seen in Figure 1, page 14 of this manual.

Expert:  Tim H. replied 1 year ago.

Called combustion air inducer in the diagram.

Customer: replied 1 year ago.
the black circle that spins? Combustion air inducter runs when the red and green lights are flashing
At the moment only the green light is flashing and inducer is not running
Expert:  Tim H. replied 1 year ago.

In that same diagram, they show a combustion air prove switch. It is round with a hose attached. Take the hose off, not on the prove switch end, but the other point it attaches and gently suck, then blow through the hose a few times.

Expert:  Tim H. replied 1 year ago.

When you're done, put hose back on and try again.

Customer: replied 1 year ago.
I don't see a hose
All I see are electrical wires
Expert:  Tim H. replied 1 year ago.

Look for a small round part with a rubber hose attached.

Expert:  Tim H. replied 1 year ago.

Probably black in color and hose may be black, too.

Customer: replied 1 year ago.
Ok I see it
Small round part (Honeywell) with two electrical wires and a red in color hose
Expert:  Tim H. replied 1 year ago.

Yes, now need you to:

Take the hose off, not on the prove switch end, but the other point it attaches and gently suck, then blow through the hose a few times.

Customer: replied 1 year ago.
Take the hose off the black small round piece?
Or on the opposite end that has the inducer spinning
Customer: replied 1 year ago.
the inducer is spinning now.. Should I power it down first?
Expert:  Tim H. replied 1 year ago.

Nope, as I said, not on the prove switch end, but the other end. If you want, turn furnace off first.

Expert:  Tim H. replied 1 year ago.

The prove switch is the small black round part.

Customer: replied 1 year ago.
Just to be clear the prove switch is the small black piece with two wires coming out of it with the hose attached in the top right part of the furnance
Customer: replied 1 year ago.
this hose is only about 2 inches in length
Expert:  Tim H. replied 1 year ago.

Yes. Leave the hose on that part, but take the other end off and gently suck then blow through the hose a few times. We are trying to "break free" the pressure or prove switch.

Customer: replied 1 year ago.
I can take it off and blow in it but not real effective if I can't take the whole part off both ends
Expert:  Tim H. replied 1 year ago.

Nope, wont' do any good if you take the other end off.

Expert:  Tim H. replied 1 year ago.

If it's too short, you may need to go up to a hardware store and pick up a longer length of hose to perform this task.

Customer: replied 1 year ago.
Anything else I can do?
Expert:  Tim H. replied 1 year ago.

If this doesn't work and inducer is running, the prove switch may be bad and would need replacement.

Expert:  Tim H. replied 1 year ago.

We are trying to do something that will prevent you from having to buy a part and has worked for me many times.

Customer: replied 1 year ago.
Ok
Customer: replied 1 year ago.
I can't seem to pull this hose off
Customer: replied 1 year ago.
I'll have to try and disassemble the price switch to get the hose off
Expert:  Tim H. replied 1 year ago.

If you need to perform this and get back to me later, that's fine. Sometimes the hose is hard to get off on the other end because it gets "welded on" due to the heat.

Expert:  Tim H. replied 1 year ago.

Why don't you go ahead and work on doing this, then post back when you get done. Otherwise, as I said, prove switch may be bad.

Customer: replied 1 year ago.
Got it off! Blew in the hose
Customer: replied 1 year ago.
Now what
Expert:  Tim H. replied 1 year ago.

Suck, then blow a few times. Gently. Then as I said previously, when you're done, replace hose, reset furnace (power off, then back on) and see if it starts.

Customer: replied 1 year ago.
Ok will turn back on
Customer: replied 1 year ago.
Green and red flashing together again
Customer: replied 1 year ago.
Still blowing cold air
Expert:  Tim H. replied 1 year ago.

Red LED off, green flashing slowly?

Customer: replied 1 year ago.
red is out now and green is flashing
Expert:  Tim H. replied 1 year ago.

OK, more than likely this prove switch will need replacement. We tried anyways.

Customer: replied 1 year ago.
Grrrr this sucks
Customer: replied 1 year ago.
No heat! Will have to continue to troubleshoot
Expert:  Tim H. replied 1 year ago.

Yup. But at least now you have a better idea of what's going on.

Expert:  Tim H. replied 1 year ago.

One other thing you can try: on the other end of the tubing on the inducer assembly, where pressure switch hose attaches, there is a small barb that hose pushes onto. Take a small drill bit and push in and ream this out, make sure its clear. Occasionally this will plug up with rust, debris. I'm not saying this will work, but worth a try.

Expert:  Tim H. replied 1 year ago.

Did you have additional questions on this? Otherwise, good luck! Thank you for using Just Answer and please don't forget to rate my service as that's how experts get paid (Thanks in advance for any bonus left). Otherwise experts work at their own time and expense. Feel free to post back with any additional questions.

Customer: replied 1 year ago.
I had to call a tech out today..
Expert:  Tim H. replied 1 year ago.

OK, I can understand your decision to call in a pro. Thanks for using Just Answer and please don't forget to rate my service.

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