Carrier Furnace question

[ragtop69gs]

I'm hoping that one of our HVAC members can help me out with my house furnace problem.

 

I have a Carrier 48GS unit, 10 years old. The problem I'm having with it is a few times a week, i'll get up first thing in the morning and it's blowing cold air, If I throw the circuit breaker for a few seconds the unit will fire up and run fine for days till the same scenario repeats it's self again. I have removed the service panel and there is no trouble codes flashing on the LED. The thermostat has fresh batteries and the filters are clean. Any idea what is going on? It was suggested that I look at the pressure switch and tube but I see nothing that looks like your typical pressure switch. This only seems to happen overnight, never during the day.

 

Mama's not happy being cold in the morning.

[Phoneman1981]

Hello: I am sure there are several members with HVAC experience. I know one of the Carrier service people here pretty well. Marc consults with me on technical phone and data wiring issues at times. I may just call him in a while and check in with him to see if he has any ideas.

[kevin-vic-b.c.]

Years gone by the old oil furnaces had a red "reset" button. This would trip when you had flame failure. The newer gas units do not have a red button but you turn off the switch to Reset the control module.

So in short for some reason the flame is not lighting.

Depending on the exact unit, I only had a bit of Carrier experience, there is a red rubber tube that proves air flow from the draft inducer, unplug it from the inducer and poke a small Allen Key through the hole and make sure the passage is clear. The Air Switch that the other end is connected to may be faulty. Or the circuit board could be defective. There is a Test capability on this circuit board, should be instructions in your owners manual, but if it is intermittent my guess would be air switch or plugged(cracked) air line.

[SilvrT]

Where's Kit Carson when ya need him? LOL!

[MiCarl]

Not Carrier specific and I'm not an expert, but I did stay at Holiday Inn Express once.....

 

 

When the thermostat calls for heat:

 

1) the blower runs to evacuate the combustion chamber.

2) the igniter and gas come on

3) the controller detects the flame and turns the igniter off.

 

If for some reason it doesn't light, or the controller doesn't know it's lit, the controller will shut down the gas and go back to step one.

 

After several tries it shuts down and just leaves the blower run.

 

So, you have several things that might cause the problem:

 

1) igniter doesn't work consistently

2) gas valve doesn't work consistently (which I doubt)

3) flame sensor bad

4) control module malfunctioning

5) improper fuel/air mixture confuses the flame sensor.

 

The furnace in my travel trailer was doing exactly the same thing you describe. Turned out to be dirt behind the gas orifice and the resulting flame wouldn't reliably trigger the flame sensor. I suspect leaves, cob webs, bug nests etc. in the combustion air intake could cause a similar problem.

[ragtop69gs]

Years gone by the old oil furnaces had a red "reset" button. This would trip when you had flame failure. The newer gas units do not have a red button but you turn off the switch to Reset the control module.

So in short for some reason the flame is not lighting.

Depending on the exact unit, I only had a bit of Carrier experience, there is a red rubber tube that proves air flow from the draft inducer, unplug it from the inducer and poke a small Allen Key through the hole and make sure the passage is clear. The Air Switch that the other end is connected to may be faulty. Or the circuit board could be defective. There is a Test capability on this circuit board, should be instructions in your owners manual, but if it is intermittent my guess would be air switch or plugged(cracked) air line.

 

Can't find anything on the unit that looks like your typical pressure switch & tube. The LED that shows trouble codes is clear, no fault codes flashing. No tube on air induction motor.

 

Not Carrier specific and I'm not an expert, but I did stay at Holiday Inn Express once.....

 

 

 

 

So, you have several things that might cause the problem:

 

1) igniter doesn't work consistently

2) gas valve doesn't work consistently (which I doubt)

3) flame sensor bad

4) control module malfunctioning

5) improper fuel/air mixture confuses the flame sensor.

 

The furnace in my travel trailer was doing exactly the same thing you describe. Turned out to be dirt behind the gas orifice and the resulting flame wouldn't reliably trigger the flame sensor. I suspect leaves, cob webs, bug nests etc. in the combustion air intake could cause a similar problem.

 

No blockage in intake, exhaust. burners are clean as well as gas orifices. inducer motor comes on every time there's a call for heat.

[campfire12]

remove an clean the flame sensor with steal wool. sensor is opsite side of burners from ignitor. sensor detects flame when last burner lites. should pull out from below burner box. all you need is 1/4" nut runner. good luck.

[syscrusher]

My Hunter digital thermostat has been confused in the morning a few times lately and just decides to run the fan on it's own. Have you ruled out something like that?

[RandyR]

I had numerous problems with a Trane control unit when there were electrical spikes/surges/brownouts.

[ragtop69gs]

Had a nice chat with Gary (Mega2500) about my furnace trouble and he gave me a few things to try, I'll report back when I see if any one of them was the fix I needed.

 

Thanks for the call Gary, Hopefully it'll work now.

[frankd]

Jay,

 

Like Campfire suggested, take the flame rod out and clean it with a Scotchbrite pad. The flame rod will probably have only 1 wire going to it, and on mine it's held in with a bolt with a 1/4" head. The flame rod is what tells the main board that the flame is lit and it's safe to leave the gas on.

 

Frank

[ragtop69gs]

I cleaned it today, we'll see if that does it. Pain in the butt to do on this furnace, I had to pull the burner rack and gas valve to get to it.

[kevin-vic-b.c.]

I cleaned it today, we'll see if that does it. Pain in the butt to do on this furnace, I had to pull the burner rack and gas valve to get to it.

So it is spark to pilot flame then? The ignition assembly 1/2 way back on the burner. Most of the units up here have been "hot surface ignition " for a long time to meet efficiency and code requirements, so never thought of spark. If the flashing code says clear then as far as I recall from doing residential work years back, that is is most likely not proving air flow.

 

Some Carrier models you could disconnect the thermostat then jumper across 2 pins on the board marked "Test" and it would run it through the pre-firing sequence

 

You do check your flashing code before you turn power off and before you open the cabinet door? The cabinet door had a switch that killed power on some models before you could read the code. We drilled a hole in the door so you could read code first.

 

Residential work was 15 years back now, I can tell you easily how to get a 35,000,000 BTU high pressure steam boiler fired if you need.