intermittent issue w/ Trane XL80 gas furnace


  #1  
Old 01-31-10, 04:00 PM
M
Member
Thread Starter
Join Date: Jan 2010
Location: california
Posts: 3
Upvotes: 0
Received 0 Upvotes on 0 Posts
intermittent issue w/ Trane XL80 gas furnace

Hi,

I have a Trane XL80 gas furnace (probably 18 yrs old - is there a way to tell ?) that for the past couple of months has been exhibiting an intermittent failure, wherein the flame fails to come on.

Symptom:
1) Thermostat sends Heat-on correctly
2) Ignitor comes on (glows red)
3) When problem occurs, the gas does not come on
4) Ignitor goes off, and cycles through on-off 3 times before shutting off until Reset.

What I have tested:
1) Pressure switch is good - verified voltage at both ends when problem occurs.
Also, found that problem occurs even if switch was shorted out.
2) When problem occurs, no voltage at the gas valve input from control board (measured using m/m).

Had a technician come and take a look but after diagnosing for 2 hrs, decided that the vacuum tube had something in it and he blew into it. The heater worked the 5-6 times he tried after that and he concluded he had fixed the problem. However, I now know that it is not the pressure switch. Also, since the technician "fixed" the vacuum tube, the heater seems to work a little more before failing. It almost certainly starts each morning, runs for ~45mins and then the flame just shuts off. Once it trips in this manner, getting it work again is very hard; time seems to be the only thing that gets it to work again.

The turning off of the flame does not seem to be triggered by the Fan Limit switch as when the flame goes off, it is not in the high-temp range.

Do you think it is the board ? Is there something else I can check before getting a new board ?

Thanks
Mehul
 
  #2  
Old 02-01-10, 01:04 PM
P
Temporarily Suspended
Join Date: Jul 2008
Location: NY
Posts: 10,265
Upvotes: 0
Received 0 Upvotes on 0 Posts
Look at the vacuum tube again. See if there is something in it.

When was the last time you changed the filter?
 
  #3  
Old 02-01-10, 04:02 PM
E
Banned. Rule And/Or Policy Violation
Join Date: May 2007
Location: Wisconsin
Posts: 7,826
Upvotes: 0
Received 1 Upvote on 1 Post
You never mentioned the inducer fan in the sequence, and what it does both when the flame goes out, or when the glow starts, and then the fire never happens.

Normally with furnaces, when the ignitor comes on(including even if it is weak), the safety switches have been proven, including the pressure switch.

IF the inducer behaves as it should, and that is an IF, at this time, without you saying yet - could be a bad electrical connection not letting the current to the gas valve, or the board is bad, would be my guesses.
 
  #4  
Old 02-01-10, 10:01 PM
M
Member
Thread Starter
Join Date: Jan 2010
Location: california
Posts: 3
Upvotes: 0
Received 0 Upvotes on 0 Posts
The inducer fan always comes on promptly when the heat signal from the thermostat comes in and it stays on when the gas turns off. The blower fan stays on until the fan limit switch reaches the lower OFF position. I think the inducer fan and the pressure switch are not the culprit since the gas turns off even w/ the pressure switch shorted out.

Before replacing the board, I just wanted to understand if there are any switches or inputs that the board receives that prompts it to turn on the current to the gas valve.

Also, the board in the furnace is not open, i.e., it is covered by a plastic box and there is some sort of wax bonding the board to the plastic cover. Are these still available or do I need to buy a new cover and some sort of adapter kit ?

Appreciate you replies.
Mehul
 
  #5  
Old 02-02-10, 04:29 PM
E
Banned. Rule And/Or Policy Violation
Join Date: May 2007
Location: Wisconsin
Posts: 7,826
Upvotes: 0
Received 1 Upvote on 1 Post
Does your furnace have a flame sensing rod? If no flame sensing rod at the end of a single wire that heads in toward the combustion chambrer, then the HSI is the flame sensor.

Do you have any pin-type connectors on the sealed control board? Unplug connectors and plug back in.

Make sure you are not losing power along your 24 volt safety circuit. Do not ONLY rely on what you call "shorting" of wires. You must know if 24 volt power is even entering a switch. Shorting a switch with no incoming power obviously does no good.

Other than these things, all I can think of is the board causing your problems.
 
  #6  
Old 02-02-10, 09:27 PM
M
Member
Thread Starter
Join Date: Jan 2010
Location: california
Posts: 3
Upvotes: 0
Received 0 Upvotes on 0 Posts
I do have a flame sensor rod and the other day I took it out, cleaned it out w/ a wire-brush and put it back in ... didnt help :-(
The flame sensor rod seems to be correctly positioned in the path of the flame. Is there anything I can do to verify that the flame sensor is good ?

As for the pressure switch, yes I did verify that there was 24V going thru it.

Once everything is going (inducer fan, gas valve, blower fan) what kind of monitoring goes on ? What other switches could trip that may cause the circuit board to stop the gas ? Is it possible to safely bypass any of the switches for further isolation ?

Thanks,
Mehul
 
  #7  
Old 02-03-10, 04:49 PM
E
Banned. Rule And/Or Policy Violation
Join Date: May 2007
Location: Wisconsin
Posts: 7,826
Upvotes: 0
Received 1 Upvote on 1 Post
Originally Posted by mdholakia
I do have a flame sensor rod and the other day I took it out, cleaned it out w/ a wire-brush and put it back in ... didnt help :-(
The flame sensor rod seems to be correctly positioned in the path of the flame. Is there anything I can do to verify that the flame sensor is good ?

As for the pressure switch, yes I did verify that there was 24V going thru it.

Once everything is going (inducer fan, gas valve, blower fan) what kind of monitoring goes on ? What other switches could trip that may cause the circuit board to stop the gas ? Is it possible to safely bypass any of the switches for further isolation ?

Thanks,
Often in furnace wiring schematics, the pressure switch is the last line of defense. If you have 24 volts going through that, yet you have failure, then it has to be ignition related, gas components related, flame related, flame sense related. That's all there is.

One has to check to make sure everything that allows the furnace to initially start up and or continue running without interuption, is right, before condeming a control board or igniton control module.

Since the pressure switch is good, even at failure, that means the furnace drafts properly and has no issues with other 24 volt safety switch parts, including the limit switch, nor is there condensate water or a plugged vent system. So we are down to that list of things I mentioned.

You have to be sure the flame sense rod is clean(Did you use one of those wire toothbrushes? I would not trust say one of those large painter's wire brushes, as the wires are not as densely packed. I'd trust say fine steel wool, better.), positioned in right place, and that the burner flame engulfs it.

If that is not it, you are now down to bad connections, a bad ground, or bad board/module not letting the sequence to continue on, to let power to the gas valve. There are no other sensors - generically speaking at least - with all the furnace brands I have worked on.
 
 

Thread Tools
Search this Thread
 
Ask a Question
Question Title:
Description:
Your question will be posted in: