# Trane XR90 furnace intermittent ignition problem



## momo608 (Oct 5, 2015)

Hello All!

Have a stumper, at least for me. I have a trane XR90 furnace. Here is the problem. Mostly after sitting idle for an extended period, my furnace will go through the start cycle and ignite. The problem is the flame immediately blows out after ignition. I do not mean in a few seconds, I mean instantly before all the burners light. I can hear the gas still coming our before the safety controls shut the gas off. The furnace will restart after the failed attempt and function normally. The flame blows out like an acetylene torch would if the oxygen side was filled with air and you opened the valve on the oxygen side while acetylene was burning. My best guess is air is getting in the burner gas line on the burner side of the gas valve. Maybe a bad gas valve? Obviously there would be a constant gas leak if there was a leak on the pressure side. Like I said, this occurs mostly when the furnace has sat idle because it's not that cold outside or I lowered the thermostat. By the way the flame sensor and igniter are new. I also checked the vacuum switch and for obstructions in the air intake, exhaust and all the drain lines are cleaned out. Otherwise there are no problems with the furnace. I find it unsettling when things are not working right and this cycling on and off is just that. 

Thank You


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## user_12345a (Nov 23, 2014)

Do you hear a click when it starts to blow out.

One would have to be there to tell what's going on.

It could be a dirty flame sensor; you can look that up and clean it. Could be many other things.

Check the board for a fault code when this happens; there should be a sight glass with a flashing led.


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## momo608 (Oct 5, 2015)

user_12345a said:


> Do you hear a click when it starts to blow out.
> 
> One would have to be there to tell what's going on.
> 
> ...



#1 No click

#2 I explained it, maybe not well enough but to the best of my ability. Please read it again, I thought it was pretty good. 

#3 as stated, flame sensor is new. The flame cuts out so fast that there is not enough time for the flame sensor to react. The farthest burner where the flame sensor lays has not even ignited yet. We are talking tenths of a second to 1 second for the blow out.

#4 No codes, slow steady flashing when the furnace is idle.


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## CompleteW&D (Sep 4, 2015)

Maybe it's time for a $69.95 clean and check from a reputable HVAC company. I'm a homeowner and am all for saving $$$ too. But, when it comes to something as combustible as a gas furnace, sometimes caution is the better choice.


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## momo608 (Oct 5, 2015)

CompleteW&D said:


> Maybe it's time for a $69.95 clean and check from a reputable HVAC company. I'm a homeowner and am all for saving $$$ too. But, when it comes to something as combustible as a gas furnace, sometimes caution is the better choice.


I will start throwing parts at it before that. I'm not afraid of my furnace nor intimidated by trying to fix things that involve explosive gases. Bring it on!

I'm leaning towards the gas valve. Having people coming over for intermittent problems is a problem in itself. They will not give you your money back if they don't fix the problem, they only charge you more when they come again.


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## roughneck (Nov 28, 2014)

Is the gas valve closing while still being powered? What's the voltage to it during a failed ignition? 
Don't throw parts at it. That's a complete waste of money, time, and energy.


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## momo608 (Oct 5, 2015)

roughneck said:


> Is the gas valve closing while still being powered? What's the voltage to it during a failed ignition?
> Don't throw parts at it. That's a complete waste of money, time, and energy.



Yes I'm going to more closely look into the gas valve function. Logic dictates the problem lays there, either with the control of it or possible wear issues, I'm leaning towards wear. While watching it do complete cycles, the call for heat up to the call has been met, i.e. it has fired long enough. The difference between a normal gas valve shut off at the end of the cycle and the flame blow out I'm asking about, is not the same. The gas shuts off cleanly on a normal cycle. On the blow out it is just that, it looks like its blowing out the flame with air in the line like I described above. I have no doubt something is blowing out the gas orifice on the first burner by the igniter before it shuts down. You can see it and hear it. I also know the solenoid on the gas valve is not shutting off at least electrically before the blow out occurs. Again, the difference between the blow out and a normal shut down is clear, the gas valve solenoid makes an audible click that I feel and hear on a normal shut down, it does this only after the blow out occurs in the problem situation.


If you figure a service call costs around $100 minus the parts at FULL retail prices, you can buy a few critical parts with that if you shop wisely. My furnace is getting several years of use on it and I like the idea of replacing aging parts that are known to go bad instead of lining the pockets of repairmen. Nothing against repairmen but I can do this stuff, why pay someone. There are not that many control components to replace anyway. The components responsible for 90% of the furnace problems that occur before a complete furnace replacement is due.


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## CompleteW&D (Sep 4, 2015)

momo608 said:


> I will start throwing parts at it before that. I'm not afraid of my furnace nor intimidated by trying to fix things that involve explosive gases. Bring it on!


You are a braver soul than I.... I tend to shy away from things that go BOOM or can send 240 volts through me.

Although, I definitely respect those of you who do work on stuff like this. I remember when I had my first house and my Dad came over to change my fuse box to a breaker box. It was my job to hold the flashlight for him. :glasses:

When he pulled the main fuse, and everything went totally dark and utterly quiet.... it was the most eerie thing I had ever experienced at that point. When he finished and threw the main breaker.... I was ready for a kaboom, but thankfully, all was well and it passed inspection.

So my hat's off to ya! :vs_cool:


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## roughneck (Nov 28, 2014)

momo608 said:


> Yes I'm going to more closely look into the gas valve function. Logic dictates the problem lays there, either with the control of it or possible wear issues, I'm leaning towards wear. While watching it do complete cycles, the call for heat up to the call has been met, i.e. it has fired long enough. The difference between a normal gas valve shut off at the end of the cycle and the flame blow out I'm asking about, is not the same. The gas shuts off cleanly on a normal cycle. On the blow out it is just that, it looks like its blowing out the flame with air in the line like I described above. I have no doubt something is blowing out the gas orifice on the first burner by the igniter before it shuts down. You can see it and hear it. I also know the solenoid on the gas valve is not shutting off at least electrically before the blow out occurs. Again, the difference between the blow out and a normal shut down is clear, the gas valve solenoid makes an audible click that I feel and hear on a normal shut down, it does this only after the blow out occurs in the problem situation.
> 
> 
> If you figure a service call costs around $100 minus the parts at FULL retail prices, you can buy a few critical parts with that if you shop wisely. My furnace is getting several years of use on it and I like the idea of replacing aging parts that are known to go bad instead of lining the pockets of repairmen. Nothing against repairmen but I can do this stuff, why pay someone. There are not that many control components to replace anyway. The components responsible for 90% of the furnace problems that occur before a complete furnace replacement is due.


I wouldn't replace parts just to replace parts. Especially judging by the reliability (or lack thereof) of some of the brand new parts I pull out of boxes. 
Something to also think about, is the fact that parts seldom just "go bad". Failure usually occurs due to another source. For example a gas valve failure due to the valve receiving low voltage from a transformer problem. Or a control board failure due to a condensate leak or moisture damage. 
Wearable items are more in line with contactors, bearings, and cheap capacitors.


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## user_12345a (Nov 23, 2014)

The gas valve is something that you really, really need to call a pro to replace if it's bad. When the valve is changed the furnace has to be checked for gas pressure and proper combustion (at least monoxide).

If the flames are going out with power being applied to the solenoid, the regulator in the valve is most likely bad. If not you have an electrical issue elsewhere.

Go ahead and deal with the electrical side yourself if you know what you're doing. Don't touch the natural gas or venting portion of the furnace.


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## momo608 (Oct 5, 2015)

user_12345a said:


> The gas valve is something that you really, really need to call a pro to replace if it's bad. When the valve is changed the furnace has to be checked for gas pressure and proper combustion (at least monoxide).
> 
> If the flames are going out with power being applied to the solenoid, the regulator in the valve is most likely bad. If not you have an electrical issue elsewhere.
> 
> Go ahead and deal with the electrical side yourself if you know what you're doing. Don't touch the natural gas or venting portion of the furnace.


I thought this was a do it yourself website? I didn't know it was a run for the hills and grab the phone book honey because I'm scared website! Look, I know I'm way ahead of most of you. I was hoping there was a real furnace guy here. I'll order up a new gas valve tomorrow. Most people probably wouldn't even have noticed the problem if my tenants are an indicator.


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## roughneck (Nov 28, 2014)

Have you tested the valve yet? Why replace it if nothing's wrong with it? Then you've spent however many $$$, plus the energy to replace it, and your back at square one with broken equipment


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## user_12345a (Nov 23, 2014)

> Look, I know I'm way ahead of most of you.


okay. go ahead and change parts until it puts out heat. 

Recommended parts try changing, one at a time, without doing any electrical or gas pressure testing, because they "could" be the issue:

- Gas valve
- Circuit board
- flame sensor
- burners
- pressure switch
- cables and molex connectors between valve and board

Let us know how it turns out and how much it costed you seem to know what you're doing.


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## beenthere (Oct 11, 2008)

You said the flame blows out. Is it that the flame is strong and then gets loud and goes out. Or it just chokes down and goes out.


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## momo608 (Oct 5, 2015)

I know you guys want to help so thanks for that! I just wish you would have taken more time to read and understand my description of the problem better. Sounds like most never got past the first sentence.

I found the cause! like I was saying, air WAS getting in the gas supply line between the gas valve and the burners. The acetylene torch example was perfect. There was enough gas in the line to ignite the burners but once the air that leaked into the gas line reached the burner it blew out the flame. The gas valve on this furnace has a connection to tee into the vacuum line for the pressure switch. This is where air was getting into the line. The tubing for the pressure switch is open at the burner combustion chamber in order to do its job, so it is open to air circulation from the venting system. I plugged the tubing connection on the gas valve and the rubber tubing that goes to the pressure switch. No more misfires. So there must be some kind of separate valve malfunctioning in the gas valve that is allowing air to leak into the gas line when the furnace is idle, hence the problem is worse the longer the idle time. I am scratching my head as to the purpose of this gas valve connection when most furnaces do not have it. It's probably some safety overkill that we are plagued with on just about everything these days. I'll replace the valve, not crazy about bypassing stuff to make things work in most cases.


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## CompleteW&D (Sep 4, 2015)

momo608 said:


> I thought this was a do it yourself website? *I didn't know it was a run for the hills and grab the phone book honey because I'm scared website! Look, I know I'm way ahead of most of you. *I was hoping there was a real furnace guy here. I'll order up a new gas valve tomorrow. Most people probably wouldn't even have noticed the problem if my tenants are an indicator.


Wow.... a little snarky are we?!? Just because you are "WAY AHEAD" of most of us or specifically me, when it comes to furnace diagnostics and repair.... several folks have tried to give you some solid advice in this thread. 

To now accuse them of basically being pansies and not thoroughly delving into your original post with proper zeal it so rightfully deserved (as mentioned in your last post), is a bit over the top and self serving. 



> *I just wish you would have taken more time to read and understand my description of the problem better. Sounds like most never got past the first sentence.*


But I digress.... I bow to your superior intelligence and valor. :notworthy:


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## roughneck (Nov 28, 2014)

DO NOT EVER BYPASS A SAFETY!!! That is an incredibly stupid thing to do. 
It's probably not "air". While very low in pressure, the gas line is still above atmospheric pressure. 
What is your gas pressure, before at and after when the furnace fires?


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## user_12345a (Nov 23, 2014)

> The gas valve on this furnace has a connection to tee into the vacuum line for the pressure switch. This is where air was getting into the line. The tubing for the pressure switch is open at the burner combustion chamber in order to do its job, so it is open to air circulation from the venting system.


This is most likely to provide compensation for the regulator to provide the right gas pressure relative to the burner compartment which runs under negative pressure. 

You most likely haven't actually solved the problem.


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