Furnace blows but does not heat (intermittent problem)

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We have a "York Diamond 90" gas-fired furnace that was working well until today.
There seems to be something wrong with the gas flow/valve, burner, or sensor.
I can see the electronic ignition element turning white hot, then hear the gas valve click on, then see the blue gas flames light for a second or two before sputtering out. This cycle repeats itself several times until, the system "gives up" (or I guess locks out) at which point the blower just continues to blow cold air forever and no further ignition attempts are made. The blower continues to blow even if I shut off the thermostat
Interestingly, if I cycle off the power (even just for a second or two), the furnace will start up normally and ignite the burners properly. The burner will continue to fire for a *couple of minutes* before blowing out at which point it goes into the (failed) cycle of trying to relight a couple of times before finally giving up and again leaving me in the stuck state with the blower blowing cold air.
Sometimes in the cycle of trying to re-light it will burn for a few seconds before sputtering out. Also, sometimes, I hear the valve clicking on-and-off a couple of times in rapid succession.
Now the other two gas furnaces are working properly, so I don't think it is a supply problem. Also, I don't think it is a ventillation problem because it seems to burn fine for a few minutes with a nice blue flame before suddenly sputtering out.
When I opened up the bottem panel (overriding the disable switch), I noticed that after it locked up in the continuous blower position, the diagnostic LED exhibited a pattern of 8 short (red) flashes -- I believe this simply indicates that the flame is lost 5 times within a heating cycle which according to the online manual indicates either: Low gas pressure (unlikely since other two gas furnaces in the house work fine, right?) Faulty gas valve Dirty or faulty flame sensor Faulty hot surface igniter (unlikely because the flame does ignite, right?) Burner problem
Any idea what might be wrong and how to fix?
Even if this is not a DIY fix, I would like to have some idea of what the potential problems could be so that I can intelligently engage the service person and make sure that I am not being taken :)
Thanks!
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blueman3333 wrote:

I believe there may be a flame sensor. Its failure could cause the symptoms you describe.
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Would this be consistent with the fact that initially the flame burns for a couple of minutes before shutting off?
Also, if it is a flame sensor, how difficult/advisable is it as a DIY repair? (I am very experienced in electrical, electronics & mechanical, reasonably experienced in plumbing, but have minimal experience in gas).
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blueman wrote:

Minutes? I thought you said "a second or two."
It's entirely consistent with "a second or two" multiple times over a span of minutes.

I've never seen your particular furnace. It might just be dirty. Look for a little gadget with a window facing the flame and a couple of wires coming out of it. Make sure the window isn't sooty and the wires are still connected to the control board. Judge for yourself how hard it would be to change it.
Of course, it could also be the associated control board electronics. Less likely, IMHO, but possible.
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It initially burns for maybe 2-3 minutes without interruption. Then it starts cycling with each subsequent flame lasting typically for 1-2 seconds or often even less (though sometimes I have seen it last as long as 10 seconds).

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blueman wrote:

Apparently I misunderstood. That doesn't sound like a flame sensor.
I think you'll need somebody familiar with your particular furnace (and its controls) to sort this out.
Good luck.

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NO SHIT!
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CJT wrote:

Another could be inducer pressure switch?
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blueman wrote:

Flame sensor is like a keavy needle which is usually located opposite side of ignitor. They does not go bad easily, but surface rust make them sluggish. Use ememry cloth or fine fine snad paper to remove rust. Most likely it'll work again,
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I just went through exactly the same thing with an Amana furnace here. It was the flame sensor and cleaning it with emory paper did the trick. It would go into lock out and then the fan would remain on until I reset the system.
Tony Hwang wrote:

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I just went through exactly the same thing with an Amana furnace here. It was the flame sensor and cleaning it with emory paper did the trick. It would go into lock out and then the fan would remain on until I reset the system.
Tony Hwang wrote:

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wack the gas valve with a piece of wood. But this occurs to me only on my first startup of my old unit
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One interesting additional (but probably unrelated fact)... The only recent change to the system is that two days ago I installed a condensate pump since previously all the condensate was just dripping vertically down a 2-story 3/4" pvc pipe onto our crawl space floor. I connected a short length of 5/8" clear flexible tube to the end of the pvc pipe which then hangs over the basin of the condensate pump. The condensate pump seems to be pumping the water fine.
Not sure how this could be related to my problem since the water is draining by gravity into a basically open reservoir so I don't see how it can be backed up two stories to the height of the furnace. However, the timing is strangely coincidental...
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blueman wrote:

while installing it?
For instance, one for the flame sensor? :-)
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Nowhere near the controller -- I was just down in the crawl space... unless somehow manipulating the pipe downstairs somehow moved the other end on the second floor bumping something electrically...
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Bad thermostat.
Change to a digital thermostat and upgrade the wiring too.
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Oscar_Lives posted for all of us...

most.
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Tekkie Don\'t bother to thank me, I do this as a public service.

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wrote:

Why exactly is it that you homemoaners think you need to "engage" the service tech? All that does is run your bill up while we waste time chit chatting on your dime. A good tech really doesnt need to hear a word from you other than, "the heat doesnt work and its down there in the basement". If he needs any more than that then he isnt much of a competent tech. If you want to fix it yourself, pull all the parts off the working furnace and start putting them on the inoperative furnace one by one till you get it. Whats the worst that can happen? Your burn yourself, you asphyxiate yourself or your family, you burn your house down or you electrocute yourself. and maybe, just maybe, you might get it right. Bubba
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Get which part right? The burning, asphyxiation, arson, electrocution, or the part about calling someone who doesn't have to guess, actually knows where the basement is?
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Mo Hoaner wrote:

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