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Furnace runs then gas shuts off but inducer blower still runs but no ignition!

Furnace runs then gas shuts off but inducer blower still runs but no ignition!


  #1  
Old 12-11-08, 07:53 AM
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Furnace runs then gas shuts off but inducer blower still runs but no ignition!

My furnace (a high efficiency) starts runs fine for several minutes then the gas shuts off and the furnace shuts down after the main blower cools it off. The temperature never reaches the thermostat setting. Meanwhile the inducer blower continues to run and the ignition sequence starts, the igniter lights but the gas valve does not open so no ignition. This attempted ignition sequence happens several times and finally the control board goes into lockout. Turning off the main power and then turning it back on will cause normal ignition sequence and operation again for several minutes then the above shut down is repeated.

I have cleaned the flame sensor. I also made sure that there is no restricted air flow and that the furnace is not over heating.

Any help would be appreciated.
 
  #2  
Old 12-11-08, 09:25 AM
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If I am understanding you correctly, it sounds like my problem which aoppears to be the igniter. See my post I placed this morning for my symptoms. http://forum.doityourself.com/gas-oi...g-furnace.html
 
  #3  
Old 12-11-08, 12:16 PM
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Decibel,

Nope the igniter on my furnace is fine and it lights the burners. It is just that after a while the gas is shut off and then the igniter, though it glows, cannot light the burners because the gas valve does not open. It will light again if I turn off the power to the furnace so that the control board resets to normal and then restore the power.

Thanks for the thought.
 
  #4  
Old 12-11-08, 12:36 PM
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Tricky, Do you know if you are getting enough gas pressure to the furnace? You should have 7 to 11 inches in water of gas pressure to the inlet of the gas valve and the valve will drop it to around 3.5 to the burners. I take it you do not have a water manometer. Try this 1st. Go out to your gas requlator next to the meter. There should be a vent on the regulator. Make sure it is clear and nothing inside it. (it's a small vent and sometimes small spiders build a web or live there in the summer they plug it up and you loose pressure). If that is not the problem we shall look into limit switches.

Good Luck
 
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Old 12-11-08, 12:50 PM
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Mr. Happy,

There appears to be plenty of pressure (judging by the strength of the burner flames). The pressure switches (there are two) seem to be working at least at the start up but I have not checked them again when the burner shuts down. This is really puzzling because the furnace will work for several cycles if it does not have to raise the temperature very much to satisfy the thermostat. But then it will do the shutdown and the temperature will fall and it goes into the failure to relight routine. It seems like the gas valve shuts down before the thermostat is satisfied and then the inducer blower runs and the cycle of ingiter with no gas released starts. After the lockout the inducer blower still runs but there is no more attempt of light the burners.

Have you ever encountered a "weak" pressure switch that drops out after a period of time??
 
  #6  
Old 12-11-08, 01:05 PM
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OK, 2 things. First yes I have had a week pressure switch that drops out. The small inducer fan has a clear plastic tube that runs to a diaphram disc. That pulls in and proves draft, and opens the gas valve. If it does not prove draft or the pressure falls after a period of time the burners go out. If you can some way check the flupe to make sure it is free of any obstructions would be the 1st thing I would check. Also with the high efficency units there is a combustion air pipe as well (normally PVC) both running outside. If the pipes are clear, I place the unit on a heat call and get as close to the unit as possible with out getting too close as to burn yourself, and listen for a slight click. If you hear a slight click it is a limit. (assumeing you do have a multimeter) there are several small disc shaped controls on the blower, around the burners and the main limit that normally looks like a black rectagle with 2 wires comming out near the center of the heat exchanger (all the limits will have 2 wires normally purple) Take your meter and see if the limits are open electricaly. Some have a manual reset small button on top and others have to cool down to reset.

Let me know what you find out, also can you give me the modle # and Ser # maybe I can find a wireing diagram and send it to you.
 
  #7  
Old 12-11-08, 02:13 PM
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Mr Happy,

OK I'll check the inducer fan for the "flap" you mention. I know that there is a short piece of clear plastic tubing attached to the bottom of the inducer fan that connects into a plastic unit that appears to be associated with collecting condensation. One of the pressure switches is also connected to this unit via another clear plastic tube. A drain tube exits this unit and is routed out of the furnace.

The furnace is:
Heat Controller INC
High Efficiency Reclaimer II
Model GWU 120 - D4

I will check for the serial # but right now I am not at the house.

I will also look for limit switches with purple wires but I do not recall any such as you describe.

Thanks for your help!!!
 
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Old 12-11-08, 02:46 PM
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If you could send me a picture of the wiring diagram on the back of the door.
 
  #9  
Old 12-11-08, 04:40 PM
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Check to see if you have incoming and outgoing power to the pressure switches when it drops out. If you lose voltage TO them, problem is upstream, like a limit switch.

But I think more likely not, since often the blower will keep going (in order to cool the hot heat exchanger down), and yours does not. I think you may be losing power on the outgoing side of the limit switch, because after the furnace runs for a bit, backed up condensation may be causing reduction in drafting.

If you can actually see the plunger move on your pressure switches, see if the plunger is fully pulling away from the switch, or if it is sort of on the cusp. If you are not sure of it's travel distance, you can tell if you remove the vacuum tube (leaving the pressure switch end connected), and sucking on the tube and watching how far that plunger travels.

You also could have a problem where you have blockage in the port hole where the pressure switch's vaccum tube originates from. You could ream that out clean, and see if that improves the ability of the pressure switch to stay operating.
 
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Old 12-11-08, 06:03 PM
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Originally Posted by ecman51` View Post

But I think more likely not, since often the blower will keep going (in order to cool the hot heat exchanger down), and yours does not. I think you may be losing power on the outgoing side of the limit switch, because after the furnace runs for a bit, backed up condensation may be causing reduction in drafting.
Actually the main blower is working correctly. When the flame goes out (during the malfunction) the main blower continues to run until the plenum cools down and the lower limit control stops the main blower. However, the inducer blower continues to run and the igniter glows etc but the gas valve does not open so no flame results.

I have tried to clear the tubes which drain the condensation but it may still have a plug somewhere up the line.
 
  #11  
Old 12-12-08, 05:10 AM
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Originally Posted by Mr Happy View Post
If you could send me a picture of the wiring diagram on the back of the door.
Unfortunately the diagram is very faded and seems to have gotten wet somehow. I do not think I can get a good photo of it. There is some rust and it appears that there has been a leak inside the the main furnace compartment.

I am beginning to suspect the flame sensor. The connector to the sensor is rusty and I am afraid I will break it off if I try to disconnect it. Still I wonder if there is enough resistance in the connection to block the micro voltage that the flame sensor puts out.

One reason I suspect the flame sensor is that sometimes the unit will start the burner will light but the gas valve will shut down within a minute or so. Other times the furnace will light and run as normal until the thermostat is satisfied.

I am about ready to call in the pros and bite the bullet while a serviceman give it a try at diagnosing the problem.
 
  #12  
Old 12-12-08, 06:13 AM
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Try to jump out the pressure switch, and then see if you can splice a new connection on the flame sensor. Pull the sensor out and clean the post with some sand cloth, Even though this is not a permenant fix it will give you an idea where the problem is.
 
  #13  
Old 12-12-08, 07:24 AM
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[QUOTE=JCCralley;1480701]
Unfortunately the diagram is very faded and seems to have gotten wet somehow. I do not think I can get a good photo of it. There is some rust and it appears that there has been a leak inside the the main furnace compartment.


This raises an eyebrow for me, if the main compartment is leaking, it could be a crack in the heat exchanger. However hopefully it is just a blocked drain either in the A/C coil leaking into the furnace or the PVC flue drain comming from the hear exchanger is blocked. At this point it is just a matter of pride in fixing it yourself. Even if you get it running, I would still have a pro come out and give it the once over with a blessing.


Good luck.
 
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Old 12-12-08, 08:30 AM
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[QUOTE=Mr Happy;1480771]
Originally Posted by JCCralley View Post



This raises an eyebrow for me, if the main compartment is leaking, it could be a crack in the heat exchanger. However hopefully it is just a blocked drain either in the A/C coil leaking into the furnace or the PVC flue drain coming from the hear exchanger is blocked. At this point it is just a matter of pride in fixing it yourself. Even if you get it running, I would still have a pro come out and give it the once over with a blessing.


Good luck.
The rust in the compartment dates back to a couple of years ago when the AC drain clogged and spilled over into the compartment. There appear to be no leaks of the PVC flue drain system.

I removed the flame sensor and cleaned up the connection. I also probed out pressure valve line and it is clear. Right now the furnace is functioning normally so I just have to wait and see what happens next.

One good thing out of this is that I discovered that the hot water heater is leaking!! So I'm off to see about getting a new one (which I will have professionally installed!!!) -- what else can go wrong???

Thanks for your advice!!
 
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Old 12-12-08, 03:22 PM
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OK. I hope this is the last post for this thread!!!

This morning I pulled the flame sensor out and gave it a thorough cleaning with a green Scotch scrub pad and also I was able to disconnect the rusted plug from the sensor. I then sand papered the plug and the contact on the flame sensor to remove most of the rust. After I reinstalled the sensor the furnace has been working perfectly since 8:30 AM (it is now 4:20 PM here). So I have my fingers crossed and am hoping for the best!! LOL

Oh yes, I also have a new water heater installed and up and running. Beer 4U2

Thanks for the help!!
 
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Old 01-13-09, 02:05 PM
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Oops guess I didn't get it fixed after all!

Well, here we are in Jan. and the problem has resurfaced. Course I was sure that it had to be the flame sensor since the furnace ran for a month after I cleaned the last sensor, so I purchased a new one and installed it. That was Thursday 5 days ago and all seemed to be well --- but last night the furnace went back into lock out with the inducer fan running as before. I shut off the power to reset it and it went back through a normal ignition and heating cycle. Then again at around 5:00 AM the furnace was back in lock out with the inducer fan running. Cut the power to reset and once again normal ignition and heating cycle. I called the pros but they will not come out until it goes back into lock out so they can "diagnose" the problem. It is now 3:00 PM and the furnace is still going through normal cycles.

I'm wondering if the hot surface igniter may be the problem. It is lit for what seems a long time before the gas valve opens.

How does the control board sense the the igniter is hot enough for the gas valve to be activated??

I have another igniter (I keep a spare on hand) and maybe I will install it and time how long it is lit before the gas valve opens verses the current igniter. Should be the same-- right???

John
 
  #17  
Old 01-13-09, 09:27 PM
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The hot surface ignitor is on a timer. Often the circuit board will leave the hot surface ignitor on for an extended period of time in order to reach maximum heat if the circuit board has earlier detected a failure of the burners to light and is repeating the ignition cycle.

That you notice the HSI stays on for a lengthy period of time before the gas turns on suggests that the furnace is shutting off because of that failure to detect the burners lighting.

Since the HSI does light the gas reliably when you observe the lighting process, that suggests to me a bad circuit board or ignition module where that flame detection circuit is failing.

I haven't seen any discussion or flashing lights on the circuit board identifying problems --- are there any such flashing lights?

Personally, I'm pretty satisfied that the circuit board needs replacement.
 
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Old 01-14-09, 06:54 AM
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Thanks for the reply,

You are likely correct that the circuit board is at fault. I have already replaced one on this furnace about ten years ago. There are no LEDs on this unit so there is no code to read.

Having said all that, the "pro" found that the heat exchanger is cracked and he could not (by law) reassemble the furnace.

Bottom line: I have a new Trane 90% single stage going in tomorrow for $2583.00. Could have saved myself some head aches last Dec if I have done a thorough inspection when the problem first came up. Luckily the house is not a tightly sealed and no one suffered CO poisoning!

Do like the "pros" say and get the heat exchanger inspected annually BEFORE the heating season.

Thanks to all who tried to help.

John
 
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Old 01-14-09, 07:39 AM
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Personally, I'd have the furnace installers leave the old furnace and expose and verify that there is indeed a hole or crack in the heat exchangers.

I'm always interested in how often that is and isn't the case when someone has told that there is a problem.
 
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Old 01-14-09, 09:59 AM
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SeattlePioneer wrote: "Personally, I'd have the furnace installers leave the old furnace and expose and verify that there is indeed a hole or crack in the heat exchangers".

That could be a good idea BUT I have already verified that the heat exchanger is cracked. I can easily see the split.

As a postscript, I have the furnace back together and in operation (with the same lock out problem) because the temperature is going down to subzero tonight!! I would rather break the law than have frozen water pipes burst (been there in the past). I am not concerned with the CO poisoning possibility because no one will be in the house overnight and I will only stop over briefly to reset the furnace if it goes into lock out. There cannot have been too much CO present up to now or we would have experienced symptoms (nausea and headaches etc.).

Anyway, the problem is going to be solved when the new furnace goes in tomorrow. By the way, the Trane furnace carries a lifetime warranty on the heat exchanger.

John
 
 

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