Gas Furnace Help!

[jo552006]

I am hoping somebody can help me with my gas furnace issue.  This has been a long running intermittent problem.  Until now, I have never had a chance to observe the exact problem (I HATE intermittent issues...cars especially), and I think I now enough of the pieces that somebody who knows more about furnaces than myself may be able to put together in order to point me in the right direction.

Background: I am familiar with working on things and fixing things.  I have tools, and multimeter for testing available.  Furnace is ~20 years old.

Occasionally the furnace does not keep the house at temperature.  I have seen this several times, usually when we get home from work and it is colder than it should be.  I have occasionally noted that the fan is still circulating, but with no heat.  Simply turning the furnace off, then back seems to cure this and allows the house to come to temp.  Today I noticed the house was set to 68 degrees, but was only at 62, after running for a few hours (if the furnace is running well, it takes minimal time to get to temp).  I went downstairs noticed the ignitor going, flame starting, and going out after approximately 4-5 seconds.  This occurred repeatedly.  I opened the blower housing to see if the LED indicator was giving me a trouble code, and it suddenly worked!  I have noticed in the past that a new filter seems to help with the issue (maybe opening the blower cover allowed extra air in?), but after looking at the old filters, I find it hard to believe they are so clogged they are causing an issue.  It's like my furnace is extremely (read: maybe a bigger problem) sensitive to dirty filters or something.  Is this because I bought cheap filters from home depot?  Was my opening of the blower housing a weird coincidence? Is replacing filters more than once a month common?  I can give more specifics if necessarily.  I would hate to call a repair man for an intermittent issue, just as much as I'd hate to take my car to a mechanic for an issue that may take days to appear again.  If anybody who knows about these systems has any ideas, I'd love to hear them.  Again, perhaps I'm mistaken, but the filters I am removing, though used never look SO clogged that I would expect an issue.  I replace my filters monthly whether they need it or not (except spring/fall when the furnace literally isn't on).  I have AC, and the unit appears to be clean (cleaned it myself).

[MDM]

Has there ever been a time when changing the filter did not solve the problem?

Exactly how do you "[turn] the furnace off then back" so that it "seems to cure this"?

"ignitor going, flame starting, and going out after approximately 4-5 seconds" sounds like a flame detection problem.  How does your unit attempt to detect flame?

[wizlem]

Agreed. Almost certainly a flame sensor problem. Sometimes they just need cleaned.

[jo552006]

MDM, in answer to your questions

1. I am not sure if a filter always solve the problem (due to the intermittent nature).  What I have noticed, is that if the problem starts occurring frequently a new filter definitely seems to reduce the frequency.  Does it solve the issue... I'm honestly not sure.  I do know that the filters I've been pulling out recently didn't seem like they were NEAR needing to be replaced.  (2 were so clean my wife put them back with the new filters because she didn't realize they were used)  They are cheap filters, but I thought this would mean LESS filtering, and thus MORE cfm through the filters compared to other filter types.

2. I have a Honeywell digital programmable thermostat (actually can be used for a heat pump...which I don't have).  While it has MANY features, I only see 4 wires hooked to the terminals in the back, as my setup is pretty simple.  When I say I shut the furnace off, I simply go to the thermostat, set it to "off", wait till the blower fan stops blowing, then turn back on.  Works like a charm until the next time my issue occurs, which could be days, weeks, months, etc.  Could also be the same day (but usually a filter reduces the frequency).  I have noticed that when the "issue" occurs, the thermostat always says "heating" or "heating on" so I do not believe it is the thermostat.

3.  I totally agree it sounds like it could be a flame sensor issue.  I have a metal rod type that detects the flame.  Previous owner had a second flame sensor in the documentation with the furnace.  Don't know if it was a spare, or one they had removed due to it being bad.  I *think* I threw it in last year to see if it made a difference, but didn't notice any so I took it out.  (Or maybe I MEANT to try that...)  Either way, is there a way to test this item?  I have a blowtorch and a multimeter...  Alternatively, I can always order a new one, and throw it in for good measure.  It should fix it or rule it out.  (I figure the total cost of parts for my debugging should still be less than a single repair bill + I learn something).  Do the metal rod ones need to be cleaned?  (I always assumed cleaning probably is for eyes that "see" the flame)  If they do, should I just use some crocus cloth or sandpaper?

Should I assume (for now) that the filters are NOT the issue, but rather a coincidence?  Like maybe when I go in to change the filter I am jarring the furnace enough to wiggle the flame sensor's sporadic connection maybe?

My biggest concern is that when it gets colder, this could happen on a long weekend and a pipe could burst.  Luckily where I live it's not too cold yet, and we only get pipe bursting weather for short time periods.

P.S. sorry for the slow reply, and thanks very much for the help!

[AccidentalMiser]

MDM, in answer to your questions

1. I am not sure if a filter always solve the problem (due to the intermittent nature).  What I have noticed, is that if the problem starts occurring frequently a new filter definitely seems to reduce the frequency.  Does it solve the issue... I'm honestly not sure.  I do know that the filters I've been pulling out recently didn't seem like they were NEAR needing to be replaced.  (2 were so clean my wife put them back with the new filters because she didn't realize they were used)  They are cheap filters, but I thought this would mean LESS filtering, and thus MORE cfm through the filters compared to other filter types.

2. I have a Honeywell digital programmable thermostat (actually can be used for a heat pump...which I don't have).  While it has MANY features, I only see 4 wires hooked to the terminals in the back, as my setup is pretty simple.  When I say I shut the furnace off, I simply go to the thermostat, set it to "off", wait till the blower fan stops blowing, then turn back on.  Works like a charm until the next time my issue occurs, which could be days, weeks, months, etc.  Could also be the same day (but usually a filter reduces the frequency).  I have noticed that when the "issue" occurs, the thermostat always says "heating" or "heating on" so I do not believe it is the thermostat.

3.  I totally agree it sounds like it could be a flame sensor issue.  I have a metal rod type that detects the flame.  Previous owner had a second flame sensor in the documentation with the furnace.  Don't know if it was a spare, or one they had removed due to it being bad.  I *think* I threw it in last year to see if it made a difference, but didn't notice any so I took it out.  (Or maybe I MEANT to try that...)  Either way, is there a way to test this item?  I have a blowtorch and a multimeter...  Alternatively, I can always order a new one, and throw it in for good measure.  It should fix it or rule it out.  (I figure the total cost of parts for my debugging should still be less than a single repair bill + I learn something).  Do the metal rod ones need to be cleaned?  (I always assumed cleaning probably is for eyes that "see" the flame)  If they do, should I just use some crocus cloth or sandpaper?

Should I assume (for now) that the filters are NOT the issue, but rather a coincidence?  Like maybe when I go in to change the filter I am jarring the furnace enough to wiggle the flame sensor's sporadic connection maybe?

My biggest concern is that when it gets colder, this could happen on a long weekend and a pipe could burst.  Luckily where I live it's not too cold yet, and we only get pipe bursting weather for short time periods.

P.S. sorry for the slow reply, and thanks very much for the help!

I suspect I know why the previous owner had a spare flame sensor with the documentation, the first thing I'd do is replace the flame sensor or at least move it (if possible) to ensure it's getting max exposure to the flame.

The filtered air doesn't directly impact the combustion air side of the furnace unless you have a hole in your heat exchanger (which could cause carbon monoxide problems) so what you are saying about changing the filter having an impact on the flame is quite concerning to me.  If you don't have a carbon monoxide detector, please get one TODAY!

Your thermo should have four wires, so I doubt it's the thermostat, particularly since the furnace is trying to light and flaming out.

Please do an inspection of the heat exchanger and ensure it's not leaking as best you can.  The filter change issue is either coincidental or indicative of a larger problem. 

[MDM]

...the first thing I'd do is replace the flame sensor or at least move it (if possible) to ensure it's getting max exposure to the flame.

Or try something similar to https://www.youtube.com/watch?v=tYwR9qL4lnk if both are already dirty.  Some form of this is where I'd place my bet, although for the flame sensing function to work with one of these rod sensors, you need at least
  - flame impinges on the sensor (as A. M. says, to get max exposure to the flame).
  - sensor is responsive to the flame heat (this is the problem addressed by the youtube video)
  - wiring is continuous from the flame sensor to the burner management logic device (this would be the sporadic connection problem you suggest)
...etc.