Weird natural gas furnace problem

We have a 25-30 year old Rheem furnace & just tired firing it up for the first time this year. Initially what happened was the flue fan kicked on them nothing. I checked it again about an hour later, the fan was still running & based on the thermostat it never lit. 
 

So I started troubleshooting. I confirmed we have 24vac(measured 28.3v with my multimeter) at the thermostat between W & R, so I jumpered them and...nothing happened. I did a bit of googling, tried jumpering them again, the flue fan kicked on & a minute or 2 later the ignigtor started glowing. I never heard the gas valve though, and the ignitor glowed once more without any gas before I pulled the jumper. 
 

So off to google for more "research" and I ohmed out the solenoid on the gas valve. It's neither open nor shorted, so I set my meter for volts & put it on the ignitor, jumpered the thermostat wires again, and still nothing. I pulled the jumpers, reset the molex connectors between the blower box & burner box, but still nothing. 
 

So I unplugged the furnace from 120v, tried again & it worked! It fired up & seemed to be working fine, but since I still had my meter leads on the ignigtor & the thermostat jumpered I pulled the jumper before anything melted. 

And it hasn't worked since, even after pulling the 120v and resetting it again. 
 

What would cause such weird intermittent & inconsistent behavior?

So I gave up, went outside to tend to the critters, and when I came back in about 10 minutes later the furnace is on & working fine...

Nice job. Ya fixed it. 

In reply to OHSCrifle :

Hopefully for the entire season. 

Furnaces with circuit boards often need shiny new circuit boards.

My 35 year old Rheen gets a new thermocouple every decade or so.  I've now spent almost $40 on maintenance since I bought it.  I know a new one would save the world, but I do like not freezing my pipes...

In reply to Streetwiseguy :

Exactly, I have no desire to replace this with one that will be less reliable over the long term. Although it's starting to get pretty rusty inside so that day is likely nearing. 

Rusty inside...make sure your running a carbon monoxide detector!  Cheap insurance against something that can kill you. 

Twenty years ago or so, I had a big scare with CO that awoke us at night and forced us out of the house until replacement could be done. 

I have a 25yrold Trane. Couple years ago, during the very last week of heating season, the heat stopped working. It would start up if I turned breaker off then back on, but once it hit desired temp, it would stop heating but the fan would keep blowing and it would not start heating without cycling power again. Got a used control board on eBay for $70 and it's been running fine the last 2 years. Also, I kept the old board installed that summer, as the AC control part of that board worked just fine.

My thermocouple goes every once in a while. Cheap and easy fix.

Similar story, my house is still on oil heat from 1983 or so.  Talked to my trusted local HVAC bro, he said unless its broke, don't replace it, just maintain it.  New furnaces are E36 M3, my old oil burner will run 100 years with just maintenance.  Considering I only run through 1.5 tanks of fuel oil a year in heat (~$600 or so, probably more this year, [flounder removed by Mod]), there is no payback on a new furnace that makes sense.  

93gsxturbo said:

Similar story, my house is still on oil heat from 1983 or so.  Talked to my trusted local HVAC bro, he said unless its broke, don't replace it, just maintain it.  New furnaces are E36 M3, my old oil burner will run 100 years with just maintenance.  Considering I only run through 1.5 tanks of fuel oil a year in heat (~$600 or so, probably more this year, [flounder]), there is no payback on a new furnace that makes sense.  

No, I don't consider this a joke, nor do I think it's funny.  Yes, I consider it a violation of the forum rules.

edit:
So I realize you've probably been out, and not checking the forum... and I'll assume my PM dropped into your spam folder.  I've given what I feel is a "good faith effort", and I've decided to remove the flounder from your post.  Feel free to follow up with me via PM, if you disagree.

In reply to sleepyhead the buffalo :

Thank you. 

My furnace is original to my 1950s house. I cut the gas to turn off the pilot every summer when I run the A/C. In the fall when I went to relight the furnace   it would attempt to fire but  failed . Come to find out, spiders took refuge in the burn tube causing the failure.

Since I partially heat with  a wood burner, I don't see the  point in replacing the furnace until I' m too decrepit to handle the firewood .

sleepyhead the buffalo said:

93gsxturbo said:

Similar story, my house is still on oil heat from 1983 or so.  Talked to my trusted local HVAC bro, he said unless its broke, don't replace it, just maintain it.  New furnaces are E36 M3, my old oil burner will run 100 years with just maintenance.  Considering I only run through 1.5 tanks of fuel oil a year in heat (~$600 or so, probably more this year, [flounder]), there is no payback on a new furnace that makes sense.  

No, I don't consider this a joke, nor do I think it's funny.  Yes, I consider it a violation of the forum rules.

edit:
So I realize you've probably been out, and not checking the forum... and I'll assume my PM dropped into your spam folder.  I've given what I feel is a "good faith effort", and I've decided to remove the flounder from your post.  Feel free to follow up with me via PM, if you disagree.

Yeah I didnt get any PMs and I was out enjoying my Saturday.  

In reply to rustybugkiller :

I'd actually read that was a possibility & found a fine wire to run through them. 

It's been working, but sometimes it seems like it isn't triggering to turn on the furnace so I'm going to pick up some fresh batteries for the thermostat next time I'm at the store. I honestly wish I'd never replace the original mercury thermostat with a programmable one, especially since we always just set it to the desired temp & leave it. 

Pardon the crappy pic but this is the setup on our Heil heater of the same general vintage.  Notice the orange vacuum hose going from the flue fan to a vacuum diaphragm with a wire attached.  When that is disconnected, the flue fan will run when the thermostat commands but the gas will not flow.  The diaphagm closes a circuit to let the gas safety valve know the fan is running and start the gas flow.  If that hose has a cut in it or it's dried out or it's not connected, you won't get the gas safety valve to cooperate. The diaphagm (or the equivalent on your heater) could be bad or the wire might be frayed and making intermittant contact.

In reply to Jerry From LA :

Thanks! I replaced our flue fan a year ago & the hose seemed ok then, but I can pull a vacuum on it & make sure the diaphragm switch is closing. 

Might the gas valve have a physical issue without an electrical fault?

Because that's exactly how my 15-year-old Goodman behaved when the gas valve was bad.

I had lots of problems with the diaphragm switch on my Goodman furnace. I replaced it two or three times in as many years then it failed again. Out of frustration I opened it up and cleaned the contacts and put it back together and it worked for years until I sold that house. I think the new switches just had some junk in them that would eventually build up on the contacts. Check out your diaphragm switch with a multimeter and make sure it's good. It's a simple single pole switch and you can actuate it by gently sucking on the tube. Do not hook any kind of pump up to it. I learned the hard way that it's measuring a very gentle negative pressure and anything approaching vacuum blows out the diaphragm. 

Faulty airflow switch?

There are a series of switches involved.  It turns on the furnace blower first.  Then there is a microswitch with a "paddle" in the airflow.  Once the airflow pushes that switch closed and knows that the blower is working, only then does it send juice to the ignitor and gas valve.  Then a temperature sensor reports after 30 seconds or so that things are actually getting warmer in the firebox which lets it know that it successfully ignited.

If your airflow switch is intermittent and only sometimes reports airflow, that would explain why you sometimes get heat and sometimes don't.

I forgot to mention that the draft sensor diaphragm switch - the one that gave me trouble in my Goodman - has the symptoms you describe. Furnace fan kicks on, draft fan kicks on, furnace doesn't light. If the draft fan is on the diaphragm switch closes which tells the fan the chimney is clear. If it doesn't close the furnace won't fire. Like I said it's a simple dry contact so you can unplug it, connect an ohm meter or continuity detector to it, and suck on the tube coming out of it (gently, it's tender!). With a tiny bit of suction there should be continuity. If it's always got continuity that's also a fail as the furnace must see it open initially then close once the draft fan comes on. 

Be careful here OK fellas?   The induced draft fan proving switch is what makes sure that the combustion gasses are heading out of the heat exchanger and up the flue.  For what it’s worth that switch is usually pretty inexpensive, if there’s any doubt look it up on line and replace it.  I wouldn’t want to take chance and risk carbon monoxide poisoning.  Oh yeah, if you don’t have a couple of CO detectors please get some.  Cheap insurance. Thanks!