Anyone knowledgeable about gas furnaces?

[Orrex]

Ours is acting a bit funny, despite the six different HVAC techs who've been to the house and assured us that it's fine.

Each of them has said some variation of:

Your furnace is too old (it isn't)
Your house is too big (it definitely isn't)
Your ducts are too big (well...)
Your ducts are too small (well...)
It's cold outside (no shit. The furnace works great in the summer, when it doesn't run.)

And each of them has superfluously cleaned the brand new flame sensor, shrugged, then handed us a bill.

I'm reminded of my late father, in whose world all problems derived from a chocolate allergy:

Headache? It's because you ate chocolate.
Stomach ache? It's because you ate chocolate.
Joints sore? It's because you ate chocolate.
Slept poorly? It's because you ate chocolate.
Leaky roof? It's because you ate chocolate.
Muffler rattling? It's because you ate chocolate.


I'll give more detail about the furnace if anyone would care to throw their hat in the ring.

[rgbecker]

1. Can't wait to hear the problem!

If it's headaches, turn it off and get a Carbon Monoxide detector to determine if you have leak. Or maybe it's too much chocolate.

Forced Air? boiler?

[Orrex]

3. Forced air

Specifically, a Goodman 80% furnace, installed in August 2013 (and still under parts warranty).

Basically, the problem can be described as "it runs too much." So I can see how a tech eager to get to lunch would dismissively answer "it's cold out," but that's like a mechanic saying "of course your car's having trouble; you're driving it uphill."

I'm sure the furnace runs very well when it's 75 degrees out, just like my car runs great when I'm driving downhill with a tail wind, but what about the other 99% of the time?

It cycles on, the burners start up, then shut off in a few seconds (hence the newly replaced flame sensor (that didn't fix it)).

Or...

It cycles on, the burners start up, then shut off after 20 - 30 seconds.

Or...

It cycles on, runs for a while, gently exhaling tepid air throughout the house

Or...

It hums for a while, doesn't cycle on for a stretch of time, then does cycle on and does one of the above

Or...

It runs just fine, heats the house to a cozy 68 or whatever, and then shuts off normally after the cycle.

That last iteration is what it's done EVERY TIME WE'VE HAD A TECH AT THE HOUSE! That's true whether it's 10 in the morning, 3 in the afternoon, or 9 at night. Somehow the goddamn furnace knows exactly when to run with clockwork perfection.

I've tentatively narrowed it down to the blower capacitor, which is easy enough to replace, but I don't feel like it addresses all of the quirky performance issues.


One thing that was significant is that our house was getting low gas pressure from the street, but this has been corrected, yet the problems with the furnace persist.

[CloudWatcher]

15. Get a video of it failing

Intermittent problems are any repairman's curse. Get a video of it acting poorly and tell the tech that it's an intermittent problem!

I put up with a boiler that was acting up for years. Intermittent issues. Finally got it on video and the slow-motion showed 1-foot flames gushing out the front of the unit (for a fraction of a second). Finally figured out it was the first cycle (after a long period of off) that would have problems. After testing it for a bit it wouldn't fail until the next day.

Decided that I didn't really want my house catching fire and got it resolved (over a month of replacing parts).

After the video, the second thing I'd do is to rule out the thermostat. The theory being that if the thermostat is busy telling the furnace to go on/off/on/off/on/off it's going to act weird. Can you trigger a cycle at the furnace? Or maybe just set the temperature to 100 degrees (for a few minutes) and see if it misbehaves?

Intermittent problems are such fun!

[snacker]

44. Something kind of similar with my Goodman furnace

My furnace guy replaced the pressure switches. One of them was bad, but I told him to replace both of them. Only took a short time and the switches weren't expensive.

Good luck.

[Thunderbeast]

2. Symptoms?

[Orrex]

4. The fix seems to be that I need to keep an HVAC tech on site at all times

The furnace runs just fine when one of them is here, thereby making us look like lunatics complaining about imaginary symptoms.

[Fix The Stupid]

5. Just spent 4 weeks working on my Goodman - ask me anything...

At the end of the day, I have to get a new one installed as I finally diagnosed a cracked heat exchanger...

The replacement part is $600 and after ripping my old one out last night, let's just say the labour to install that $600 part is definitely not worth it.

But..I kept it going for 4 weeks after it started acting wierd...

All I ask is that if your furnace is starting to act up, please make sure you have a WORKING Carbon Monoxide detector...very important if there are serious issues with your furnace.

Other than that, I have replaced the following on a furnace

motherboard
presssure swtiches
roll out switches
etc...

Tell me some symptoms and I'll try my best to steer you towards the probable issue...

[Orrex]

7. I have two CO detectors in the basement and two upstairs I'm positively paranoid about that.

[vanlassie]

31. Well... since you asked! I don't really have a problem

(I don’t think, anyway!) but. I got a new 35 ton 14 seer unit last summer. Works fine. It’s just that the heating starts up in a way that seems different from past units. It will start (I will hear it). Then, it seems to cut out for a few seconds. Like maybe 5-8 seconds? Then it comes on fully. Is this a pattern that makes sense to you?

[Fix The Stupid]

6. Ok.. I was typing that bit up there when you replied...

Is the LED light on the motherboard flashing any codes when it starts acting up?

[Orrex]

8. LOL. That's the other thing. Thanks for asking.

Solid red. So even while it's going through its crazed spasms, the LED says "feelin' fine."

[Fix The Stupid]

9. Mine did that too.. then the tech said I needed a new motherboard...

Which he promptly quoted me $700 to install...

I bought one for $100 and it took me 10 minutes to install and did not fix my issue either.

Another thing - which I am sure you have already done but will repeat for anyone reading - is to make sure you have a clean, new air filter. A clogged or dirty filter can make your furnace schizophrenic.

So..let me ponder this - wish you were closer, I would just send you a replacement motherboard to try out...

Cheers.

[Orrex]

11. Good tip on the filter, but it's likewise brand new

Even when I couldn't get out to buy a new one early last month (thanks to covid), I simply took out the dirty one for an hour or so, with no noticeable change in function.

Thanks for giving it some thought. I'm sorry that you had to go through that with your furnace, but I'm grateful to benefit from your recent experience.

[tikka]

26. If it has a drum fan, check to make sure the blades aren't clogged with gunk.

The reduced airflow can cause problems like short cycling.

[Emile]

10. Vent pipe clear? I had a bird nest in mine. Your heat exchanger

could be going out.

[Orrex]

12. Good thought on the vent pipe, but I think we're ok

This furnace vents via our chimney, for which the installer put in a brand new metal sleeve with cap, to prevent nesting.

I wondered about the heat exchanger, but I confess that that diagnosis is a bit beyond me. Also, I'd hope that an experienced HVAC tech would have considered it, but who knows?


Thanks for your input--all suggestions are welcome!

[Emile]

16. I had a cap on mine too. How a bird got in there was

beyond me. I could smell propane outside when the furnace was running and it was coming from the vent on top the house. If it ends up being your heat exchanger, you might as well replace the furnace for what they cost. Good luck!

[Orrex]

18. That sounds like one determined bird!

Happily, there's no smell of gas, and the actual gas company has been out for a separate but related issue and thoroughly swept the basement for leaks.

[mopinko]

13. how's the flue damper?

i've had that a few times. sometimes they drop dead, but sometimes they flake for a while.
next most common is gas valve.

i have a 2 yo boiler that was having trouble keeping up. running constantly. turned out there was an efficiency setting that was on min efficiency. set it closer to max and it's doing fine.

[Orrex]

14. One guy checked the gas valve, which is how we learned of the bad gas line to the house

Otherwise the valve is apparently fine.

Not sure about the damper. I'll look into it.

[rgbecker]

17. I'm thinking main gas valve is sticky.

These two symptoms would suggest this.

"
It cycles on, runs for a while, gently exhaling tepid air throughout the house

Or...

It hums for a while, doesn't cycle on for a stretch of time, then does cycle on and does one of the above

Or... "

A sticky valve would be intermittent, working only when tech is there watching it! It you could catch it humming, but not lighting, a tap with light hammer might free it open...that would confirm the diagnosis. Had one like this. The hum would be the solenoid trying to open the valve.

Maybe the high gas pressure that you reported did something to change the way it was working before?
Maybe the running too much is because it is blowing without lighting.

[Orrex]

20. It was actually low gas pressure to the house, due to an ancient and cracked pipe in the yard

It's now been fixed, which wasn't cheap (and was also a surprise, since we thought we'd purchased the protection plan for that).

I can try the gas valve suggestion.


One thing that I can add, after staring at it since I first posted, is that it seems to run ok once it gets started. That is, once it manages to get its shit together, it seems more or less normal.

But then that cycle finishes, and the next time around it's another dice roll to see what result we'll get.

[Kaleva]

19. What actually hums for awhile?

Your comment:

"It hums for a while, doesn't cycle on for a stretch of time, then does cycle on and does one of the above"

Is it the exhaust blower? The blower must be running up to speed in order to close the vacuum switch which then allows the ignition sequence to begin.

"It cycles on, runs for a while, gently exhaling tepid air throughout the house"

What do the flames look like when it's doing the above. Nice and blue or mostly yellow and some orange?

One problem I often ran across with 80% gas furnaces that had a exhaust blower was there'd be crude in the vacuum lines or in the ports where the vacuum line connected to.



The service techs you had there should have checked the lines and connections for blockages and restrictions.

[Orrex]

21. I can offer a little more detail after further observation

I'm not honestly sure where the hum comes from. It's very noticeable upstairs (apparently transmitting the sound through the ducts) but not as obvious right at the furnace.

The flames are consistently blue.

I just watched the cycle and, though I apologize for my likely incorrect terminology, here's about how it goes:

Option 1
A fan starts running (not sure where; I don't want to pop the bottom door & kill the cycle)
The igniter glows a lovely bright yellow for several seconds
The gas comes on and ignites a pure blue, and less than two seconds later it shuts off.
The igniter goes dark
Fan still running.

A few seconds later the process repeats more or less exactly

It does this four or five times over the course of a minute or so, then:

Option 2
The igniter glows a lovely bright yellow for several seconds
The gas comes on and ignites a pure blue
The main(?) blower comes on
The gas burns for 30 seconds or so then shuts off
The main(?) blower stays on
The igniter glows a lovely bright yellow for several seconds
The gas comes on and ignites a pure blue
The gas burns for ~20 seconds then shuts off
The main(?) blower stays on

And this repeats, going through one option or the other until, gradually, the house is warmed to the desired temperature, and the thermostat temporarily shuts off the furnace.

This is what kills me about the HVAC techs: There is absolutely no way that this is normal function. Because of my job I work in a great many basements, and I have never once heard a furnace behaving like mine, even though Goodman is apparently the brand of choice in this part of PA and nearby Ohio. If it were so common, and if my furnace were truly "working fine," you'd think that I'd run into other furnaces "working fine" in the same spastic, stop-and-start way during my many hours spent in basements.

[Kaleva]

23. does your furnace have a fan like in the pic I posted?

and what is the model # of your furnace?

Goodman is a very good brand. I've installed several years ago.

[Orrex]

27. Yes, very similar

I also just noticed that the flame does have little streaks of yellow/orange but is about 98% blue.

Nothing clearly indicates "model number," but I do see a label with the following:

GMH80803BN
1212102195

Anything?

[Kaleva]

30. the 1st number is the model

I'll do some research this evening.

[Orrex]

33. Thanks--much appreciated!

Of course, it's been running more or less normally since I posted.

I think it's spying on me.

[Orrex]

22. One other thing that vexes me:

If the blower capacitor were bad, I would (perhaps incorrectly) expect it to be consistently bad, not "perfectly fine some of the time" and "kind of shitty the rest of the time."

Am I wrong in that? That is, if the capacitor is bad, does it result in inconsistent function, or does it die altogether?

[Kaleva]

24. I don't think your main blower is the problem

If I recall correctly, it is controlled by a timer. which explains why your furnace sometimes puts out tepid air. The burner are cycling on and off but your main blower is running steady while there is a call for heat.

[Paha Sapa]

25. My two cents.

It still sounds like a flame sensing issue. If you only replaced the sensor one time you may have received a bad one from stock. They are very sensitive and can be easily damaged by touching them with your hands. Some are easy to get and easy to get to. I have a Lennox and it is relatively easy to R and R. Check YouTube.

[Orrex]

28. I can try that

We actually ordered a two-pack, so I can try the other. I made a point of not touching the metal part for exactly the reason that you describe.

[Kaleva]

32. A faulty flame senor wouldn't prevent a furnace from attempting to fire.

""It hums for a while, doesn't cycle on for a stretch of time, then does cycle on and does one of the above""

[tikka]

29. Usually induction motors start to make noise when they start to go out

[Bantamfancier]

34. Worth a look.

Check the rubber hose that runs from the switch to the blower motor. There should be no moisture at all in it. Also check for cracks or pinholes. Our goodman would collect moisture in there and the furnace would try and try to start but without throwing that switch wouldn’t fire. Pull the hose, blow it out and bob’s your uncle.

[Orrex]

35. I was looking at that earlier, in fact

The next time the furnace cycles off, I'll check it out.

[Paha Sapa]

36. Pressure switch issue?

Just throwing this out there. Some of the symptoms could be the same.

[Paha Sapa]

37. Hear is some more generic troubleshooting. Seems informative.

Just putting this out there.

[Kaleva]

38. According to this, your furnace stores diagnostic codes

Goodman GMH80803BN

"Features
10 Year Parts Warranty
Lifetime Heat Exchanger Limited Warranty
Patented TuffTube dual-diameter tubular heat exchanger with lifetime limited warranty plus10-year limited furnace replacement warranty
Two-stage gas valve with revolutionary new convertible technology that allows installer to activate two stage operation with the flip of a dipswitch
Silicon Nitride igniter with patented adaptive learning control for maximum igniter life
Furnace control board with self-diagnostics, color coded low-voltage terminals and provisions for electronic air cleaner and 24 volt humidifiers
Control board stores the last five diagnostic codes in memory; simple push-button activation outputs the fault history to a flashing red LED
Low constant fan allows homeowner to activate the low heat speed to efficiently circulate air throughout the home
Self-adjusting feature automatically adjusts furnace to high or low stage based on outside temperature without an outdoor temperature sensor"

https://www.supplyhouse.com/Goodman-GMH80803BN-90000-BTU-80-Efficiency-Two-Stage-Burner-3-Speed-Blower-Upflow-Horizontal-Application-Gas-Furnace

Page 26 gives you info on the safety circuit and diagnostic chart

Page 29 and page 30 is the troubleshooting chart:

https://s3.amazonaws.com/s3.supplyhouse.com/manuals/1249544712473/Goodman-GMH8-GDH8-DualSaver-Gas-Furnace-Installation%20Instructions-Instructions.pdf

[Orrex]

39. Thank you for that--I have a furnace guy here now

Not to disparage the guys before him, but they were not as seasoned in the field as this fellow, so I'm more optimistic this time around.

Updates to follow.

[Kaleva]

40. Don't hesitate to ask him questions

Learn all the steps that need to happen in order for you to have heat. Beginning when the tstat makes a call for heat till the thermostat is satisfied and the furnace begins the shutdown sequence.

[Orrex]

41. He was, in fact, very cool about the whole thing

He was the first tech who actually seemed to listen to us as we described the problem, and he didn't shut us down simply for not knowing the technical terms or the precise timing sequence, etc. He also dismissed those other techs' explanations about the furnace being too old or the ducts being too small. The furnace isn't too old, he assured me, and if the ducts were too small they'd have been too small since day one, whereas the problems began the winter before last.

Also, he had a more discerning ear that the previous techs, and he seemed to make a preliminary diagnosis fairly quickly.

He walked me through the steps and showed me why it couldn't be this or couldn't be that, until finally he settled on the pressure switch. He didn't have a Goodman unit on him, but apparently they're all pretty similar, so he installed another brand to see if the problem improved. This switch doesn't secure to the furnace housing, but if it works, he'll be back to replace it with a Goodman. He wanted to see how it behaved over the weekend and will return Monday.


Fast forward to now, and the furnace has gone through about a dozen cycles smooth as silk, for the first time in several seasons.

[Kaleva]

42. That guy sounds like a keeper

The duct size being too large or too small is irrelevant. And your furnace isn't too old.

With your symptoms being erratic, the furnace works when a tech is there, he's doing what I've often had to do. Using what I observed and my experience, making a best educated guess and replacing a part that is most likely to be the cause with the understanding it might not be.

By observing and listening to the customer, one can eliminate all that can't be the problem and narrow it down to what could be the problem. Sometimes you can narrow it down to the exact problem .

If you learn how your furnace works and what should happen at certain points during the heat cycle, you can really help the tech when he arrives or you may be able to fix it yourself.

[Orrex]

43. This was definitely a learning experience, both in the basement and on DU!

We had this company out on Wednesday, but it was a newer tech who couldn't find anything. Also, I was at work, so I couldn't talk to him about any of it. I was off today, so I called the company, not to complain, but to ask if they could get someone else out.

They sent the owner, who's also the most senior tech.

Certainly jumped to the top of my list, so we'll call them for future issues. And when it's time for a new furnace, we'll see what they have to offer.


Thanks again for your guidance here!