Furnace is Hiccuping - Help Before it dies!

Factual Questions
1

Raindog talked me through replacing the guts of my furnace a few years back, for which I will be eternally grateful.
Now, we have another issue with the aging box.

Lennox Elite natural gas updraft.

The little motor which checks for a clogged flue is starting, stopping, starting again.

About 3 times before the burners kick in.

Is this more likely a clogged flue (we have had nasty storms lately, but I removed most of the trees around the house, so there aren’t any large limbs to hit it or deposit part of itself) or a defective motor or vacuum switch?

Is there a way to clear a flue without getting on top of the house? Between weather and a bad back, am not looking forward.

And, as always, money precludes the easy way of calling somebody, pointing to the box and saying “fix it”. Damn, I miss the days when that was all there was to it.

I replaced the entire circuit board (this is an early model when everything is on a single board), so it shouldn’t be the cute toy relays this time.

2

That’s the inducer motor.
So, it starts up, then it stops. Does anything else happen before it shuts off? Specifically, I want to know if the gas turns on and attempts to fire.

Also, since you said it happens three times then quits, you’ll probably have a trouble code flashing on the board. If you pull the lower panel you should see a flashing light. You might have to open yet another panel to access it and find the legend.

3

Also, you said ‘hiccup’. I assume that was just a word you used, but is it making a gurgling sound? Is there a drain hose coming off the bottom of the furnace (as opposed to just coming from the top)? If there is, you need to make sure it’s clear. If it isn’t that’ll cause a similar problem and it’s an easy fix.

4

I’ll check back on this in the morning, but two quick thoughts…

If it fires and then the flame goes out after a second or two, then it could be your flame sensor. Some people have luck with sanding it a bit to clean it, but they’re easy to replace. The circuit board will let you know that the flame sensor is throwing a code.

If you have a hose coming from the bottom of the furnace (and hoses inside the furnace), you need to make sure they’re not full of water. Especially the one attached to the bottom of the inducer motor. Pull it off and see if water comes dumping out behind it. If it does, you have a clogged hose that you need to deal with, that’ll cause the problem you’re describing as well. Just blowing through it may clear enough water out of it to let the furnace run long enough for you to deal with it. A still wire. Flushing it with bleach. Running to Home Depot or an auto parts store and replacing all the hoses, whatever you need to do, but you can’t have clogged hoses. They’ll shut down your furnace in winter and your AC will make a mess in summer if it shares the same plumbing.

5

Thanks!

No - the gas valve does not open - I am sitting 10’ from the unit, so I get to hear each noise it makes.
The is just the induction motor coming on and then shutting off. The 3 cycles take about 3-5 seconds.
If I weren’t so close, I would not know it’s happening.

The only plumbing is a PVC drain in the A/C condenser.

We are having a cold snap and I have the poor thing just barely able to maintain the heat setting, so it is coming on frequently.

I notice that the last 2 cycles have gone without the start/stop routine, so I"m thinking maybe the vacuum switch, but it cuts out much more quickly than I’d think it would. It seemms to stop before it has a chance to set up an updraft - like a doc saying “open your mouth, say 'aaahh” - from 5’ away - not enough time to know if it is or isn’t going to work.

Yes, I’m going to turn it down and retire myself.

I’ll play with it tomorrow and see if I can get it to flash a code.

6

on the induction motor should be a thin rubber tube, ensure the tube is clear and theat where both ends go are clear as well. I watched the furnace guy last time and he stuck a long thin allen key into the hole to clear it out. Might help.
Youtube has some good furnace maint/trouble shooting vids too.

Good luck

7

It probably is not your flue (or an obstruction in the flue anywhere, including the chimney), the rubber tube or the pressure switch, IMO.

Probably not.

Think of the “sequence of operations” as a series of dominoes that must fall for the furnace to fire and the blower to come on distribute the heat.

Each domino must essentially 'prove" itself, before the next is allowed to fall. In this sequence, typically the inducer continues to run for some period. If it ran for 1 minute or so before stopping I would indeed to be looking at the flue, or pressure switch etc.

If, however, the inducer is stopping after 3-5 seconds I’d typically looking at the inducer itself, or the board.

The inducer because it may be getting a continuous 115V and stopping. Most of them don’t have capacitors so if the inducer is bad, it will need to be replaced.

A couple things you can do to trouble shoot the inducer:

If you have an amprobe, meter or hot stick, you can see, “is voltage dropping in concert with the inducer stopping?” If so, the inducer is likely fine; it’s just getting an intermittent signal. If, on the other hand, the inducer is getting a continuous 115V and still stopping, it’s likely bad.

A tech without tools will sometimes cut the wires coming off the inducer, strip them and carefully insert them into the end of an extension cord. Then (after putting the wires in the end) plug it into a 115V wall socket. You have now taken the board out of the equation and by hot wiring the inducer you can see, “will it run continuously now, without stopping?” If it’s stopping while being hard wired, it’s likely bad.

One other thing to check… sometimes a bird or animal will get all the way down the chimney and all the way to the inducer and die. Take the flue loose and inspect it; make sure there is nothing in the mouth of the inducer, or actually *in the inducer wheel. * (I’ve seen small sparrows get all “Cuisinart”'ed")

…trouble shooting the board/stat in another post…

8

3-5 seconds total or 3-5 seconds for each attempt?

I’m not familiar with vacuum tubes in a furnace. If you can get me a model number I can do a bit of reading and try to find some pictures online unless raindog shows up first. Are you sure they’re vacuum tubes?
Also, what do you mean when you say the last 2 cycles have gone without the start/stop routine? Do you mean that it fired up just fine?

ETA, and there he is.

9

As to the board/stat…

The inducer is 115V, but it is being controlled by a 24V signal that starts at the board, goes up to the stat, and back to the board.

On the board is a relay that will close, allowing 115V to pass through to the inducer; all from that 24V signal.

If the inducer is getting an intermittent signal, that may be the cause of the problem.

To troubleshoot that, take the stat out of the equation: Put a jumper “R” to “W”. You’ve now bypassed the stat and made a continuous call for heat. (Remember the door switch if there is one)

If everything works fine, it looks like your stat is sending an intermittent signal. Replace the stat. (although most people accidentally replace stats that are fine. It is overwhelmingly not the stat that s the problem)

To troubleshoot the board…make the same “R” to “W” jumpered call on the board. This should cause the relay that controls the inducer to close.

Is the 115V continuous? A tech would use the same wires I had you cut earlier. Previously, I had you use the wires that came from the inducer to hot wire the inducer. The other side of those wires, naturally, are from the board.

A tech would reason: “the inducer was dropping out after 3-5 seconds. I’m going to put my meter leads on the 2 wires coming from the board and see; is the voltage actually dropping out after 3-5 seconds?”

While that seems to be the flip side of hot wiring the inducer (and it is) it gives definitive proof that the board is wierded out.

10

As funny as it sounds, the time it takes something to fail is a telling fact.

People say, “It stops after 30 seconds or so.”

And I respond, “Are you sure? Could it have been, 3 or 4 seconds, for example?”

A common answer is “I’m really not sure.”

But the difference between a unit that fails after 3 seconds, as opposed to one that fails after 30 seconds, determines where the tech starts looking.

11

As always…

You’re on the internet. If Joey or I were at your home we would likely see things that you didn’t see and make observations that you didn’t think were pertinent, but were.

More importantly, we don’t know your skills. 115V can hurt/kill. If this is way over your head, it may be best to call a pro.

12

ETA…

There are many different things that could cause this, or most problems. Sometimes it may be simple as a loose low voltage wire.

13

Also, don’t forget, it’s Sunday. It may be better to let it hobble along for the next 24 hours then to completely break it and not have access to parts until Monday or end up paying someone double or triple time to come out today.

14

Might be worth checking the battery in the tstat as well. I’m not sure if that can cause this, but it can’t hurt to check it.

15

True. I’ve never seem low batteries do that, but there’s no reason why they couldn’t. Good thing to check.

16

ETA

And this is an open offer…

Sometimes a phone call is a superior way of communicating for the sheer amount of information possible, and the immediacy of feedback.

If usedtobe or any of the regular board denizens has an HVAC need, you can PM me for my # and I’ll help as I’m able.

I can’t guarantee I can help, but I’ll try.

17

Thanks and thanks and thanks

Sorry about the late post - I’m nocturnal and sleep in a lot.

Just opened it and watched the led’s. With nothing happening, they both (1 red, 1 green) flash at a moderate rate. This time it came up with only 1 false start, so I didn’t see a change - it might have started to flash something, but the re-try was successful, so it resumed its “nothing to see” mode.
The induction motor runs for about 1 second - not enough time to tell anything, so I doubt this is by design - e.g. not a foreign object causing a designed response - if there were an obstruction, shouldn’t the glitch show up every time?

The vacuum hose is good; the crimped clips on the ends are intact.

The problem is not the motor - I can hear the relay click on and off - the motor is stopping only (immediately) after that second “click” from the board.

I am desperately hoping this is just the vacuum switch - luckily, the symptoms are pointing in that direction.

The t’stat is 3’ from the furnace.

I’ll get out the old patch wires and play between the t’stat (new batteries about 4 years ago) and vacuum switch.

My skills: re-wire houses, replace components on circuit boards, routinely plays with live bare 115. Wears non-conductive shoes to prevent grounding. Has ratchet terminal crimpers for insulated and uninsulated terminals, as well as the terminals. At one time had MIL-SPEC wire on hand.
I trust me to play with anything inside a furnace

What I screw up is wet stuff - I cannot apply paint to anything larger than a breadbox without painting myself.

18

The “vacuum switch” is called a “pressure switch” and it doesn’t control the inducer. It controls what’s downstream of the inducer. (usually the ignitor)

So…the pressure switch is tied to the inducer, but it doesn’t control the inducer. Whats controlling the inducer is whats immediately upstream of the inducer.

Usually that’s the circuit board.

19

Sometimes a faulty relay will still click.

I’d be applying 115V to that inducer with an extension cord to see if it’s the inducer, or what’s controlling the inducer.

20

So far, it is either an intermittent connect from the t’stat or a bad relay - it goes click click click in rapid succession for 4-5 times, then will pause for a couple of seconds, then repeat the cycle. This time it caught on the second such cycle.

My back and shoulder argue strongly against getting into it tonight.

My notes from last time show red-to-green = call for heat - correct? I’ll do that short and see if it’s the relay or t’stat signal.

I could dig out one of the relays I used when I was trying to salvage the original board and use it to switch on the inducer whenever there is a call for heat. but that would have it running for the duration of the burn. It looks to be a heavy-duty motor, but I don’t want to have it running 100’s times as long as designed.

I’m just going to keep a jumper handy and short across the pressure switch if it fails completely.
I’m not up for tearing a furnace apart when temps are in the 30’s - 60’s (eat your hearts out, people with real weather).

While we’re on HVAC:

When the old board finally burned itself up, I looked it to see what all it was doing - and found a crystal - you don’t see those real often. I’m guessing it is used to create a square wave to match the flame sensor - correct? If not, what was it doing there?