I have a 1.5 year old Armstrong Furnace that seems to break and fix itself...A couple weeks ago we woke up to a cold house and the furnace wouldn't stay on for more than a few seconds (inducer would start, igniter heat up, gas fire but then shut off after at most a minute or two). I turned the furnace off and back on and it started up no problems and the house heated back up.

A week ago, the same thing happened except this time off/on didn't fix it. Taking the advice from another thread in this forum I checked the outside vents, changed the filter, cleaned the flame sensor, changed batteries in my thermostat, but nothing helped. The error it gave was "Pressure switch open with inducer on." Had a tech come and he checked everything and found a bunch of dirt in the drain coming off the furnace. After he cleaned that out it started up no problem. He had thought it might be a faulty pressure switch but it was opening/closing when he used suction on the little tube attached to it.

Yesterday I woke up to a cold house again, checked the drain and everything else, no problems. A couple hours later, it started up and stayed on (I had taken a break from looking at it, I did nothing between trying it and it not working and then it starting up.) Worked fine all day yesterday and then this morning the house was cold again. Turned it off/on and it started up and has kept the house warm so far.

My thermostat is set to just hold a temperature, no schedule. There is a humidifier attached to the furnace that's been there for about a year. There's also an HRV. If there's any other info you need let me know. I feel a little strange calling a tech to come look at a furnace that is working fine at the moment, but it seems pretty clear something is wrong.

Any help would be appreciated!
Steve

 

Welcome to the forums.

I would doubt the pressure switch is bad, especially with the age. Usually it turns out to be a vent issue. Did he check the pressure, at the inducer, with a gauge when he was there.

 

I can't say for certain but he did attach some kind of gauge in that area. I was watching my daughter while he was here so was in and out a bit. I've been doing some reading online and read that closing vents can cause some furnace issues. We'd blocked a few to try to get some rooms warmer (seemed like a reasonable idea :/), could that cause this kind of problem?

 

Check the intake & exhaust piping. Look for any sags or low spots. These pipes need to slope upward from the furnace to the outdoors at a slope of at least 1/4" of rise per foot of run. Installers often use plastic plumber's strap to hold the pipes up & it will stretch especially if the supports are too far apart.
You can also take the hoses off the inducer & probe up thru the nipples with a snug fitting wire or drill bit turned by nothing more than your fingers. Sometimes rust will bulid up & block the hole.

 

Thanks for your reply. The problem continues, with us often waking up to a cold house and then at some point during the morning it kicks on and stays on, usually after a couple failed attempts. I've tried cleaning the hoses/nipples but that had no effect.

I've attached a few pictures of the intake/exhaust piping, to me it looks like there are a couple flat parts but it's difficult to tell as 1/4" per foot seems like a pretty small slope. I also attached a picture of the exterior where I noticed today there's a pretty significant buildup of ice around the exhaust pipe.







Thanks again for the help.
Steve

 

Nice pictures. The pipe you are more interested in is the one with the ice on it. That would be the discharge line. I have to wonder if that icing is part of the problem. If it's icing where you can see it.... it must be doing the same thing at the bottom on the inside.

Can you post the model number for the furnace.

 

Usually you can measure off the floor joists to get a pretty good idea as to slope. A 4' or longer level is ideal but most homeowners don't have a level that long.

 

Thanks to both of you for your replies.

re: icing. I went out and took a look tonight and both chunks of ice had fallen off. The furnace also came on while I was outside and there was pretty significant air flow coming out of the exhaust. I did knock a similar chunk off when the problem first occurred and I'd gone to look outside, so the ice formation is a recurring issue. The model # is A95UH1D090C16A

re: sloping. I took a level and the exhaust piping has a very significant slope. The intake piping has very little slope to it - the bubble is still inside the middle section of the level, but shifted to one side. By my rough measurements from the floor joists have it sloping at very close to 1/4" per linear foot. This is true on two sections, the one in the picture below with the red oval around it, and then the same ~1/4" per foot slope from the top of the elbows to the exterior. I also wonder about the elbows themselves, the junction on the intake piping seems a lot messier than the exhaust (see the green ovals on the picture)



Steve

 

In an ideal situation the intake & exhaust would be virtually identical but in the real world it doesn't often happen. I presume the exhaust is the upper pipe in the last picture. It's a less restrictive path than is the bottom pipe. Are those pipes 2" or 3"? Should be 3".

 

Yes the exhaust is the upper pipe. Both pipes are 3".

 

Like Pete, I'm thinking there is ice in the bottom of that ell on the exhaust pipe. IF that's the case, then the question is what to do about it. I'm open to ideas.

 

I have no ideas but I'm willing to try your suggestions

I was out there today and the ice had started reforming. There was also a fair bit of water around both connectors on the exterior exhaust pipe. I assume that shouldn't be happening?

Also it seems to have pretty much settled into a pattern of stop working sometime in the night, start working around mid-morning, occasionally stop during the day but work enough to keep the house at a reasonable temperature all afternoon/evening, repeat.

 

The lack of exterior elbows is one reason I am a big advocate for concentric vents.

If you have an electric outlet close, maybe installing a heat tape approved for use on PVC might be worth a try.