Anybody up for a furnace diagnosis?

toxcrusadr

Omelette au Fromage
I'm about to call the service guy so I'm grasping at straws. But sometimes you guys have good straws. :thmbsp:

York Stellar Plus 90% efficient gas furnace ca. 1989. One day in the spring it didn't fire up and I just, well, procrastinated. The small combustion air blower (vent fan) wasn't running. I pulled it out, verified it wasn't running and found a replacement ($274 for the darn thing, but two other schmucks wanted $500 so I did OK. Bastids.). It turns and blows air, but the light-up cycle does not proceed. No glow from the hot-surface ignitor.

The old fan had bad bearings AND there were little pebbles of hard stuff in the squirrel cage. I heard that dust and dirt can get into the system, go through the flames and make these kinds of cinders. They do look like cinders. I also found, inside the burner box, a bunch of what looked like tiny white poop from some critter. Pencil lead sized. Very odd. No sign of a nest or anything.

Just to be sure I checked the inlet/outlet pipes which go out of the back of the house. The inlet pipe had a small mud dauber nest which I vacuumed out. I pushed a flashlight 15 ft back to the first turn in the pipe and saw no blockage. Same for the outlet pipe. There is about 10 ft of pipe at right angles on both before you get to the furnace. It's not accessible due to turns in the lines.

I pulled the hot surface ignitor and it looks OK - not burned open.

The gas is on, all the condensate lines are hooked up so no holes in the boat. All wires reconnected and multi plugs plugged in.

I'm thinking maybe there's a relay out somewhere, or one of the many safety interlocks is shutting her down for some reason. I'd like to check the inside of the heat exchanger for debris (or waxy yellow buildup? :D ) but not sure if it's even accessible. Maybe there's an entire groundhog nest in there.

Ideas?
 
going from on how mine works it sounds like the combustion air speed sensor isn't doing its thing . i use a vacuum cleaner on mine .
 
Hmm...one thing I did not report was that I put a piece of fiberglass window screen over the inlet to the new ventilator fan. That was to keep out any of those gravelly bits that might jam it up if they are still coming out of the combustion chamber. It did cross my mind that this will block off maybe 10-20% of the area and could cut down on the air flow rate. Maybe I will pull that back out and see if it has any effect.
 
Well, dammit, it's fixed! I cut that piece of screen out of there and it fired right up. Pete, you're a genius.

Fussy lil thang, complainin about a little piece of window screen. :tongue:

Once again, AK rocks! :thmbsp:
 
We have Carrier high efficiency furnaces here at work. One failed in a similar way to yours, combustion fan motor ran but nothing else. Finally I chased it to a failed over-temperature sensor on the firebox. The air motor was running to cool the thing down, but it was keeping the gas shut off. I actually flicked the thing with my finger and it went back to working for a few days till the replacement part came in.
 
Well, dammit, it's fixed! I cut that piece of screen out of there and it fired right up. Pete, you're a genius.

Fussy lil thang, complainin about a little piece of window screen. :tongue:

Once again, AK rocks! :thmbsp:

I was just about to say that I had a similar problem upon furnace start up last year. Restricting air flow can do all sorts of negative things. I was working on my basement all year, and put a 1" thick piece of filter material over the downstairs intake to keep construction dust down. After awhile that was enough to cause the reset switch to kick in which kept my gas furnace from igniting. High efficiency filters can do that as well. Yeah, they trap smaller particles, but that also causes less air flow over the combustion chamber which the safety controlls don't like. I bought a $50 cleanable filter which I am going to clean EVERY month...time will tell ...but if I get 2 years out of it then it paid for itself.

FWIW...those blue spun filters that you can get for $1 are not good for your HVAC system. You can pour salt right through them which means that a lot of particles are getting through to clog up your coils. Filters are kind of strange...to efficient and they clog fast causing the fan to work harder, more current draw or in my case furnace shutdown.....Not efficient enough, they don't filter, and cause debris to accumulate in the furnace or be sent back into your air. BTW ...I took HVAC in college
 
Those blue filters look worthless. I use the mid-grade white ones. We burn wood and have a woodshop too, plus we garden and like to keep the windows open so there is plenty of dust. I do change the filter often. I have to say that I never thought about dust accumulating in the 'house' side of the heat exchanger - yikes! I wonder if the guys who replaced our AC coil (above the furnace heat exchanger) in 2006 got a look at it and whether they would have cleaned it.
 
Most people don't think about it, but I can promiss you it is true.

If "stuff" gets by the filter it either accumulates somewhere in the furnace or is sent back into your environment through the ducts. If it collects on your AC coil ( I don't have central air BTW) then it reduces its cooling effect AND causes higher current draw due to poor air flow. The condensate (water on the coil) will carry some of the "stuff" to the drain where it can eventually cause a clog. I clean carpet, and had many jobs where people left for vacation with their AC running only to come home to a flood and find stinky, browned out carpet because their condensate drain was stopped up. Not cheap to fix, and all because of improper filtration.
 
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Another place the dirt that bypasses the filter can end up is in the blower wheel. The curved vanes can get filled with dust that over time hardens and will turn the vane that was once curved into an inefficient flat that will overheat the motor. :nono:
 
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