trouble shooting gas warm air unit

lchmb

ok..so i have a 90 plus York warm air furnace that's giving me the fits. The unit has been installed 3 months......I adopted this one after many calls and honestly cant figure it out...(I hate to give up). The complaint is the unit will rumble a couple times a day on start up. I found the unit was not vented per specs so changed that. Changed the first and second stage regulators (LP) and set the flow to 11" wc. I adjusted the manifold pressure to 9.8" wc. I redid the air intake thinking it was causing an issue where it was located above the burner thinking it may be causing the issue. Tech support is at a loss as am I. After redoing everything the unit ran about 24 hours with no issue and then 3 times in a row the customer reports it rumbled...granted not as bad as before....I cannot see an issue with the burner tray, the igniter is original to the unit and I tested it to verify it is good...Any thoughts? suggestions...and yes shooting this is a possibility...
I did have the unit fire back through the venturi prior to doing the work listed. After I was unable to recreate it but my guess is that is the cause of the rumble...

Jamie Hall

Draught and combustion air come to mind. Particularly, is there enough combustion air? Are there any other exhaust fans in the house that might be causing a slight negative pressure and back draught until the unit is up and running?

Jamie Hall

To which I might add -- in other words, it isn't always the furnace or boiler that is the problem. Just as we say in steam, it sometimes pays to wander out of the boiler room...

lchmb

I have no doubt your right Jamie..funny thing is, this is a direct vent unit BUT it will burn in the venturi with the door off or on....

Jamie Hall

lchmb said:

I have no doubt your right Jamie..funny thing is, this is a direct vent unit BUT it will burn in the venturi with the door off or on....

They're not really supposed to do that... all the connections tight? All the seals intact? I'm shooting in the dark...

qctechfred

Hi,
Couple of things I tought you could check,
Distance between intake and exhaust outside is in spec? I suppose you calculate every elbow to choose the right size exhaust
Do this furnace needs mixer screws in each burner ( I know ICP needs some when on Lp)
Good size orifice? If you say it burns in venturi maybe you don't have the right orifice/pressure
Gap between igniter/spark rod and ground ok?

Also what were your combustion numbers when doing your analysis?

It's just a couple of idea to start from

Thanks

rick in Alaska

Is the blower fan coming on immediately on burner light off? I did not think this unit did this, but if it did, maybe compromised heat exchanger?
Rick

Aaron_in_Maine

I would check orifice size as well also check manifold pressure. Does it burn in just one Venturi or all of them?

captainco

Without combustion numbers it is kind of tough to diagnose a combustion problem. If you are burning in the venture you are not pulling air into the burner. Is their some kind of blockage, is the inducer weak, is there water building up and not draining. Does this furnace have baffles in the burner chamber that don't belong (NOX rods)? All these things can be somewhat determined with combustion readings. Having taken multiple 90% furnaces and blocking the air intake anywhere from 80% to 100% most still have enough combustion air on the intake side. Therefore the problem is on the discharge side assuming the inducer is correct. Low O2, low flue temperature, high and rising CO would definitely indicate a restriction somewhere. Overfired would rarely cause burning in the venturi.

ratio

My Old Guy once had a white grocery bag end up in the PVC intake pipe of a 90%'er. Static would hold it to the side of the pipe wherever the inducer was off, but whenever it came on the bag would billow out & obstruct the airflow. It took him a while to figure it out--it was just about invisible when it wasn't actually causing a problem!