Gas Furnace Shutdown

ggerg

I have a gas furnace that shuts down due to rollout switch open. I've cleaned out all burner tubes and any rust buildup around the manifold and ports. The air flow in and out seem unrestricted. I noticed that the flame was not focused 100% down the burner tube. Forgot to mention its a condensing furnace. Why does it continue to trip the rollout switch?

Jamie Hall

What is your draught at the breech? When starting, when running?

EBEBRATT-Ed

@ggerg

I would check the flue pipe and draft inducer of an obstruction like dead mice or a bird. Very common

HVACNUT

How old is the furnace?
Do you know at what point in the cycle the rollout occurs?
The inducer proves draft by means of a pressure or centrifugal switch, and the flames are straight down the tubes. But what happens to the flames when the blower motor comes on?
Could be a cracked heat exchanger. 
A manometer would see this right away.

ggerg

Thanks guys for those tips, Would a clogged AC coil overheat the furnace. The top of the furnace gets to be about 200 degrees above the burner manifold, the flame was sloppy and spreading everywhere so I removed the manifold and cleaned all the tubes and diverters that were clogged with rust. It seams much better the flame isn't spreading but the rollout keeps tripping. What temp do those rollout switches open typically?

EBEBRATT-Ed

@ggerg

A clogged ac coil would overheat the furnace but would not cause flame roll out.

I would start the furnace and watch the flames closely. When the blower starts any change in the flames means the HX is cracked

JUGHNE

If this is a condensing furnace with PVC venting, then it has a secondary heat exchanger that could be partially plugged not allowing the inducer fan to pull the fire up as it needs to be.

Do you have any pressure switch problems with this?

ggerg

A change in flames is a broad answer, what exactly am I looking for. It's all PVC piping about 25' for both lines w/ell's. Why would the heat exchanger be clogged? There was a little gunk in the trap assembly, how do you flush a secondary heat exchanger. Pressure switches close and it fires up fine. Seems to overheat and open the rollout safety switch. Could the rollout switch be played out?
How can I check for a cracked heat exchanger? Thanks for your input guys it's helpful!

Lance

I admire your DIY activity, but be cautioned, we tag out these furnaces until repaired. I recommend a licensed gas fitter with experience in this type of furnace. I question the rollout switch label. Perhaps it is simply the high temp limit? Limits usually are in series and any one of three can open the circuit. A high temp limit tripping can be faulty, or if due to high a temps indicates one of two conditions: Over firing of the burner, or restricted airflow which can be caused by dirty filters, closed air dampers, dirty indoor ac coil or furnace condenser coil. Over firing could mean a failed gas regulator or incorrect adjustments. A pressure switch monitoring the vents could indicate clogged vent, condensate problems. An induced draft fan can also be failing. We have found corroded fans when drains back up. Remember a proper combustion test will determine what the fire is doing. A proper temperature difference between the return and supply plenums is necessary. See Mfg. specifications for critical measurements. Good luck.

HVACNUT

The rollout switches are in the burner compartment and are manual reset. Is that what we're talking about?
Rollout usually occurs at the time of ignition, not mid cycle. 
It might be time for a pro.

Gregg\"the egg\"Foley

Still having issues with this furnace shutdown on rollout switch opening. Luckily homeowner doesn't use this furnace much, being a 8000sq ft. home they can live without for now.
Anyway I looked at it again , here's what I checked out. Both fresh air and makeup air piping is clear, no restrictions. I opened up the ductwork and examined the heat exchanger and underside of the AC coil, no cracks clean as a whistle. There are CO detectors in the house which are functioning,which should be going off if the exchanger was cracked. What I discovered while looking at the burner manifold while firing was flames licking out from in between the manifold and not shooting down the heat exchanger tubing in various spots creating blacking in between the exchanger ports.So i decided to take temperature readings of the cabinet in various spots and found the top of the burner box to be about 240degrees and the sides of the box about 180-200 degrees exactly where the rollout switch is located. So after about 15min of running pop goes the rollout switch again. I've come to the conclusion the burner assembly is faulty and allowing the flame to spread in between the assembly and not going 100% down the burner tubes. Any more thoughts on this? I'm at my wits end but hopeful this is the answer. Thanks for your input your all appreciated out there, thanks. Gregg

mattmia2

Sounds like the combustion is improperly adjusted.

The CO detectors aren't likely to go off with a cracked HX for a number of reasons. 1. If the combustion is proper there isn't much CO being produced. 2. The blower keeps the HX under positive pressure so it it more likely to add excess air out the vent or to the combustion side than to have CO come out of the HX. 3. UL listed CO detectors need high sustained levels of CO to alarm.

You need to find someone that knows what they are doing to help you look at this before someone gets hurt.

realliveplumber

Is this a horizontal furnace.? Could be condensate (water) in the lowest tube.

EBEBRATT-Ed

You have a potentially serious issue. Roll out switches don't trip for no reason.

Read the other current post"Please read the instructions"

You don't want that to happen. Find a professional and get him in there