HVAC-Central furnace problem, burner not heating.

[Vern_in_IL]

Coleman
Presidential 3 7966-856
Propane furnace.

Pilot light is a 50% yellow and 50% blue, does not hardly move when blower is on.

When the Thermostat calls for heat, it does this every time:

woshwoshshshshshshshshshshshhshsh, while it makes this noise, the burner is lit, but the plenum is not heating up to temp to kick on blower switch.. and it keeps this up until you turn off the thermostat.

If you turn the thermostat off, and then back to the setting, the burner shuts off, then on, it does NOT make the whooshing sound, and it runs normally.

Now it is inconsistent. I took out the burner assembly and cleaned up the pilot orifice, the main jet also. Noticed that Asian beetles had been sucked into the chamber, cleaned them out.

When I hear it in "woshwosh wooshshshs" mode, I can often just turn off the thermostat, and turn it right back on, and it works fine then. I am puzzled.

I have verified the integrity of the plenum: There are no holes or cracks, and the pilot does not dance when blower is on.

A good solid 28 volts is present in the gas valve circuit.
The only variables I can think of is a drafting issue or a gas pressure issue. If it was a drafting issue, would it not be consistent?(roof jack plugged with beetles)

Now I do have a Manometer, so this Sunday I will be measuring the Water Column at the gas valve fitting, and the Tank regulator, performing a lock up test.

Any suggestions what to look for? I'll have to call in a HVAC tech if I cannot resolve this, I really don't want to put anymore investment into this furnace(going to replace it in two years with a 90% efficiency model)

 

[CaptainFreon]

OK. I run my own biz, been doing Refirg, heat and A/c this since late 70's.
What you are probably hearing is the LP burning inside of the burner tube venturi, instead of on top where it should be. The bang ba-woosh is delayed ignition, where gas builds up and ignites all at once, instead of a smooth fast light across the burners.

The plenum is the sheet metal box on top of the furnace, the heat exchanger is where combustion takes place, where cracks and holes can happen.

LP will not burn as pretty blue as natural, but mostly blue. LP is not as forgiving as natural. MAKE SURE there are no leaks, it will fill up the basement, and you won't smell it upstairs.... because LP is heavier then air, keep that in mind. Natural gas is lighter then air.

Check your nameplate, all the LP furnaces I've seen are designed to work on 11 inches of manifold pressure. That manifold pressure does NOT mean before the gas valve, at the tank, or before/after the reg on the outside of the house. The manifold is the header pipe that all the furnace burners are attached to. there is a brass orifice that meters the gas, they are screwed into the manifold and are directed into the center of the burner venturi. Centering the orifices is important, When I've encountered a troubling ignition and exhausted all else, I've had to get 1/8 inch pieces of pipe and screw them into the orifice tap on the manifold, and then add flat washers to the manifold mounting feet to center it in the venturi.

The gas mixes with air in the venturi before leaving out the top for the burn.

Gas valves have an allen wrench plug you can take out, THAT is where to read the manifold pressure, the threads are 1/8 inch IP. IP = iron pipe thread, not the material it's made of. NPT (national pipe thread) standardization. Some older furnaces have a 1/8 inch IP plug on the manifold itself that you can pull for the same pressure measurement.

The gas has to flow through the gas valve first, before it gets to the manifold. There is a minimum of 2 inches (often 2.5) of pressure drop across that valve. THAT is the catch. the LP dealers in my area are ignorant, they supply 11 inches to the house. I tried cranking their supplied reg all the way down as far as it would go, and barely got the 11 I needed, But that made the stove burners like a bonfire.

The local LP providers just can't seem to grasp the concept of pressure drop across the mechanical inefficiencies of the gas valve. I proved it to one group, it was like I blew away their mind. anyway, sorry... Just aggravating. If I had a house that required LP, there would be two lines from the tank, one to satisfy the furnace.

The lower gas pressure under fires the furnace. Premature heat exchange failure can happen. Sooted furnaces can happen.

I have seen them run OK with 9 inches. It's just not right.

I think you may have dirt and rust to clean out of the burners. Most burners have cross over tubes, where the one pilot lites off one burner and the cross over tubes carry ignition to the rest of the burners. If that cross over tube is gummed up, you will have a ba-woosh. Some cross over tubes are nothing more then hollow flat plates welded to the front of the burners, making a continues mini burner all the way across.

If you get them out, shine a lite down the venture, You may find a bug nest in there from the summer. Coat hanger it out. I have flushed sooted burners out with water too, let them dry out.

the pilot will get crusted over inside. That will come apart. Ash from dust burning and falling down inside of it. There is a little tiny hole in the pilot orifice in the pilot, make sure that is open and don't make it bigger.
Good luck. This is written as IMO. What you do is your own liability. I recommend a licensed and trained HVA/C company to look at your furnace.

 

[user52165]

Kudos to CaptainFreon who went to great lengths in his post. His last line is telling. I didn't think he would get to that.

I am the ultimate DIYER.

But............... there are few things in life that should be left to an experienced pro. This is one of them.

 

[SumpChump]

Now THIS is the POWER of BITOG. I have yet to find ANY forum as deep as active as instant with expertise to share in any area such as paint hvac legal etc.

CaptianFreon with that one post has helped probably hundreds of future readers and Google searchers. Its just amazing.

 

[Vern_in_IL]

Thank you CaptainFreon.

Very well could be a dirty burner, some alignment issue with the burner sounds about right, a lot of variables here, pointing to the burner, I'll for sure get a tech out here.

I don't believe the furnace has ever had a seasonal checkup, it has only been serviced by homeowner when the thermo coupler fails, and all the service calls have been for the AC system.... then I moved in....

 

[Vern_in_IL]

Just had a technician out... he adjusted stuff and has come to the conclusion that my roof jack may be full of debris right above the furnace.
He said the pilot looks right FOR THAT FURNACE. The burner flame is all yellow, he believe it's a drafting issue, partial blockage of intake air.

Not allowing enough draft to the flame. The Roof Jack is stuck really good to the plenum. Technician attempted to remove the top cap...

...but the cap screws are rusted solid, they might end up destroying the roof jack trying to open it up.

They will use a 3/4" pipe and a vacuum to clean it out. He does not want to mess with it right now, until he has a backup roof jack. The furnace is almost 30 years old. he does not want to go further until he has a replaceable roof jack. He pointed out while on roof that the cap covers was thin(and rusted) and if the top cap screws are difficult to remove, the second cover will be even worse, as it goes though more sheet metal.