Whooshing noise and plentum is not heating up-inconsistant

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Vern in IL
Posts: 3
Joined: Mon Nov 09, 2015 10:33 pm

Presidential 3 7966-856

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 so often:

woshwoshshshshshshshshshshshhshsh, while it makes this noise, the burner is lit, but the plenum is not heating up to temp to kick on blower.... and it keeps this up until the thermostat stops calling for heat. (vertified- fan switch is new)

Now it works normal most of the time, it is inconsistent. I took out the burner assembly and cleaned up the pilot orifice, the main jet also.

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.

It is the only appliance I have on propane right now.. could my 500 gallon propane tank regulator be bad?

There is a air shutter adjustment on the burner assembly, I am not sure how to set or check it.
Vern in IL
Posts: 3
Joined: Mon Nov 09, 2015 10:33 pm

The furnace is propane.
Vern in IL
Posts: 3
Joined: Mon Nov 09, 2015 10:33 pm

If anyone else is wondering, this is a reply I got elsewhere on this issue. Hope it helps someone out there!
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.

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.

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.
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