Any furnace experts out there?

[jbarker496]

Hi fellas-
Got some trouble with my furnace here. I have a wood stove and electric heat for backup, so my problem is not super urgent, but my wife doesn't like fussing with the wood stove when I'm gone, and I think the electric heat is kind of expensive.
Anyways, here's what's happening....
Furnace is a Ruud 90 plus running on propane, installed in the mid 90's.
Thermostat calls for heat, exhaust blower fan turns on, hot surface ignitor glows red, propane valve opens, and most of the burners light. I believe that there are about 7 burner tubes. The problem is that sometimes the flame does not travel down to the the last burner tube, where the flame sensor is. When the flame does not travel down to the last burner, of course the flame sensor senses no heat and it shuts the flow of propane off after about 3 seconds. Sometimes the flame travels to all burners and it works fine. Sometimes it malfunctions. When it does work, all flames are nice uniform blue color. No yellow flames.
I took the burner out and cleaned it very well. There was some ash and a little bit of rust, but no obvious obstruction in the crossover ports between burner tubes. It malfunctions less now, but still has the same problem on occasion.
So.......do I need a new burner, need to check out something else, or what? Thanks in advance for any help.
-Jay

 

[DAKSY]

Hmmm. First guess would be something blocking the burner orifice.
There's a small brass "Jet" that regulates the amount of gas getting
into the end of the burner tube.
I'd pull that to see if there's debris or a spider's nest behind it, limiting
your gas flow...
HTH

 

[jbarker496]

Well, I guess we think alike. Since I posted this question, I got to thinking the same thing so I took the manifold off again and checked all jets. All are clean and free of obstruction. Let me know if you can think of any other potential problems. Thanks for your time.
-Jay

 

[fbelec]

sounds like a pressure problem. might be time to call a licensed gas guy. i think you should have 5 inchs water column of pressure. might have a bad regulator.

 

[Gooserider]

Moving to the "boiler room", see if we can get some more suggestions from the experts there...

Gooserider

 

[colebrookman]

sounds like a pressure problem. might be time to call a licensed gas guy. i think you should have 5 inchs water column of pressure. might have a bad regulator.

I agree with"fbelec" There are times to bring in the pros with needed gauges. Better than big boom! Be safe.
Ed

 

[jbarker496]

Moderator- Thanks for moving this to appropriate place. I wasn't really sure where to post it.
Furnace specs are as follows: line pressure should be 11-13" WC, and manifold pressure should be 10. (Pressure is higher cause of propane).
I checked the pressures today with manometer. Line pressure is 14.5, a little high. . But the gas valve seems to be compensating well, as manifold pressure is right on at 10 " WC. I observed this manifold pressure both when furnace was functioning properly and also one time when burners did not completely light.
But now there is a new wrinkle...
Up until now, once the furnace lit, it would always run until it hit the thermostat temp and the thermostat shut it off. Now, it shuts off before room reaches set temperature. It tends to shut off after running about 5 - 8 minutes. I had a manometer on it during this problem. Manifold pressure instantly drops from 10 to 0. Flames go out immediately, no sputtering or any sign of a problem before this happens. Clearly, the valve is closing abruptly. The blower motor runs for a few minutes as it always does once the flames go out. When blower motor shuts off, furnace immediately cycles back on and will repeat this cycle as long as thermostat is calling for heat.
All advice is greatly appreciated, including the advice to stay safe. Thanks!
-Jay

 

[heaterman]

If that's the little 34" tall model that furnace is known for the problem you have going on. Those burners corrode over time and the dimensions of the "wings" on each tube go south. This causes the failure you are experiencing where the flame doesn't carry from one to the next. Even if they are clean it'll still mess up. You can try to isolate which one in particular is causing the problem by interchanging the tubes and observing where the flame stops but most often it means a new set of burners. The way they have that manifold tucked up in there makes a kind of a PITA to change 'em but it can be done. If you have had them out to clean you already know what I'm talking about.