Pilot goes out, burner not lighting

[suninthepines]

Nordyne model M1GH 070A BW: Furnace has been in use (LP gas) several years with no problems. This summer I turned the pilot off because of hot weather. Now having trouble getting it to stay lit - I light it according to directions, set control to on and it goes out after about a minute; turning up thermostat while pilot is still lit (before it goes out) and neither light (burner seems to blow out pilot). Pilot adjustment makes little difference in pilot flame.

First tried taking burner assembly out of furnace - pilot will stay lit with assembly out but when I replace assembly into furnace with pilot still lit, it weakens and goes out after about a minute. Replaced thermocouple with new and still have the problem.

Next, got up on roof and found that several inches of soot had accumulated in bottom of flue blocking it so I vacuumed it all out hoping that would fix the problem. No such luck - still have the problem. Suggestions?

BTW the gas orifice has the red band marked LPG installed closest to the valve body and is on the left side of the valve body - I'm wondering if it should be on the up side? Is valve installed wrong or doesn't that matter?

[Robert]

Hi,

The only things that control the pilot light are the thermocouple,pilot tubing and the gas valve.

With new thermocouple, make sure the tubing is clear and unkinked. If still a no go, replace thermocouple again (they're cheap and you do get bad ones sometimes0.

If still a no go, call an hvac tech to test the gas valve.



Thanks,
Robert

[suninthepines]

No kinks in thermocouple tube - I figured it was ok since pilot stays lit when burner assembly is outside the exchanger - tried again today; pilot seemed to stay lit (although it appears weak and can't be turned up), but when thermostat called for heat pilot was blown out - no heat/no go.
I have a local tech I guess I'll have to call in.

Its ok that the valve seems to be (to me anyway) installed sideways according to diagram on furnace? Or perhaps the orientation of the valve doesn't matter?

[Robert]

Pilot tubing, not thermocouple tubing. Gas valves sometimes can be mounted/installed different ways. Cannot say on yours as I have not seen it.

However, IF the pilot is getting blown out, would be wise to check the heat exchanger for a crack.



Thanks,
Robert

[suninthepines]

I will check the pilot tubing and orifice today. Valve is pretty much like a 720-007. Valve is installed so gas orifice and thermocouple inlet are on left side when facing the furnace; not on top side. Pilot doesn't really blow out, just weakens and goes out, but ONLY when inside of exchanger. Its as if the pilot ends up starved for oxygen and so goes out.

I don't think the exchanger has a leak. Am still wondering if the flue is still blocked with some more soot or other blockage.

[Robert]

Do a more thorough inspection of the roof jack and flue piping.



Thanks,
Robert