Furnace won't lite have 3.5 wc to gas valve every thing seems ok was told possible cob web in burner?

I am not a furnace tech but, there are several inputs to a gas valve. There must be an 1) electrical/mechanical indication that there is a draft fan running (to move potentially explosive gas so it doesn't accumulate), 2) adequate gas pressure to the valve, 3) the thermostat must be calling for heat 4) and electrical indication that there is an ignition source. In your scenario, you have the source of ignition, the gas valve is picking up the burner but the gas valve believes there is something missing so it shuts off the gas to the burner. In this case, I suppose you might not have enough gas pressure (is another gas appliance running or is the line to the furnace too small?), a thermocouple probably isn't the problem since the burner wouldn't pick up if the gas valve didn't believe the ignition source wasn't good enough. You could have a plugged chimney or exhaust pipe if it is direct vented since there is also a pressure switch that detects whether there is enough draft once the burner picks up. Really should call a furnace person especially if you don't have a carbon monoxide detector that you trust. If you think you are capable of continuing, you could jumper the various inputs the the gas valve one by one until the burner picks up and stays picked up. Whichever terminals you jumper that accomplish this, is the culprit. Look at the schematic on the side cover to the furnace for this information. Hope this helps...

Do you mean it won't match light or the ignitor won't spark and light the burners? Or both?

If it won't match light or the ignitor sparks, but the burners don't light, that means you're not getting gas to the burners.

Check to the tank valve and make sure it's turned ON. Also, make sure that there's gas in the tank.
If both of those check out as OK, turn the gas tank valve OFF. Disconnect the hose from the tank and wait 5 minutes. Reconnect the hose to the tank and turn the tank valve ON VERY, VERY SLOWLY (Slow motion, slow). Once the valve is fully open, light the burner farthest from the tank first and turn it to HIGH. Did the burner light and the flame height looks correct? If so, light the remaining burners following the same procedure. Now, are they all lit and the flames heights look correct? If so, you solve the problem. Just remember to always open the tank valve very, very slowly.

if the burners failed to light. Remove and clean all the burners, as well as the venturi valve on the open of each burner. Spiders like to build nest in the venturi valves.

if cleaning the burners did not solve the problem, you will need to replace the grill's hose/regulator assembly. You can find it in any home center or hardware store that sells gas grills and accessories, for about $30.00. Take you old one with you for comparison purposes.

Hope this helped you troubleshoot and solve the problem. Please be kind enough to let me know. Thanks and Happy grilling!!

I've got an easy way for you to check, and then we can help you from there.
First pop open both lids to your furnace (this may require a screwdriver). Turn up your thermostat, push the safety interlock button in if it has one (it's a button that turns off the power when the doors are removed), and watch.

On most modern furnaces (past 20-25 years) you'll hear/see:
1. Inducer fan
2. orange glow (ignitor)
3. Gas valve opening and ignition (flames)
4. Main Fan.

that's normal heater operation.
The most common issue here is a burned out ignitor, in which case you wouldn't see the orange glow but the sequence would attempt to continue anyway (the gas valve will open and hiss gas, but it won't lite, then it'll give up and try again a few minutes later).

On older furnaces there's a pilot light to start with, the click of the gas valve and boom... ignition.

On new furnaces it's
Inducer fan
gas valve opening
spark - ignition
Main fan.

and there are some oddballs in between, particularly the Lennox Pulse. If that's your furnace just back away slowly and call a professional... some guys won't even touch those, so be sure to mention what it is... if you call a pro.

So go ahead and watch it run, then tell me/us what you see.

Depending on the age of the unit the gas pressures should be fine. The first thing I would do is to give the burners a thorough cleaning and make sure pilot is clean if so equipped. I would shut furnace off and remove the burners for easier and better cleaning. Use a wire brush to lightly clean openings on the burners,and gently tap inlet end of burners on soft wood block or thick cardboard pad to clean soot and rust from inside the burners. Also blow out with air if you can.
I always do one burner at a time and put them back in the order they were removed in. When you get everything back together and furnace lit then you can loosen retaing screws on air baffles if needed to get the best looking cleanest flame possible.Flames should be tight on the top of the burners with nice blue flame with sharparrow points with just a little yellow on the very tip.
I hope this helps you. Thank you

Hi am welcome to FixYa. I am Kelly.

1. Does the LPG tank have gas in it?
2. Is it one gas control or more that gives you this symptom? If it is more than one and there is gas in the LPG tank then the LPG regulator can be the problem.
3. There can be debris on the face of the gas orifice where the burner mates to to the gas control selector assy. (Spider webs / grease /dirt)
4. The air hole in the burner can be partially blocked with something (same as above Grease, dirt, cob webs)
5. The holes in the burner itself can be corroded or blocked with something. (This one would be pretty obvious)
6. The gas valve 'heat control may not be functioning properly.
7. There is a very remote possibility that the LPG hose has collapsed.

I would check the orifice and the burner holes for obstructions after verifying the LPG tank is usable. Then the next suspect would be the LPG regulator.

Thanks for using FixYa,

Kelly

You could have the gas co to check to preassures at or before the unit. Too low or high a preassure and the gas valve cant regulate the proper flow>>Natural gas to the house needs at least 5"-wc psi ---to the gas valve needs 7.5"--wc and while its calling for heat the orrifice needs 3.5" -wc psi! Propane needs 11" wc psi to the valve on the unit and 11' -wc psi to the inside furnace.

If you dont hear the WHOOSH of the heater firing, which may be briefly before it shuts off, the first thing that you should do is clean out the gas nozzles. If you follow the gas line coming from the valve it should lead you to a line of brass-ish pieces (probably 8-12 of them). Sometimes cob webs can accumulate in them and block the flow of gas to the burner.

if the gas pressure is good on inlet and outlet of valve u might wanna chk the burner trays and manifold for flakes, dirt, spider webs ect... once u kno the passages are clear from obstruction you can focus on the mechanical. (sounds like gas valve may be getting weak and not able to open fully)