Low propane at furnace

[obscenic]

With a U-tube manometer I have measured 11" WC at the propane regulator output. When I fire up the furnace the WC drops to just about 10.5". When I measure at the furnace, at the post-valve gauge output, I see just 6" WC when the furnace fires. Any idea what could be causing this?

[GASMAN6674]

4" Drop across the valve is terrible. Should be .5. Change the gas valve. You might want to also look for a kink in the line to the furnace as 1" drop is pretty severe.
Hope this helps

[obscenic]

Quick rundown of where I'm at.
At regulator gauge port: 12" WC idle, 11" WC with furnace on
At gas line into furnace: 10" WC idle
At furnace valve gauge port: 6" WC with furnace on.

Does that get any gears turning?

[Harvard]

Not sure if this has been mentioned BUT there could be an oil trap in the copper line to the furnace which allows gas flow when warm but not when cold due to change in viscosity.

[obscenic]

The service manual for my furnace is here: http://bryantrv.com/docs2/docs/hflamefurn04.pdf

It's a 7900 series (7916-II to be exact). The manual doesn't give a manifold pressure, it simply says to check for 11" WC in the "propane system"... which I seem to have, unless there's a blockage between the regulator and the furnace?

I should note at this time that the furnace ignites only about 10% of the time, which is why I'm exploring gas pressure issues.

[kampinguru]

Is there a way to test a regulator that has been taken out of service to see if it can still be kept as a spare? Is there any point in trying to blow oil out of all the propane lines as a PM thing? Other than doing a leak test, if you replace the regulator and two hoses to the tanks, is there anything else I should be doing as PM? Thanks

[GASMAN6674]

You should maintain minimum of 11" wc at the inlet of the appliance gas valve with the appliance running and when it shuts off the regulator should "lock up" at no more than 120% of flow pressure. If you check after the appliance gas valve refer to manufactures specs for manifold pressure.

[Garry_Gayle]

I read in the Dometic manual that you need 50% of the gas appliances operating then set the regulator to 11"WC. When I did my testing I had 8" at the Refrigerator test port and the reefer would not cool on gas so I replaced the regulator with 1 that I could adjust at the tanks I got it set to 11"WC with all 3 burners on the stove top on max and the reefer on, now the reefer cools to 34 degrees.

[dougrainer]

Static w/c should be 11.5. You should NOT see a drop below 11 inches when an appliance is running. 6 inches at the furnace is strange. Regardless, IF you have to adjust a LP regulator that has been in use for years, it is BEST to replace the LP regulator. This is because a Regulator can get a weak spring/diaphragm and you can never get the W/C to stay on target as it will hunt and surge up and down. Doug

[nomad297]

A half-inch drop is the design.

Bruce

[Golden_HVAC]

This is normal.

Fred.