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Gas manifold pressure in 1950's furnace?

jduncan Member Posts: 5
This old system had been super reliable for 60 years and then the gas valve failed (refused to close reliably without beating on it). The obvious solution was to swap out the valve and this is what the hvac guy did with me watching over his shoulder. Then he set the pressure to 3# because this was "standard." This however greatly increased the gas flow and did not look OK with me. He then left. Now perhaps 10 years later, I am getting this grainy soot that I never saw in any gas system anywhere. This needs to be set up correctly. Any suggestions? I know that some vintage systems regulate the pressure as low as 8oz.
The furnace was mfg by United States Radiator Corporation. There is no rating nameplate that I can locate and I'm sure that the company no longer exists.


  • mattmia2
    mattmia2 Member Posts: 7,207
    Is the LP or natural gas? The size of the orifice(s) and the input rate of the appliance will tell you the manifold pressure. It still should have a combustion analysis after replacing the vale (i assume with a modern total shutoff combination valve). Some pictures to make sure it isn't something weird like a modulating valve would be good too.
    EBEBRATT-Ed Member Posts: 13,149
    3.5 " wc is the standard setting for natural gas. That is on the gas valve outlet or on the manifold.

    LP gas is usually 11" wc

    Should be set up with a combustion test and gas pressure readings. Should be on the furnace name plate the required gas pressure at the burner
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 4,991
    edited August 2022
    3# means 3 pounds. and that is way too much. 3 inches water column is the standard. Are you sure the 3 that you read on the gauge was not Inches WC? 3.5"WC might be the max rating and 2.5" WC might be where the old valve was set. Either way as long as the temperature at the exhaust vent is at least 350° and not much higher than 500°, you should be fine. I prefer 450 myself. That temperature measurement is before the draft hood, not after it (the big opening in the vent pipe that looks like an upside-down funnel).
    did the Technician do a combustion test? or just set the pressure? Call the boss and ask!

    O yea... and what @mattmia2 said
    Edward Young Retired HVAC Contractor & HYDRONICIAN Services first oil burner at age 16 P/T trainer for EH-CC.org
  • jduncan
    jduncan Member Posts: 5
    Sure wish my dad was still with us--he would know what to do. We installed countless heating systems together--many good memories. Will add some photos...
  • jduncan
    jduncan Member Posts: 5
    edited August 2022
    Install date: 1950, valve replacement date: 2012 (old valve NA)
    Natural gas, HW system
    Combustion test: yes, it lights! (what does a bona fide combustion analysis involve?)
    Valve type: basic non-modulating type
    Pressure: obviously not 3 lb (perhaps 3"), but flame dimensions much larger than previous setup...
    Still looking for the nameplate...
    Many tnks for the responses so far. I have to be wasting tons of fuel...
  • pedmec
    pedmec Member Posts: 713
    What was the model number of the old valve? If you have the old valve you can cross reference a new valve. Not all gas valve are the same.

    3.5" wc is the normal manifold pressure rating for the old atmospheric furnace or boiler.

    Combustion test should have absolutely been done to confirm safe operation of furnace. Just setting the pressure to a standard doesn't guarantee your safety.
  • mattmia2
    mattmia2 Member Posts: 7,207
    Was the old valve a separate valve and regulator? Some valves are slow opening to allow the burner to light smoothly.
  • 109A_5
    109A_5 Member Posts: 484
    @jduncan, Maybe look at it a bit differently. Maybe your dad set it up a bit down fired or the original valve was never set up correctly from the factory. I don't think modern combustion analyzers were available 60 years ago so down firing a bit may have been a strategy to avoid excessive CO.

    So the present soot accumulation is ten years worth of accumulation ? Or has something else recently changed causing the soot accumulation ?

    3.0 inch of water column = 1.73 ounces per square inch
    3.5 inch of water column = 2.02 ounces per square inch
    jduncan said:

    I know that some vintage systems regulate the pressure as low as 8oz.

    13.83 inch of water column = 8 ounces per square inch
    National - U.S. Gas Boiler 45+ Years Old
    Steam 300 SQ. FT. - EDR 347
    One Pipe System
  • jduncan
    jduncan Member Posts: 5
    The replacement gas valve is the common White Rodgers 36C03-300. It has operated flawlessly since installation with the exception of a pilot flameout every two or three years.
    I suggest that the gas pressure may have been throttled back years ago to improve system efficiency--if so, the original gas pressure info is history. All I know is that fuel consumption seems excessive, especially since fuel prices have skyrocketed. Judging from the low duty cycle in cold weather, this furnace has far more capacity than what is actually required.
    Then there is the issue of "soot" deposits that were not present when the valve was replaced--I was the one that cleaned out the system at that time--birds & squirrels, but no soot. Now lots of gritty soot on the top of the heat exchanger and in the horizontal smoke pipe.
    Another item of interest is the secondary manual gas valve that splits the gas flow--it is turned on, but what is its actual purpose? My take is that it could put additional thermal stress on the cast iron heat exchanger via heating only half the structure. This valve is barely visible on the right side of the photo.
    Then there is the potential issue of the gas jets getting loaded with 70+ years of sediment--is this ever an issue? However, there seems to be no gas flow limitations or obvious variations--breaths well. Regarding the air adjustment inputs, they have this info cast into them: "11-A-3" --They seem quite loose.
    In the one photo, I removed the pilot access cover so we could see inside.

  • jduncan
    jduncan Member Posts: 5
    Replacement gas valve is the common W-R 36C03-300. It has worked OK, but pilot extinguishes every other year or so.