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  1. #1
    Join Date
    May 2007
    Posts
    32

    inches of water column

    I'm trying to adjust a gas valve on a RUDD roof top unit. 7.5 ton.
    I know for natural gas you should have between 7 and 8 for your inlet pressure and 3.5 (single stage) for your outlet. This unit has been changed over to propane, what is the proper inlet and outlet pressure I should have in inches of water column? I know propane is higher than natural gas but I'm not sure of the exact numbers.

  2. #2
    Join Date
    Aug 2006
    Location
    Vancouver , Canada
    Posts
    310

    Hmm Lpg

    Propane supply pressure is 11 inches water column, and MOST of the time the manifold pressure is 7 inches, but I have seen factory specs call for as high as 10 inches water column burner manifold pressure.

    Get the Ruud model number and check to see if it is a factory retrofit to LPG. Propane is a bit more finicky to set up than natural gas, and I have found, from considerable experience, alot more prone to burn back in the venturi ( bugleing ) . The solution is to ensure the gas pressure is correct to the manifold and cut back on the air to the burner A BIT. You can soot up a furnace real quick, proceed with caution.

    Good luck!
    Superheat and subcooling tell it all !

  3. #3
    Join Date
    Jan 2004
    Posts
    23
    i have just conerted a unit to propane and it called for 10" manifold and 11" inlet(recommended), they also wrote in the book, anywhere from 11 to 14 is o.k..but each manufacturer may be different

  4. #4
    Join Date
    May 2007
    Posts
    32
    My other problem with this unit, I forgot to mention previously. When I first checked the pressure on the regulator on the supply line, it was so high my meter couldn't even read it. My meter only goes up to 19.95. So I adjusted the pressure down at the regulator, but I could only get it down to 14.95. If I turned the adjustment screw anymore it would have fallen out. Even at 14.95, I couldn't get the cap back on so I had to turn it back up to like 16.5. So I guess all I can do is adjust the pressure outlet on the main gas valve, and just have a high supply pressure?????

  5. #5
    Join Date
    Aug 2007
    Posts
    971

    Propane

    Most propane systems have two stepdown regulators. One at the tank and the other at the Bldg. Supply pressure at the unit should be 11" wc (with the unit firing). A lot of gas valves actually have conversions kit that bypass the regulator in the in the valve and only rely on the proper size orifice.


    Expect nothing, yet expect the unexpected.
    Press on Regardless, Endeavor to Persevere.

  6. #6
    Join Date
    Feb 2004
    Posts
    2,479
    Make sure you know the mfg rating. Some units call for 3.5 not higher like we are used to.

  7. #7
    Join Date
    Aug 2006
    Location
    Vancouver , Canada
    Posts
    310

    Propane gas

    Quote Originally Posted by Defrost View Post
    My other problem with this unit, I forgot to mention previously. When I first checked the pressure on the regulator on the supply line, it was so high my meter couldn't even read it. My meter only goes up to 19.95. So I adjusted the pressure down at the regulator, but I could only get it down to 14.95. If I turned the adjustment screw anymore it would have fallen out. Even at 14.95, I couldn't get the cap back on so I had to turn it back up to like 16.5. So I guess all I can do is adjust the pressure outlet on the main gas valve, and just have a high supply pressure?????
    Defrost:

    As noted, there should be 2 stage reguation, tank pressure to 10 psi ( hi pressure regulator, usually red in color ) and then 10 psi to 11 inches WC ( low pressure regulator, usually green in color ). I had a trailer furnace that had the low pressure regulator with brass threads that were cut off ( over tightened ) by an over zealous fitter , keeping the reg plunger off the seat, subjecting the gas valve to 10 psi ( red regulator outlet pressure ).

    I came along on a no heat call - the tank had run out and there was air in the line - The unit pilot lit, the main burner came on for 5 seconds, then went out due to air ( ?? ) - I opened the inspection door, verified the pilot was out, let it sit for a few minutes - opened the inspection door, no gas smell, pushed the piezo button and BOOM!! Singed me real good and made the straight sided round heat exchanger look like a wine barrel.

    Inspection of the gas valve showed it was bypassing ' enough ' even when no power to it to flood the heat exchanger with propane - the chimney effect held the propane in the burner chamber till I lit it. I found the regulator problem when I installed the new furnace and the oil blew out of my manometer on a pre - startup test.

    Bottom line - DO NOT leave the inlet pressure high - rectify the problem!! Also find the Ruud published manifold pressure.

    Good Luck !
    Superheat and subcooling tell it all !

  8. #8
    Join Date
    May 2007
    Posts
    32
    Thanks Freon Guy. Someone else had told me not to worry about the pressure on the regulator. He said if it's to high, just adjust your MGV to a lower pressure going into your manifold?????????

  9. #9
    Join Date
    Dec 2001
    Location
    Southern Tier, NY
    Posts
    6,066
    Quote Originally Posted by Defrost View Post
    My other problem with this unit, I forgot to mention previously. When I first checked the pressure on the regulator on the supply line, it was so high my meter couldn't even read it. My meter only goes up to 19.95. So I adjusted the pressure down at the regulator, but I could only get it down to 14.95. If I turned the adjustment screw anymore it would have fallen out. Even at 14.95, I couldn't get the cap back on so I had to turn it back up to like 16.5. So I guess all I can do is adjust the pressure outlet on the main gas valve, and just have a high supply pressure?????

    your MGV is only rated at 14" wc ...
    get the right spring for the appliance regulator

  10. #10
    Join Date
    Aug 2006
    Location
    Vancouver , Canada
    Posts
    310

    Thumbs up Inches water column

    Defrost:

    If you convert inches water column to PSI, you multiply by a conversion factor of .03613 - so, the propane pressure out of the second stage regulator of 11 inches x .03613 = .3974 psi.; if you look at almost any LPG or natural gas valve, it says not to subject the inlet to in excess of 1/2 PSI.

    Even though most gas valves have a redundant safety configuration, when this style of low pressure valve is subjected to excessive inlet pressures, it can be damaged - that is what we suspected happened to the valve that caused me to get scorched. DO NOT let your last line of defense be the regulator in the gas valve - it is not meant to be the ' system ' regulator.

    I have seen three gas valves leak through in over 25 years of HVAC service - the first two were on natural gas systems that we could not find any explanation for the failure - but sitting on the bench with no power, they were wide open - we never found out the reason.

    Dave B
    Superheat and subcooling tell it all !

  11. #11
    Join Date
    May 2007
    Posts
    32
    Freon Guy - do you have any suggestions on how I can fix the problem with the regulator on the supply line. I tried turning the adjustment screw with my manometer hooked up, when I got to about 14.95 the screw was hanging on by one thread! impossible to put the cap back on! Maybe I need to go to the source and check the manifold at the supply tank. For that matter, now I'm wondering if I should just check the pressures at all the units. There's 40 RTU's! This all started with just a regular maintenance and I came across a unit that wouldn't run in heat mode so I went from therel, and so on and so on.....

  12. #12
    Join Date
    Aug 2007
    Posts
    971

    Does your insurance LPG coverage

    When in doubt let the LP supplier worry about the regulators. Most suppliers install and check LP runs.

    The insurance companies differentiate between LP and Nat.

    Who owns the the tank and the regulators?


    Expect nothing, yet expect the unexpected.
    Press on Regardless, Endeavor to Persevere.

  13. #13
    Join Date
    Aug 2006
    Location
    Vancouver , Canada
    Posts
    310

    LPG questions

    Snoring Beagle brings up some good points - the LPG supplier usually spots the tank and supplies a first stage regulator here in Canada; the customer takes care of it from there. I don't know about the US of A.

    If you are unsure of the piping / regulator ' rules ' in your jurisdiction, find out before doing any adjusting - if there is any kind of incident, the last guy working on the system - - - is where they look first.

    Is there a regulator at each appliance? Is there a primary regulator? Is it rated for the number of BTUs hooked up to it? What was the design pressures on the initial install? Regulators are not always stamped with their capacity - often only an orfice size is noted, you have to then find out how much gas it will pass at a certain pressure - contact the LPG Company.

    Good luck and be careful!
    Superheat and subcooling tell it all !

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