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  1. #1
    Join Date
    Jul 2010
    Location
    upstate new york
    Posts
    5

    purging lines when converting from lp to natural?

    Hello, longtime hvac tech/electrician in the industry here.
    We have a 14 unit apartment complex that was rehabbed by us 5 years ago with all new hvac equipment for each apartment.
    The local utility was to provide natural gas service so we piped everything for that day.In the meantime all furnaces/boilers/ H W tanks were converted to LP.
    We finally have the natural gas switchover scheduled for next week and I have all the conversion kits to convert back to natural gas.

    My question is : do I need to purge the lines of lp before introducing the natural gas(after the conversions of course)?

    My initial reaction would be no but a couple of local hvac guys said the 2 gases might not get along well. So I would be better off purging the lp before introducing the natural gas. I can do that but it would consume a lot more time.
    I googled the question on the internet but didn't get a single hit.
    Thanks , Bob

  2. #2
    Join Date
    Aug 2007
    Location
    Northern VA 38 degrees N by 76 degrees W
    Posts
    5,060
    Quote Originally Posted by just robert View Post
    Hello, longtime hvac tech/electrician in the industry here.
    We have a 14 unit apartment complex that was rehabbed by us 5 years ago with all new hvac equipment for each apartment.
    The local utility was to provide natural gas service so we piped everything for that day.In the meantime all furnaces/boilers/ H W tanks were converted to LP.
    We finally have the natural gas switchover scheduled for next week and I have all the conversion kits to convert back to natural gas.

    My question is : do I need to purge the lines of lp before introducing the natural gas(after the conversions of course)?

    My initial reaction would be no but a couple of local hvac guys said the 2 gases might not get along well. So I would be better off purging the lp before introducing the natural gas. I can do that but it would consume a lot more time.
    I googled the question on the internet but didn't get a single hit.
    Thanks , Bob
    IFGC 406.7 Purging. Purging of piping shall comply with section 406.7.1 through 406.7.4

    NFGC NFPA 54 Purging 7.3

    Which ever code you are under should be followed.

  3. #3
    Join Date
    Jul 2010
    Location
    upstate new york
    Posts
    5
    Those codes talk about purging lines for the first time to get the air out.
    Nothing about the need to purge out the LP when converting to natural gas or vice versa.

  4. #4
    Join Date
    Dec 2004
    Location
    maple ridge, bc
    Posts
    107
    purging gas piping...
    hard to believe there is any quarrel regarding purging lp or natgas piping. ask yourself this...'what do i have to lose by purging and how much am i looking forward to failing equipment or worse, failing with fire'. there is no room with this issue to guess. purge always.

  5. #5
    Join Date
    Nov 2000
    Location
    Eastern PA
    Posts
    68,962
    The worse that would happen is the burners would burn a little hot for a minute or so until the LP gas is burned off. Up to 20% of LP is mixed with natural gas during really cold weather in some areas.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  6. #6
    Join Date
    Dec 2004
    Location
    maple ridge, bc
    Posts
    107
    with respect roboteq..you need to hit the books friend.

  7. #7
    Join Date
    Nov 2000
    Location
    Eastern PA
    Posts
    68,962
    Quote Originally Posted by deerwhistle View Post
    with respect roboteq..you need to hit the books friend.
    Please elaborate with real life, real conditions information....with all due respect...
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  8. #8
    Join Date
    Jul 2010
    Location
    upstate new york
    Posts
    5
    I didn't mean to offend anyone with the question.

    I truly wanted to know if anyone had knowledge or actual experience with this type of situation of the two different gases not "getting along well" for the 60 seconds or so while the natural gas makes it's way through 40-60' of 1" pipe to each of the furnaces.

    Of course the new pressure to the equipment is 6 or 7" w.c. not the old pressure of 11" w.c. and the burner orifices will all have been changed.

    Keeping in mind that these are existing lines that have been in service for years now, I would feel comfortable just converting the units and firing them up if I didn't hear from you guys that it would NOT be ok to do that.

    I don't have a problem purging the LP out of the lines,it would require some logistics to safely vent it out of the 14 individual enclosed furnace rooms,but if it isn't necessary why expose the tenants to the potential odor situations.

    As always I appreciate any input/knowledge that you can share!
    Thanks , Bob

    ps- With all the storm damage from Sandy the Utility Company has postponed the conversion switchover date by 2 weeks.

  9. #9
    Join Date
    Nov 2000
    Location
    Eastern PA
    Posts
    68,962
    Quote Originally Posted by just robert View Post
    I didn't mean to offend anyone with the question.

    I truly wanted to know if anyone had knowledge or actual experience with this type of situation of the two different gases not "getting along well" for the 60 seconds or so while the natural gas makes it's way through 40-60' of 1" pipe to each of the furnaces.

    Of course the new pressure to the equipment is 6 or 7" w.c. not the old pressure of 11" w.c. and the burner orifices will all have been changed.

    Keeping in mind that these are existing lines that have been in service for years now, I would feel comfortable just converting the units and firing them up if I didn't hear from you guys that it would NOT be ok to do that.

    I don't have a problem purging the LP out of the lines,it would require some logistics to safely vent it out of the 14 individual enclosed furnace rooms,but if it isn't necessary why expose the tenants to the potential odor situations.

    As always I appreciate any input/knowledge that you can share!
    Thanks , Bob

    ps- With all the storm damage from Sandy the Utility Company has postponed the conversion switchover date by 2 weeks.
    Don't worry about us bickering. It's all part of our teaching and learning process. Deerwhistle is correct if we only go by code regulations which are designed to prevent LP being used on systems designed for natural gas and vice versa. It is better for regualations to not allow for possibilities of abuse by just nixing what could be a potential issue.

    LP is part of natural gas before processing and is seperated during the processing of natural gas for sale as a Liquid Petroleum product. In fact, LP is considered a green gas, while natural gas is not. The two do mix well and as I noted before, natural gas suppliers add a certain amount of LP to natural gas during colder months.

    It would be disasterous to continually operate a furnace designed for natural gas, on LP. However, the amount of LP gas left in the gas lines is not going to do any harm to furnaces converted over to natural as long as the natural gas is coming up right behind. The furnaces may fire off a little noisy and hot, but no damage is going to occur from such a short usage of LP gas.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  10. #10
    Join Date
    Apr 2012
    Location
    winnipeg
    Posts
    1,330
    purge them.... appliances will be happier.... or don't.... all will be fine....
    it was working.... played with it.... now its broke.... whats the going hourly rate for HVAC repair

  11. #11
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    68,320
    Quote Originally Posted by RoBoTeq View Post
    The two do mix well and as I noted before, natural gas suppliers add a certain amount of LP to natural gas during colder months.
    Yep, or they wouldn't be able to maintain gas pressure in their main lines.
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  12. #12
    Join Date
    Nov 2000
    Location
    Eastern PA
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    68,962
    Quote Originally Posted by beenthere View Post
    Yep, or they wouldn't be able to maintain gas pressure in their main lines.
    I didn't even think about that aspect. I always thought they did it just to raise the Btu rating of the gas a bit. Maintaining line pressure makes a lot more sense.

    Also, it's not like many of us have not accidentally fired up furnaces with LP before we changed out the orifices or adjusted the pressures....NOT ME....OR YOU....of course.....
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  13. #13
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,320
    Quote Originally Posted by RoBoTeq View Post
    I didn't even think about that aspect. I always thought they did it just to raise the Btu rating of the gas a bit. Maintaining line pressure makes a lot more sense.

    Also, it's not like many of us have not accidentally fired up furnaces with LP before we changed out the orifices or adjusted the pressures....NOT ME....OR YOU....of course.....

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