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IFGC Gas Pipe Sizing - Pressure Drop

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niklaw1
niklaw1 Member Posts: 9
Im sorry guys and gals, I need to kick a dead horse. Ive searched post history and found a couple old threads about this but Im still not wrapping my thick skull around it.

As pressure drop increases, maxium btu capacity increases?
I work on low pressure residential systems (mostly using this for converting from tanked to tankless dhw) and I understand the incoming pressure from the regulator is a constant. Ive read through various manuals, and tables. In my head it feels like its saying "If you need more gas capacity at your set length and pipe diameter, just add a larger appliance.... or more 90's. Which makes no sense to me at all."

For one instance, I added a 199k btu tankless to a house and missed total btu capacity of the line by 5k btu at 0.3" wc pressure drop, but hit it with a 0.5" wc pressure drop. How can this make sense to a non engineer?

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  • ratio
    ratio Member Posts: 3,696
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    The pressure drop listed in the tables is a theoretical number. It is entirely dependent on the details of the material used and installation procedures & practices. IOW, it's best case using quality materials, best practices, & installation skill. Basically, the greater the length (& fittings) you have, the more resistance to the gas flow. The more volume of gas you flow, the more that resistance to flow influences the pressure at the appliance. To offset that pressure drop you must size the pipes larger.
  • Zman
    Zman Member Posts: 7,605
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    The code requires that the pressure drop from the meter to any appliance should not exceed 0.5" WC. The chart tells you what size pipe you need to install to meet that requirement.
    A typical tank water heater requires around 40 k/btu, a tankless unit is usually around 200k/btu. For most tank to tankless conversions, the pipe size needs to be increased.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
    niklaw1
  • niklaw1
    niklaw1 Member Posts: 9
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    > @Zman said:
    > The code requires that the pressure drop from the meter to any appliance should not exceed 0.5" WC. The chart tells you what size pipe you need to install to meet that requirement.
    > A typical tank water heater requires around 40 k/btu, a tankless unit is usually around 200k/btu. For most tank to tankless conversions, the pipe size needs to be increased.

    Yes, I understand and completely agree. This job in particular the customer had already had an Eternal tankless that failed and I replaced it with a Navien. The inspector failed it because of lack of btus. (missed it by a gnats ****) I just increased gas pressure at the regulator to just above 8" and he passed it. I've always just sized off 0.5" pressure drop, but I never understood why and how. When trying to self learn, I went down a rabbit hole I don't understand now.
  • Snowmelt
    Snowmelt Member Posts: 1,418
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    think of it even with domestic water, if you have to many hose bibs on a 3/4 inch line or for the sake of argument a 1/2 inch line. the water isn't going to come out full force it will trickle out. ( i had this happen with a tank-less when they where trying to run 7 fixtures 4 showers 2 kitchen sinks and a slop sink in basement at the washing machine.) you have to increase the line to 1 or maybe 1 1/4 to get the flow gpm up. when your doing it with gas your increasing your btu with same gas pressure
  • niklaw1
    niklaw1 Member Posts: 9
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    > @Snowmelt said:
    > think of it even with domestic water, if you have to many hose bibs on a 3/4 inch line or for the sake of argument a 1/2 inch line. the water isn't going to come out full force it will trickle out. ( i had this happen with a tank-less when they where trying to run 7 fixtures 4 showers 2 kitchen sinks and a slop sink in basement at the washing machine.) you have to increase the line to 1 or maybe 1 1/4 to get the flow gpm up. when your doing it with gas your increasing your btu with same gas pressure

    Yes but gas pressure drops say the more pressure drop, the more btu capacity in the same sized line which is the exact opposite of the water example.
  • niklaw1
    niklaw1 Member Posts: 9
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    I completely understand how to size a gas line, I have to submit an isometric to the city every time I pull a permit. What I'm trying to understand is the science of how more drop = more btu capacity
  • Zman
    Zman Member Posts: 7,605
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    You might be over thinking this a bit. If you are allowed more pressure drop, you will get more BTU's through the pipe. This is why commercial jobs run at 2 psi (or higher). In low pressure applications, you are only allowed 0.5"

    Not sure what the inspector was thinking. Was he just wanting you to a have a minimum at the appliance? Many inspectors don't really understand gas.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • ratio
    ratio Member Posts: 3,696
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    You don't get more BTUs per se, with a higher flow rate the pressure sags further. If you were burning 40 kBTUh at ½" drop at the appliance, with everything remaining the same, if you were to burn 80 kBTUh of gas you'd see a pressure drop of say 1". That sort of game is ok if the regulator only serves a single appliance, but once you get three or four the gas pressure is all over the place depending on what else is burning.
    niklaw1
  • Tim McElwain
    Tim McElwain Member Posts: 4,629
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    When you look at piping charts each chart has a different "allowable pressure drop" for example chart 6.2(a) in NFPA 54 (all charts in different codes are the same by the way) allows .3 while chart 6.2.b allows .5. When you look a what that creates it causes you typically to use larger pipe with the .3 allowable loss versus .5 allowable loss. No matter how you look at it under 2 lbs psi you have to take into consideration total load on the system (all appliances), what is your pressure at the outlet of the gas meter usually 6 to 7 inches W.C. (water column) keeping in mind appliance regulators for gas systems (natural gas) usually reduce pressure to the manifold to 3.5" W.C. It is then up to the orifice sizes of the burners. Keeping in mind regulators control pressure orifices control flow (BTU). The piping plan has a big affect on what is available at the end of the farthest run. It is always better to oversize pipe than to undersize.

    Think about this when we were using storage type water heaters 30, 40 and 50 gallon usually BTU 30,000 to maybe 50,000 we usually could get away with 1/2" pipe. Now you install a wall hung on demand which is usually 199,000 BTU's that 1/2" pipe may not handle it.

    Go to Annex "C" in NFPA 54 for some good info on pipe sizing. Then it matters what method you are going to use such as Longest Length Method, Branch Length Method, Hybrid Pressure Method and finally Pressure Drop per 100 feet method all discussed in Annex "C" NFPA 54.

    It can get confusing so my attitude is it is better to over size than to under size.

    I would also find out what the local inspector is using for a sizing method.
    rationiklaw1
  • Tim McElwain
    Tim McElwain Member Posts: 4,629
    edited January 2019
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    By the way we teach all of this in our courses so contact me at gastc@cox.net
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,829
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    The longer the length of run, the more fittings you use the more pressure drop you get for a given volume of gas. Remove fittings or length of pipe your pressure drop goes down. Add fttings or pipe it goes up.

    If your pressure drop is too high you need to remove pipe or fittings or install a larger pipe.
    niklaw1