Gas meter size vs. demand

Discussion in 'Plumbing Forum, Professional & DIY Advice, Tips & ' started by jadnashua, Oct 18, 2010.

  1. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
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    I've got more, and larger gas appliances in my home since when the place was built and was wondering if the gas meter is still sized properly. If I add up the BTU needs for everything (unlikely to all be on at once, but you never know), I come up with 245K BTU. That includes a 75K boiler, 60K stove, 65K grill, and 45K dryer. The meter says 175 cu ft max at 0.5pound drop in pressure (nominal 5#) (think I got that right, it's dark, I'll look again, if you think I've goofed). I'll call the gas company, but was wondering if this is too small.
  2. Thatguy

    Thatguy Homeowner

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    Location:
    MD
    I can't get your numbers to work but if your appliances need 2.45 input therms per hour and you give them half that, the cooking time will be about twice as long.
    My meter says 275 CFH and our appliances use ~200 KBTU/hr.

    You could run one of your appliances and clock your gas meter to check how many cu. ft. per minute it uses.
    It takes 830 BTU of energy to raise 1 gal. of water 100F. If you can do this in 6 minutes you must have put in >8300 BTU/hr of power, so you may have used >8.3 cu. ft. of NG.

    "A cubic foot of natural gas gives off 1000 Btu, but the range of values is 500 to 1500 Btu". My gas company claims that 100 CF of their gas gives off 1 therm, but who's to know?
    Last edited: Oct 19, 2010
  3. Dano6401

    Dano6401 Banned

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    Location:
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    Thatguys figgering is pretty close but you nailed it when you said the chance of everything running at once is pretty small. Might not be a bad idea to call the utility though and see what they say
  4. nukeman

    nukeman Nuclear Engineer

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    Location:
    VA
    Remember that this is the max flow at a certain dp. If you pull ~175k BTU, the pressure drop across the meter will be 0.5 psi. You have a total of 5 psi to play with, so you could pull more BTUs, but the pressure drop will be higher. What you can actually pull depends on the length/size of your lines, fittings, etc.

    I wouldn't worry about it. You may be able to feed the full 245k BTU if you had to, but the probablity of all of those on at the same time and all running at full capacity is pretty slim.
  5. gator37

    gator37 Retired prof. engr.

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    108
    Location:
    Alabama
    For a natural gas meter, 245000 btuh total input is approximated as 245 CFH demand. (~1000btu per CF)
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Location:
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    So, for worst case, I probably need a new meter...wonder if this is a free thing, or will cost me if needed.

    Waited on hold with the utility company for awhile, and decided to send them an e-mail instead. See how long they take to respond. I'll let you know what they say. the original unit probably was marginal if everything was firing on full. Wonder how well these things regulate when they get older...mine is now probably close to 30-years old. I don't seem to remember them ever changing it, but they could have. Certainly looks like it's been there for 30-years!

    the original unit had a 60K BTU boiler, fixed burner (my new one is modulating), a WH at probably 18K or so (it's gone, replaced by an indirect), a basic stove with maybe 35K, and while plumbed for a dryer, no dryer installed, so 175 cu/m probably worked okay. My stove, boiler, are both higher output capable, and added a big NG grill (but lost the WH) - I've had a dryer all along, but recently got a new one. Not really sure if this dryer uses more or less than the old one...guessing that part's proably a wash.

    I know each appliance has it's own regulator, but how would they function if the supply pressure dropped?
    Last edited: Oct 19, 2010
  7. shacko

    shacko Master Plumber-Gas Fitter

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    Location:
    Rosedale, Md
    >>>jadnashua
    So, for worst case, I probably need a new meter...wonder if this is a free thing, or will cost me if needed.<<<

    Every area I've worked in to upgrade the meter always costs you.

    It doesn't matter if you use everything at the same time or not, the gas has to be figured on that assumption.

    You will have to check with your local jurisdition; a lot of them require the pipe to be the right size ( that means ALL the pipe ) before they will install a new meter
  8. Dano6401

    Dano6401 Banned

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    Location:
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    Yes, it will cost you and if your utility co. is like ours it will take the better part of a year to get it
  9. jimbo

    jimbo Plumber

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    Location:
    San Diego
    Yes, gas, like water, you pay a fee for your maximum potential draw, because they have to size their mains and systems to support that. Assuming maybe 4 psi off the street to a gas company regulator, probably all you need is a new meter, but you will pay .

    The cost difference between a 5/8 or 3/4 water meter and a 1" meter is $$$$$$
  10. Thatguy

    Thatguy Homeowner

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    Location:
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    Run everything at once and see if the effect is noticeable.

    If it is, if you convert any two items to electric to avoid this occasional inconvenience, and if elec. costs more per therm than gas in your area, then there will be some breakeven point in the future where it would have paid you to convert your meter.
    Last edited: Oct 19, 2010
  11. nukeman

    nukeman Nuclear Engineer

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    709
    Location:
    VA
    I still think you'll be fine. If the pressure drops too much, the regulators on the devices will just go wide open to try to compensate. You might get a weaker flame at the grill, etc., but no big deal.

    Also, think about it this way..say you pull the full 245k BTU

    (245/175) = 1.4x rated flow

    Double the pressure drop gives about 1.414x the flow [sqrt(2)]. This means that at 245k BTU, the pressure drop across the meter would be ~1 psi instead of 0.5 psi. Starting with 5 psi nominal pressure, this means that you would have 4 psi after the meter instead of the rated 4.5 psi value (175k BTU). The more important thing is that the piping after the meter is properly sized to support the demand. As long as that is the case, there are no issues, even if everything is running full bore at the same time.

    The 175 CF (~175k BTU) is just a rating and not a hard limit. They have to rate it based on a given pressure drop to compare apples-to-apples. They couldn't really rate it on the full capacity since the nominal pressure can vary with location not to mention that the total pressure drop depends on what is downstream of the meter. Sure, if this was a new install, it would be better to maybe go one step up on the meter, but it isn't worth it in a retrofit situation.

    Besides, you'll probably never pull more that the 175k rating anyway. You might have the stove/boiler/dryer going full bore in the winter (Christmas/Thanksgiving), but you probably won't be using the grill at that time. And when you are using the grill, you probably won't be using the stove to full capacity and may not be running the boiler either.
  12. jimbo

    jimbo Plumber

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    Location:
    San Diego
    As noted by many, we think you are probably ok. One problem with the numbers being thrown around: the number of 0.5 PSI drop is tossed around. But in most residential, the regulated input pressure to the regulator is in the neighborhood of 11" WC, or about 0.4 PSI
  13. zl700

    zl700 DIY Senior Member

    Messages:
    237
    Location:
    Texas
    Add one more thing

    A meter can run beyond its rating, with a appreciable pressure drop, however, you potentially are running it outside of its calibrated range, resulting in an overcharge of gas actually used.
    (It never under meters but always over)
  14. hj

    hj Moderator & Master Plumber Staff Member

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    quote; But in most residential, the regulated input pressure to the regulator is in the neighborhood of 11" WC, or about 0.4 PSI

    That is not true. The INPUT to a regulator is ALWAYS in the multipound range, often about 10 psi. Your number is the OUTLET press and 11" w.c. is a propane pressure. Natural gas always in the 6-8" w.c. The regulator has much more capactiy than the meter so often all that is needed is a different meter which may or may not have an additional charge. Even increasing the line size might not cause a charge if it means you would be buying more gas. The .5" drop is a nominal number. The system will work adequately at a larger drop if it is not too extreme. Meters typically undermeasure, usually because of "slippage", unless there is an improper gear train.
  15. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    21,824
    Location:
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    I'm told we are in a high-pressure gas line area, so supply should not be a problem. Haven't heard from the gas company yet. It's hard to get everything cranked up at once. Just one of those things as I sit here after retirement, jogging my mind a bit with 'what-ifs'. Thanks for your thoughts. I'll let you know what the gas company says, and if I end up doing anything (probably not!).
  16. Winslow

    Winslow Plumber

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    Location:
    Hawaii
    you need to add up the total btu demand of all the appliances on the line then devide that number by the heating value of the type of gas you are using. This will give you the demand in cubic ft/hr. Then you need to measure the distance from the meter to the furthest appliace and look up the required pipe size in a sizing chart. The meter may supply enough for your demand but the increased demand might be too much for the existing pipe size.
  17. jimbo

    jimbo Plumber

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    Location:
    San Diego
    I meant to say the regulated input to the METER since they had been talking about drop through the meter.
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