Clocking a Gas Meter

  After writing last week's post: "High CO Reading, Now What?" it occurred to me that not as many people out there: technicians, raters, or auditors; are up to speed on the age old firing rate calculation.  I hope this simple procedure is easily understood as you may find it useful if you are working with negative pressure gas valves, with Energy Star Homes, or just simply forgot your manometer at the shop.  I know most inspectors and raters avoid attaching a manometer to measure gas pressure, particularly since some states even require a license to service gas appliances.  By clocking the gas meter, one can tell if the British Thermal Units (Btu's) being consumed matches the input of the furnace, boiler, or even a water heater.
  1. Turn off all Gas Appliances in the home.
  2. Turn on the appliance being tested, to the highest firing rate (be careful of two-stage furnaces and variable capacity boilers, etc.)
  3. Once at steady-state, use a stopwatch (last check there are about 219 Apps for that) to time how long it takes the smallest unit of measure (typically the 1/2 Cubic Foot) dial to make a full revolution on the gas meter.
  4. Cubic Feet per Hour (CFH) = (3600 x Dial Sze) / Time (seconds)
  5. CFH x 1000 Btu's = Input Btu/hr
  6. Remember to relight any standing pilots that are burning up to $20/year!
  Based on the ACCA Quality Installation Specification, the basis for Energy Star Homes V3 Checklist, the calculated input btu/hr must be within 5% of the data plate.  If a technician properly adjusted the manifold gas pressure and adjusted the airflow to ensure the temperature rise is within Manufacturer Specification, at worst the input will be within a couple of percent of the data plate.

A couple of tips if this is new for you:
  • When turning appliances off, I mean off!  A couple of standing pilots can throw off your calculations when you have a 40K btu/hr furnace.
  • Know how to override any outdoor reset control on a condensing boiler.  The starting and stopping of these burners are the least efficient operation and will consume high CFH if short cycling, never mind the inflated Carbon Monoxide (CO) and Carbon Dioxide (CO2).
  • For a more accurate calculation, replace the 1,000 Btu in the formula with the actual amount of Btu's per Cubic Foot.  This can be obtained by contacting your gas supplier.  Otherwise, it would be very tough to account for altitude.  For instance, I heard Denver operates about 860 Btu's per cubic foot or so.
  • Also, I would recommend you clock (3) revolutions of the meter, then divide the time by three for an average reading in seconds.
  • When working with propane, it helps to temporarily pipe in a meter to accurately clock as most tanks only have regulators.  Also, propane has approximately 2,500 Btu's per cubic foot.
  Has anyone ran across a digital gas meter yet?  I have not had the privilege of clocking one of these and would love to hear how you are doing so!

3 comments:

  1. Since the burner needs suitable amount of heat to produce steam for fulfilling the needs of commercial organization. Therefore, if you want to produce desired amount of heat, you should check the supply of gas or fuel by using gas meters . These meters measure exactly the heat needed to perform operations.

    ReplyDelete
  2. I checked my meter and it took 24 seconds for 1 full rotation around the 1/2 cubic foot dial. Is this normal? My gas meter read went from 2094 to 2707 in a month in a two person household.

    I turned off all appliances then turned on the furnace by itself and timed the 1/2 CF at 24 seconds. That sounds pretty fast to me, but I have nothing to gauge it from. Asked my gas company to come out and re-read the meter, but I am still waiting on them. The meter appeared to have been correctly read as it is now at 2843 from the last read of 2707.

    Now I'm wondering if I should be concerned about a gas leak with the furnace.

    ReplyDelete
  3. Hi Rubes! Thanks for reading my blog! That math works out to be about 75,000 Btu/hr. I guess the answer is if you have a furnace or boiler with that size Input? If so, sounds like it is normal. A 75K Gas Heating Appliance is pretty normal. If the unit is much smaller, than yes I would be asking for a service call from your local mechanical company as an adjustment might be needed. The other possibility is that the system may not be operating efficiently. Just beause the gas is adjusted correctly, does not imply the unit is efficient - the service compmany could tell you more!
    Hope this helps, and thanks again for this real world example!

    ReplyDelete