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!

http://excessair.blogspot.com/2013/01/clocking-gas-meter.html

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Comment by Leo Rainer on January 17, 2013 at 7:58am

In your last bullet you mention "propane has approximately 2,500 Btu's per cubic foot". And that "approximately" has been my problem in the past. I have piped in gas meters for submetering LPG equipment before, but I have never been able to get any good information from the gas supplier as to what their energy content is. Any thoughts on how to get a better number than ~2,500?

Comment by Steve Waclo on January 15, 2013 at 11:32am
Chris,

Excellent article and wish I had it a few years back when checking the input of my furnace. Finally worked it out.

Numbers were very close but as Jim pointed out, an accurate system output figure is a bit more involved.

BTW, for applications with dial type electric meters, there is a similar technique for timing the disk that produces an accurate, instantaneous kW reading. Does not reflect kW demand for commercial applications unless the load remains constant for the utility's demand period (typically 1/4 or 1/2 hour).

Best wishes.
Comment by Christopher Morin on January 11, 2013 at 3:02pm
Hi Jim! Thanks for taking a moment to read and comment. I would never discount the safety procedures of High CO readings, as I know how harmful and quick the "bad air" can be. Thanks for providing a great tip about testing the CO across the entire cycle.
It is amazing to me, and I should have made it more obvious in my blog, that the Quality Installation standard does not put a max CO or other combustion analysis numbers, but instead relies solely on Clocking the meter...
Comment by Jim Tenhundfeld on January 10, 2013 at 12:51pm

I agree that clocking the gas meter may indicate the proper gas pressure, but a true combusion anlysis measuring the O2, CO & stack temperature and draft pressure will let you know if the the appliance is operating at its highest efficiency, and I don't mean the irrelavant efficiency numbers on the combustion analyser. Gas pressure in most residential water heaters cannot be adjusted so the pressure is what it is.

As far as CO testing, I was originally trained over 20 years ago to test from light off to shut down to see how the system operated during it entire cycle, especially for gas ovens.  Testing one time at steady state does not tell me much.  It is also much safer for the auditor to test during the entire time so he/she does not inhale combustion by products if the appliance is back drafting.

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