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.
- Turn off all Gas Appliances in the home.
- Turn on the appliance being tested, to the highest firing rate (be careful of two-stage furnaces and variable capacity boilers, etc.)
- 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.
- Cubic Feet per Hour (CFH) = (3600 x Dial Sze) / Time (seconds)
- CFH x 1000 Btu's = Input Btu/hr
- 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!