Gas Rate Calculator App

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HVAC Gas Rate Calculator (Meter Clocking)

Calculate gas flow rate (CFH) and appliance heat input (BTU/hr) by timing your gas meter.

0.5 ft³ Dial 1.0 ft³ Dial (Most Common) 2.0 ft³ Dial 5.0 ft³ Dial Select the volume of the proving dial you are timing.
Use a stopwatch to time exactly one full rotation of the dial.
Standard Natural Gas is typically between 1000-1050 BTU/ft³.
function calculateGasFlowRate() { var dialSizeElem = document.getElementById("dialSize"); var revolutionTimeElem = document.getElementById("revolutionTime"); var calorificValueElem = document.getElementById("calorificValue"); var resultElem = document.getElementById("gasRateResult"); var dialSize = parseFloat(dialSizeElem.value); var revolutionTime = parseFloat(revolutionTimeElem.value); var calorificValue = parseFloat(calorificValueElem.value); if (isNaN(revolutionTime) || revolutionTime <= 0) { resultElem.style.display = "block"; resultElem.innerHTML = "Please enter a valid positive number for time in seconds."; return; } if (isNaN(calorificValue) || calorificValue <= 0) { resultElem.style.display = "block"; resultElem.innerHTML = "Please enter a valid calorific value."; return; } // Calculate Cubic Feet Per Hour (CFH) // Formula: (Dial Size / Seconds taken) * 3600 seconds/hour var cfh = (dialSize / revolutionTime) * 3600; // Calculate BTU Per Hour (Heat Input) // Formula: CFH * Calorific Value var btuPerHour = cfh * calorificValue; resultElem.style.display = "block"; resultElem.innerHTML = "

Calculation Results:

" + "Gas Flow Rate (CFH): " + cfh.toFixed(2) + " ft³/hr" + "Appliance Heat Input: " + Math.round(btuPerHour).toLocaleString() + " BTU/hr"; }

Understanding Gas Rate Clocking for HVAC

For HVAC technicians and informed homeowners, ensuring a gas furnace, boiler, or water heater is firing at its rated capacity is crucial for efficiency, safety, and longevity. This process, known as "clocking the meter," calculates the actual gas flow rate into the appliance.

By comparing the calculated BTU input against the manufacturer's rating plate on the appliance, you can determine if the unit is under-firing (insufficient heat, potential condensation issues) or over-firing (overheating, potential heat exchanger damage, safety risks).

How to Use This Gas Rate Calculator

  1. Ensure Only the Test Appliance is Running: Turn off all other gas appliances in the house (stove, dryer, fireplace, etc.) so the gas meter is only measuring the fuel going to the unit you are testing.
  2. Identify the Proving Dial: Locate the smallest revolving dial on your gas meter. It is usually marked with its volume per revolution, typically "1 Cubic Foot", "2 Cubic Feet", or sometimes "0.5 Cubic Feet". Select this size in the calculator block above.
  3. Time One Revolution: Using a stopwatch, measure exactly how many seconds it takes for that specific dial to make one complete rotation while the appliance is running at steady-state full fire. Enter this value into the "Time for One Revolution" field.
  4. Verify Calorific Value: This is the energy content of the gas. For standard natural gas, this is typically around 1000 to 1050 BTU per cubic foot. You can often find the exact current value on your gas utility bill. If unsure, the default of 1050 is a standard industry estimate.
  5. Calculate: Press the button to see the results.

Interpreting the Results

  • Gas Flow Rate (CFH): This is the volume of gas flowing through the meter expressed in Cubic Feet per Hour.
  • Appliance Heat Input (BTU/hr): This is the total energy input rate. Compare this number directly to the "Input BTU" rating found on your appliance's data plate. Generally, a variation of ±5% from the rated input is considered acceptable by many manufacturers, but always consult specific appliance documentation.

Note: This calculator provides estimates based on the inputs provided. Altitude and specific gas composition can affect exact readings. For precise adjustments (like adjusting manifold pressure), always rely on trained HVAC professionals using calibrated combustion analysis tools.

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