1. What is Gas Rate?
Gas rate (also called heat input) is the amount of energy a gas appliance consumes per unit of time, measured in kilowatts (kW). It tells you how much gas an appliance is burning and whether it's operating within its rated capacity.
As a Gas Safe registered engineer, calculating gas rate is essential for:
- Commissioning - Verifying new installations meet manufacturer specifications
- Servicing - Checking appliances are operating efficiently
- Fault-finding - Diagnosing under-firing or over-firing issues
- Safety checks - Ensuring appliances aren't exceeding their rated input
2. The Metric Gas Rate Formula
For metric gas meters (measuring in cubic metres, m³), the gas rate formula is:
Metric Gas Rate Formula
Volume (m³) = End Reading - Start Reading
Flow Rate (m³/h) = Volume × (3600 ÷ seconds)
Corrected Flow = Flow Rate × 1.02264 (volume correction factor)
Gross kW = Corrected Flow × 39.5 ÷ 3.6
Net kW = Gross kW ÷ 1.11
Key values explained:
- 39.5 MJ/m³ - Standard UK calorific value for natural gas
- 1.02264 - Volume correction factor (accounts for temperature and pressure)
- 3.6 - Conversion factor from MJ to kWh
- 1.11 - Gross to net conversion (accounts for latent heat in water vapour)
3. The Imperial Gas Rate Formula
For imperial gas meters with test dials (measuring in cubic feet, cu ft):
Imperial Gas Rate Formula
Volume (cu ft) = Test dial size (0.5, 1, 2, or 5 cu ft)
Flow Rate (cu ft/h) = Volume × (3600 ÷ seconds per revolution)
Flow Rate (m³/h) = cu ft/h × 0.0283168
Gross kW = m³/h × 1.02264 × 39.5 ÷ 3.6
Test dial sizes: Most imperial meters have test dials of 0.5, 1, 2, or 5 cubic feet. The size is usually marked on the dial face. Time one complete revolution of the test dial.
4. Step-by-Step Method
Follow these steps to perform a gas rate calculation:
Step 1: Isolate the appliance
Turn off ALL other gas appliances in the property - including pilot lights, gas fires, hobs, and any other gas appliances. Only the appliance you're testing should be consuming gas.
Step 2: Record starting reading
For metric meters: Note the meter reading including ALL decimal places (e.g., 12345.678 m³). For imperial: Identify your test dial size.
Step 3: Fire the appliance
Turn on the appliance at MAXIMUM rate. For boilers, this usually means hot water mode at maximum temperature. Start your timer at the same moment.
Step 4: Time the consumption
Metric: Run for at least 2 minutes (120 seconds) for accuracy. Longer is better for low-output appliances.
Imperial: Time one complete revolution of the test dial.
Step 5: Record and calculate
Note the final reading and time. Apply the formula above, or use our gas rate calculator for instant results.
Step 6: Compare to data plate
Compare your calculated NET kW to the appliance data plate. A variance of ±5% is typically acceptable. Larger differences may indicate a fault.
5. Worked Example
Scenario: You're commissioning a new 28kW combi boiler and need to verify its gas rate.
Example Calculation (Metric)
Given:
- Meter start reading: 12345.678 m³
- Meter end reading: 12345.834 m³
- Time elapsed: 180 seconds (3 minutes)
Calculation:
Volume = 12345.834 - 12345.678 = 0.156 m³
Flow Rate = 0.156 × (3600 ÷ 180) = 0.156 × 20 = 3.12 m³/h
Corrected Flow = 3.12 × 1.02264 = 3.191 m³/h
Gross kW = 3.191 × 39.5 ÷ 3.6 = 35.0 kW gross
Net kW = 35.0 ÷ 1.11 = 31.5 kW net
Result: The boiler is operating at approximately 31.5 kW net, which is slightly above the rated 28kW. This indicates the boiler may be over-firing and the burner pressure should be checked and adjusted.
6. Common Mistakes to Avoid
Not isolating other appliances
Forgetting to turn off other gas appliances will give you an inflated gas rate reading. Always isolate before testing.
Running the test too short
Short test periods magnify any timing errors. For metric meters, always run for at least 2 minutes, preferably longer for low-output appliances.
Comparing gross to net values
Most appliance data plates show NET kW. Always compare your calculated NET value (not gross) to the data plate.
Not accounting for modulating appliances
Modern modulating boilers adjust their output. Ensure the appliance is firing at maximum rate during the test - usually by running hot water at maximum temperature.
Using wrong calorific value
The standard CV of 39.5 MJ/m³ is used for most calculations. Regional variations (37.5-43.0) can affect accuracy. For precise work, check the current CV with your gas transporter.
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