PlumbReviseCalculate appliance heat input from a timed gas-meter reading, with metric, imperial, smart-meter and LPG revision presets.
Heat input from a timed meter-dial reading.
Method: BS 6798 (gas rating)
Gas rate is the volume of gas an appliance burns per hour. Time how long the meter takes to pass a known test volume, then scale that volume to one hour: gas rate (m³/h) = (volume ÷ seconds) × 3600.
For a metric meter, enter the cubic-metre movement directly. For an imperial meter, choose the imperial mode and enter the cubic-feet movement; the calculator converts it to cubic metres before calculating heat input.
For a smart meter, use the interval method: record a start reading, run the appliance steadily, record the end reading and enter the volume difference and elapsed seconds. For LPG propane revision examples, choose the LPG CV preset; for live work, use the supplier or appliance procedure data.
Heat input is gas rate multiplied by calorific value and divided by 3.6: heat input (kW) = gas rate (m³/h) × CV (MJ/m³) ÷ 3.6. Compare the result against the appliance data plate and manufacturer tolerance before deciding what the reading means.
Mind the gross/net basis. A declared calorific value such as 39.5 MJ/m³ is a gross figure, so this method gives gross heat input — while many modern data plates quote net input. For natural gas, net ≈ gross ÷ 1.1 (the calculator shows both), so check which basis the plate uses before calling a reading high or low.
A combi rated at 24 kW passes 0.01 m³ in 16.4 seconds at CV 39.5. Gas rate = (0.01 ÷ 16.4) × 3600 ≈ 2.20 m³/h; heat input = 2.20 × 39.5 ÷ 3.6 ≈ 24.1 kW gross (≈ 21.9 kW net basis). On an imperial meter, the same heat input is roughly 47 seconds for 1 ft³.
Approximate timings for common heat inputs using natural gas CV 39.5 MJ/m³. Use it for revision and quick sense-checking, then use the calculator above for the exact timed volume.
| Rated input | Gas rate | 0.01 m³ time | 1 ft³ time |
|---|---|---|---|
| 12 kW | 1.09 m³/h | 32.9 s | 93 s |
| 18 kW | 1.64 m³/h | 21.9 s | 62 s |
| 24 kW | 2.19 m³/h | 16.5 s | 47 s |
| 30 kW | 2.73 m³/h | 13.2 s | 37 s |
| 35 kW | 3.19 m³/h | 11.3 s | 32 s |
| 40 kW | 3.65 m³/h | 9.9 s | 28 s |
Times are rounded and assume steady full-rate operation. Manufacturer instructions, appliance tolerance and current calorific value still decide live work.
Choose the imperial meter mode, enter the volume passed in cubic feet and the timed seconds. The calculator converts cubic feet to cubic metres before calculating heat input.
Use a measured interval. Record the start reading, run the appliance steadily, record the end reading, then enter the difference as the volume and the elapsed time in seconds.
For revision examples, choose the propane LPG preset. For live LPG work, use the supplier/appliance data and current procedure because the exact fuel and installation context matter.
For natural-gas revision examples, 39.5 MJ/m³ is a common default. For real readings, use the declared or supplied calorific value for the situation.
The declared-CV method gives gross heat input. Many appliance data plates quote net input instead; for natural gas net is roughly gross divided by 1.1, and the calculator shows both figures.
Method: BS 6798 (gas rating)
Training & revision aids — live installations follow the full standard, the manufacturer’s instructions and calibrated instruments.