Pressure tells you gas is getting to an appliance. The gas rate tells you how much the appliance is actually burning. Converted to heat input, it's the figure you compare with the manufacturer's data plate. It's one of the checks GSIUR requires after work on an appliance: operating pressure or heat input, or both where necessary (reg. 26(9)).

Measuring the gas rate

Run only the appliance you're testing, at the rate you want to check (usually its maximum, or as the manufacturer's instructions say). Then read the meter over a timed two minutes.

Metric meters (electronic, smart or mechanical) read in cubic metres (m³):

1. Note the meter reading at the start.
2. Time exactly 2 minutes.
3. Note the reading at the end.
4. Subtract the first from the second. That's the gas used in 2 minutes.

Electronic and smart meters don't always show the reading straight away, and each make has its own way of displaying it. Gas Safe Register's Technical Bulletin 112 gives each manufacturer's procedure. In general, the reading is taken from the moment you first press the button, even if the display takes a moment to appear. Some meters can only be read within 30 seconds of waking. On a prepayment meter, make sure there's enough credit (or emergency credit) for the whole test. If the display shows "OFF", gas won't flow until the credit or a tamper alarm is dealt with.

Older imperial meters read in cubic feet, using dials. Few are still in service, and this course doesn't cover them.

Working out the heat input

• × 30, because there are 30 two-minute periods in an hour.
• ÷ 3.6, because 1 kWh is 3.6 MJ.
• Calorific value: BS 6891 uses 38.9 MJ/m³ (gross) for natural gas, which works out at about 10.8 kW for every 1 m³/h.
• Gross and net: for natural gas the ratio is about 1.11 to 1, rounded to 1.1 in practice. Check whether the data plate gives gross or net before you compare.

Example. A boiler is run on its own. The meter reads 1234.567 m³ at the start and 1234.667 m³ two minutes later.

• Gas used: 1234.667 − 1234.567 = 0.100 m³ in 2 minutes
• Gas rate: 0.100 × 30 = 3.0 m³/h
• Gross heat input: 3.0 × 38.9 ÷ 3.6 = 32.4 kW
• Net heat input: 32.4 ÷ 1.1 = 29.5 kW

Compare this with the manufacturer's rated input. If it doesn't match, investigate before leaving the appliance in use, for example by checking its operating and burner pressures against the manufacturer's instructions.

Range-rated appliances

A range-rated appliance can be set anywhere within a narrow band of heat inputs to suit the heating system. When you've set it, BS 6891 requires you to mark permanently on the appliance the heat input it's set to. Gas-rate it against that figure, not the maximum.

Sources: GSIUR 1998 reg. 26(9)checks after work Gas Safe Register TB 112 (2017)electronic meters BS 6891:2015 Annex A.2, Table A.1CV, gross:net BS 6891:2015 10.1range-rated marking CCN1 Issue 16 §10assessed