What standing, working and burner pressure each show, the 20 and 21 mbar figures, and how to find a pipework pressure loss.
A pressure reading means nothing until you know where it was taken and what was running. Record both.
The meter regulator sets the starting point
The regulator (governor) at the meter reduces the higher service pressure to the operating pressure the installation needs, and holds it broadly steady as flow and inlet pressure change. It cannot boost pressure, and emergency isolation is the emergency control valve's job, not the regulator's. For domestic natural gas the meter outlet working pressure is nominally about 21 mbar, a little above the 20 mbar appliances are designed around.
Standing pressure: no gas flowing
Taken with every appliance off. The regulator has closed, so the gauge shows the pressure it locks up at. That makes standing pressure a check on the regulator, and the baseline for later readings.
- A small rise as demand stops can be normal: the regulator needs some closing force to seat.
- A pressure that keeps rising after the setup has been checked and conditions have settled suggests the regulator is not locking up and its seat may be passing. Investigate through the authorised route.
- A good standing pressure does not show the supply holds up under demand.
Working pressure: gas flowing
Taken with the appliance running under the specified load, it shows the pressure actually reaching the appliance. Compare it with the inlet pressure the manufacturer states for that appliance and test point: 20 mbar is the nominal figure, not an exact pass mark.
LPG appliances are designed for higher pressures: 37 mbar for propane and 28 mbar for butane. Never judge an LPG reading against the natural-gas figures.
Finding a pressure loss
Compare meter-outlet and appliance-inlet working pressures at the same load. Subtracting working from standing pressure at one point does not measure the pipework loss.
- Natural-gas installation pipework is designed to lose no more than 1 mbar at full demand.
- Example: with every appliance at full rate, 20.6 mbar at the meter outlet and 19.3 mbar just before the boiler's isolation valve is a loss of 1.3 mbar. That is too much: check for undersized or restricted pipe, even though each reading looks close to normal.
Trace the test points: meter outlet → installation pipework → appliance isolation valve → internal appliance pipework → gas-valve inlet → burner. Mark each reading on that route before interpreting the difference.
At the same demand, a good meter reading with a low reading just before the appliance isolation valve puts the excess loss in the installation run. Check its capacity, restrictions and valve positions. If the meter reading is already low, investigate the supply or regulation through the authorised route. A burner restriction lies beyond the inlet, so it cannot explain a large loss along the run before it.
A maker's limit measured at the gas-valve inlet can include losses inside the appliance. That allowance does not apply to a reading taken before the appliance isolation valve. LPG has different criteria.
- One boiler maker, for example, allows up to 2.5 mbar from the meter outlet to the test point on the boiler's gas valve: 1 mbar for the installation pipework plus up to 1.5 mbar through the boiler's own isolation valve and internal pipe. A reading taken just before the appliance's isolation valve is still judged against the 1 mbar pipework figure.
Burner pressure
Taken at the specified burner-pressure test point, it shows what the gas valve delivers to the injectors. Where the instructions specify a burner-pressure setting, compare it at the required firing condition. An appliance with specified minimum and maximum burner pressures moves between them as demand changes, so a reading below its maximum-rate figure can be right at low fire.
- Too low for the required firing condition: on an atmospheric burner this reduces gas flow and heat input; the flame may become unstable or go out. Too low a flow velocity can allow strike-back.
- Too high for the required firing condition: over-gassing can cause flame lift, noise, sooting and overheating.
An appliance-inlet reading is not a burner pressure, and it does not give the gas rate or heat input: check those by the manufacturer's procedure. Some appliances are not set by burner pressure at all, and a correct setting does not prove combustion is safe.