Gas Pipe Sizing.

All the facts and questions around gas pipe sizing explained; 
New boiler, Property renovation, or adding appliances etc. Common problems and common misinformation.

To understand the need for correct gas pipe sizing it must be understood that burners in gas appliances are fed with a pre-mix of gas and air. 80% air and 20% gas produces a clean and efficient flame. 

Insufficient air supply can lead to carbon monoxide production due to vitiation (lack of oxygen). 

Insufficient gas supply can lead to flame lift off. Without enough gas the air-gas ratio will cease to be combustible, the mix then "lifts off" of the burner. The low gas mix will not burn. Safety features on modern appliances will cause the appliance to shut down, e.g the flame supervision device will read no flame and send a signal causing the appliance to shut down at the gas valve. (In older appliance like simple gas fire burner this is potentially extremely dangerous).


There are two overruling factors on gas pipe sizing for supplying domestic appliances.

  1. The supply should at no point be smaller than the inlet size of the gas pipe. E.g. if you have a boiler with a 22 mm gas pipe feed the entire supply line from the gas meter to that appliance must not include any pipe fittings or valves of less than 22 mm, a gas fire with a 12 mm inlet cannot served by an 8 mm gas valve. Restrictions to the flow of gas other than those in the design of the appliance can lead to an incorrect gas-mix and potentially unsafe situations. When the appliance are expecting to consume a certain amount of gas, the correct air supply will be added to the burning mix

  2. The gas pipe system of any individual system must be capable supply all the gas appliances with sufficient pressure of gas to enable correct consumption, and correct gas-air mixes. In a domestic natural gas installation the pressure drop along the gas pipe system including maximum potential demand i.e. all the appliances in simultaneous maximum consumption should be no more than 1 millibar pressure drop. This is ensured by the use or installation of correct pipe sizing diameters with considerations taken in respect to the distances and restrictions on supplies combined with the volume of gas required by the appliances without excessive pressure loss. 

In Effect an appliance gas pipe size is  determined by the appliances and the pipe system, what distances from the inlet pressure regulator (by the gas meter) and the amount and type of fittings used (as the fitting add friction and therefore restriction to maintaining pressures. E.g. the same appliance further from the meter may need a larger size supply line.


Gas Pipe sizing can be determined using the following gas pipe sizing tables, or calculations.



Gas Safe Reg no 305929    Company No. 07980305    VAT No. GB172932986    D-U-N-S Number: 21-827-8135

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