The main purpose of gas rating is to confirm that the appliance is operating properly and in accordance with the manufacturer’s technical instructions. 

If the stated working pressure to the appliance is correct, this test will confirm if the appliance is consuming the correct amount of gas in relation to the stated operating pressure.

The outcome of the test will give an indication on the correct size and condition of injector. 

If an injector is too small or partially blocked the volume of gas passed will be less than stated, and if the volume of gas is more, the injector  could be too large or worn.

Key point: The pressure could be identified as correct but the volume of gas consumed could be different that stated by the manufacturer of the appliance.

If too much gas is being passed, the appliance could be over-gassing and there are dangers of the flame on the burner being too large and the possibility of impingement (ie  touching the surfaces of surrounding metal surfaces), thereby chilling the flame and, in the end, producing carbon monoxide.

Imperial meter                                                                                                               Metric meter 

When carrying out gas rating in domestic premises, it is possible to come across to specific types of gas meters. One will be metric and other will be in imperial. Although the purpose of the test remains the same the means of calculating the volumes will be different.

Let's have a look at a Metric meter and calculate

The modern metric meters measure gas in meters cubed per hour m3/h  This meter display shows that in total 14m³ 396 litres (14.396) have been consumed by the appliances connected to the system.

During a gas rating test only a very small amount of gas will be measured over a two-minute period. Once the appliances reaches its operating temperature, the meter reading is confirmed and recorded while the appliance is working. After two minutes, the reading is taken again. For example, if we take the meter reading, shown in the image (Metric) as the first reading: 14.396m3/hr and the second reading as 14.414m3/hr taken after the two minute period, the volume of gas can be calculated.

Meaning of the figures from the calculation

0.018 = cubic metres of gas per hour

3600 = number of seconds in an hour

10.76 = kw of energy from a cubic metre of natural gas (approx)

120 = the amount of seconds in two minutes

Calculations-Imperial

Once the time it has taken for the pointer to complete one revolution of the dial then the gas rate can be calculated, For example, if it takes 38 seconds to pass 1ft³ of gas, the following calculation applies:

3600 x 1040*      =  3744000   =    98526 Btus/h

      38                           38

(98526 ÷ 3412 = 28.87kW (gross)

28.87 ÷ 1.11 = 26kW net