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Is Your Gas Boiler Costing Too Much to Run?

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Is Your Gas Boiler Costing Too Much to Run?

This post is put together to help you identify if your central heating/hot water gas boiler is consuming too much gas. You do not need any specific skills other than being able to record gas meter values and work with the values shown on your gas bill.

All work on gas appliances must be undertaken by a competent person registered with Gas Safe. The following link will help you confirm the competency of anyone undertaking gas appliance work at your property.


Get to Know Your Gas Meter

Measurement is management. The biggest part of energy efficiency is knowing how much energy is being used. Your gas meter may be either imperial or metric. A metric meter has m3 next to the number counter.

A metric meter displays gas consumed in m³. The following shows the changes in reading for consumption of 1.5 m³.



If your gas meter is imperial [not showing m3 next to the number counter; the following factor can convert cubic feet cubic meters.

100 ft³ = 2.831684659 2 m³

The following link gives details for all heating and hot water boilers


Detailed below are the published values for two Alpha boilers.

Boiler Heat input Heat output Gas Flow
Alpha CB24 12.1 to 28.4 kW 9.3 to 23.3 kW 1.16 to 2.71 m³/h
Alpha CB28 14.3 to 34.0 kW 11.2 to 28.0 kW 1.37 to 3.25 m³/h

This section contains the calculations to convert the published values into cash terms.

Calorific value (CV) is a measurement of the amount of energy contained in gas. CV is usually quoted in mega joules per cubic metre (MJ/m³). Gas has a CV of between 37.5 MJ/m3 and 43.0 MJ/m3.

One cubic meter (1 m³) of gas is equivalent to 11.06kWh.

From the above table it can be estimated the CB24 boiler will consume between 12.83 kW to 29.97 kW

If we base the calculation on a gas price of 5 pence per kWh the CB24 boiler will cost as follows:

Published consumption value x 11.06 x cost of gas (per 1m³)

For the CB24 the calculation is

The minimum gas flow rate calculation is 1.16 x 11.06 x 0.05 = £0.64

The maximum gas flow rate calculation is 2.71 x 11.06 x 0.05 = £1.50

You can use the above calculation, adjusted to the published values contained in your boiler manual. To identify how much your boiler costs to run.

Step 2 Compare Gas Bill to Calculated Data

You can check your gas bill total against the calculated data in the following ways:

Central Heating/Hot Water Timer Control Fitted

  1. Check details for your heating/hot water timer and note total daily hours of operation
  2. Multiply daily values by the time period on your bill.

Central Heating/Hot Water Timer Control NOT Fitted

  1. Divide bill total (minus any standing charges) by the number of days in the time period of the bill.
  2. Divide the daily total by the approximate number of hours of operation per day.

It would be more energy efficient to fit time/temperature control equipment. If not you could note approximate times of operation to compare published consumption data against the cost of your gas bill.

There are many variable conditions that affect boiler performance. A summary is listed below:

  1. System type [Combi, sealed system, open vented]
  2. Size of boiler & central heating system
  3. Age of boiler & central heating system
  4. Ambient temperature
  5. Thermal efficiency of the space being heated. A cold drafty house will cost much more to heat.
  6. Time of operation per day [timed, continuous or manual operation]
  7. Type of central heating controls, if fitted.

Please contact me if you have any experiences of checking your gas bills and if your bill is correct against manufacturers published data.

I would also appreciate if you have any ideas for energy efficiency, or questions to ask. Please let me know.


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