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Energy in buildings
Energy in buildings

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3.1 Gas boilers

Figure 29 shows a simplified diagram of a typical 1980s domestic boiler. Combustion air enters at the bottom from the interior of the house to feed a large gas burner heating a set of finned heat exchangers. Water is pumped through these and circulated out to the heating system. The burnt gases exit at the top into a flue to the outside air. Such a boiler might have an output of 15 kW and an overall thermal efficiency of 65%.

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Figure 29 Schematic of a typical 1980s gas boiler
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This is shown as a rectangular white box. In the centre is a labelled ‘finned heat exchanger’. A blue pipe labelled ‘water return from heating system’ is shown feeding cold water to the top of the heat exchanger. A red pipe labelled ‘water flow to heating system’ is shown carrying hot water away from the heat exchanger. Inside the boiler a gas burner with four blue flames is shown heating the heat exchanger. At the right is a smaller flame labelled ‘pilot light’. Beneath the burner an upward pointing vertical blue arrow is labelled ‘combustion air from house’. At the top of the boiler a vertical flue is shown with an upward pointing vertical arrow labelled ‘hot exhaust gas’.

Figure 29 Schematic of a typical 1980s gas boiler

More recent gas boiler designs have incorporated a number of improvements:

1 Balanced flues

Here the burner and heat exchanger are totally sealed off from the interior air of the building and the combustion air is blown through the boiler using an electric fan. Air is drawn in from the outside and the burnt gases are blown out again using a pair of concentric pipes via a single flue terminal on the outside of the building (Figure 30). Although this design does require a certain consumption of electricity, it allows the building to be made more airtight.

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Figure 30 A boiler with a balanced flue
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The boiler is shown as a white box mounted on a cavity brick wall which is shown on the right-hand side of the picture. The boiler assembly is fixed to the left side, on the inside of the house. At the left there are pipe connections to the heating system. The air supply and exhaust flue pass through the wall to the outside. This takes the form of two concentric pipes. Fresh air is shown being drawn into the boiler through the outer concentric pipe. It then travels down to the bottom of the boiler and upwards on the left-hand side. Near the bottom of the left side of the boiler is a burner shown with blue flames and labelled ‘gas’. The heated air then travels up the left-hand side of the boiler to a set of finned pipes labelled ‘heat exchanger’. From these hot water is shown flowing through a red pipe labelled ‘water flow’ out to the left to the heating system. Another blue pipe labelled ‘water return’ feeds cold water from the left back to the heat exchanger. The combustion air continues rising to the top of the inside of the boiler and across to the right-hand side. It is pumped by a fan inside the boiler housing driven by a labelled electric motor shown mounted in a grey area inside the centre of the boiler. The air then exits from the boiler to the right through the inner concentric pipe where it is labelled ‘hot exhaust gas’.

Figure 30 A boiler with a balanced flue

2 Electronic spark ignition

This has replaced the permanently burning pilot light used in many older boiler designs which could consume 10% of the total gas used.

3 Condensing gas boilers (see Figure 31)

The main component of natural gas is methane, CH4. When this burns it produces a large amount of water vapour:

  • CH4 + 2 O2 → CO2 + 2 H2O

    methane + oxygen → carbon dioxide + water vapour

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Figure 31 Condensing gas boiler
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The boiler is shown as a rectangular box. It is mounted on a cavity brick wall which is shown on the right-hand side of the picture. The boiler assembly is fixed to the left side, on the inside of the house. At the left there are pipe connections to the heating system. Its air supply and exhaust flue pass through the brick wall to the outside. This takes the form of two concentric pipes. Fresh air is shown being drawn into the boiler through the outer concentric pipe. It then travels down to the bottom of the boiler and upwards on the left-hand side. Near the bottom of the left side of the boiler is a burner shown with a blue flame image and labelled ‘gas’. The heated air then travels up the left-hand side of the boiler to a set of red pipes labelled ‘heat exchanger’. From these hot water is shown flowing through a red pipe out to the left to the heating system. The air continues rising to the top of the inside of the boiler and across to the right-hand side. It is pumped by a fan inside the boiler housing driven by an electric motor shown mounted outside the boiler at the right hand side. The air then descends through a second heat exchanger coloured light blue. The pipework for this is connected at one end to the first heat exchanger. At the other end it is fed by a blue pipe from the heating system and labelled ‘return’. Water droplets labelled ‘condensed water vapour’ are shown falling from this second heat exchanger. They fall into a container and then move down through the bottom of the boiler in a pipe labelled ‘condensate drain’. The air then exits from the boiler through the wall in the inner of the concentric pipes. Its flow is shown with an orange arrow labelled ‘warm exhaust gas’.

Figure 31 Condensing gas boiler

In a non-condensing boiler this water vapour is lost with the flue gases. In a condensing boiler the heat exchanger is made sufficiently large so that the return water from the heating system, which may be at about 50°C or lower, can cool the flue gas sufficiently to enable the water vapour to be condensed out recovering its latent heat of vaporization. The energy content of a fuel can be quoted as ‘higher’ or ‘lower’ values, depending on whether or not this latent heat of vaporization is recovered. The potential energy savings in using a condensing boiler depend on the difference between the gross and net calorific values for the fuel burned (also called the higher and lower heating values, HHV and LHV). Table 10 gives sample figures.

Table 10 Higher and Lower Heating Values of Fuels
Higher heating value/

MJ kg–1

 Lower heating value/

MJ kg–1

 Ratio
Natural gas 53.1 47.8 1.11
Liquefied petroleum gas (LPG) 49.2 46.0 1.07
Light heating oil 45.3 42.6 1.06
(Sources: BEIS, 2017a)

Where natural gas is the fuel, using a condensing boiler can increase the amount of heat extracted from the gas by up to 11% and the overall boiler efficiency to 90% or more.