Costs: Condensing gas boilers

The April 2005 amendments to Part L of the Building Regulation require new or replacement boilers to meet a SEDBUK energy efficiency rating of A or B, which equates to an average annual efficiency of greater than 86% in typical domestic conditions. This effectively means specifying a condensing boiler in most circumstances.

Condensing boilers are more effective than conventional boilers at recovering heat from hot flue gases. This is because they have larger heat exchangers and can recover the latent energy present when water vapour, produced as a by-product of combustion, condenses into a liquid. When this happens the boiler is operating in “condensing mode” – this heat is used to pre-heat the water before the burner heats it.

Condensing boilers are most efficient when operating in condensing mode. Even in non–condensing mode they are more efficient than traditional boilers. Using condensing boilers should reduce fossil fuel use, which in turn reduces carbon emissions.

Whole-life performance and cost issues

Both system and combination boilers are available in condensing versions. The problem for the specifier or purchaser of a condensing boiler is how to evaluate their whole-life performance.

Boilers contain many sub–assemblies and parts which can be replaced if they fail. In theory, this means that the boiler has an unlimited life. The whole-life costs of a boiler relate to the reliability of the individual parts, their performance as a complete system and fuel costs.

There are dozens of ways a condensing boiler can fail, and their makers rarely provide information on reliability or component performance, which makes it harder to make decisions based on whole-life performance.

BS EN 677, the European standard for condensing boilers, requires that all parts of the heat exchanger and other parts likely to come into contact with the acidic condensate produced by the boiler should be sufficiently corrosion-resistant to ensure that it has a reasonable life expectancy.

The most common heat exchanger material options are stainless steel and cast aluminium alloy. Condensing boilers designed with two heat exchangers, cast iron and aluminium alloy, may benefit from the proven performance of cast iron. However dual heat exchanger boilers tend to have lower SEDBUK ratings.

Where boilers have been type-tested to meet the requirements of the boiler standards (BS EN 483, BS EN 297 and BS EN 625) specifiers have the assurance that:

• Heat exchanger materials meet minimum standards
• Component assemblies have passed endurance tests. For example, the burner control system or spark generator will have cycled through 250,000 operations.

Boilers consist of much more than the heat exchanger. Whole-life value can be enhanced by minimising the risk of boilers needing repairs, and ensuring maximum efficiency is extracted from the heating system. Some options include:

Design issues

• Design the central heating system for low temperature return flow (around 40°C) so that the boiler operates in condensing mode to maximise efficiency. Common techniques include underfloor heating systems or radiators oversized by 10-12%.
• Avoid specifying a boiler with an output higher than the demands of the heating and hot water system.
• Check how long manufacturers will stock spare parts.

• Specify boilers designed for quick installation.
• Condensate drainage must be installed with a slope of at least 2.5°.

• Include a frost thermostat to avoiding risk of frost damage where the boiler is sited in cold locations.
• Include an anti-scale device or water treatment in hard water areas.
• Use boilers with electronic systems featuring downloadable operating history or remote access for targeted and effective maintenance.
• Pump protection automatically starts pump for a few seconds if the boiler is not being used to prevent seizure.
• Scale or sludge build–up can be removed by appropriate chemicals or a power flush.

• Burner modulation so that the gas burned is directly related to the heat demand.
• Burner anti–cycling controls.
• Pre-mix burners for efficient combustion.
• A variable-speed pump reduces associated electrical energy costs.
• Weather compensation control and optimum start controls to manage heat demand in response to weather.