Condensing boilers (by a London Plumber)

How does a condensing boiler work?                                                             
Condensing Boiler

This is a design of boiler which can have an increased efficiency over the more traditional boiler. The efficiency of a typical non-condensing boiler is around 75%, whereas with condensing boilers it can be over 87%. This increased efficiency is due to the extraction of heat from the otherwise wasted flue gases. Most boilers have a single combustion chamber enclosed by the waterways of the heat exchanger through which the hot gases can pass. These gases are eventually expelled through the flue, located at the top of the boiler, at a temperature of around 180°C.

Condensing boilers, on the other hand, are designed first to allow the heat to rise upwards through the primary heat exchanger; when at the top the gases are rerouted and diverted over a secondary heat exchanger. These can reduce the flue gas temperature to about 55°C. This reduction of temperature causes the water vapour (formed during the combustion process) to condense and, as the droplets of water form, fall by gravity to collect at the base of the flue manifold. The remaining gases are expelled to the outside environment through a fan-assisted balanced flue. The condensation produced within the appliance should be drained as necessary into the waste discharge pipework or externally into a purpose-made soakaway.

It is only possible for a condensing boiler to work to these very high efficiencies if the flow and return pipework is also kept below 55°C. The flow & return temperatures need to be maintained for the heat transference to occur from the flue to the water (i.e. heat transference goes from hotter to cooler materials).

Many people are installing condensing boilers in homes which are fitted with radiators and a primary flow and return to the hot water. Some of these people may be under the impression they are getting more for their money; unfortunately, as stated above, low flow and return connections are essential; therefore they are not making the vast savings they are led to expect. For a central heating system to work with radiators and hot water primary circuits, flow temperatures need to be around 82°C, so in fact the installer has put in an expensive condensing boiler which gives only slightly improved efficiency over the more traditional boiler. The appliance basically only works in its condensing mode during initial heat-up.
To achieve a system which will function in its condensing mode the installer needs to consider a suitable system of radiant heating.

Boiler repairs can be expensive.

Condensing boilers an alternative view
As the cost of fuel increases and the limitations of sources of energy become more apparent. fuel suppliers and boiler manufacturers are diverting more time and thought to improving the efficiency of their appliances. One of the more recent products of this research is the condensing boiler. The reader will appreciate that 100% efficiency is not possible with any burning appliance. This is mainly due to the fact that products of combustion carry away some of the heat given off by the burning fuel up the chimney and into the atmosphere. The actual efficiency of a boiler varies considerably depending on a number of factors. some of which may be listed as follows: the age of the appliance. whether it is oversized in relation to the scheme it is heating and whether or not it has a permanent pilot or is fitted with electronic (spark) ignition. A modern boiler of the conventional type is said to be approximately 75 per cent efficient. the remaining 25 per cent of the heat passing up the flue. To achieve greater efficiency some of this heat must be reclaimed. but this poses a serious problem. There must be a considerable difference between the ambient air temperature and that of the products of combustion in order that convection takes place.
The second difficulty is that water vapour is formed during the process of combustion. and if the flue temperature is lowered significantly the vapour will condense and turn to water. The first problem can be overcome by using a flue extractor, indeed some non-condensing boilers already use this method to reduce flue sizes. The difficulty of disposing of the condensate (water) has to some degree been overcome by allowing it to collect in the base of the boiler where it can be disposed of into the drainage system. Although the condensate is acidic in nature containing traces of nitric and sulphurous acids.

Tests have been conducted on materials from which domestic drains are constructed and the following conclusions reached. Drains made of plastic material and clayware showed insignificant damage while cast iron is likely to be affected in the long term and gives rise to staining. Cement and concrete products appeared to be affected more seriously
than other materials. This could lead to problems in older properties having salt-glazed drain pipes with cement joints. In practice, however, the adverse effects due to condensation are unlikely to be serious as it will be appreciated that it will be
diluted very quickly by the discharges from sanitary appliances.

There are two basic types of condensing boilers using either wet or dry heat-transfer principles. The wet type requires a purpose-made system which at present is only produced for commercial boiler plant and is still being developed. Dry condensing boilers are more suitable for domestic work. Unlike the wet system where the circulating water is in direct contact with the combustion gases, they operate on the more conventional principles of traditional boilers where the products of combustion are separated from the circulating water via a heat exchanger.

It will be seen that the combustion gases pass through the primary heat exchanger in a similar way to boilers of a traditional pattern, but at this point the similarity ends. Instead of passing the combustion products directly into the flue, bearing in mind they are at a temperature of approximately 200-250 °C, they are circulated around the secondary heat exchanger where more heat is given up to the cooler return water entering the boiler. Heat is extracted from the combustion products in two ways.

(a) In the form of sensible heat. i.e. the transmittance of heat from a hotter medium (in this case the combustion products) to the cooler return.
(b) By the latent heat of evaporation.

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The reader should understand that the water vapour in the combustion products reverts to a liquid when condensed, giving up the heat that caused it to become a vapour. These two sources of heat can be usefully employed in properly designed boilers to increase dramatically their efficiency, even when the electrical energy necessary to operate the extractor is taken into account. Obviously such a component is necessary on all boilers of this type as the temperature of the outgoing products of combustion are so low that the convective effect of natural draught is impossible. The water produced by condensation in the boiler is collected in the base and discharged into a drain, preferably via a gully where the acidic nature of the condensate will be diluted. One further point is worth noting, the relatively small quantity of water produced when the boiler is in operation would, in frosty weather , cause the discharge pipe to freeze. To prevent this a device similar to that employed in automatic flushing cisterns is used which allows a body of condensed water to build up in the base of the boiler until it operates what the plumber would call an automatic siphon. The relatively large body of water thus discharged is then unlikely to freeze.

The lower the temperature of the return water in relation to that of the products of combustion, the greater will be the heat transference to the water. Some authorities have suggested that by using much larger radiators to permit lower operating temperatures with consequently lower boiler return temperatures, greater efficiency can be achieved.
In practice, however, such a design concept would only fractionally increase operating efficiency and any saving would be offset when compared with the initial cost of installing much larger heat-emission equipment such as radiators. A more realistic approach would be to increase the heating flow and return differential from the recommended 11 °C to approximately 20 °C. This would have the effect of reducing the average or mean temperature of the heat emitters, but if the radiators are accurately sized this might not be a practical proposition,It is, however, worth checking on a refurbishment
job where a condensing boiler is replacing an older model, as it is quite common for existing radiators to be oversized.

The initial cost of condensing boilers is more than that of conventional types, and although the 92-94 per cent efficiency claimed by manufacturers is attractive, as already explained this depends largely on operating temperatures as illustrated by the graph in Fig. 5.9. Dewpoint is the term used to signify the temperature at which the water vapour condenses and reverts to water. The ideal temperature at which this occurs is 59 °c when the air-to-gas ratio is just sufficient to cause complete combustion of the gas. In practice, however, to ensure safe working conditions, a quantity of air in excess of the ideal requirements must be provided which has the effect of lowering the combustion temperature and the dewpoint of the combustion products. Despite this it will be seen that with a dewpoint of approximately 53-54 °C very high efficiency can be obtained with low return-water temperatures. With regard to the viability of changing an existing boiler for a condensing type, several factors must be considered. If the existing boiler is of the old pattern with an open burner, giving efficiencies of only 60-65 per cent, and taking into account its remaining service life, it will be an economic proposition. If, however, it is of the new pattern with a closed combustion chamber giving an average efficiency of 75 per cent, despite the savings on operating costs, the initial cost of the condensing boiler would not be viable, bearing in mind that a modern conventional b9iler is likely to be operating a heating scheme where the return temperature is relatively high. Condensing boilers are, however, an economic alternative to a conventional boiler in terms of running costs
when fitted with new installations designed to
give a 20-22 °C drop between the flow and return temperatures. Radiant heating schemes employing a system of pipework embedded in walls, floors or ceilings, operating at lower temperatures and heated by a condensing boiler, would show significant savings on heating costs.

When upgrading an existing boiler one of the practical aspects that must be considered is the position of the flue outlet. Due to their efficiency the flue gases from condensing boilers tend to produce a greater degree of 'plumbing' than those of the traditional type. Flue termination under windows or adjacent to doors, car ports or opposing walls, must be avoided.

Makes of condensing boilers

Potterton condensing boilers Potterton High Efficiency Combination Boilers
Baxi condensing boilers Baxi High Efficiency Combination Boilers
Worcester condensing boilers Worcester High Efficiency Combination Boilers
Ariston condensing boilers Ariston High Efficiency Combination Boilers
Vaillant condensing boilers Vaillant High Efficiency Combination Boilers
Vokera condensing boilers Vokera High Efficiency Combination Boilers
Glow-Worm condensing boilers Glow-Worm High Efficiency Combination Boilers

Combi Boiler

Combination boilers

Combination boilers provide both hot water to a sealed heating system and a separate supply of instant hot water directly to taps and showers. The advantages are ease of installation {there are no tanks or pipes in the loft}, space- saving {there's no hot-water storage cylinder} and economy {you heat only the water you use}.'.
The main drawback is a fairly slow flow rate -so it takes longer to fill a bath, and it's not usually possible to use two hot taps at the same time. Combination boilers are therefore best suited to small households or flats. However, to overcome these problems, the newer generation of combination boilers incorporate a small built-in hot-water storage tank.

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