Condensing Boiler’s

New energy efficient condensing boilers not only save money but reduce CO2

A condensing boiler is a water heating device designed to recover energy normally discharged to the atmosphere through the flue. When a condensing boiler is working at peak efficiency the water vapor produced by the burning of gas or oil in the boiler condenses back into liquid water - hence the name "condensing boiler".

The boiler uses a heat exchanger so that incoming air or water cools the exhaust, forcing the condensation of the water vapor it contains; this heats the incoming air (if an air-to-air heat exchanger is used) or pre-heats the water (if an air-to-water heat exchanger is used). A small proportion of the extra efficiency of the condensing boiler is due to the cooling of the exhaust gases, but the majority of the energy recovered is from the condensation of the water vapor in the exhaust gases. This releases the latent heat of vaporization of the water (970btu/pound) of condensate (the water vapor released whenever one burns fuels containing hydrogen).

 

The actual operating efficiency of a condensing boiler depends on the ambient air temperature and the relative humidity. If the incoming air is at 100% relative humidity, the condensing boiler will operate at its maximum efficiency (since it can - in theory - condense all the extra water vapor introduced by combustion). As the relative humidity falls, so will the actual efficiency of the condensing boiler (because less of the water vapor produced can be recovered from the exhaust).

Usage

Condensing boilers are now largely replacing earlier, "conventional" designs in powering domestic central heating systems in Europe and, to a lesser degree, in the US. In Europe, their installation is strongly advocated by pressure groups and government bodies concerned with reducing energy use. In the United Kingdom, for example, since 2005 all new gas central-heating boilers fitted in England and Wales must be high-efficiency condensing boilers unless there are exceptional circumstances, and the same applies to oil-fired boilers from  April 2007. In the United States there is a tax rebate for the installation of condensing boilers. The increase in natural gas and propane prices in the US has encouraged the retrofit of existing boiler installations with condensing equipment.

Efficiency

Condensing boiler manufacturers’ claim that up to 98% thermal efficiency can be achieved, compared to 70%-80% with conventional designs (based on the higher heating value of fuels). Typical models offer efficiencies around 90%, which brings most brands of condensing gas boiler in to the highest available categories for energy efficiency. In the US they typically receive an Energy Star Certification.

When installed in real houses, the performance of condensing boilers is typically 4-5% lower than in laboratory tests. Part of the efficiency drop is because boilers are often oversized for the thermal demand of the property, giving return temperatures over 50°C from the heating system, which prevents significant condensation in the heat exchangers. Better education of both installers and owners could be expected to raise efficiency up towards the reported laboratory values. Eco Depot USA suggests one way to make better use of these boilers is by combining space and water heating systems.

Reliability

They have a reputation for being less reliable and require professional installation and regular service and may suffer for the lack of familiarity with them among installers and plumbers. In reality, gas condensing boilers have only one extra element to be installed - the drain pipe for the condensate collected during operation. This comprises a short length of inexpensive plastic waste pipe.

Exhaust

The water that condenses out tends to be mildly acidic (pH between 3 and 4) because of the impurities in the fuel, chiefly sulfur and nitrogen, but no special treatment is necessary in most applications. The relevant parts of the boiler have to be constructed of materials that will withstand this acidity, typically aluminum. Since the final exhaust from a condensing boiler has a lower temperature than the exhaust from a conventional boiler a fan is always required to expel it, with the additional benefit of allowing the use of low-temperature exhaust piping (typically PVC in domestic applications) without insulation or chimney requirements. This allows for the added benefit of flexibility of installation location.

Cost

Condensing boilers are up to 50% more expensive to buy and install than conventional types in the US. However, as of 2006, the extra cost of installing a condensing boiler should be recovered through lower fuel use, in around 2-5 years at US prices. Obviously the exact figure will depend on the efficiency of the original boiler installation, boiler utilization patterns, and costs associated with the new boiler installation.

 

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