Condensing Boilers

High-efficiency condensing boilers feature additional advanced heat exchanger designs and materials that extract more heat from the flue gases before they are exhausted. The temperature of the flue gases is reduced to the point where the water vapour produced during combustion condenses back into liquid form, releasing the latent heat, which improves energy efficiency. With some 12 percent of the energy of a gas-fired boiler tied up as latent heat, this represents a significant energy-savings potential. A side effect is that this condensate is usually acidic and has to be piped to a drain.

Modern condensing boilers have energy efficiencies of 90 to 96 percent. New conventional non-condensing models have energy efficiencies of only 70 to 85 percent. Many boilers over 20 years old typically operate at only 60 to 70 percent efficiency, making them good candidates for upgrading or replacement. A number of natural-gas-fired condensing boilers are available, but very few oil-burning ones are on the market.

An important point is that for the water vapour in the flue gases to condense, the temperature of the flue gas must be reduced to below the water dew point of the flue gas. For this to occur, the return water temperature to the boiler proper must be below 60°C. If there are no heat exchange surfaces at the back of the boiler below this dewpoint, condensing will not occur, and this energy opportunity will be lost, even if the boiler is a “condensing” boiler.

In retrofit applications where you wish to retain your existing boiler, boiler efficiency can be improved by adding an economizer, which is a heat exchanger that utilizes the waste heat from the flue gas to preheat the boiler feedwater. A condensing economizer improves the effectiveness of reclaiming flue gas heat by cooling the flue gas below the dewpoint. The condensing economizer thus recovers both the sensible heat from the flue gas and the latent heat from the moisture that condenses. You do have to ensure, however, that the condensate does not enter the boiler, as the condensate is highly corrosive.

Oil condensing boilers are more expensive, and it is much harder for them to actually achieve condensing because:

* Sulphur in the oil turns the condensed water into sulphuric acid that must be neutralized. The heat exchanger must be of very high quality to prevent corrosion by the acid.
* Oil has 50 percent less energy tied up in latent heat, compared with natural gas.
* The dew point for oil is low – 47°C compared with 60°C – for natural gas, making the water vapour in the flue gas very difficult to condense.