Condensing Boilers

    How does a boiler condense?

    A by-product of the combustion process is water vapor (steam). This is due to the combustion of the hydrogen content of the fuel and not from the water from within the vessel. As the exhaust cools the water vapour turns from a gas to a liquid. If the return water piping is below the dew point (~140 F) this can cause condesation of the water vapor. As the temperature water of the water decreases there is more oxygen within the water that aids in the corrrosion

    Why is condensation bad for a non-condensing boiler?

    The condensate from the flue gas is acidic, it has a pH between 3 and 4. Combine that with hot water being an ideal solvent and over time the mixture can be very corrosive to the boiler and breeching. It will drasctically reduce the life of the boiler.

    Also the temperature change can induce thermal shock to the boiler weakening the metal even further.

    How does a condensing boiler work?

    The goal of the condensing boiler is to reuse the hot exhaust gases to preheat the return water to the boiler so that less energy and thus fuel has to be expended to raise the water temperature. Condensing boilers are designed to specifically to withstand the effects of the corrosive condensate. They are normally made of stainless steel or aluminum and involve the use of a large heat exchanger to transfering heat from the exhaust to the returning water. Even though the condensate is acidic in most cases it is no worse than tomato juice and is safe to drain.

    Whenever a condensing boiler is installed the chimney lining needs to be re-examined. In almost every case the chimney needs to be re-lined with stainless steel to deal with the more acidic exhaust.

    I hear condensing boilers can reach 98% efficiency, how can they do that.

    Combustion efficiency can effectively be that high under very tight restrictions. The burner has to be running at around 20%, the return water needs to be below 100 F depending on the make of the boiler. A better estimate is 84% - 92% seasonal efficiency. This is still much better than the tradional boiler efficiency of ~82 % for new non-condensing boilers and 55% - 65% for older boilers. This makes a condensing boilers one of the first energy saving steps that is thought of when reducing facility operating costs.