All boilers heat water by burning gas or oil under a boiler chamber to heat the water within. To simply burn fuel under the chamber, though, causes considerable waste. About 25% of the heat generated simply escapes out the flue in the form of hot exhaust (the exhaust is a hot 150°C). If the heat in the exhaust could somehow be used, it would help take a large chunk off your fuel bills. Using condensing boiler technology is currently the most accepted way to achieve these savings.
How do they work?
Regular gas burning boilers only have one water heating chamber or heat exchanger. The gas burners are placed under it. The heat that the burning natural gas generates heats the water chamber as much as it can before escaping out of the flue. Condensing boilers make use of a secondary water chamber to make use of the heat before it escapes.
In this design, the hot gases and vapors escaping the flames under the first boiler are directed to a secondary water chamber equipped with an efficiently designed heat exchanger. It absorbs most of the heat energy that the gases and vapors contain. In a well-designed condensing boiler in a properly installed setting, the heat exchanger on the secondary water chamber absorbs so much heat from the exhaust that it is a mere 55°C when it finally leaves the flue.
You might wonder why these appliances are called condensing boilers
f you’ve ever taken a cold pot of food from the refrigerator and placed it on a lighted natural gas stove, you’ve probably noticed that the sides of the pot right away show beads of condensation. The water condensation comes from the water vapor that the burning natural gas releases.
In a this type, the secondary water chamber contains relatively cool water. When the hot, water vapor-rich exhaust passes over the heat exchanger of the secondary water chamber, the vapor loses its heat to the heat exchanger and turns into droplets of condensate. It’s the condensation process that delivers the surplus heat in the exhaust into the water.
The droplets of condensation drip down into a collection pan at the bottom of the boiler and are drained out.
You need a precisely tuned system to make sure that you actually reap the savings promised
When you install a condensing boiler in your home, its main application is feeding the network of pipes that goes through all the heating radiators in the home. After hot water from the boiler goes through the network of radiators, it returns to the boiler with considerable heat still left over. While this isn’t a problem to conventional boilers, it’s often a deal breaker to condensing boilers.
They can only efficiently extract heat out of hot exhaust waste from the gas flame if the secondary heat exchanger is fed cool water. Unfortunately, the way these boilers are designed, the secondary heat exchanger is only fed the water returning from the hot water radiator system. Since this water is often very warm. It doesn’t manage to absorb much heat from the exhaust.
To work properly and deliver all the savings promised, it must be part of a precisely tuned and balanced radiator heating system that is guaranteed to return cool water back to the secondary heat exchanger. Very often, plumbing contractors installing such systems are only asked to replace an old boiler with a condensing model. It isn’t part of their contract to look at the entire radiator system in the house. For this reason, a large number of condensing boilers simply deliver no added benefit, at all.
Condensing boilers are usually a good idea only in new home constructions
In new home constructions, they can be built into a fully tuned and balanced system of radiator pipes. If this isn’t possible, it’s usually a good idea to stick with one’s old boiler.
Energy Efficient Condensing Boiler
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(price as of Oct 28, 2013)