The water that is discharged from a condensation boiler is water that is already contained in the atmosphere. It’s the same water that’s in the air that we breath and it’s the same water in the air that the boiler breathes.

To keep it simple your gas boilers burns a combination of natural gas and oxygen to produce fire. That fire heats water as it is circulated through a heat exchanger contained within the boiler. A heat exchanger is just a large hollow piece of cast iron, cast aluminium or stainless steel that has a fire burning under it while water flows through it. As the water flows through the heat is exchanged from the hot metal to the cool water. That water is then circulated throughout the property usually through radiators where the heat is transferred in the reverse order into the room. The cool water then goes back to the boiler and the process starts all over again.

Approximately 30% of the heat energy was wasted in old boilers. In other words for every pound of gas that you bought 70 pence heated the water and 30 pence went up the chimney. That’s a lot of money over an entire year! The wasted money not only hurts the pocket book but also adds to environmental waste, pollution and uses up our non-replaceable supply of natural gas.

So along came condensing boilers which use energy much more efficiently. In effect they squeeze a little extra heat out of the same amount of energy that was wasted in the old boilers. As the water passes through the heat exchanger it is re-routed through a secondary heat exchanger that the old boilers did not have. The process removes more usable heat energy before the exhaust leaves the boiler. This final removal of latent heat has two noticeable by-products. The first is that a plume of what is often mistaken for steam can be seen coming from the flue. The plume is not steam but is actually a miniature cloud being formed. The same effect hat causes a cloud to form on the lee side of a hill as the air is pushed up the hill is what’s happening in the boiler. The temperature of the air drops in the process and the water vapour can be seen in the form of a plume. The second by-product is water that collects inside of the boiler during this process and must be disposed of. The best solution they came up with was to connect the the boiler to the waste system of the house or office.

So that’s the long non-technical answer. The short answer is that the water from the boiler is the left over moisture after the heating process.

It should also be noted that the acid content of the waste water is higher than normal and the disposal should be carried out as per the building regulations. See my original article here.

Hope that helps.
Canadian Dave