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A combination boiler can use one of two different fuels to heat the water. Common combinations include natural gas/propane and oil, or wood and oil combinations.

A condensing boiler uses a secondary heat exchanger to condense water vapour (steam) in the products of combustion down to water. Here's what that means:

When any hydrocarbon (gas, propane, oil, even wood) burns, it gives off at least two "products of combustion. The carbon combines with oxygen to create CO2, carbon dioxide. The hydrogen combines with oxygen to create water vapour, or steam. (If combustion is "incomplete," there will be other trace products as well.)

It's the water vapour or steam we're concerned with here.

To boil 1 pound of water requires 960 BTUs of heat. Conversely, if you condense steam down into 1 pound of water, you will recover 960 BTUs.

The fuel is burned in the "primary heat exchanger," where of course it releases a tremendous amount of heat into the water we're warming up. Then the "products of combustion" are drawn by a fan into the "secondary heat exchanger." When the water vapour or steam component of the products of combustion hits the relatively cool walls of this second heat exchanger, they condense down into liquid water. As they do, they release heat into the boiler water through the walls of the secondary exchanger. The liquid water then goes down a drain.

Obviously, the boiler water has to be cooler than the steam component of the products of combustion in order for the heat to travel into it. So we make sure that this water is what's coming back to the boiler from the heating system. Again, for this method to work well, it's best applied in a "radiant in-floor" type heating system, where we don't heat the boiler water up to as high a temperature in the first place.

--The HVAC Veteran

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Q: What is the difference between a condensing central heating boiler and a combination boiler?
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