my husband wants to know!
Remember that Pulse furnace a few years back? That was a true condensing unit. A regular boiler operates at a HIGH exhaust temperature, thereby exhausing all condensate products up the flue. Natural gas or coal have a byproduct of combustion of water and Co2 and some Co A condensing furnace has MORE And LONGER tubes in it and it allows the gasses inside the boiler to reach a much much LOWER exhaust temp up the stack. Thus CONDENSING takes place. This is a much more efficient process as more heat is extracted from the fuel and less goes up the chimney. Problem is that the CONDENSATE forms on the inside tubes of the furnace, it is acidic and eats away at the tubes. The new furnaces are designed to take that corrosion. An old boiler or furnace is about 75 to 80 % eff. A condensing new boiler or furnace is about 92% efficient. These new ones have longer tubes and more of them and can take corrosion and they of course cost a lot more.
the combi boiler is as loobyloo says however for the condensing boiler it does NOT recycle its fumes and reuse any excess oil or gas, it DOES extract much of the heat energy in the gases produced from combustion so that when they leave the flue outside they are much cooler than non-condensing boilers. the word condensing means that water droplets, which are slightly acidic, are formed from the gas due to the gas temperature dropping and this is why a pipe is needed to take the water to a waste pipe. so its like condensation from warm air on a cold window or water vapour from a cold car exhaust. Also some condensation seen as a plume coming from the flue. taking the heat from the gases that would otherwise go out of the flue increases efficiency. the heat taken goes into the hot water in the pipes for heating and hot water. as condensing is just a method to increase efficiency you can have a condensing combination boiler or a condensing conventional boiler i.e. one with a hot water cylinder. or a non-condensing combination/non condensing conventional as one answer undoubtedly knows but omitted for ease of reading no doubt not all boilers have to be condensing, there is the ability to use non-condensing if certain criteria are met
