A condensing boiler is a modern day boiler that integrates an extra heat exchanger. This makes it possible for the extremely hot waste gases to be used to heat the water that needs to be heated. Some of these gases are allowed to condense back to water inside the boiler.
Condensing boilers normally produce some acidic condensate which should not be allowed bank into the boiler. Instead, there should be a drainage system that caries this condensate out of the boiler. These condensing boilers are normally mounted on the wall in such a way that it will be easy for the exhaust fumes form the boiler can exit without a problem. Hot water should also be easy to access from the placement of the boiler. You do not need an oversized boiler to keep your house warm during the cold season. If your house is properly insulated, a small condensing boiler will do just fine.