What's The Big Deal About Condensing Boilers"

By Alan Willi

Everyone seems to be talking about condensing boilers. Actually, we can make a boiler condense quite easily. Boilers condense, that is the flue gases condense, when the return water temperature is low enough to cool the flue gas temperature below its dew point; condensing point. For general conversation, let's say return water temperature’s 130ºF, 130º is a fairly high temperature for radiant heating system, so the flue gases would condense all of or most of the time. We've discussed at other times the 3/1 rule: for every 3º we lower the water temperature we save 1% fuel. The low water temperature appears to be a good thing. The problem is that the condensate is quite acidic; it will eat up your boiler, and it usually will only take a few years. In addition, very cold return water temperature may crack a conventional cast iron boiler. When you install a conventional boiler, e.g. non- condensing, you have to protect it from low return water temperature. You can do this by several different ways: injection station, flat place heat exchanger, a storage tank, or a 4-way valve. Note: a 3-way valve will control the water temperature to the distribution system but IT WILL NOT PROTECT THE BOILER. Another point about condensing boilers. They will only condense when they operate below the condensing temperature we mentioned earlier, around 130º. Above that temperature they will operate around 87% efficiency. The same is true of condensing warm air furnaces but they operate below the condensing point all the time. One boiler manufacturer lists their condensing boiler at 93º DOE. In small print, return water temperature 90º, supply water temperature 110º. The point is to apply our products properly and tell the homeowners what their system will deliver.