What's The Big Deal About
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
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
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
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.