Environmental Engineering Reference
modified flame retention burners create a blue flame in the combustion
chamber. This is done by recirculating unburned gases back through the
flame zone. This produces more complete burning of the fuel and results
in lower soot formation. These flame systems are available as a burner for
retrofit to furnaces, or as a complete burner and boiler system for hot wa-
ter distribution systems.
Variable fuel flow is used in burners to throttle or cut back the fuel
flow rate, reducing flame size, as the system heating load varies. These
burners have conventional steady-state efficiencies and higher seasonal
efficiencies. They are available for large apartment boilers and furnaces.
There are also burners that can burn either oil or gas. They offer no
efficiency advantages, but the ability to switch fuels in the event of a short-
age or price differences is an advantage. They are available as combination
burner and boiler units.
Tankless boilers offer some advantages in seasonal efficiencies, com-
pared to conventional units, since there is less water to heat up and cool
off. The savings are similar to that of using an automatic flue damper.
Flue economizers include small auxiliary air-to-water heat exchang-
ers that are installed in the flue pipe. The unit captures and recycles the
usable heat that is usually lost up the flue. The recaptured heat is used to
prewarm water as it returns from the distribution system. If the flue tem-
perature is lowered too much, moisture, corrosion and freezing may oc-
cur in the flue pipe. Depending upon the age and design of the boiler and
burner, a flue economizer can provide annual fuel savings of 10 to 20%
and a payback of 2 to 5 years.
Air-to-air flue economizers are also available for about 1/5 the cost
but these save much less energy and are usually not tied into the central
heating system. They are best for heating spaces near the flue.
The technologies that are well suited to groups of buildings include
cogeneration, district heating and seasonal energy storage systems. Co-
generation involves the simultaneous production of both space heat and
electricity from an electrical generating system. A district heating system
supplies heat and hot water from a central production facility to a number