Retrotec USACE - Manual (Page 186)

D56 ENERGY & PROCESS ASSESSMENT PROTOCOL
D.4.1.22 Use of Continuously Lit Pilots (Waste)
Figure D98. Boiler similar to this found cycling
in dining facility heating hot water in summer. Figure D97. Check old burners for continuous
lit pilots versus electronic ignition of pilot.
Older boilers and furnaces may have pilots that are continuously lit even
when the boiler is not fi ring. They are used to light the main burner when
the boiler is brought back online. These pilots can be replaced by spark
ignition pilots, which save fuel when the boiler is not heating (Figures D97
and D98).
D.4.1.23 Boiler Cycling On and Off at Low Loads (Ineffi ciency)
Boilers have a limited low operating level that they can still fi re without supply-
ing too much heat. This is called the “turndown ratio,” and a value of 15–20%
of the rated output is a common minimum fi ring rate. If the heating demand
is less than this percentage of the rated output, the boiler will cycle on and off
to satisfy that demand. This mode of operation is wasteful because of extra
combustion air fl ow through the boiler at startup and on shutdown. This air
fl ow gets heated in the boiler, reducing its temperature. Extra fuel is required
to bring the boiler temperature up to operating levels when the unit is started
again. This condition is often found during the nonheating season with boilers
that are used to provide process heat.