PB Heat Gas/Oil Boilers - Manual (Page 24)

b) Temporary low temperature operation is
acceptable within limits. For occasional cold start-
ups condensation will occur, but will have limited
effects. If the system is frequently allowed to cool
to room temperature, such as on night set-back
systems or energy management systems, cold
start-ups will occur often. These systems require
a Variable Low Temperature piping and control
arrangement, described below.
2. Constant Low Temperature
a) For systems with a relatively constant low
operating temperature (such as heat pump
systems), you can pipe a fixed flow by-pass
arrangement as shown in Figure 3.3. This piping
will not work for variable low temperature
systems such as outdoor reset systems or
primary/secondary systems with a large primary
circuit temperature drop. See Figure 3.4 for
multiple boilers.
3. Variable Low Temperature
a) When the return water temperature from the
system will vary (outdoor reset, snow melt, etc.) a
fixed bypass will not work. Fixed bypass piping
works by setting a high temperature rise through
the boiler. As the system return temperature rises
(during primary heating months, for example)
the boiler will cycle on the limit control
frequently, causing poor performance and
excessive cycling. To protect the boiler and
provide proper operation, install a temperature
22
PIPE THE BOILER
Figure 3.4: Fixed By-Pass Piping, Multiple Boilers
Figure 3.3: Fixed By-Pass Piping, Single Boiler