Installation

© 2001 D 364 - 07/01 8 of 36
Boiler and Mixing Priority (DHW PRI = B+M)
It can be selected that the DHW tank has priority over the boiler and
mixing zones. If this option is chosen, the mixing device is throttled
back and the boiler system pump (P2) turns off on a call for DHW.
DHW PRIORITY OVERRIDE
To prevent the building from cooling off too much or the possibility of
a potential freeze up during DHW priority, the 364 limits the amount of
time for DHW priority. As the outdoor air temperature becomes colder,
the length of time that the 364 provides DHW priority is reduced. Once
the allowed time for priority has elapsed, the 364 overrides the DHW
priority and operates DHW and heating simultaneously.
CONDITIONAL DHW PRIORITY
If the boiler supply temperature is maintained at or above the required
temperature during DHW generation, this indicates that the boiler has
enough capacity for DHW and possibly heating as well. As long as
the boiler supply temperature is maintained near its target, DHW and
heating occurs simultaneously. This feature is only available if the Boil
SENS item is set to SUP.
DHW POST PURGE
After the DHW demand is removed, the 364 performs a purge on the
boiler. This feature is only available if the Boil SENS item is set to SUP.
The 364 shuts off the boiler and continues to operate either the DHW
pump or the DHW valve and the primary pump (P1). This purges the
residual heat from the boiler into the DHW tank. The 364 continues
this purge for a maximum of four minutes or until the boiler supply tem-
perature drops 20°F (11°C) below the DHW Boil TRG temperature.
The 364 also stops the purge if the boiler supply temperature drops
below the current Boil TRG temperature.
DHW MIXING PURGE
After DHW operation, the boiler is extremely hot. At the same time,
the heating zones may have cooled off considerably after being off
for a period of time. To avoid thermally shocking the boiler after DHW
priority, the 364 shuts off the boiler, but continues to operate the DHW
while restarting the heating system. This allows some of the DHW
return water to mix with the cool return water from the zones and
temper the boiler return water.
If DHW is to be incorporated into a low temperature system such as a radiant heating system, a mixing device is often installed to
isolate the DHW supply temperature from the lower system supply temperature. If a mixing device is not installed, high temperature
water could be supplied to the low temperature system while trying to satisfy the DHW demand. This may result in damage to the low
temperature heating system. The 364 is capable of providing DHW in such a system while ensuring that the low temperature in the
heating system does not exceed its allowed maximum setting.
To prevent high temperature water from being introduced into the heating system, the boiler system pump (P2) must be turned off
during a call for DHW. To do this, the DHW PRI item must be set to B + M. If the Boil SENS item has been set to SUP, the Boil MIN
item must be set to OFF.
Note: This feature is only available if the Boil SENS item is set to SUP. This feature is not available if the Boil SENS item is set to
RET or NONE.
On a call for DHW, the 364 provides DHW priority by backing off the mixing device and by shutting off the boiler system pump (P2)
for a period of time. This time is based on the outdoor air temperature as described in the DHW Priority Override section. If the DHW
demand is not satisfied within the allotted time, the boiler shuts off and the boiler’s heat is purged into the DHW tank.
Section C3: Low Temperature Boilers
Increasing Air Temperature
Increasing Time
DHW priority demand time limit
Outdoor Air Temperature
DHW Priority OverrideDHW Priority Override