tekmar 423 Universal Reset Module - Manual (Page 27)

27 of 36 © 2007 D 423 - 08/07
DHW
Pump
Primary
Pump
OFF
DHW Priority Override Time
Increasing Time
Decreasing Outdoor Air Temperature
DHW Priority Override
DHW Priority Override applies to DHW MODE 2 and 4.
To prevent the building from cooling off too much or the
possibility of a potential freeze up during DHW priority, the
control limits the amount of time for DHW priority. As the
outdoor air temperature becomes colder, the length of time
that the control provides DHW priority is reduced. Once the
allowed time for priority has elapsed, the control overrides
the DHW priority and resumes space heating.
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 the target, DHW and heating occurs
simultaneously.
DHW Post Purge
After the DHW Demand is removed, the control performs
a purge on the boiler. The control shuts off the boiler and
continues to operate the DHW Pump and DHW Valve and
the primary pump if applicable. This purges the residual heat
from the boiler into the DHW tank. The control continues
this purge for a maximum of four minutes or until the boiler
supply water temperature drops 20°F (-6.5°C) below the
boiler target temperature during the DHW operation. The
control also stops the purge if the boiler supply temperature
drops below the current boiler target 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. When restarting the
heating system after a DHW Demand with priority, the
control shuts off the boiler and continues to operate the
DHW Pump and DHW Valve while the primary pump is
turned on. This allows some of the DHW return water to
mix with the cool return water from the zones and temper
the boiler return water.
DHW with Low Temperature Boilers Section I
If DHW heating is to be incorporated into a low temperature
system such as a radiant floor heating system, a mixing
device is often installed to isolate the high DHW supply
temperature from the lower system 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 control is capable of providing DHW heating in such a
system while minimizing the chance that the temperature
in the heating system exceeds the design supply water
temperature. In order to do this, the control must be set
to DHW MODE 2 or DHW MODE 4 and Boil MIN must be
set to OFF.
On a call for DHW, the control provides DHW priority by
sending a message on the boiler temperature bus to the
tN4 thermostats to shut off the heating zones for a period
of time. The length of time is based on the outdoor air
temperature as described in the DHW Priority Override
section. However, if the DHW Demand is not satisfied within
the allotted time, the boiler shuts off and the heat of the
boiler is purged into the DHW tank. A DHW mixing purge
occurs in order to reduce the boiler water temperature and
once the boiler supply temperature is sufficiently reduced,
the DHW Pump and DHW Valve contacts shut off. The
heating system zones are allowed to turn on for a period
of time to prevent the building from cooling off. After a
period of heating, and if the DHW Demand is still present,
the control shuts off the heating system and provides heat
to the DHW tank once again.