tekmar 420 Boiler Reset Module - Manual (Page 18)

© 2007 D 420 - 08/07 18 of 24
DHW MODE 2 - DHW in Parallel with Priority
When a DHW Demand is present, the DHW Pump contact
closes. The primary pump (P1) can operate when a boiler
demand is present. If the boiler is unable to maintain the
boiler target temperature, space heating zones are shut
off sequentially using tN4 communication in order to
provide priority to the DHW tank. This is known as zone
load shedding.
It is assumed that the DHW pump will provide adequate
flow through the heat exchanger and the boiler.
DHW MODE 4 - DHW in Primary / Secondary with
Priority
When a DHW Demand is present, the DHW Pump contact
is closed and the primary pump (P1) is operated. Priority
over space heating zones is achieved by shutting off the
zone pumps or zone valves through tN4 communication.
This is known as zone load shedding.
Mode = 2
DHW
Pump
Primary
Pump
OFF
Mode = 3
DHW
Pump
Primary
Pump
Mode = 4
OFF
DHW
Pump
Primary
Pump
DHW MODE 3 - DHW in Primary / Secondary no
Priority
When a DHW Demand is present, the DHW Pump contact
is closed and the primary pump (P1) is operated.
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.
Decreasing Outdoor Air Temperature
DHW Priority Override Time
Increasing Time
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 the primary pump
(P1) 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
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
Pump
Primary
Pump
OFF
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