©
2012 274_D
-
11/12
12
of
44
DHW Priority Override Setting in Adjust Menu
DHW Priority Override applies to DHW MODE 2 and 4, as
well as Mode 5 if there is a tN4 device with DHW. It prevents
the building from cooling off too much or the possibility of
a potential freeze up during DHW priority.
When set to auto, the priority time is calculated based
on outdoor temperature. At or below the design outdoor
temperature, 15 minutes are allowed for DHW priority. At
or above 70°F, 2 hours are allowed for DHW priority. The
time allowed for DHW priority varies linearly between the
above two points. There is a manual setting also available
in the adjust menu.
The priority timer does not start timing until priority is selected
and both a DHW Demand and a Boiler Demand exist together.
Once the allowed time for priority has elapsed, the control
overrides the DHW priority and resumes space heating.
Automatic Priority Override
15 mins
2 hours
Design Temperature
70°F (21°C)
Conditional DHW Priority
If the boiler supply temperature is maintained at or above
the required temperature during DHW generation, this
indicates that the boilers have 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. The control shuts off the boilers and continues to
operate the DHW Pump and the primary pump if applicable.
This purges the residual heat from the boilers into the
DHW tank. The control continues this purge until one of
the following occurs:
1. A Boiler Demand is detected
2. The boiler supply drops 20°F (11°C) below the DHW
target temperature
3. The DHW tank temperature rises above the DHW
setpoint plus 1/2 DHW Differential
4. Two minutes elapse
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 with Low Temperature Boilers
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 following must be true:
tN4 Present
DHW MODE 2 or 4
Boil MIN 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 contact
shuts 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.
DHW Boilers Setting in Adjust Menu
Select the number of boilers to use for DHW generation.
•
•
•