Watts tekmar 294 - DHW Operation - Priority and Purge; DHW Priority Settings; DHW Post Purge Function; DHW Boiler Limiting

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IOM-T-294 2205 © 2022 tekmar
DHW Priority
The Indirect DHW Priority setting selects priority over the cen-
tral heating system. This allows for quick recovery of the indi-
rect DHW tank.
Off
DHW priority is not provided. The system pump operates
when a Central Heating call is present.
On
When there are simultaneous DHW and central heating calls,
the system pump shuts off to provide priority to the DHW tank.
When there are simultaneous DHW and tekmarNet calls, the
system pump can continue to operate. If the boilers are unable
to maintain the boiler target temperature, the tekmarNet zones
are sequentially shut off using tekmarNet communication to
provide priority. The sequence order shuts off the thermostat
with address 24 first and address 1 last.
Priority Override
The control includes an Indirect DHW Priority Override to
prevent the building from cooling off too much or a potential
freezing condition in the event of a faulty DHW aquastat or
sensor. The time is adjustable from 20 to 240 minutes. Once
the allowed time for priority has elapsed, the control overrides
the DHW priority and resumes central heating. DHW Priority
does not resume until the central heating or DHW calls is
removed and then reapplied.
Conditional DHW Priority
If the boiler supply temperature is maintained at or above the
boiler target temperature during DHW operation, the DHW and
central heating occurs simultaneously.
DHW Post Purge
After the DHW call is removed, the control performs a post
purge. The control shuts off the boilers and continues to oper-
ate the DHW pump and the system pump to purge residual
heat from the boilers into the DHW tank. The control continues
this purge until one of the following conditions is met:
1. A heat call is detected.
2. The boiler supply drops 20°F (11.0°C) below the DHW
target temperature.
3. The DHW tank temperature rises above the DHW setpoint
plus ½ DHW Differential.
4. Two minutes elapse.
DHW Mixing Purge
After DHW operation, the boilers may be extremely hot. At the
same time, the heating zones may have cooled off consider-
ably after being off for a period of time. When restarting the
heating system after a DHW Call with priority, the control shuts
off the boiler and continues to operate the DHW pump while
the system 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 Radiant Floor Heating
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 call. 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 conditions must be met:
• All available boilers are set to condensing.
• DHW Type set to System.
• Indirect DHW Priority set to On.
• tekmarNet system is present (If system pump during DHW
operation is set to On)
On a DHW call, the control provides DHW priority by sending
a message on the boiler temperature bus to the tekmarNet
thermostats to shut off the heating zones for a period of time.
The length of time is based on the outdoor air temperature, or
selectable time, as described in the DHW Priority Override sec-
tion. However, if the DHW call 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 call is still present, the con-
trol shuts off the heating system and provides heat to the DHW
tank once again.
DHW Boiler Limiting
The Indirect DHW Boiler Limiting settings restricts the number
of boilers available for DHW heating to prevent unwanted boiler
short cycling. This only applies when there is a DHW Call. If there
is a Central Heating, Constant Setpoint or Remote Setpoint Call,
the boiler limit is removed and all boilers are available.
DHW Type Side Arm Operation
The indirect DHW tank is piped in parallel within the near
boiler piping of boiler 1. A DHW tank aquastat is used to cre-
ate the DHW call, and a DHW sensor is used to regulate the
boiler heat exchange water temperature, in order to prevent
the boiler from tripping on it’s internal maximum temperature
limit. When a DHW call is present, the DHW pump turns on
and boiler pump 1 turns off. This piping configuration provides
simultaneous central heating and DHW tank heating with the
DHW tank having the full capacity of boiler 1 heating output.
Once the DHW tank call is satisfied, boiler 1 is shut off and
there is a 2 minute DHW pump post purge, after which boiler 1
is available to resume central heating if required.
Side Arm