tekmar 374 Universal Reset Control - Manual (Page 10)

© 2009 D 374 - 03/09 10 of 36
Section C1
Domestic Hot
Water (DHW)
Section C2
DHW with Low
Temperature
Boilers
Section C: DHW Operation
Section C1: Domestic Hot Water (DHW)
DHW DEMAND
A DHW Demand is required in order for the control to provide heat to the DHW system. A
DHW aquastat or setpoint control is used as a switch in the DHW demand circuit. Once
the control detects a DHW demand, the
DHW Demand pointer turns on in the LCD and the
control operates the boiler to provide a sufficient boiler supply water temperature to the
DHW tank. The control operates the pumps as described below.
The control registers a DHW Demand when a voltage between 24 and 230 V (ac) is applied
across the DHW Dem and Com Dem terminals (29 and 31).
BOILER TARGET DURING DHW GENERATION
The boiler target temperature is at least as hot as the DHW exchange setting (DHW XCHG). The DHW demand overrides the boiler
reset target temperature, except when the boiler reset target is higher than the DHW exchange setting.
DHW MODE AND PRIORITY OPERATION
The control has four different settings available for DHW MODE. The required DHW MODE setting will depend on the piping
arrangement of the DHW tank. It is often desirable to limit or even stop the flow of heat to the heating system when the DHW tank
calls for heat. This allows for a faster recovery of the DHW tank.
DHW MODE 1 - DHW in Parallel no Priority
When a DHW Demand is present, the DHW Pmp / Vlv contact closes.
The primary pump ( Prim P1) does not turn on, but may operate based
on either a Boiler Demand, Mixing Demand or a Setpoint Demand.
Refer to sections B and D.
It is assumed that the DHW pump will provide adequate flow
through the heat exchanger and the boiler.
DHW MODE 2 - DHW in Parallel with Priority
When a DHW Demand is present, the DHW Pmp / Vlv contact closes
and the primary pump ( Prim P1) contact is opened.
It is assumed that the DHW pump will provide adequate flow
through the heat exchanger and the boiler.
DHW MODE 3 - DHW in Primary / Secondary no Priority
When a DHW Demand is present, the DHW Pmp / Vlv contact is
closed and the primary pump ( Prim P1) is operated.
This mode can be used if a DHW tank is piped in direct return and
a DHW valve is installed.
30
Boil
Dem
31
Com
Dem
29
DHW
Dem
24 to 230 V (ac)
Aquastat
DHW Pump
Primary
Pump
DHW Pump
Primary
Pump
DHW Pump
Primary
Pump