SECTION E: DOMESTIC HOT WATER OPERA- TION
SECTION E1: DOMESTIC HOT WATER (DHW)
DHW DEMAND
A DHW Demand is required for the control to provide heat to the DHW system. A DHW aquastat or set point control is used as a switch in the DHW demand circuit. Once the con- trol 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 Setp / DHW and Com Dem terminals (8 and 7).
BOILER TARGET DURING DHW GENERATION
The boiler target temperature is at least as hot as the DHW exchange setting (DHW XCHG). The DHW demand over- rides the boiler reset target temperature, except when the boiler reset target is higher than that of the DHW exchange setting.
DHW MODE & PRIORITY OPERATION
The control has five 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 have a priority for the DHW allowing for quick recovery of the DHW tank temperature. This is achieved by limiting or even stopping the flow of heat to the heating system when the DHW tank calls for heat see Figure 18.
DHW MODE OFF - NO DHW
The DHW feature is not selected. This allows for Set Point operation as described in section F.
DHW MODE 1 - DHW IN PARALLEL NO PRIORITY
When a DHW Demand is present, the Relay 9 / DHW con- tact (terminals 30 and 31) closes and the primary pump con- tact is opened. It is assumed that the DHW pump will provide adequate flow through the heat exchanger and the boiler.
FIG. 18
DHW MODE 2 - DHW IN PARALLEL NO PRIORITY
When a DHW Demand is present, the Relay 9 / DHW con- tact (terminals 30 and 31) closes with the DHW demand. The primary pump does not turn on, but may operate based on a Boiler Demand or External Input Signal. Refer to sec- tions E and G. It is assumed that the DHW pump will pro- vide 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 Relay 9 / DHW con- tact (terminals 30 and 31) is closed and the primary pump contact is closed. This mode can be used if a DHW tank is piped in direct return and a DHW valve is installed.
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