Domestic Hot Water Priority

When the buffer temperature of the domestic hot water system (DHWS) drops below a certain limit, the DHWS can request the heating circuits to close their valves in order to have the maximum capacity available to warm-up the DHWS. If very large domestic hot water storage tanks are installed, the duration of this request can be very long. The maximum duration of this DHWS request can be limited in the heating circuit, so that room conditions are not disturbed by this function.

When fast heat exchangers are used in the application, fluctuations in temperature and mixed water may occur and cause unstable behavior of the system supply temperature. Fluctuations in temperature and mixed water occur during the DHWS priority function. To minimize these fluctuations, an additional ramp down of the mixed water temperature setpoint can be configured as soon as the DHWS priority function starts.

The different behaviors can be set using the parameter SYS-03 mode DHWS priority function.

The next diagram shows the mixed water temperature setpoint during DHWS priority function. If the DHWS priority occurs the water temperature setpoint will be decreased to a minimum value, defined by the parameter MWT-02 minimum mixed water temperature. When the DHWS priority signal returns to zero, the water temperature setpoint will be ramped up to its normal setpoint with a ramp function defined by the parameter MWT-04: setpoint raise per minute.

Fig. 24.   DHW priority

Table 25.   Parameter List

Description

Value Range

Default Value

Unit

MWT-02: minimum mixed water temperature

0…150

0

°C

MWT-04: setpoint raise per minute

1…2000

25

K/min

SYS-03: mode DHWS priority function;

0 = disable

1 = enabled

2 = enabled, maximum duration defined by SYS-05

3 = enabled, down ramp enabled

4 = enabled, down ramp enabled + maximum duration defined by SYS-05

0…4

2


SYS-04: MWT setpoint decrease time

0...3600

600

sec

SYS-05: max. duration DHWS priority function

0…18000

3600

sec