If optimum start is enabled, the heating starts earlier as required by the time schedule to reach the setpoint at the beginning of the occupied period.
The time between the actual start of heating and the defined switching point in the time program is called pre-heat time.
During un-occupied periods the pre-heat time will be calculated regularly. The pre-heat time is calculated using building models. The calculated pre-heat time depends on the outside air temperature, room temperature, the maximum permissible mixed water temperature and by predicting the response of the building according to the building models.
Optimum Start can happen only if all the following conditions are true:
Optimum Stop can happen only if all the following conditions are true:
Fig. 15. Optimum start/ stop
Optimum start without room temperature sensor
During preheat time, the mixed water temperature setpoint is calculated using the heating curve equation. The room temperature setpoint is equal to the required “RoomTempSp” (next occupied switch point) + PREH-03.
CalcMWTempSp = HC (next RoomTempSp + PREH-03, FilteredOATemp, HCA-01, HCA-02)
HC – means heating curve equation
The preheat start point is a function of the outdoor air temperature.
tVV = PREH-02 * (1 – FilteredOATemp/ RoomTempSp)
The pre-heating starts at = time of the next RoomTempSp - tVV
At an outdoor air temperature of 0 °C, the preheat time starts 2 hours (PREH-02) before reaching the corresponding switch point in the Time Schedule.
If the outdoor air temperature is equal to the room temperature setpoint the preheat point is not advanced.
The value of tVV is limited to a maximum value of 48 hours.
Fig. 16. Pre-start time without room temperature sensor
Fig. 17. Optimum start without room temperature sensor
Table 18. Parameter List
Description |
Value Range |
Default Value |
Unit |
PREH-02: pre-start time at OAT = 0°C. (if RMT = 999) |
0…1440 |
120 |
min |
PREH-03: pre-start constant (if RMT = 999) |
0…20 |
10 |
K |
Optimum start with Room Temperature Sensor
The pre-heat time calculation starts as soon as the room temperature setpoint from a Time Schedule changes from a higher to a lower value. The calculated pre-heat time depends on the outside air temperature, room temperature, and the boost factor PREH-04 and by predicting the response of the building by using the building model.
If the calculated pre-heat time is longer than the minimum pre-heat time, defined by the setting PREH-01, then time-variable pre-heating is active. If the calculated pre-heat time is smaller than the minimum pre-heat time, defined by the setting PREH-01, then temperature-variable pre-heating is active. The maximum pre-heat time is 48 hours.
If the value of the minimum pre-heat time is not bigger than the time till the next higher room temperature setpoint, the heating circuit starts with time-variable pre-heating.
Time variable Pre-heating with Room Temperature Sensor
During the setback period, the outdoor temperature, the current room temperature, and the boost temperature are used to decide the time at which to begin the pre-heating phase. If the calculated pre-heat time according to the building model would start earlier than the minimum pre-heat time, time variable pre-heat is used.
Pre-heating starts in this case with the boost temperature and is limited by the maximum mixed water temperature as defined in MWT-03.
In order not to boost always with the maximum mixed water temperature MWT-03, we will boost with the value defined in the heating curve using the next room temperature setpoint – see equation below: The user can influence this boost temperature with the parameter PREH-04.
T2boost = HC (next RoomTempSp + PREH-04 * (next RoomTempSp - actual RoomTempSp), FilteredOATemp, HCA-01, HCA-02)
First the heating curve equation is used to calculate the asymptotic room temperature limit T1lim (using the heating curve).
T2boost = HC (T1lim, FilteredOATemp, HCA-01, HCA-02)
During the preheating time the mixed water temperature setpoint is repeatedly calculated from the heating curve to allow for variations in the outdoor temperature. The estimate of T1lim remains unchanged from the value calculated at the beginning of the pre-heat.
If the room temperature setpoint is reached earlier than the target point in time, room temperature control takes over.
If 30 minutes before the target point in time is reached the room temperature deviation is more than 0.5 K, then room temperature control takes over.
After the target point in time is reached, the room temperature control is enabled for another 30 minutes. The room temperature control is meant to compensate for the cooling effects caused by walls and furniture after boost heating so that after switch over to weather responsive control the room temperature remains nearly constant.
Fig. 18. Time variable pre-heating
Table 19. Parameter List
Description |
Value Range |
Default Value |
Unit |
0…1440 |
120 |
min |
|
PREH-04: boost factor |
0..30 |
1 |
- |
RCTR-01: P-band room control |
0..50 |
10 |
K |
RCTR-02: I-time room control |
0...7200 |
300 |
sec |
1…60 |
5 |
min |
|
30…1440 |
300 |
min |
|
1…60 |
5 |
min |
|
30…1440 |
600 |
min |
|
- |
- |
hhmm |
|
DISP-07: end time optimum start |
- |
- |
hhmm |
- |
- |
hhmm |
Temperature variable Pre-heating with Room Temperature Sensor
If the time variable preheat has not started when the minimum preheat time is reached, then temperature variable preheating begins immediately.
Example: Calculated pre-heat time = 60 min, minimum pre-heat time = 120 min, switch-on point in time program 8:00 => actual switch on: 6:00.
Since for temperature-variable preheating, the pre-heat time is fixed, the required mixed water temperature can be calculated by first finding the required room temperature limit (asymptotic) T1lim using the building model defined by the parameters BMOD-01 to BMOD-04.
Then the required mixed water temperature setpoint can be calculated from the heating curve equation.
CalcMWTempSp= HC (T1lim, FilteredOATemp, HCA-01, HCA-02)
The maximum permissible mixed water temperature demand is limited by MWT-03 maximum mixed water temperature.
Fig. 19. Optimum start - temperature variable pre-heating
Table 20. Parameter List
Description |
Value Range |
Default Value |
Unit |
0…1440 |
120 |
min |
|
PREH-04: boost factor |
0..30 |
1 |
- |
RCTR-01: P-band room control |
0..50 |
10 |
K |
RCTR-02: I-time room control |
0...7200 |
300 |
sec |
1…60 |
5 |
min |
|
30…1440 |
300 |
min |
|
1…60 |
5 |
min |
|
30…1440 |
600 |
min |
|
- |
- |
hhmm |
|
DISP-07: end time optimum start |
- |
- |
hhmm |
DISP-08: date last optimum start |
- |
- |
hhmm |
Room Temperature Control
If the room temperature reaches the room temperature setpoint defined in the time program before the start of the occupied period, room control takes over. After the target point in time is reached, room control is active during the occupied period. This room temperature control compensates for cool-down effects caused by walls and furniture after pre-heating.