RTHLD: Thermal room load analysis

Block RTHLD (FB861) calculates the thermal room load condition based on part heat and refrigeration demand signals as well as past minimum and maximum room temperatures.

Application: The determined thermal room load condition (load, unload) is used in particular for blinds control in the room to passively use solar heat or to protect against solar heat (e.g. Chart {RThLd01}). RTHLD_SiUn (FB861), RTHLD_UsUn (FB799)

 

Principle of operation

Step

Process

1

The last heat state [HCSta] = Heating is evaluated.

  • Time [TiEldHeat] since last [HCSta] = Heating is determined.
  • heating demand during the past time period [TiPrd] is calculated:
    If [TiEldHeat] < [TiPrd], then a heating demand was/is present.

The last cooling state [HCSta] = Cooling is evaluated.

  • Time [TiEldCool] since last [HCSta] = Cooling is determined.
  • Cooling demand during the past time period [TiPrd] is calculated:
    If [TiEldCool] < [TiPrd], then a cooling demand was/is present.

2

Deviation of the minimum room temperature [TRMin] to the heating setpoint for comfort [SpHCmf] is calculated during the past time period [TiPrd]:

  • Δϑr,H = [TRMin] – [SpHCmf]
  • Δϑr,H is positive, if [TRMin] during [TiPrd] was always greater than [SpHCmf].
    Δϑr,H is negative, if [TRMin] during [TiPrd] was less than [SpHCmf].

Deviation of the maximum room temperature [TRMax] to the cooling setpoint Comfort [SpCCmf] is calculated during the past time period [TiPrd]:

  • Δϑr,C = [SpCCmf] – [TRMax]
  • Δϑr,H is positive, if [TRMax] during [TiPrd] was always less than [SpCCmf].
    Δϑr,H is negative, if [TRMax] during [TiPrd] was less than [SpCCmf].

 

The thermal room load condition is calculated (see Function description).

The following values are outputted as the result:

  • thermal room load condition [RThLdCnd] (recommendation for supporting heating or cooling)
  • the expired time since the last heating [TiEldHeat] and cooling [TiEldCool] (for diagnostics purposes).

Function description of thermal room load condition

 

Heating demand

The thermal room load condition is set to load if only one heating demand (without cooling demand) existed within [TiPrd]. Even if no heating demand currently exists, the function assumes that a heating demand will occur in the future (persistency forecast).
Systems such as blinds can thus support heating.

Cooling demand

The thermal room load condition is set to unload if only one cooling demand (without heating demand) existed within [TiPrd]. Even if no cooling demand currently exists, the function assumes that a cooling demand will occur in the future (persistency forecast).
Systems such as blinds can thus support cooling.

No heating demand, no cooling demand

Room temperature measurement determine the thermal room load condition if no heating demand and no cooling demand existed within [TiPrd]:
The thermal load condition is set to unload if the room temperature is in the upper range of the Comfort room temperature setpoint range (condition Δϑr,H ≥ Δϑr,C).
At Δϑr,H < Δϑr,C, the thermal room load condition is set to load.
Systems such as blinds can thus affect maintaining the room temperature in the middle of the Comfort room temperature setpoint range as much as possible.

heating demand AND cooling demand

The thermal room load condition is determined by last demand if both a heating demand AND a cooling demand existed within [TiPrd]:

 

Example: Using blinds control to implement thermal load condition (not a component of the function).

If thermal load state is equal to load, blinds take advantage of solar radiation during the day (i.e. blinds should be opened as much as possible) and prevent heat flow from the room to the environment during the night (i.e. blinds should be closed as much as possible).

If thermal room load state is equal to unload, blinds should prevent solar radiation during the day (i.e. blinds should be closed as much as possible) and permit heat flow from the room to the environment during the night (i.e. blinds should be opened as much as possible).

Pin description

Pin

Description

HCSta

Heating/cooling state

Neither nor

No (present) heating or cooling demand

Heating

(Present) heating demand

Cooling

(Present) cooling demand

TiPrd

Time periods
Past time periods that are evaluated.

SpCCmf

Cooling setpoint for comfort

SpHCmf

Heating setpoint for comfort

TRMax

Maximum room temperature
Maximum room temperature occurring during past time period [TiPrd].

TRMin

Minimum room temperature
Minimum room temperature occurring during the past time period [TiPrd].

RThLdCnd

Thermal room load condition

Load

The room has the thermal room load condition load:
The room is thermally loaded using free heating (e.g. blinds positions) or prevented from quickly discharging the room.

Unload

The room has the thermal room load condition unload:
The room is thermally unloaded using free cooling (e.g. blinds positions) or prevented from quickly loading the room.

TiEldHeat

Expired time heating
Time since last heating demand.

TiEldCool

Expired time cooling
Time since last cooling demand.

 

Input values

Pin

Description

Data type

Standard value

Engineering unit or text group

Min.

Max.

HCSta

Heating/cooling state

INT

1

Neither heating/cooling.

1

3

TiPrd

Time period

TIME

T#1D_0H_0M_0S_0MS

 

 

 

SpCCmf

Cooling setpoint for comfort

REAL

24.0

°C

10.0

35.0

75.0

°F

50.0

95.0

SpHCmf

Heating setpoint for comfort

REAL

21.0

°C

10.0

35.0

70.0

°F

50.0

95.0

TRMax

Maximum room temperature

REAL

22.5

°C

10.0

35.0

72.5

°F

50.0

95.0

TRMin

Minimum room temperature

REAL

22.5

°C

10.0

35.0

72.5

°F

50.0

95.0

Output values

Pin

Description

Data type

Standard value

Engineering unit or text group

Min.

Max.

RThLdCnd

Thermal room load condition

INT

2

Load, Unload

2

1

TiEldHeat

Expired time heating.

TIME

T#0D_0H_0M_0S_0MS

 

 

 

TiEldCool

Expired time cooling.

TIME

T#0D_0H_0M_0S_0MS

 

 

 

Engineering

Typical interconnections to other blocks

Pin

Description

HCSta

The present heating/cooling demand of the room is reported to the block via this input. Heating/cooling demand for the zone can be interconnected if heating and/or cooling is not based on individual room control.

TiPrd

This input is not typically interconnected. The setting value is usually left at the default value of 24 hours.

SpCCmf

The present, valid comfort room temperature setpoint for cooling is interconnected to this input.

SpHCmf

The present, valid comfort room temperature setpoint for heating is interconnected to this input.

TRMax

This input must be interconnected with the maximum room temperature of the past time period [TiPrd].

TRMin

This input must be interconnected with the minimum room temperature of the past time period [TiPrd].

RThLdCnd

This output is typically further interconnect to the control logic for the blinds.

TiEldHeat

Output is used for diagnostics and typically not interconnected.

TiEldCool

Output is used for diagnostics and typically not interconnected.

 

Use

The function is used in HVAC applications to support heating and cooling and their operation (at nearly) no cost. Such systems include blinds or heat recovery units in ventilation plants. These type of HVAC applications can also control active heating and cooling systems.

The function ensures for present heating and cooling demand that such systems support the present demand. Moreover, the function ensures that such systems operate in a supporting manner if no present heating and cooling demand exists.

Note: The function should not be used, if operating the supporting systems is less expensive than active heating or cooling, but not free of charge. Other calculations are generally required for such systems, e.g. free cooling using a cooling tower.

Process response

Standard (see General rules and information).

Troubleshooting

Standard (see General rules and information).