Calculation Options - Heating Design

Heating Design tab on Model Options dialog and General tab on Heating Design dialog.

Calculation Options

Calculation description

Enter some text to identify the calculation. This will be used in reports and in the filename for automatically generated ESO files.

Temperature control

Heating and cooling systems control internal temperatures to meet the setpoint temperatures specified on the Activity tab. These setpoint temperatures can be interpreted as air, operative or some other radiant fraction and DesignBuilder provides corresponding options to allow HVAC systems to be controlled by:

• 1-Air temperature - control the zone mean air temperatures to the heating and cooling setpoint temperatures specified on the Activity tab.
• 2-Operative temperature - control the room temperature using 0.5 radiant fraction. See notes of operative temperature control below if you plan to use this option.
• 3-Other- you can enter the radiant temperature control fraction.

When using the 3-Other option the radiant fraction should be less than 0.9 and the minimum is 0.0. A value of 0.0 is the same as controlling on only zone air temperature. If air velocities are higher than 0.2 m/s, then lower values for radiative fraction might apply. Niu and Burnett (1998) cite International Standard ISO 77300 in recommending the values for this fraction listed in the following table.

Radiative Fraction vs Air Velocity

Note: The temperature control settings used in heating design and cooling design will also apply in simulation autosizing calculations for the winter and summer design days respectively.

Reference: J. Niu and J. Burnett. 1998. Integrating Radiant/Operative Temperature Controls into Building Energy Simulations. ASHRAE Transactions Vol. 104. Part 2. page 210. ASHRAE. Atlanta, GA.

You can think of the Temperature control option as:

• The radiant fraction sensed by the room thermostat, or,
• Given a room thermostat that mainly senses air temperature, the extent of automatic modification of the setpoint temperature to ensure occupant comfort.

Note: This option does not affect natural and mechanical ventilation setpoints - these always use air temperature set points.

Tip: See the equivalent topic for Simulation for an advanced discussion on Operative vs Air temperature control.

System Sizing

Design margin

The Heating Design margin  is used to multiply calculated steady state heating loads in each zone to give a recommended heating system size. It accounts for the additional heat required to bring the building up to temperature in a reasonably short preheat period and allows you to be confident that comfort conditions will be maintained in all but the most extreme winter conditions.

The default heating design margin is 1.25 which results in heating systems being oversized by 25% relative to the steady state load calculated by EnergyPlus.