System Design Load (SDL)

How Does It Work?

System-based design is a technique which considers specific HVAC system features when performing load estimating and system sizing calculations. This is important because many systems have unique features which require special sizing procedures. For example, consider Single Zone CAV, VAV, and Fan Powered Mixing Box systems. All contain different components and operate in different ways. To provide high quality system sizing data, the special features of each must be considered when sizing the system.

In the past, HVAC load estimating programs calculated room loads and airflow rates, and system airflow rates and coil loads without considering system specifics. This data is useful for general studies and for preliminary design when the system type is not known. System Design Load still offers these capabilities, but also provides the option of going a step further to tailor system sizing data to specific air handling designs and controls.

In System Design Load, general load calculation and system-based design start with the basics. Access to design weather data for thousands of cities is at your fingertips. The ability to define specific wall, roof, and window constructions. Features for defining building design, layout, and operation in as much or as little detail as necessary. The process then continues by allowing the user to specify the type of system and controls used to deliver cooling and heating to the building. As much or as little detail as desired can be used. Sizing procedures then are tailored to the system choice to provide high-quality output listings for room loads and airflow rates, system coil loads and fan airflow rates, and component load details.

System Design Load Features

• System Design Load v6 offers the same 3D building modeling features as HAP v6. Please see the HAP v6 web page for more extensive discussion of those features.
• System-based design loads for sizing and selecting system fans, central cooling and heating coils, air terminal equipment, space heating coils, preheat coils, and central chiller.
• Uses ASHRAE® Heat Balance cooling load calculation procedure and ASHRAE clear sky solar algorithms.
• Uses ASHRAE procedure for peak heating load calculations.
• Calculates outdoor ventilation airflow rates per ASHRAE Standards 62.1 (2004 though 2019 editions).
• Permits scheduling of cooling and heating equipment operation from 1 to 24 hours per day.
• Performs complete psychrometric analysis with conditions adjusted for altitude.
• Calculates zone and coil loads for 12 monthly design cooling days, and 24 hours per day.
• Searches for maximum zone and cooling coil loads.
• Analyzes sloped roofs and skylights.
• Analyzes building orientations at any of 16 principal exposures.
• Analyzes external shading due to overhangs, reveals, and fins.
• Analyzes effects of thermostat throttling range on zone cooling loads.
• Analyzes morning pulldown loads.
• Analyzes return plenum loads.
• Analyzes the impact of duct leakage.
• Computes supply and return fan brake horsepower and heat gain.
• Analyzes draw-thru and blow-thru fan arrangements.
• Analyzes dehumidification controls, economizers, exhaust heat reclaim equipment and humidifiers.
• Permits hourly scheduling of lights, occupancy, electric appliances, and other miscellaneous sensible and latent heat gains or losses.
• Calculates true zone relative humidity.
• Design weather for over 7000 cities worldwide.
• Analyzes effects of common interior shading devices in reducing solar and transmission loads through windows.
• Analyzes effects of common window framing in reducing solar loads and heat transmission through windows.
• Provides data for common wall and roof constructions. Permits entry of other wall, roof construction data.
• Provides data for common windows. Permits entry of other window performance data.
• Provides global editing features.
• Permits rotation of building easily.
• Outputs can be directed to a file for use in word processors, spreadsheets, etc.
• Models common air conditioning systems including: constant volume, VAV, variable refrigerant flow (VRF), induction, mixing box, VVT, fan coils, PTACs, water-source heat pumps, ground source heat pump systems, induction beams, active chilled beams.
• Offers part-load performance models for common fans such as: forward-curved, backward-inclined, airfoil, and vane-axial. Part-load performance data for other types of fans may be entered.
• Permits hourly scheduling of thermostats, cooling and heating equipment, and fans. Permits seasonal scheduling of cooling and heating equipment.
• Extensive use of presentation-quality graphical output.