Heating & Cooling Load Analysis

This section explains how xStar estimates heating and cooling loads.

Overview

xStar currently performs a heating and cooling load analysis using a simplified balance point method. This will be expanded in the near future to a full hourly simulation.

Run an accredited hourly simulation for full confidence in results. Our analysis is still undergoing testing and is not yet accredited software.

Heating and cooling degree day analysis

The analysis works by calculating the balance point for the building based on the thermal envelope’s UA (minus floors with ground contact), average infiltration rate, solar heat gains, and average internal gains (people + plug loads + lighting). The balance point is the exterior temperature at which the building heat loss equals the heat gain. The difference between the balance point and the interior setpoint temperature determines how much heat must be added (or removed) by the heating (cooling) system to achieve thermal equilibrium at the setpoint temperature.

Note that this is a heating and cooling energy load calculation. The actual energy use of the building will be higher due to the need to supply energy for internal gains (e.g. equipment & appliances) and reduced efficiencies of realistic HVAC systems, etc.

Current Limitations

Please be aware of these technical constraints:

• Ground heat transfer is ignored.

• Latent heat gains are ignored.

• Internal gains are modeled using REScheck’s internal gains model which scales with occupancy.

• Average monthly data is used.

• Balance point method is known to be less accurate than an hourly simulation.