Waterside HVAC assumptions for early-stage models

Commercial buildings often include a heating hot water system, chilled water system or condenser water system.

Heating hot water system

When defining a heating hot water system for a simple box model there are several decisions to make: heating source type and efficiency, loop pressure loss, flow control and temperature control.

The most common heat source is a natural gas boiler, but electric heat pump boilers are becoming more common. Heat pumps may be air-source or water-source. Efficiency for a gas boiler is typically about 80% except for condensing boilers operating in condensing mode, which have efficiencies above 90%. If modeling a condensing boiler, then make sure to understand how its performance is represented in your simulation tool; condensing boilers operate at higher efficiency only when the return hot water temperature is low enough for condensation to occur, and the rated efficiencies are based on an 80°F return temperature. Electric heat pump boiler efficiency varies with hot water supply temperature and with outdoor air temperature (for air-source units) or with evaporator entering temperature (for water-source units). Heat pump boilers are typically designed to provide lower hot water temperature than gas boilers.

The pressure loss in a hot water loop will vary by system size and layout. The baseline system for ASHRAE Standard 90.1-2019 Appendix G has hot water loop pump power of 19 W/gpm, which is equal to pressure loss of 70 to 75 ft w.c.

Typical choices for hot water loop control inputs include variable flow control with variable speed pumps and hot water supply temperature reset control based on outdoor air temperature.  

Chilled water system

A primary choice when modeling a chilled water system is whether to choose air-cooled or water-cooled chillers. Smaller systems are more likely to use air-cooled chillers due to lower cost and complexity. Water-cooled chillers are more common in central plants of 100 to 200 tons or larger capacity, which corresponds to building floor area in the range of 50,000 to 100,000 ft2 and larger.

A second choice is the type of compressor used in the chiller, because different compressors have different performance characteristics. ASHRAE Standard 90.1-2019 Appendix G calls for the baseline building model to use one water-cooled screw chiller for capacity up to 300 tons, two water-cooled screw chillers for capacity between 300 and 600 tons, and two or more centrifugal chillers for plants larger than 600 tons.

Part-load efficiency of a chiller is affected by the type of unloading control. Variable-speed compressor control is usually the most efficient option and is a common choice. Most simulation programs represent chiller performance with a set of polynomial curves, with different coefficients to represent different types of compressors and unloading control. Pay attention to the chiller inputs to ensure that an appropriate set of curves is used.

Chiller efficiency inputs also warrant attention. A chiller efficiency value is valid for a specific set of operating conditions. Efficiency values listed in energy codes are specified at standard rating conditions. Manufacturers may provide efficiency values at rating conditions or at other conditions. Simulation programs typically expect efficiency inputs at rating conditions, but sometimes accept additional inputs to describe the conditions that correspond to the rating values. The important point is to make sure that efficiency values are entered properly in the simulation tool.

Select Chiller Efficiencies from ASHRAE Standards 90.1-2019

Type	Cooling Capacity	90.1-2019
Air-cooled chiller	Any	10.1 EER
Water-cooled scroll or screw	< 75 tons	0.75 kW/ton
	≥ 75 & < 150 tons	0.72 kW/ton
	≥ 150 & < 300 tons	0.66 kW/ton
	≥ 300 & < 600 tons	0.61 kW/ton
	≥ 600 tons	0.56  kW/ton
	< 300 tons	0.61 kW/ton
Water-cooled centrifugal	Any	0.56 kW/ton

Chilled water temperature setpoint affects chiller performance. Chillers operate less efficiently when producing colder water. Typical design setpoints are in the 40 to 45°F range. Some system types, such as radiant cooling systems or active chilled beams, can operate with higher chilled water temperature, so attention to the chilled water setpoint is important if the simple box model is being used to compare different HVAC system types.

Most chilled water systems have either primary-only or primary-secondary piping configurations. Primary-only systems can be more efficient if they have variable flow controls because they avoid constant flow primary pumps in a typical primary-secondary system. ASHRAE Standard 90.1-2019 Appendix G describes baseline inputs for a primary-secondary system based on chilled water plant capacity.

Condenser water system

Water cooled chillers typically reject their heat to a condenser loop. Other system types, such as water-source heat pumps, also have a condenser water system. In most cases, heat in the condenser loop is rejected by a cooling tower. In some cases, more often for water-source heat pumps, heat is rejected via a ground loop.

The common choice for cooling tower type is an axial (propellor) fan unit with variable-speed fan control. Condenser water flow may also vary, but is commonly constant flow except for plants with multiple chillers when the condenser water pumps stage with the chillers. ASHRAE Standard 90.1-2019 Appendix G provides a reasonable set of efficiency and controls assumptions for cooling towers and condenser water loops.