HVAC Systems and Title 24
The typical new California home in the central valley and the desert has a gas furnace and a split system air conditioner. In some areas, a heat pump provides both heating and cooling, eliminating the furnace. In coastal climates and in the mountains, air conditioning is rare and most new homes are heated by gas furnaces.
Heating and cooling is typically distributed to each of the rooms through air ducts. Most of the Title 24 mandatory measures and prescriptive requirements are based on this type of system. Although the Title 24 standards focus on the typical system, they also apply to other systems as well, including hydronic systems where hot water is distributed to provide at least some of the heat to conditioned space; in contrast with ducted systems that distribute heated air to heat the space.
Electric resistance systems are also used in some areas and applications, although it is difficult for them to comply under the Title 24 standards. Ground-source heat pump (geo-exchange) systems are also used, especially in areas where there is no gas service.
The Title 24 residential standards do not set limits on the sizing of heating equipment, but they do require that heating loads be calculated for new heating systems. Oversized equipment typically operates less efficiently and can create comfort problems due to excessive cycling and high airflow. Just as for heating equipment, the Title 24 standards do not set limits on the size of cooling equipment, but they do require that cooling loads be calculated for new cooling systems. Avoiding oversizing is especially important for cooling equipment because efficiency degrades when the system cycles on and off frequently.
Air Distribution Ducts and Plenums
Air distribution system performance can have a big impact on overall HVAC system efficiency. Therefore, air distribution systems face a number of mandatory measures and prescriptive requirements. The mandatory measures require air distribution ducts be sealed and HERS tested for leakage in all climate zones even when the performance method of compliance is used. This is a big change from the 2008 Title 24 standards where duct testing was a prescriptive requirement that could be waived by using the performance method of compliance. There are also a number of compliance credits available related to duct system design.
In all cases, unless ducts are enclosed entirely in conditioned space, the minimum allowed duct insulation value is R-6 in most climate zones and R-8 in zones 11 and 14-16.
Automatic setback thermostats can add both comfort and convenience to a home. Occupants can wake up to a warm house in the winter and come home to a cool house in the summer without using unnecessary energy. A setback thermostat is always required for central systems whether the prescriptive or performance compliance method is used. An exception is allowed only if the building complies using the performance method with a non-setback thermostat and the system is one of the following non-central types:
- Non-central electric heaters
- Room air conditioners
- Room air conditioner heat pumps
- Gravity gas wall, floor, or room heaters
- Wood stoves
- Fireplace or decorative gas appliances
When it is required, the setback thermostat must have a clock or other mechanism that allows the building occupant to schedule the heating and/or cooling set points for at least four periods over 24 hours. If more than one piece of heating equipment is installed in a residence or dwelling unit, the setback requirement may be met by controlling all heating units by one thermostat or by controlling each unit with a separate thermostat. Separate heating units may be provided with a separate on/off control capable of overriding the thermostat. Note that thermostats for heat pumps must be “smart thermostats” that minimize the use of supplementary electric resistance heating during startup and recovery from setback.
An energy compliance credit is provided for zoned heating and air-conditioning systems, which save energy by providing selective conditioning for only the occupied areas of a house. A house having at least two zones (living and sleeping) may qualify for this compliance credit. The equipment may consist of one air-conditioning system for the living areas and another system for sleeping areas or a single system with zoning capabilities, set to turn off the sleeping areas in the daytime and the living area unit at night.
(Excerpted from the CEC Title 24 Residential Compliance Manual)