Page 118 - Designing for Zero Carbon-Volume 2_Case Studies of All-Electric Multifamily Residential Buildings

CASE STUDY NO. 5
CASA ADELANTE AT 2060 FOLSOM
Heating, Ventilating and Cooling Systems
The heating and cooling of each apartment is carried out by an independent system for each unit, as opposed to a centralized system that serves multiple apartments. The temperate climate and the energy-efficient design of the building envelope (well-insulated walls and window shad- ing) minimize the cooling loads as well as the heating loads. As a result, there is no need for mechanical cooling via a heat pump system, which ordinarily would be used.
For heating, each unit has electric resistance heaters, which are located in ceiling coves.21 This was the least expensive option. In addition, electric resistance heaters are easily maintained and replaced at minimal cost. The ceiling location was chosen rather than floor-level baseboard in order to avoid potential blockage by furniture and the resultant inefficiency or potential damage. Furthermore, the high-quality building envelope in the marine climate of San Francisco mini- mized the normal thermal comfort advantage of floor-mounted heaters below windows.
Although mechanical cooling is not utilized, in order to ensure good indoor air quality and fresh air supply, each individual apartment is supplied with a Heat Recovery Ventilator (HRV) system22. The HRV system not only provides heat recovery during the heating season, but can be used for night ventilation cooling when needed. The night flush operation precools the concrete structure for the next day during rare times of peak weather events.
The HRV is either in “heat recovery mode” or “bypass mode”. It is always operating silently. Fresh air is always entering the unit through the HRV air intake. Normally, if heating is required, the heat recovery mode is operating and the windows would be closed. If neither heating nor cooling is required, windows can be opened if the electric resistance heaters are turned off.
When the kitchen fan or bathroom fan is turned on by the occupant, the HRV switches into by- pass mode and its internal exhaust shuts off. The kitchen or bathroom fan then becomes the exhaust route of the incoming fresh air. (The bathroom fan is always operating at a low exhaust level but the occupant only hears the bathroom fan when the bypass fan switch is engaged.)
Because each apartment has its own HRV system, the air exhausts only through the adjacent building façade. A large mechanical unit serving many apartments would require exhaust ducts to the roof, with fire dampers and an emergency generator to maintain the air exhaust. All this is avoided if the air system is contained in a single apartment, thereby saving extra costs and loss of floor space. Furthermore, using the individual HRV units means that the use of shared ductwork between apartments is avoided, which prevents a possible migration of insects and other pests.
Two conventional air-source heat pumps23 are used for the common spaces on the first two levels of the building. These are DOAS systems24 with Variable Refrigerant Flow (VRF)25. One system serves the ground-floor spaces plus the child-care center on the second floor. The sec- ond system serves the remaining tenant common spaces on the second floor.
21 King KCV1202 cove heater
22 Zehnder ComfoAir 350-R
23 Mitsubishi PUZ-A30NHA7
24 For a description of a DOAS system, see: https://en.wikipedia.org/wiki/Dedicated_outdoor_ air_system
25 VRF works well in heat pumps for heating and cooling in buildings. See: https://en.wikipedia. org/wiki/Variable_refrigerant_flow
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Designing for Zero Carbon: Volume 2