Zoning Doesn't Save Energy - Right? Wrong!
By Hal Alles
Last month I wrote about zoning a residential forced-air heating and cooling system by sensing the temperature in every room and controlling the airflow to every vent &mdash even for bathrooms. A central computer controls everything to keep every room the "right" temperature all of the time, no matter the conditions inside or out. Most people agree this would be comfortable, but will it save energy? Studies using typical two- or three-zone systems show only small savings at best, sometimes even small increases.
But a control zone for every room is different, especially when used with ducts sized to satisfy the peak loads for each room and properly sized two-stage equipment that adjusts airflow to match capacity. Up to 40% savings can be achieved in larger single-family homes (verified by an independent study of before/after utility bills).
How is this possible? Assume just the right volume of conditioned air is delivered to each room to exactly meet its needs and that the efficiency is constant. (Contact me if you want details about how this is done.) There are three primary ways energy is saved.
1. The thermostat in a home is usually set to keep the hardest-to-satisfy person comfortable in the hardest-to-condition room. In larger homes, the cooling temperature may need to be set to 73º to get 77º in rooms that get sunshine, while those rooms without sunshine will be only 69º. While they may be satisfied with 77º, they think they need 73º. If all the rooms are controlled to 77º, then the energy savings is equivalent to setting the central thermostat 4º higher. Likewise, the heating temperature may need to be set to 72º to get 68º in rooms with two or three outside walls. Maintaining 68º in every room is equivalent to setting the central thermostat 4º lower. Setting the thermostat to use less conditioning saves energy.
2. Circulation can move heat by mixing air from a warm room with air from a cool room, helping balance temperatures without using conditioning. Selectively controlling circulation to just the warmest and coolest rooms is as much as five times more effective than normal circulation. For example, if a fireplace heats a room while other rooms need a little heating, directing circulation to just the coolest rooms and the fireplace room will cool the fireplace room and warm the coolest rooms. When a heat cycle is needed, the fireplace room receives no airflow. Directed circulation saves energy.
3. When rooms are not occupied, they can be conditioned less. An interior wall provides a layer of R-4 insulation plus the dead air space in the room. When every room is controlled, bedrooms can be set back during the day, whether they are located upstairs or downstairs. The nursery or a bedroom used as a home office can be conditioned while the other bedrooms are not. Special purpose rooms for exercising, hobbies, home theaters, etc., can be conditioned a few hours per day, or even a few hours per year. Only the occupied bedrooms need to be conditioned at night. Reducing conditioning when rooms are unoccupied saves energy.
Motivating homeowners to "program" conditioning schedules requires ease of use. A well-designed interface using a graphics touch screen can make it easy. Reporting the actual cost of the energy used and predicting how setting changes will affect their utility bill can also provide the motivation.
Controlling the temperature in every room provides incredible comfort for everyone in the family &mdash and saves energy for the environment. It is possible to be "green" without compromising comfort.
September 2007 Builder Architect Edition Issue