(The following is taken from Manual RS: Comfort, Air Quality and Efficiency by Design, published by the Air Conditioning Contractors of America.)

In residential applications, a good number of "cold room, hot room" complaints are caused by the performance limitations of single-zone conditioning systems. No matter how carefully this type of system is designed and installed, space-temperature excursions with two or more rooms are subject to an incompatible set of load conditions. These conflicting load patterns may be caused by the architecture, the position of the sun (time of day), or the type of load (heating or cooling).

The information in this section can be used to decide if a single-zone system is an appropriate choice for a particular home. And, if a multizone system is required, this information can be used to identify the rooms that have similar load characteristics (so they can be grouped into zones).

Control strategy

A single-zone-control strategy is appropriate for single-story homes that have a compact floor plan, providing that the rooms are "open" to each other. In this case, natural convention currents tend to minimize room-to-room temperature differences. For small rooms, an open door is adequate establishing the required connectivity. For large rooms, an opening that is equal to or greater than 25 percent of the partition area is required.

Created by isolation

When rooms are isolated from each other (by a closed door or an inadequate opening), room-to-room temperature differences cannot be moderated by natural convection. In this case, the desired temperature can only be maintained in the spaces that are under direct control of the thermostat and in other rooms that happen to have a similar load condition.

Incompatible load patterns

Incompatible load patterns cause the thermostat to be satisfied when one or more rooms still require heating or cooling. This condition will occur whenever an isolated room has a load pattern that is not synchronized with the load that affects the thermostat (as the day progresses from morning to evening or as the season changes from cooling to heating).

Time of day

Conflicting time-of-day load patterns are caused by the variation in solar gains as the sun moves across the sky. Temperature-control problems should be anticipated if the thermostat is placed in a room that is subject to a large solar-heat load, because the conditions in that room are not usually representative of the conditions in all the rooms.

Conversely, if the thermostat is placed in a room that is not subject to a large solar gain, temperature-control problems can occur in the rooms that have significant solar gains. It follows that the most severe temperature-control problems will occur when isolated rooms and the room that is under thermostatic control are affected by large solar gains that reach a peak during different hours of the day.

Seasonal issues

Conflicting seasonal load patterns are a result of envelope designs that cause some rooms to require relatively more winter heating than summer cooling, while other rooms require more summer cooling than winter heating.

For example, north-facing rooms with a large glass area can have a relatively large heating load and a moderate cooling load, while west- and south-facing rooms with large glass areas can have a relatively large cooling load compared to the heating load.

Large heating loads and small cooling loads also are typical of rooms that have large exposed surface areas and relatively small solar gains. For example, a north-facing room that has a slab floor will have a larger heating requirement and smaller cooling requirement than a west or south-facing room located over a basement.

Incompatible seasonal-load patterns can be discovered by comparing the percentage of the blower cubic feet per minute that is required to heat or cool a room. An example of this type of data is provided by Figure 3-1.

Reference to Figure 3-1 indicates that the thermostat is located in Room No. 4 and that 24 percent of the blower cfm is required to neutralize the Room No. 4 cooling load during a hot summer day. However, on a cold winter day, Room No. 4 only requires 17 percent of the blower cfm. Therefore, if the airflow has been adjusted for cooling, Room No. 4 will receive 7 percent more heating capacity than it needs, but Room No. 4 will not overheat because the thermostat will compensate for the excess supply cfm by reducing the equipment's run time. However, the temperature in some of other rooms may drift out of control - for example, consider Room No. 3.

Room No. 3 requires 12 percent of the cooling capacity. On a summer design-day, the temperature in this room will be maintained because Room No. 3 and Room No. 4 both receive the correct portion of the blower cfm.

However, in the winter, Room No. 3 requires 18 percent of the blower cfm. This means that on a cold winter day - even with the equipment operating continuously - Room No. 3 will be 6 percent short of heating capacity. Also note that the heating problem in Room No. 3 will be acerbated by the behavior of the Room No. 4 thermostat, which tends to reduce the equipment run time.

Of course, the Room No. 3 temperature problem could be resolved by moving the thermostat to Room 3, but this strategy will create an overheating problem in Room No. 4. (Since Room No. 3 is 6 percent short of heating capacity, a Room No. 3 thermostat will call for continuous operation during a very cold day, even though Room No. 4 is receiving more supply air than it needs.)

(Reprinted with permission. All rights reserved. To order Comfort, Air Quality and Efficiency by Design, write to the ACCA, 2800 Shirlington Road, Suite 300, Arlington, VA 22206; call (888) 290-2220; see www.acca.org on the Internet.)