Typical residential ducted central air conditioning-heating systems are a compromise between comfort and practicality.

When technicians are on a furnace service call, customers may comment on home comfort conditions being less than ideal. Techs should be aware of the conditions created by older systems, and possible solutions that a company salesperson may be able to follow up with.

Most systems use a constant-output furnace, a single-capacity air conditioner, and one centrally located thermostat, although the amount of heating or cooling needed in a home varies widely with outdoor ambient conditions.

Single-capacity furnaces and air conditioners are sized to meet the most severe heating-cooling conditions. As a result, they are oversized most of the time.

Solar effects and thermal stratification further compound the problem of system design by shifting the relative distribution of load within the structure over the time of day and the change in seasons. The ideal residential system would be able to sense heat losses-gains and provide the space conditioning back at about the same rate.

Solution: variable capacity

A variable-capacity furnace and air conditioning system can adjust to the rate of loss-gain of the house. Therefore, temperatures in the house are more constant and the furnace is more efficient.

Another characteristic of an ideal residential space conditioning system is the ability to provide heating or cooling to individual rooms as needed to maintain comfort.

Consider, for example, a sunny summer day. Morning sunlight striking the east side of a house causes rooms on that side to heat much more rapidly than rooms on the shaded side. In the afternoon, the opposite is true.

Ideally, the system supplies cooling only to the areas that really need it. With the typical, one centrally located thermostat, comfort conditions are maintained at that thermostat, but not necessarily throughout the remainder of the house.

Split-level and multilevel homes are additional examples of where controlled distribution of heating-cooling is more effective.

Heat in the house rises to the higher floors. In summer and winter, it is common for the upstairs rooms to be too warm when the thermostat controlling the system is downstairs.

A system with the flexibility to adjust the distribution of heating-cooling in the house can solve this problem.

Types of zoning

By equipment — Residential space conditioning with zoned air distribution and variable capacity has been available for some time. Historically, the best approach has been to “zone by equipment,” which involves installing several independently operated systems to serve different portions of the house.

The drawback is that many homeowners feel that the increase in comfort is not worth the incremental cost over a conventional central system.

By controls — Another approach has been to use off-the-shelf zoning controls that control duct dampers to distribute conditioned air to individual spaces. These systems are retrofitted to any standard air conditioning-heating equipment.

They provide additional comfort, but tend to shorten the life of the equipment. A thermostat is installed in each individual zone to maintain temperature, but the heating-cooling equipment is sized to provide for the entire home. So, when an individual thermostat calls for heat, the equipment is oversized.

As a result, the equipment is providing oversized capacity to an undersized space, which results in tripped high-temperature limits for furnaces, electric heat units, and heat pumps, and frozen evaporator coils for air conditioners.

Also, since these standard systems have a fixed amount of air delivered during heating and another during cooling, the extra air for the individual zones is usually rerouted back through the air handler with a bypass damper, or is dumped into “dump zones.”

(The bypass damper is one way to short circuit the air from the supply side back through to the return side controlled by a static switch.)

While both traditional methods are aimed at reducing the amount of air delivered to the zone in order to reduce the air noise at the register, they both have drawbacks.

The bypass approach intensifies the limit-tripping and coil-freezing problem, since the short-circuited air is heated or cooled. The dump zone approach usually dumps the excess capacity into an unoccupied or uncontrolled space at the expense of comfort and economy.

New advances

A better solution may be to offer a single unitary modulated heating-cooling system and a variable-volume airflow system. To address the gas heating delivery side, there would be a variable-speed, two-stage gas furnace with variable-speed motor.

The motor would provide constant cfm under variable static conditions and is controlled with a pulse-width modulated (pwm) electronic signal from the control center. The pwm signal is adjustable for each zone on the control center by moveable jumpers.

It allows the installer the flexibility of selecting the exact amount of air needed to cool or heat each zone in the field. The contractor also could change continuous air delivery levels and air temperature to the space.

With these product advancements, it is possible to offer homeowners the comfort they desire.