How Home Thermostats Work

System Zoning

A programmable thermostat can help regulate the temperature during different times of the day.
A programmable thermostat can help regulate the temperature during different times of the day.

A lot of times, there are rooms in your house that are always warmer or colder than others are. There can be many explanations for this. For one, heat rises, so rooms on second or third floors are often too warm. In turn, basement rooms are typically too cold. Rooms with vaulted ceilings have a difficult time retaining heat, while rooms that receive long hours of sunlight are often difficult to cool down. These are just a few reasons, but regardless of why a room's temperature is uncomfortable, there's only one surefire way to even out your house's temperature: system zoning.

System zoning is pretty simple. It involves multiple thermostats that are wired to a control panel, which operates dampers within the ductwork of your forced-air system. The thermostats constantly read the temperature of their specific zone, then open or close the dampers within the ductwork according to the thermostat's settings. Not only is system zoning helpful for houses with inconsistent room temperatures, but it's also great for heating or cooling individual bedrooms based on the desired temperature setting. If you have a usually empty guest room, just shut the door and close the damper.

If used properly, system zoning can help you save money on your energy bills. According to the U.S. Department of Energy, system zoning can save homeowners up to 30 percent on a typical heating and cooling bill. Those savings can add up to quite a sum -- the Department of Energy also estimates that heating and cooling account for 40 percent of the average household's utility costs. Because guest rooms and other seldom-used rooms don't require constant heating or cooling, system zoning allows you to save money by running temperature-controlled air to those rooms only when it is necessary.

Many homeowners are hesitant or unwilling to make the transition to programmable thermostats and system zoning because of the initial cost of installation. This is an understandable concern for anyone who's not building a new home or replacing an old HVAC system, but there are other options available. Even though installing a typical zoned system is not a do-it-yourself project, the Department of Energy's Inventions and Innovation Program funded the development of a damper system that can be retrofitted to existing ductwork. The system combines RetroZone's flex damper air control inserts with an electronic controller and air pumping system. There are no heavy motors involved, so existing ductwork does not need to be altered or supported.

The flex dampers, which come in circular and square duct models, fill with air to constrict or block the airflow within the duct. They're resistant to heat, aging, moisture, airborne chemicals and ozone, and even if they're punctured, which is unlikely, most holes will not affect the performance. Flex dampers should be installed in steel or flexible ducts. The dampers can be serviced easily by gaining access through a register. Flex dampers also work with most brands of zone-control panels.

If you're planning to install a retrofitted zone-control system, here's what you'll need to put on your shopping list:

  • thermostat for each zone
  • solenoid pump
  • solenoid panel
  • zone control panel
  • plenum tubing
  • transformer
  • fire rated tape
  • control limit switch
  • flex dampers

The number of zones your home needs will affect the way you set up the system. In a two-zone system, with the zones being fairly equal in size, each zone's ductwork must be capable of handling up to 70 percent of the total CFM (cubic feet per minute) of air produced by your HVAC system. In a three-zone system, the zones need to be as close in total area as possible. In this case, each zone's ductwork should be able to handle up to 50 percent of the total CFM. Installing a four-zone system requires a bit more work. The ducts need to be enlarged by one inch, and they require a static pressure relief damper and high- and low-limit protection. To avoid major damage, be sure not to completely cut off the airflow over the heat exchanger or coil of your HVAC system.

Now we'll look at another home-thermostat innovation -- the talking thermostat.