A “zone” is a room or group of rooms controlled by a single thermostat. “Zoning”
refers to the practice of having two or more thermostats controlling a home.

In addition to improving comfort, zoning can save money: The individual zones can
be setback on different schedules, providing the capability to further lower average
temperature in the winter, and further raise average temperature in the summer
(see “

Multiple zones are required in any of the following situations:
  • A multistory home; each floor must have a thermostat.
  • A room with a disproportionate glass area (as compared with the other
    rooms in the home) must have its own thermostat.

This produces a number of permutations:
  • A conventional single story home requires a single thermostat.
  • A single story home with one room having a disproportionate glass area
    requires two thermostats.
  • A conventional two story home requires two thermostats.
  • A two story home with one room having a disproportionate glass area
    requires three thermostats.
  • Etc.

The easiest way to incorporate zoning is by providing multiple systems, but that’s
becoming a problem, because the smallest system is 1-1/2 tons; under today’s
Energy Codes, a well built, well insulated two zone home may not require 3 tons
of cooling (and a three zone home might not require 4-1/2 tons, etc.).

Enter the “Zone Damper” system: A single central system is divided into multiple
zones by providing thermostatically controlled dampers in the supply ducts;
multiple room thermostats control respective dampers and feed back to a “Zone
Control Panel”, which operates the central system.


Do you remember the evils of low system airflow? Well, they will rear their ugly
heads in a Zone Damper System in a New York Minute: When all zones are
calling, there’s no problem; BUT as soon as one shuts down, system airflow
decreases, often drastically. That’s why many manufacturers require two safety
features: A “discharge sensor” and a “bypass damper”. The former alerts the
control panel of unusually high or low discharge temperatures, and the panel
deactivates heating or cooling (but continues to run the blower) until temperature
returns to the safe range, when the heating or cooling is reactivated. The latter
consists of a counterbalanced or motorized damper in a duct between the supply
and return; it senses the change in duct pressure when a zone damper closes,
and opens enough to maintain relatively constant airflow through the equipment.
These two devices protect equipment against failure due to extreme temperatures
and low airflow.

IMPORTANT POINT: Manufacturers often list these two as “options” or
“accessories”; contractors often use that as a reason not to spend their money (it
is theirs, you know) on them.

IMPORTANT POINT: Always insist on a discharge sensor and a bypass damper.

As we said, there are two types of bypass damper: Motorized or counterbalanced;
the counterbalanced type is our first choice, for its lower cost and higher reliability.

A client sent us this image of a counterbalanced damper that wasn’t working,
hoping we could troubleshoot the installation without actually going to the home;
we checked and noted the damper was installed upside down and the shaft
wasn't even horizontal; they had the installer correct the installation and it worked
much better.

IMPORTANT NOTE: We’ve seen many instances of inoperable bypass dampers;
there are contractors out there who don’t know how to install and/or adjust them.

FACT: The bypass damper must be closed when all zones call, must be partially
open when one zone calls and other zones do not; you should be able to see the
bypass damper open when one zone closes after all zones were calling.

FACT: You should be able to move a counterbalance damper by pushing the
counterbalance with your finger.
One Contractor's
Attempt at Zoning

Here we see a
rectangular zone
damper (upper right),
a round zone damper
(upper left) and a
bypass damper

Note the connection
between the furnace
outlet and the
horizontal supply main.
This arrangement is
called a "Bull Head",
and it's the single most
restrictive fitting there
is (EL = 120 feet).
Inside a
bypass damper.

If you can't move the
counterbalance by
pushing it gently with
your finger,
something's wrong.

We believe this
damper had been
purposely disabled by
removing the gasket
used to seal around
the damper and
jamming it under the