The Zoning and Variable Speed Solution
In the past, Forced Air
Zoning was not readily accepted by the HVAC industry. Numerous concerns and
misconceptions, some valid and some not, prevented a true appreciation of the
great benefits of forced air zoning. As each myth or concern was cleared away
and resolved, another would take the place. But thanks to the devotion and
dedication of those involved in its conception, refinement and growth; forced
air zoning has over-come many obstacles, and has proven these misconceptions and
concerns to be false or no longer of valid concern. However misunderstood,
Forced Air Zoning has become a widely accepted, energy saving and comfort
Forced Air Zoning has certainly matured and adapted over the years. Older relay
based logic control panels were large, reliable, and worked very well, but
lacked the versatility and intelligence that is available in today’s
microprocessor based zone control systems. Modern zone control panels are
programmed with intuitive firmware. You can select the system type options and
staging options. Automatic over-current protection is standard including
integrated safety features like built in time delays and system temperature
monitoring. Dampers are now equipped with advanced and reliable optical
circuitry. A single forced air zone control system is now compatible with most
residential and commercial HVAC systems. Against heavy odds, Forced Air Zoning
has come a long way and yet, mysteries and myth still abound.
The most recent misconception we encounter when performing field training, or when discussing
system specifications, includes the use of variable speed indoor blower motors
with a forced air zoning system. “I can’t install a zoning system with a
residential variable speed air handler or variable speed furnace.”
The confusion behind this statement is that many do not realize that variable speed motors are
controlled internally via the air handler’s integrated motor control (ICM).
These integrated logic controls can accept but do not require an external pulse
width modulation (PWM) or 4-20ma input signal, to control the motor’s speed and
ramp up rate. The only signal required to operate these variable speed motors is
the standard (G) input from a conventional thermostat. Just as a standard 24vac
thermostat connects to and operates a variable speed furnace, so does a zone
control panel. Both simply output a demand for the fan to operate. The variable
fan speed operation is determined internally by the system mode and stage of
Example: FLA = Motor
full load amp rating
Y2= 100% FLA, Y1= 90% FLA, W2= 80% FLA, W1=70% FLA, G= 25% FLA.
The ramp up rate of these ICM’s, are usually fixed at 1% per second. Variations
on this basic control scheme exist, but all of them simply require a (G) signal
from the thermostat or zone control panel to operate. Advanced variable speed
ICM’s provide for an “Enhanced” setting. These settings provide a gradual
(timed) ramp up rate during cooling mode. This gradual ramp rate starts at 70%
of full load and takes five minutes or more to reach 100% full load. This slow
ramp rate increases the latent cooling effect of removing moisture from the air,
and results in lower than normal supply air temperatures. Just like a
conventional zoned air system with a bypass damper has been doing for forty
years. Simply adjust the low temperature limit potentiometer setting on your
zone control system to avoid a “nuisance” trip of the freeze protection setting.
Another misconception we hear in reference to variable speed blowers and zone
control systems is “I will no longer need a by-pass damper when I install a
variable speed furnace and a conventional zone control system.” This is a false
assumption. A by-pass damper should never be left out of any zoning system. The
latent cooling effect that a bypass damper provides alone, is enough to justify
the cost of the zone system. The bypass damper will not open or activate unless
excessive static pressure makes it open.
The fact is that a variable speed fan motor operates at full load (FLA) or near
full load at different times, depending on the mode or stage of operation. A
variable speed air handler or furnace is not aware of how many zones are
demanding conditioned air, or how many dampers are open or closed. In the same
way, it has no idea if the connected ductwork has been properly sized and
installed. The use of a barometric or preferably an electronic, static pressure
operated by-pass damper prevents over-pressurizing the ductwork, noisy
registers, and the negative effects of excessive air velocities. When installed
in a variable speed system, the by-pass damper simply may not open as often as
it does in a constant volume system. However, it still proves to be an
indispensable component in any zoned forced air system. As to the never-ending
dilemma of using a dump zone instead of a bypass damper, we respond again with
this question: Why spend money to condition the air, and then dump it where
nobody benefits from it?
The more advanced and costly zone control systems do provide an external pulse
width modulation (PWM) or 4-20 ma output signal, to the air handler’s integrated
motor control. These zone systems can control and modulate the PWM output, in
direct response to the number of zones that are demanding conditioned air, and
the pre-programmed CFM requirements of each zone. These systems may not require
a bypass damper but they are very unforgiving with poor duct design and layout.
These systems could probably benefit from a bypass damper, just in case.
The diagram above reflects a typical motorized bypass arrangement based on
static pressure. The static pressure control is field adjustable from .01” w.c.
- 4.0” w.c. The control is multi-positional. That is, it can be mounted in any
position and will still function properly. Note that the supply air sensor is
installed upstream of the bypass takeoff, and wires back to the zone control
panel. This is to ensure that the zone control microprocessor can accurately
monitor the supply air temperature, regardless of the position of the bypass
damper. The ability of the zone control system to properly monitor the supply
air temperature is critical to any successful installation. Some zoning
manufacturers provide the supply air sensor as an option only. Other
manufacturers include the sensor in every zone control package. The ability to
adjust the cooling and heating limit set points is built right into the zone
control panel, which makes it very easy to fine tune the zoning installation.
Older style zone control systems will not accept a supply air sensor at all.
External field installed temperature limit controls must be used. The field
installed temperature controls, (external or non-integrated) can be expensive,
bulky and difficult to install and wire. Utilize the supply air sensor provided
with these state of the art zone control systems. They are accurate, easy to
install and provide superior supply air temperature monitoring directly to the
A properly designed and installed forced air zone system can provide exceptional
equipment control as well as energy savings. Specifying variable speed equipment
adds an even higher degree of control and comfort. The two systems compliment
each other and do function very well together. They are definitely not as
complicated to install and setup, as some would have you believe.
John Phillip Brown
Chief Engineer, EWC Controls, Inc.