Understanding system pressure and its effect
on system performance is an important consideration when designing and
installing forced-air zoned systems.
It's true that system pressure can be greater in zoned forced-air systems, but
how zoning systems are designed by their manufacturer and installed by the
local contractor have a definite impact on how the system operates and the
level of comfort the home occupants enjoy. There are a number of different
methods for providing pressure relief when utilizing forced-air zoning to
control air distribution of conditioned air by means of motorized or pneumatic
dampers. Many installing contractors have been reluctant to use zoning because
of the added duct system pressures related to the closing of these zone dampers
and the effect this has on the system. This article will help you understand
the alternatives available when it comes to providing pressure relief.
With forced-air zoning, the amount of system
static pressure will vary depending upon the size and quantity of zones calling
for conditioned air at any given time. How this pressure is relieved will be
based upon the way the ductwork is sized and the method of pressure relief the
contractor has designed into the system.
The Oversize Ductwork Option
The oversizing method for providing
pressure relief refers to drastically oversizing the system ductwork. This
method involves sizing the supply duct for each zone to handle approximately 75
percent of the total system airflow. While this method does maintain adequate
airflow across the evaporative coil or heat
exchanger of the system, it also increases ductwork costs, reduces
throw (velocity) when multiple zones are calling and can be prohibitive due to
space available for system components.
The Bypass Damper Option
A more common method of providing pressure
relief is to size the ductwork as if no zone control is being used and include
either a motorized or a barometric style bypass damper. The bypass damper is
designed to direct excess airflow to either a "dump zone" (an area
where the comfort of the occupants won't be compromised) or into the return
duct.
When using this method, a lot of thought
should be put into where to dump the conditioned air. Some common areas to use
as dump zones include: hallways, basements, foyers, bathrooms, or mechanical
rooms. A dump room or zone can be any non-critical area where the conditioned
air can mix with a larger volume of air so the overall
temperature of that area won't be adversely affected. The use of a dump
zone also avoids the negative and possibly damaging effects of bypassing
conditioned air back into the return. The greatest drawback associated with
using a dump zone is the typical drop in overall system efficiency. By dumping
air into a non-critical area, the energy used to condition this air is mostly
wasted. As a result, the energy-saving benefit in this application is
significantly reduced or even eliminated.
When bypassing excess air back into the
return, it is best to introduce the conditioned air into the return ductwork as
far upstream from the heat exchanger/cooling coils as possible. This allows the
air to mix well with the return air before entering the air handler. Air that
is too cold or too hot that is being bypassed to the return reduces the
temperature differential across the heat exchange or A-coil, causing a
reduction in equipment efficiency. Additionally, excessive short circuiting of
conditioned air can cause the furnace to shut off on high limit or cause the air conditioning
coil to freeze. A freeze control should be used to prevent coil freeze-up
when bypassing cooled air into the return.
An Innovative Option
A unique method of
zoning which gives the energy-saving benefits of a return-type bypass system,
equipment protection, and proper air flow characteristics involves the use of
dampers specifically designed with controlled pressure relief built in. By slightly oversizing the ductwork based on the
manufacturer's specifications, these dampers will relieve pressure by bypassing
a small amount of air into the closed zones. The combination of duct sizing and
preset dampers assures a minimum of 85% of design airflow across the
evaporative coil/heat exchanger at all times, but not enough to affect the
temperature of a non-calling zone by more than 1°F. ASHRAE ( https://www.ashrae.org/ ) Standard confirms that a person does not react to a
change of 2°F during an equipment cycle when stating. There are no restrictions
on the rate of temperature change if the peak to peak is 1.1°K (2°F) or less.
Therefore, the comfort of the occupants is not compromised.
Benefits of circulating air within the
non-calling zone(s) include:
Humidity Control - allows for humidity to
equalize more quickly throughout the building when using a whole-house
humidifier during the heating season. Air Cleaning-allows for more air movement
to carry particulate from the living space back to be trapped in a
high-efficiency air cleaner.
Minimizing Negative Pressure - allows a small amount of air into the zone that
is closed. This will help reduce the possibility of a zone being put under
greater negative pressure when compared with other means of pressure relief.
Energy Savings
In addition to the comfort advantages of
forced-air zoning there is an added benefit, energy savings. Provided that the
system is designed with proper duct sizing and bypass relief, a typical
homeowner can save up to 20 percent on energy costs. However, following a good
setback strategy and only conditioning the areas of the house that are occupied
provides the major portion of the energy savings that can be realized. Only
conditioning the areas of the home when it's needed just makes good sense.
An Easy-to-Use Guide
Duct sizing
calculations are based on ACCA ( https://www.acca.org/ ) and SMACNA ( https://www.smacna.org/ ) recommendations of
maximum velocities of 900 fpm in the main ducts and 650 fpm in the branch ducts to
avoid air noise. What this means is a barometric or motorized bypass damper is
not required, thus reducing the system cost and installation time. A design
guide provides step-by-step instructions through the process of sizing the
ductwork. The savings that can be realized with this type of system design
range from 25 percent on a three-zone system to 33 percent on a two-zone
system, compared to a zone system utilizing a bypass damper.
Source: https://dticorp.com/understanding-system-pressure-and-residential-zone-controls/
