ZONING SYSTEM DESIGN
Zoning any forced air system is easy once you know a few of the basic rules. The main consideration is to maintain a constant amount of
air ow (CFM) through the HVAC Unit. This needs to occur when only
one zone is open and if the zones are of varying size when the smallest zone is open. The other consideration is not to oversize a duct
system too much in order to maintain adequate velocity and airow
when all zones are open and may be calling for conditioning on those
design temperature days.
The design of the duct system for today’s zoning is an important
factor to a comfortable and efcient zoning system. The number of
zones, along with their size, often determine the best type of design.
There are scenarios for zoning. The rst, which is typically on new
installations where the duct work can be designed for zoning would
be to oversize the ducts for each zone in order to get more air to the
zone when it may be the only one calling. The scenario for all others
would be to use a by-pass damper to relieve the excess air pressure
in the duct system when a minority number of zones are calling.
The reason for each zone duct being the same size is that any zone
could be the only zone calling and therefore that zone must handle
the CFM of the HVAC Unit. When the duct is sized for 2/3 of the total
CFM the smaller size does restrict the airow and forces the air at
a higher pressure and velocity, however it does not increase the air
typically over that static pressure rating of the blower motor, usually
0.5”W.C. This also keeps the air velocity from being noticeably noisy.
Below is a quick guide to determine the minimum equivalent size of a
zone duct for each size HVAC Unit.
ZONING
DESIGN
In retrot and systems with 4 zones or more, over sizing the ducts is
not practical. In these instances a by-pass damper is used to relieve
the excess air back into the return air duct or dump the air into a
central area of the building, such as a hallway, where often there is
a common return. In this instance try to locate the by-pass air as far
away from the return air intake as possible.
In new installations where ducts are being added it is recommended
to size each zone duct the same and to size the duct for approximately 2/3 of the total HVAC System CFM. This is practical on systems
with 2 or 3 zones and when all zones are approximately equal in size.
This is NOT practical in an installation where 80% of conditioned area
is one zone and 20% is the other zone.
The key to a good zoning system is to deliver the conditioned air to
the calling zone as fast and quietly as possible in order to satisfy the
demand. Whatever air cannot be directed into the zone must then by
by-passed. This develops the formula for calculating the size of the
by-pass damper.
Total CFM - Smallest Zone CFM = By-Pass CFM
Once the amount of by-pass air is known it is just common sense to
size a duct adequately to handle the amount of air. White-Rodgers
has both round and rectangular/square by-pass damper sizes.
Systems over 5 Tons typically are commercial and would use a by-
pass damper to relieve the excess air pressure when the majority of
zones shut down.
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ZONE 1 CLOSED
DUMPER DAMPER
DUMP ZONE
ZONE 3 CLOSED
ZONE 2 OPEN
ZONING
DESIGN
The diagram above shows a typical three zone damper system with
a barometric by-pass, Model CSPRD, between the supply air and return air duct. A round take-off is also shown as an uncontrolled dump
zone delivering air continuously to a non-critical temperature area,
such as a basement or hallway as another method of relieving excess
air. Either a by-pass or a dump zone is used but rarely ever is there a
need for both.
MINIMUM POSITION DAMPER
Another form of by-pass is using minimum position dampers, such as
the CZD and CRDS dampers. Setting the larger zone dampers to a
minimum position can also be a method of relieving excess air pressure. This should be used when only small amounts of air need to be
by-passed as the minimum position air in some cases can cause a
zone to overshoot its comfort zone. Caution must be exercised when
using minimum position dampers for by-pass.
ZONING HEAT PUMPS
Heat Pumps are a very popular form of heating and cooling in the
milder climates of the country. These efcient units use the compressor for both heating and cooling and have a reversing valve that
reverse the ow of refrigerant in order to switch between heating and
cooling modes.
Heat pumps, while very efcient, are also most efcient in milder
climates where often there is more of a cooling demand than a heating demand, such as the southern United States. A heat pump is also
rated based upon its cooling capacity and not the heating capacity
and rely typically on electric resistance heating to supplement the
heat pump in colder weather, often less than 35°F to 40°F. When this
back-up form of heat is used to supplement the heat pump compressor, the cost to heat the home rises dramatically. It is this reason that
zoning should be installed with every heat pump.
Heat pumps with their limited capacity for heating cannot afford to be
run on the colder climates heating the entire home or building. However if the heat pump is zoned, most likely less than the total building
requires heat at any one time. The zone calling typically requires less
than the total BTU capacity of the heat pump, more evenly matching
the heat pumps capacity to the load of the calling zone. By doing so
this lessens the need for the supplemental heat to come on, therefore
providing substantial energy savings.
For example, a typical home may have a 3 Ton (36,000 BTU) heat
pump. The total heating load for the home at heating design temperature maybe 60,000 BTUs or even more. Obviously with only 36,000
BTUs the heat pump can never keep up at design temperatures.
However during milder temperature days, those above 45°F, the heat
pump will often be more than adequate.
Heat pumps most efcient outdoor temperature, typically 45°F to 50°F
and the amount of BTUs produced by the heat pump decreases as
the temperature moves further below this temperature. This is when
the supplemental electric resistance heat is often needed. As the
heat pump compressor heats the air, the electric resistance heaters,
located downstream of the heat pump coil, can come on to supplement the heat pump air. However if the system is zoned and heating
is required for typically only one room or zone, the capacity of the
heat maybe equal to or greater than the heating load of that zone
and even as the output of the heat pump diminishes as the outdoor
temperature falls, the capacity of the heat pump, (BTU output) is
adequate for the zone(s) calling therefore not requiring the use of the
supplemental heating.
The combined use of zoning and heat pumps allows more equally
matched capacity to match the load of those zones calling allowing
the most efcient form of heating to be used. Even during the colder
times when the supplemental electric heat is needed, it is important to
remember that a smaller amount of heating is always required for one
room or zone vs. the whole house. Zoning allows the heating to be
directed only to those areas needing it.
FOSSIL FUEL FURNACES AND ADD-ON HEAT PUMPS
When a heat pump is added on to a fossil fuel (gas or oil) furnace, the
sequence of operation is somewhat different than using electric resistance heating as supplemental heating. The heat pump coil is often
on the supply side of the furnace and both the heat pump and furnace
cannot be run together as the high temperature from the furnace will
cause the heat pump compressor safeties to engage.
All heat pump manufacturers use a fossil fuel kit in order to make
the most effective use of the heat pump and furnace. Simply this
fossil fuel kit uses an outdoor thermostat to switch between the heat
pump and furnace based upon the outdoor temperature or balance
point. The balance point is calculated to determine the most effective
temperature to operate the heat pump vs. the furnace. See the heat
pump manufacturer’s information to calculate the balance point.
When using a White-Rodgers System with an add-on heat pump it is
recommended to use the manufacturer’s fossil fuel kit in order maintain the manufacturer’s warranty. The zone control panel equipment
terminal block will be wired to the thermostat connections on the fossil
fuel kit.
When using any heat pump it is important to remember that with a
limited amount of heating capacity, ZONING is imperative in order to
make maximum use of the heat pump and where the individual zone
load is more closely matched to the capacity of the heat pump.
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238
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ZONING
DESIGN
ZONING COMMERCIAL SYSTEMS
Zoning for commercial and light commercial ofce buildings makes
even more sense as every person has their own idea of their own
comfort level. The number 1 and 2 complaints in any ofce is its either
TOO HOT or its TOO COLD. Being able to provide each ofce with its
own thermostat to control the temperature is a simple and cost effective way to solve this problem.
Here is a small professional ofce with 4 zones. One zone would be
the waiting room, reception area, another be the receptionist’s ofce,
another be the boss’s/doctor’s ofce, or conference room and the
other smaller rooms be ofces or examining rooms all on the same
zone.
In commercial buildings the need for zoning is increased by the build-
ing exposure when ofces facing south and north are controlled by
the same thermostats. Those south facing ofces on a bright sunny
day may be needing cooling while ofces on the northern side with no
solar gain still need heating. In the morning the entire building may
need heating for a morning warm up and soon after only the perimeter ofces need heating while the interior core needs cooling. Conference rooms that go for hours unused can be shut off and then when
there is a meeting and the ofces are not being used, the conference
room can be controlled comfortably by having its own thermostat.
Zoning also helps the landlord in tenant improvement costs by
eliminating the need to ad separate air conditioning units in order to
satisfy multiple tenants on one HVAC system. While one tenant in a
professional suite maybe an attorney and have more normal working
hours, the doctor and dentist with after hours patients on evenings
and Saturdays can still get conditioning by having there own temperature control.
Commercially, zoning is a very economical alternative to the more
sophisticated VAV and computer controlled HVAC systems. Zone
Control in commercial buildings can range from installing a single
motorized damper and thermostat to control an over-conditioned or
seldom used room or ofce; such as a conference/training room to
zoning every room on the HVAC System.
There is really little difference in zoning a residential 5 Ton Split HVAC
System and a commercial 5, 7-1/2, 10, 15 or 20 Ton Package Rooftop
HVAC Unit. The system components of the dampers, thermostats and
control panel can be the same in many instances. The only difference is the commercial rooftop maybe 2 stage heating and/or cooling
and there may be more zones and the duct sizes may be larger,
however the basic operation remains the same.
The Number 1 and 2 complaints in ofce buildings are people are
either Too HOT…..or Too COLD. Zoning is the less expensive alterna-
tive to the more commercially used VAV Systems and provides just as
good temperature control as those expensive system for a fraction of
the cost.
The design of a commercial zoning is also the same as a residential
system where a by-pass is most typically used. Small commercial
systems that may be just two zones might be able to get away without
a by-pass provided they are only two zones and the ducts for each
zone are large enough to handle 60% to 70% of the total airow.
Commercial zoning systems of three zones or more will need a
by-pass. The by-pass for commercial is often easier as many times
the false ceiling space is used as a common return. Here again it is
always important for the by-pass damper to be located as far away
from the blower as possible. Barometric by-pass dampers can be
used up to 7.5 Tons (3,000CFM). Over 10 Tons (4,000 CFM) should
use a motorized by-pass and a static pressure control.
White-Rodgers Systems can automatically direct the ow of the conditioned air to those zones needing it and automatically switch over
and provide the opposite mode to the other zones eliminating the
need for on site techs to constant balance and adjust outlets based
upon the ever changing conditions.
Light Commercial and Commercial systems are basically just bigger
residential systems with more capacity. Duct design for these will
almost always include a by-pass system, especially those over two
to three zones. The commercial systems are easier to retrot as the
false ceiling space if often used as a common return and a great
place to by-pass the air. Wiring is a snap as well as damper installation all taking place in the false ceiling.
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ZONING versus MULTIPLE UNITS
For years many HVAC Installers and Home Builders have used
multiple HVAC units in order to accomplish zoning. Installing a unit for
the upstairs and another for the downstairs is typically most common.
While effective to achieve zoning, using multiple units is often an unnecessary and substantially higher added cost.
There are good reasons for using multiple units. They would be that
the load of the home is so big that multiple units are needed. Homes
continue to grow in size and on average homes over 3,000 square
feet will typically require more than 5 Tons of cooling. In these larger
homes, multiple units are necessary. Typically trying to use one
central unit also creates long duct runs that may not properly get the
airow to all areas. Using multiple smaller units can be more effective
however in these instances a zoning system should still be looked at
as each smaller unit could still be subdivided into small zones.
The case for multiple units is also used in existing homes where
add-on air conditioning may be installed and duct work cannot be run
throughout the house. Example a older home without ductwork, that
may utilize steam or hot water heating and air conditioning is being
added. In order to add central air conditioning for both levels ductwork
cannot be run from the basement to the second oor or from the attic
down to the rst oor.
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TECHNICAL HELP
ZONING
DESIGN
However when two units are installed side by side in the same area
and both units add up to less than 6 Tons, this is the case for using
one unit and zoning.
Anytime a contractor can install one unit versus two, or more, the
homeowner wins. Maintenance costs are cut with every unit saved.
Remember the maintenance of air lters, electronic air cleaners,
humidiers, electrical requirements and the life expectancy of these
HVAC units as well, are unnecessary added costs that can be saved
by using one unit and zoning.
Another reason for using one larger unit with zoning is to economical-
ly obtain zoning and the highest efciency HVAC equipment. Often
when multiple HVAC units are installed these units are typically on the
lower scale of efciency. For heating this is an 80% efcient furnace.
For cooling this is a 10 SEER air conditioning unit.
ZONING EFFICIENCY AND DOWNSIZING
The standard way of not living in your whole house all of the time and
mostly occupying one zone of the home at a time proves the need
for zoning. Zoning makes the use of the heating and cooling more
effective by only conditioning those zones being occupied or that may
need it. Therefore typically the majority of the time, even on a two
zone system only one zone is typically calling.
When this occurs the furnace and air conditioner is oversized in BTU
capacity when supplying only a single zone. Due to this many HVAC
Installers will downsize the heating and/or cooling units based upon
the use of zoning. On a two zone system this many not be recommended, however when using 3 or more zones it is practical to
downsize to the next lower capacity unit.
An HVAC Unit should be sized to heat and cool the home at design
temperatures (the hottest days in summer and the coldest days in
winter). Realistically how often do design conditions occur. Of course
this depends upon where you live. In the milder climates downsizing
is much more of a possibility than in the severe heating climates of
Northern states or the southern cooling climates.
HVAC installers have been quite successful in going to the next
smaller size unit when using zoning. In many cases heating and cooling units are often oversized, especially on older homes. Doing a heat
loss and heat gain calculation is very important in determining the
size of the heating and cooling unit. Once the loads are determined
for the overall structure, the affect that zoning will have on the overall
load can be determined. Seldom used zones such as basements
or extra bedroom loads can be looked at as not always needing
conditioning at the same time as more often used zones. In instances
where 3 or more zones are used may be advantageous to downsize
to the next smaller size of heating and cooling unit.
NOTE: There are times during extended periods at design temperatures where it will take longer for the zones to satisfy as the
heating or cooling. It is important to note that some zones would
have to be adjusted in order to direct more BTUs to the more
important zones.
Down sizing while also increasing efciency is often the best way
to go. When considering the alternative of using 2 separate lower
efciency units in order to zone vs. one high efciency unit with zone
damper system, the cost difference is negligible if not sometimes
less. Take for example the following scenario and price out the costs
TECHNICAL HELP
comparison of the following example.
A 100,000BTU Home with 2 Zones.
Two Units @ 80% Efciency vs. One Zoned 90% Efciency
2Units 1 Unit
x 50,000 BTU Each (100,000 Total) x 90,000 BTU (Downsized)
x .8 (80% Efciency) x .9 (90% Efciency)
= 80,000BTU vs. = 81,000BTU
When you look at the two options you can still get more output, with
higher efciency and still provide zoning. Combine this with the potential utility rebates for the higher efciency and that often offsets the
added cost of the zoning system. Even going to one 80,000BTU, 90%
efciency unit will only decrease the total output by 10% . In milder
climates this can be an approach to lower HVAC installed costs when
quoting against the competition with 2 lower efciency units.
Applying higher efciency and zoning to cooling can also increase the
effectiveness of the cooling. Here again if utility rebates are offered
for the higher efciency this can further offset the added cost of zoning and possibly add other comfort options.
Typically the more zones you have the wider the diversity in the use
of the zones. This factor can help in downsizing the unit. Take for
example an exercise room that may only be used a hour or two a day.
The family recreation room that is only used for a few hours in the
evening and not when all are sleeping in the bedrooms or eating in
the kitchen dining room zones.
Zoning and high efciency equipment can increase the overall energy
performance of your home and keep rising energy costs down to
manageable level.
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