Design Considerations - York Meridian Technical Manual

036-21478-001 Rev. A (0802)

Design Considerations

Load Diversity
A zoning system is designed to improve tenant comfort
by dynamically rebalancing the air distribution when
used with a typical constant volume rooftop heating/
cooling unit. If zones with extremely different load con-
ditions are serviced by a single rooftop unit, the result
will be poor control and excessive wear due to cycling
of the equipment.
It is especially important to avoid mixing interior zones
(which require cooling all year) with exterior zones
(which may require constant heat during winter months).
If you must mix zones under these conditions, consider
using either VAV boxes with heat or separate external
heat on perimeter zones. Meridian
offer a variety of methods to control additional zone
heat to help you avoid problems.
Group similar loads on an individual unit and use more
than one zoned unit if required. Any special loads can
be handled by using separate constant volume units.

The Meridian Plus system offers the designer consid-
erable flexibility by allowing both multiple-zoned units
and single-zone units to be connected within a single
simple system.
Cooling - Partial Load Conditions
The engineer must be aware of several potential prob-
lems when applying a zoning system for cold weather
operation.
1.) Low Ambient Temperature Lockout. During very
cold weather it is common for mechanical systems to
have "low temperature lockouts" which protect equip-
ment from damage if operated under these conditions.

Meridian also provides user programmed lockouts for
protection purposes, although mechanical safeties
should always be used as the final stage of protection.
If the rooftop unit services interior zones with thermal
loads, which require cooling when outside temperatures
are below the safe operating limits for your equipment,
you should seriously consider installing an economizer
on your rooftop unit. The Meridian
designed to take advantage of an economizer if it is
installed. The use of an economizer will save money on
Meridian Systems

utilities and provide comfort under conditions when it
is not possible to operate the mechanical cooling sys-
tem.
2.) Low Supply Air Temperatures. Under lightly
loaded conditions much of the supply air may be by-
passed back into the return air side of the HVAC unit.
This bypassing will result in the lowering of the supply
air temperature, which may result in the supply air tem-
perature reaching the low temperature safety limit. If
the supply air low temperature safety limit is exceeded,
the control system will "cut off" the mechanical cool-
ing to protect it from damage. Excessive cycling of the
mechanical system will result if this condition persists.
Comfort may also suffer if the system cannot run long

Zoning systems
enough to satisfy cooling demands.
A number of things can be done to reduce this problem.
Some of these things depend upon the type of installa-
tion.
Avoid oversizing the unit. Do your all load calculations
carefully. Since the zoning system directs the heating
or cooling to the zones which require it, you may find
that you can use a smaller unit in many cases. Oversizing
is the number one cause of excessive low supply air
temperature cycling.
Use an economizer. Although this is not a cure-all, it
greatly improves operation during cool weather when
cooling loads are minimal. Using an economizer also
improves ventilation and lowers operating costs.
Increase cooling minimum airflow. Increase your cool-
ing minimum airflow or damper position settings to al-
low more air during cooling operation. Be careful to
avoid minimum settings that are so high they may cause
over cooling of the spaces.
Bypass the air into the ceiling plenum. If you have a
system without ducted return, bypass the air into the
ceiling plenum instead of into the return air intake. Be
careful if you use this method since you may get "dump-
ing" of cold air from your return air grilles. This method
works best with plenum returns. Do not use this method

control system is
with ducted returns.
Zoning Design Guide
11