When designing a comfort system, it is not adequate to merely produce a heat loss and heat gain estimate. Heat loss
and heat gain estimates are part of a design procedure that flows from system selection decisions and the actual load
calculations to equipment selection procedures, placement and selection of air distribution hardware, duct routing and
airway sizing.
Documents such as ACCA Manual RS provide valuable information about zoning, system concepts, equipment capability,
and design procedures. It is strongly recommended that system designers be familiar with the material in Manual RS.
Manual J or equivalent load calculations affect every aspect of the system design procedure. The calculations must be as
accurate as possible.
- Equipment capacity that matches the size of the applied heating and cooling loads will deliver comfort, efficiency,
and reliability over the entire range of operating conditions.
- Heating and cooling loads determine the total air delivery requirement (blower CFM) and the airflow requirement for
each room (room CFM). This airflow information is then used to select supply air outlets and to size the duct runs.
- Load information also is used to estimate purchased energy requirements and to estimate annual operating cost. In
this regard, the energy and operating cost estimates will only be as accurate as the load estimate.\ The design concept must be suitable for the application:
- Contemporary architecture tends to produce dwellings that require a zoned system and/or variable capacity
equipment.
- Custom homes that feature a large amount of architectural glass that provides a panoramic view or architectural
theme may not have internal shade, or the shading device may be completely open when the room is occupied. In such cases, the performance of the glass (U-value and solar heat gain coefficient) has a significant
effect on comfort, equipment size, and energy use. If there is a large amount of south glass, cooling may be
required during cold weather. These dwellings must be carefully zoned and may require year-round cooling.
- People may be uncomfortable when bathed by sunlight pouring through a window. During cold nights or cold
overcast days, radiation from the occupant’s skin to cold glass surfaces may cause discomfort.
- External overhangs or some type of internal shading device are desirable because they provide comfort for the occupants
(overhangs provide shade without interfering with the view).
Manual S (or Equivalent) and Manufacturer’s Data to Select Equipment
In general, the effective capacity of heating and cooling equipment shall, as closely as possible, match the load when the
equipment is subjected to design conditions. For instance, Manual S explains how to use Manual J output and manufacturer
performance data to obtain this result. Manual S also provides guidelines pertaining to the acceptable amount of
excess capacity and manipulating heat pump balance points.
ACCA Manual T (or Equivalent) and Manufacturer’s Data to Select Supply Outlets and Return Grilles
Supply outlets (grilles and registers) shall be the appropriate style and size for the application and shall be in an appropriate
location for the application.
- Supply outlets shall not produce objectionable noise. Design guides and manufacturers’ information establish limits
for face velocity.
- Supply outlets shall provide the appropriate throw for the installed location. Floor outlets shall throw the supply air
to the ceiling; ceiling outlets shall throw the supply air to the wall, etc. Size depends on product performance, the
supply CFM value, and the face velocity limitation.
- Never blow supply air directly into the occupied zone. Occupants will complain about drafts.
- Floor outlets that blow air straight up the exposed wall are best for cold-climate heating and, if properly selected,
adequate for cooling.
- Ceiling outlets are best for cooling but will not warm slab or exposed floors during the winter.
- If high sidewall outlets are used for cooling, supply air shall not drop into the occupied zone during cooling. These
devices will not warm slab or exposed floors during the winter.
- The relation between supply CFM, throw, face velocity, and drop is established by manufacturer performance data.
Performance is very sensitive to size, and devices that appear to be generally similar can have substantially different
performance characteristics.
- A low-resistance return path shall be provided for every room that receives supply air—a wall opening with no door,
a transfer grille, or a ducted return. Door undercuts are not acceptable.
- Return grilles shall be the correct size for the grille flow rate. Filter grilles have a lower face velocity than plain
grilles.
- The location of the return grille does not affect room air patterns, which are controlled by the supply outlets and will
not have a significant effect on pockets of stagnant air. Low returns do pull warm air down to the floor, and high
returns do not pull cool air up into the occupied zone.
Manual D (or Equivalent) to Size the Duct Runs
The resistance (inches water gauge of static pressure) of the longest circulation path (longest supply run plus longest
return run) shall be compatible with the performance of the blower that is supplied with the heating-cooling equipment.
Airway sizes that are compatible with the blower performance shall be increased if airflow velocity creates a
potential noise problem. All systems shall have adequate provision for balancing airflow.
- The length of the longest circulation path and the available static pressure determine the friction rate used for airway
sizing.
- The length of the circulation path includes the straight runs and the equivalent length of the fittings along the path.
One fitting can add from 5 feet to more than 60 feet to the length of the path.
- External static pressure is determined from the equipment manufacturer’s blower performance data, preferably for
medium-speed operation.
- The available static pressure equals the external static pressure minus the pressure drop through all the air-side
devices in the circulation path. Refer to blower table footnotes and manufacturer pressure drop data for devices that
were not in place when blower performance was laboratory-tested by the equipment manufacturer.
- Accessory or after-market filters (or any device) that produce a substantial increase in system resistance shall not be
installed if the blower cannot accommodate the increased resistance by speed change. An arbitrary increase in system
resistance may cause low airflow to rooms, a high temperature rise across a furnace heat exchanger, or low suction
pressure at the cooling coil.
- The room heat loss and heat gain estimate (Manual J or equivalent) and the heating and cooling factors (Manual D
or equivalent) determine the design value for room airflow.
- Airway size is determined by sectional flow rate and the design friction rate value.
- The friction chart or duct slide rule used for airway sizing shall be technically correct for the type of duct material.
- Airway velocities shall not exceed specified design limits.
- Branch (runout) ducts shall be equipped with a hand damper (for balancing).
Related Comfort Conditioning System Design Considerations
Impact of Incorrectly Sized Heating and Cooling Equipment
- The obvious problem with significantly undersized equipment is that it will not maintain the desired set-point temperature
when a passing weather system imposes a design load on it. However, slightly undersized cooling equipment—
by a margin of 10 percent or less—may actually provide more comfort at a lower cost.
- Oversized equipment causes short cycles, marginalizes part-load temperature control, creates pockets of stagnant air
(unless the blower operates continuously) and degrades humidity control during the cooling season (more information
on this subject is provided below). Oversized equipment also requires larger duct runs, increases installed cost, increases
operating cost, increases the installed load on the utility grid, and causes unnecessary stress on the machinery.
Humidity Control During the Cooling Season
- Sensible and latent cooling loads are imposed on dwellings in climates that have a substantial amount of moisture in
the outdoor air during the cooling season (wet-coil climates). When the summer design condition occurs, properly
sized equipment will operate continuously or almost continuously, both loads will be completely neutralized, and the
occupants will be comfortable. But, the design condition occurs for only a few dozen hours per season.
- Reduced latent capacity at part load will cause the indoor humidity to drift above the design value, which is acceptable,
providing the relative humidity stays below 60 percent. The possibility for experiencing comfort problems at
part-load conditions is minimized by using the default indoor and outdoor design conditions recommended by the
design manual, providing a code or regulation does not specify a different set of conditions.
- Some climates are too dry to produce a latent load on the indoor coil. In this case, the indoor humidity depends on
the moisture content of the outdoor air, the infiltration rate, and the amount of moisture generated by the occupants. If the outdoor air is very dry, these factors will combine to produce an indoor relative humidity of less than 50 percent and could even be lower than 40 percent. But if the relative humidity stays above 30 percent, the indoor air condition will be in the comfort zone.
Humidity Control During the Heating Season
During the heating season, dry air causes a sensation of coolness, a desire to increase the thermostat set point, problems
with static electricity, and dry sinuses. Adding a humidifier to the heating system moderates these problems, but if a
humidifier is installed, it must not produce a visible or concealed condensation problem. (See the unabridged version of
Manual J for more information on this subject.)