In HVAC, Bigger Is Frequently Not Better!

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Recently a Wrightsoft customer asked us for a write up to share with their customers who ask, “Why is my unit smaller than my neighbor’s? Should I have a bigger unit?” We wanted to share this with you.

Building Codes Determine HVAC Unit size
Modern houses need efficient HVAC systems and quality contractors design houses that take maximum advantage of modern technology. To supply systems that provide comfort and efficiency, current building codes require that heating and cooling units be sized to match the house. Just as building codes determine structural requirements of houses, building codes determine the choice of equipment size in today’s houses.

Why Codes Insist on Size
A major reason for this is that nationwide experience with equipment sizing has taught that when HVAC cooling equipment is sized too large, moisture control problems can develop, because the cooling unit only runs for a short time (‘short cycles’), even when the cooling load is at the maximum. And when cooling equipment ‘short cycles’, it doesn’t have time to get cold enough to remove moisture from the air – it just cools the air without extracting much moisture and shuts off. Consequently, oversized cooling equipment can result in cool houses with high humidity, and that’s a dangerous health condition – molds can easily grow and have grown in houses with oversized cooling units.

The effects of oversizing an HVAC system is similar to driving a big car in rush hour traffic. The more often a car engine ramps up and down in a short span of time, the less MPG the car achieves. When the cooling system turns on and off in short time spans, the system does not have the chance to reach its peak efficiency therefore costs more money to operate and may not remove excess humidity.

There are several parts to an HVAC system:

  • The central heating equipment, typically a furnace or an air-to-air heat pump.
  • The central cooling equipment, typically an air conditioner or an air-to-air heat pump.
  • The distribution system, typically an air duct system.
  • The registers and grilles, that point air flow into a room

Each of these parts are selected by engineering methods developed by ACCA, and required by national and state building codes. These methods (Ref 1, 2, 3, 4) are detailed engineering calculations that quality contractors follow rigorously.

How Calculations Are Done
ACCA has developed a technical manual for designing each part of the system. The first step in design is to calculate how much heat should be added (for heating) or extracted (for cooling) to keep the house temperature constant. ACCA’s Manual J must be used to compute the amount of heat required for those days when the climate is at the most common extreme outside temperature. This calculation takes into account the shape and
size of the house, the windows, roof, insulation, air leakage, and a host of other variables that change from house to house. In other words, a Manual J calculation must be done for each house, and even houses that look the same, may have different heating and cooling loads. The load is expressed in BTU’s (British Thermal Units). This number is used to describe how much heat the building losses in the winter and gains in the summer.

Once the heating and cooling loads for a particular house are obtained, the central heating and cooling equipment can be selected, and once again building codes require an ACCA method, Manual S, to be used to restrict equipment size to be within a narrow band of capacity to match the house
requirements. The Manual S method compares the heating and cooling capacity of the unit to the Manual J heating and cooling load. If the load of the house exceeds the largest residential cooling capacity limit determined via Manual S, then an additional system is needed. However, when a single
system meets the load of the house and complies with ACCA’ s Manual S standards, the next step is to design the duct system.

Designing the duct system and verifying that it passes ACCA’s Manual D is the third step. Designing the system according to Manual D is absolutely necessary to be sure the ducts fan has enough pressure to deliver the air to all parts of the house. It is very important that each duct layout is carefully sized. A common oversight is not taking into account all components that restrict airflow such as filters or lossy fittings and the end result is less airflow than loads require. Manual D provides quantitative limits for available pressure, duct velocity, and measures of airflow.

Once the duct system has been designed, the last step is to select and size registers using ACCA’s Manual T (Ref 4). For registers to properly project air into the room at the rated velocity, the technical characteristics of the registers have to be used for matching the register placement and size to the design airflow.

Careful calculation using ACCA’s four residential design manuals result in systems matched to their houses like engines are matched to cars. In both cases, the result is a smooth system providing comfort and efficiency.

Bill Wright

Posted In: ACCA Now, Technology

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