HVAC SYSTEM

HVAC SYSTEM

 For proper operation, a HVAC system must be properly designed, sized and installed. A proper HVAC system will provide an improved indoor environment and minimize the cost of operation. In the planning process for an energy efficient home, everything should be done to reduce the heating and cooling load on the home before the HVAC system is designed.

   SIZING

When considering a HVAC system for a residence, remember that energy efficient and passive solar homes have less demand for heating and cooling. Substantial savings may be obtained by installing smaller units that are properly sized to meet the load. Because energy bills in more efficient homes are lower, higher efficiency systems will not provide as much annual savings on energy bills and may not be as cost effective as in less efficient homes.

Not only does oversized equipment cost more, but also it can waste energy. Oversized equipment may also decrease comfort. For example, an oversized air conditioner cools a house but may not provide adequate dehumidification. This cool, but clammy air creates an uncomfortable environment.

Many contractors select air conditioning systems based on a rule, such as 600 square feet of cooled area per ton of air conditioning (a ton provides 12,000 Btu per hour of cooling). Instead, use a sizing procedure such as

·        Calculations in Manual J published by the Air Conditioning Contractors Association;

·        Similar procedures developed by the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE); or

·        Software procedures developed by electric or gas utilities, the U.S. Department of Energy or HVAC equipment manufacturers.

The heating and cooling load calculations rely on the outside winter and summer design temperatures (see the appendix for a definition) and the size and type of construction for each component of the building envelope, as well as the heat given off by the lights, people, and equipment inside the house. If a zoned heating and cooling system is used, the loads in each zone should be calculated. Table 7-3 compares the size of heating and cooling systems for the homes in Table 2-2. The more efficient home reduces the heating load 35% and the cooling load 26%. Thus, the $600 to $1,000 savings from reducing the size of the HVAC equipment offset the additional cost of the energy features in the more efficient home.

Table 7-3 Equipment Sizing Comparison
Type of House	Code Home HERS=98	ENERGY STAR® Home HERS=85	Exceeds ENERGY STAR® Home HERS=70
HVAC System Sizing
Heating (BTU/hour)	52,200	38,800	25,700
Cooling (BTU/hour)	31,700	25,700	19,800
Estimated tons of cooling*	3.0	2.5	2.0
Square feet/ton	667	800	1,000
*Estimated at 110% of calculated size. There are 12,000 Btu/hour in a ton of cooling.

Oversimplified rules-of-thumb would have provided an oversized heating and cooling system for the more efficient home. The typical rule-of-thumb in Kentucky has been to allow for 600 square feet per ton of air conditioning. Since the home has 2,000 square feet of conditioned space, HVAC contractors could well provide 3.5 to 4 tons of cooling (2,000 ÷ 600 = 3.33, then round up.) The oversized unit would have cost more to install. In addition, the operating costs would be higher. The oversized unit would suffer greater wear and may not provide adequate dehumidification.

Proper sizing includes designing the cooling system to provide adequate dehumidification. In a mixed- humid climate, it is important to calculate the latent load. The latent load is the amount of dehumidification needed for the home. If the latent load is ignored, the home may become uncomfortable due to excess humidity.

The Sensible Heating Fraction (SHF) designates the portion of the cooling load for reducing indoor temperatures (sensible cooling). For example, in a HVAC unit with a 0.75 SHF, 75% of the energy expended by the unit goes to cool the temperature of indoor air. The remaining 25% goes for latent heat removal—taking moisture out of the air in the home. To accurately estimate the cooling load, the designer of a HVAC system must also calculate the desired SHF and thus, the latent load.

Many homes in Climate Zone 4 have design SHFs of approximately 0.7. This means that 70% of the cooling will be sensible and 30% latent. Systems that deliver less than 30% latent cooling may fail to provide adequate dehumidification in summer. It takes 15 minutes for most air conditioners to reach peak efficiency. During extreme outside temperatures (under 32°F in winter and over 88°F in summer), the system should run about 80% of the time. Oversized systems cool the home quickly and often never reach their peak operating efficiency.

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