8.21   Evaluating Ducted Central Air-Conditioning Systems

An energy-efficient home shouldn’t need more than a ton of air-conditioning capacity for every 1000 square feet of floor space. Window shading, attic insulation, and air leakage should be evaluated together with air-conditioner performance to size an air conditioner.

The following four installation-related problems are characteris­tic of central air conditioning systems.

1.      Inadequate airflow

2.      Duct air leakage

3.      Incorrect charge

4.      Oversizing

Refrigerant-charge testing and adjustment come after duct test­ing and sealing, which comes after airflow measurement and improvement.

The recommended airflow rate for central air-conditioning sys­tems is between 350 CFM and 450 CFM per ton of refrigeration capacity. Heat pump airflow rate should be between 400 CFM and 450 CFM per ton.

8.21.1   Central A/C-Heat Pump Inspection and Maintenance

Air conditioners move a lot of air, and that air contains dust. The filter in the air handler catches most large dust particles. However some dust travels around or through the filter, depending on the filter and its mounting assembly. The con­denser coil outdoors isn’t protected by a filter and is usually quite dirty.

Cleaning the Condenser Coil

Dirt enters the coil from the outside. The goal of this procedure is to drive the dirt out by spraying inside to outside. With high-pressure water, however, you can drive the dirt through the coil and into the cabinet where it drains out through drain holes.

✓     Inspect the condenser coil and know that it is probably dirty even if it looks clean on the outside. Take a flat tooth­pick and scrape between the fins. Can you scrape dirt out from between the fins?

✓     Use a stiff-bristle brush to remove visible surface from the outside of the condenser coil.

✓     Apply a biodegradable coil cleaner to the outside of the coil. Then spray cold water through the coil, preferably from inside the cabinet. Many coils can tolerate a high-pressure spray but others need low-pressure spray to avoid bending the fins.

✓     Straighten bent fins with a fin comb.

Cleaning the Evaporator Coil

Dirt enters the filter, blower, and coil from the return plenum.

✓     Inspect the filter slot in the air handler or the filter grille in the return air registers. Do the filters completely fill their opening? Are the filters dirty?

✓     Inspect the blower in the air handler after disconnecting power to the unit. Can you remove significant dirt from one of the blower blades with your finger? If the blower is dirty, then the evaporator coil is also dirty.

✓     Clean the blower and evaporator. Rake surface dirt and dust off the coil with a brush. Then use an indoor coil cleaner and water to clean between the fins.

✓     Straighten bent fins with a fin comb.

8.21.2   Air-Conditioner Sizing

Calculate the correct size of an air conditioner before purchas­ing or installing it. The number of square feet of floor space that can be cooled by one ton of refrigeration capacity is determined by the home’s energy efficiency. Air-conditioners provide cool­ing most cost-effectively when they are sized accurately and run for long cycles.

The cooling-cost reduction strategy should focus on making the home more energy efficient and making the air conditioner work more efficiently. Making the home more efficient involves shading, insulation, and air-leakage reduction. Making the air conditioner more efficient involves duct sealing, duct insulation, and a quality installation.

8.21.3   Duct Leakage and System Airflow

Unfortunately, duct leakage and poor airflow afflict most air-conditioning systems. The testing and mitigation of these prob­lems was covered earlier in this chapter.

1.      See “Air Filter Effectiveness” on page 339.

2.      See “Ducted Air Distribution” on page 321.

8.21.4   Evaluating Air-Conditioner Charge

Air-conditioning replacement or service includes refrigerant charge-checking. HVAC technicians evaluate refrigerant charge by two methods depending on what type of expansion valve the air conditioner has.

1.      If the expansion valve has a fixed orifice, the technician performs a superheat test.

2.      If the valve is a thermostatic expansion valve (TXV), the technician performs a subcooling test.

These two tests indicate whether the amount of refrigerant in the system is correct, or whether there is too much or too little refrigerant. The amount of refrigerant is directly related to the efficiency of the air-conditioning system.

Perform charge-checking after the airflow tests, airflow adjust­ments, and duct-sealing are complete. Do charge-checking during the cooling season while the air conditioner is operating.

Note: In multifamily buildings, install locking refrigerant caps on refrigerant access ports, if required by code, and as required by the SWS.