Air conditioning

Editor's note: This story is adapted from theU.S. Department of Housing and Urban Development's Residential RehabilitationInspection Guide, 2000. Clickhere for other stories in this series.

Looking down into an outdoorcompressor unit with the top cover removed. The compressor is on the upperright, controls are on the lower right, and the fan and condenser coils are tothe left.

The capacity of an existing heating or cooling system, as measured by itsability to heat or cool a specific building or space, can be determined ineither of two ways:

1) Field test. Properly sized heating and cooling systems shouldoperate at full capacity at normal yearly outside temperature extremes andshould be slightly undersized for unusual outside temperature extremes. It israre, however, that they can be checked under such conditions.

This procedure, while inexact, may provide some idea of the system'spotential capacity. When the system has a history of continuous use,maintenance, and repair, it can be assumed to have sufficient capacity. However,check with present or former building tenants on this matter. Of more concern isthe fuel efficiency of the system. Ask the local utility company or fueldistributor for records of past fuel consumption and consider this in theoverall assessment of the HVAC system.

2) Design calculation. An HVAC system's capacity can be moreaccurately determined by noting its heating or cooling output (in tons or BTUs)from information on the manufacturer's data plate and comparing it to thebuilding's heating and cooling loads. These loads can be calculated using theAir Conditioning Contractors of America's Manual or similar load calculationguide.

A rough estimate of a building's required heating equipment size in BTUsper hour (BTUH) can be obtained by using the following formula: BTUH= .33 x [square footage of building to be heated] x [difference between outsideand inside design temperatures]

The factor of .33 in this formula is based on R11 exterior walls, anR19 ceiling at the top floor or roof, and double-glazed windows. A roughestimate of a building's required cooling equipment size, in tons, can be madeby dividing the floor area by 550 (each ton equals 12,000 BTUH). Tonnage is notan adequate measure of cooling capacity in a dwelling of three or more floorswith the air handling unit located on the lowest floor, with such a layout, thetop floor can never be properly cooled. These estimates should be followed by acomplete load calculation after rehabilitation needs are firmly established.

Central air conditioning systems are defined here as electrically operatedrefrigerant-type systems used for cooling and dehumidification. Heat pumps aresimilar to central air conditioners, but are reversible and can also be used asheating devices. Air conditioning systems should be tested only when the outsideair temperature is above 65 F (18 C); below that temperature, the systemswill not operate properly and may shut down due to safety controls.

There are two types of central air conditioning systems: integraland split.

In the integral system, all mechanized components -- compressor,condenser, evaporator, and fans -- are contained in a single unit. The unit maybe located outside the building with its cold air ductwork extending into theinterior, or it may be located somewhere inside the building with its exhaustair ducted to the outside.

In the split system, the compressor and condenser are located outside thebuilding and are the evaporator is placed either directly above or below thefurnace, depending on the furnace design. Assess the condition of central airconditioning systems as follows:

Compressor and condenser. The compressor pumps refrigerant gas underhigh pressure through a condenser coil, where it gives up heat and becomes aliquid. The heat is exhausted to the outside air by the condenser fan.Compressors have a service life of 5 to 15 years, depending on the maintenancethey receive, and are the most critical component in the air conditioningsystem.

If the compressor, condenser, and condenser fan are part of a splitsystem and are located in a separate unit outside, check the air flow around theoutside unit to make sure it is unobstructed. Look for dirt and debris insidethe unit, particularly on the condenser coils and fins, and inspect allelectrical wiring and connections. The unit should be level and well-supported,and its housing intact and childproof. An electrical disconnect switch for useduring maintenance and repairs should be located within sight of the unit.Integral systems located somewhere on or in the buildng should have theircompressors placed on vibration mountings to minimize sound transmission toinhabited building spaces.

Refrigerant lines. Refrigerant lines form the link between theinterior and exterior components of a split system. The larger of the two linescarries low pressure (cold) refrigerant gas from the evaporator to thecompressor. It is about the diameter of a broom handle and should be insulatedalong its entire length. The smaller line is uninsulated and carries highpressure (warm) liquid refrigerant to the evaporator.

Check both lines for signsof damage and make sure the insulation is intact on the larger line. On itsexterior, the insulation should be protected from ultraviolet damage by acovering or by white paint. Sometimes a sight glass is provided on the smallerline; if so, the flow of refrigerant should look smooth through the glass.Bubbles in the flow indicate a deficiency of refrigerant in the system. Frost onany exposed parts of the larger line also indicates a refrigerant deficiency.

Seismic vulnerability. If the building is in seismic zones 3 or 4(California and portions of Alaska, Arkansas, Hawaii, Idaho, Missouri, Montana,Nevada, Oregon, Utah, Wyoming, and Washington), check roof-mounted compressorand condenser units for the presence of seismic bracing to the structure.

Evaporator. The evaporator is enclosed in the air distributionductwork and can only be observed by removing a panel or part of the furnaceplenum. High pressure liquid refrigerant enters the evaporator and expands intoa gas, absorbing heat from the surrounding air. Air is pushed past theevaporator coil by the system's circulation blower; in the process, watervapor from the air condenses on the evaporator coil and drips into a drain pan.From there, it is directed to a condensate drain line that may sometimes includea condensate pump. The drain line empties into a house drain or directly to thebuilding's exterior.

Examine the ductwork around the evaporator for signs ofair leakage and check below the evaporator for signs of water leakage due to ablocked condensate drain line. Such leakage can present a serious problem if theevaporator is located above a warm air furnace, where dripping condensate watercan rust the heat exchanger, or above a ceiling, where it can damage thebuilding components below.

Follow the condensate line and make sure that itterminates in a proper location. If there is a pump on the line, check itsoperation. In split systems where the evaporator is located in an attic orcloset, the condensate drain pan should have an auxiliary condensate drain linelocated above the regular drain line or an auxiliary drain pan that isseparately drained. The connection of a condensate drain line to a plumbing ventin the attic may violate local codes. Check for such violations.

A leaking condensate tray

Central air conditioning systems can be tested by an HVAC service technicianto determine their overall condition and operational efficiency. This testrequires a variety of specialized equipment and involves: 1) testing thepressure in the refrigerant lines, 2) taking amperage readings on thecompressor, and 3) taking temperature readings of the air passing over thecondenser and the evaporator coils, and correlating these readings with ambientoutside temperature conditions.

Distribution system

Cool air distribution ductwork and controls, including zone controls, shouldbe inspected similarly to those for forced warm air heating systems. Thedistribution system is made up of supply and return ducts, filters, dampers, andregisters. Supply and return ducts may be made of sheet metal, glassfiber, or other materials. Glass fiber ducts are self-insulated, but sheet metalducts are usually not insulated except where they pass through uncooled (orunheated) spaces. Sheet metal ducts are occasionally insulated on theinside; determine the presence of insulation by tapping on the duct andlistening for a dull sound.

Check ducts for open joints and air leakage wherever the ducts are exposed.Examine them for dirt build up by removing several room registers and inspectingthe duct. Ducts can be cleaned by a heating contractor. Supply ducts are oftenprovided with manual dampers to balance air flow in the distribution system.Locate them by looking for small damper handles extending below the ductwork.Check their operation. In zoned systems, automatically controlled dampers may belocated in the ductwork.

Check the location of supply and return registers in each room. Cold airregisters are most effective when located high on the walls or in the ceiling(warm air registers, when positioned low on the exterior wall). Return registersshould be on opposite sides of the room from supply registers. If returnregisters are located in a hallway or a different room, make sure interveningdoors are undercut by about one inch.

DON'T DO THIS: Installation of thisair conditioner destroyed the structural integrity of the bearing wall betweenthe two windows.

Window units are portable, integral air conditioning systemswithout ductwork. Inspect their overall condition and checkthe seal around each unit and its attachment to the window or wall. Ensure thatit is adequately supported and look for obstructions to air flow on the exteriorand for proper condensate drainage. Make sure all electrical service is properlysized and that each unit is properly grounded. Bent fins on the condenser coilsmay be "combed."