Cooling Systems: Understanding the System

Inspection scope

A cooling system is not required. Some houses do not have one, and the lack of a cooling system is not a deficiency. Home inspectors should consider reporting that the house has no cooling system, especially in markets where cooling is customary and a client would assume one is present.

A cooling system may be one central system, or more than one, that cools the whole house. It may instead be one or more appliances that each cool a single room. The scope rule turns on permanence. Permanently installed cooling systems are in scope, and that includes units installed through an opening in the wall. Portable units, such as an air conditioner set in a window, are considered personal property and are out of scope. Some home inspectors elect to inspect window units anyway. Best practice either way is to report that a window unit was present and whether it was inspected. That way the client is not surprised later.

Cooling system components that are in scope of a home inspection include the air handler, the evaporator coil, the refrigerant tubes, the condenser, and related components such as condensate disposal tubes, auxiliary drain pans, and standard filters.

Two groups of add-on devices are out of scope. Special filters, such as electrostatic filters, and air purifiers (sanitation devices such as ultraviolet lights) are not part of a home inspection. Home inspectors may report that these devices are present, but they do not evaluate them.

Cooling system terms

The cooling system has many terms that a home inspector should know and be able to use. The manual defines about thirty of them. The full list appears in the Key Terms section at the end of this lesson.

Air conditioning is a broad term. Strictly speaking it covers heating, cooling, humidifying, dehumidifying, ventilating, and filtering air. In residential work it usually means just the cooling, dehumidification, ventilation, and filtration functions.

A central air conditioning system includes an air handler or a furnace, an evaporator coil, and a condenser unit. It cools the house by pulling heat out of the indoor air and moving that heat outside. When the evaporator coil and fan are inside and the condenser is outside, it is called a split system. This is the most common type of cooling system.

The air handler is the indoor unit of a heat pump. It holds a fan (the blower), the evaporator coil, and the controls. People also use "air handler" loosely for a fuel-fired forced air furnace.

The condenser unit is the outdoor unit of a split system. Inside it are the compressor, the condenser coils (the tubes around the outside of the cabinet), a fan, and the controls. The compressor squeezes the refrigerant gas into a superheated gas and provides the push that moves refrigerant around the whole system.

A heat pump is a system that both heats and cools. In cooling mode it removes heat from inside the house. In heating mode it moves heat from outside the house to inside. A package system puts the evaporator coil, fan, and condenser in one cabinet. Package units are usually outside on the ground or on the roof, but they can be installed inside.

Cooling capacity is described in Btu (British thermal units) or tons. One Btu is the heat needed to raise one pound of water one degree Fahrenheit. One ton of refrigeration equals 12,000 Btu per hour. A 3-ton system has a capacity of 36,000 Btu per hour.

Two efficiency ratings appear on data plates. SEER (seasonal energy efficiency ratio) measures the cooling efficiency of an air conditioner or of a heat pump in cooling mode. It is the cooling output over a season divided by the electricity used. The minimum allowed SEER is currently 14 for most of the United States, the highest available is around 20, and older systems may be 10 or less. Some states set different minimums. HSPF (heating seasonal performance factor) measures a heat pump's efficiency in heating mode and typically ranges between about 7 and 10.

The refrigerant (also called coolant) is the substance that flows through the system. Older systems used Freon, which is R-22. Freon has been phased out of production because of its environmental effects. The manual identifies R-410A as the current refrigerant. Newer systems use other refrigerants. Exam answers follow the manual. The two copper tubes that carry refrigerant between the condenser and the evaporator coil are the line set. The suction line is the larger tube and is insulated; it carries cool low-pressure gas back to the compressor. The liquid line is the smaller tube and is usually not insulated; it carries warm high-pressure liquid to the metering device. Missing suction line insulation is a reportable defect. Schrader valves on the condenser are where a technician attaches a gauge set to read pressures. They work like the valves on a car tire.

Distribution terms

The duct terms below are defined in this chapter of the manual.

Equipment classifications

Cooling equipment is classified in several ways.

By function. Some equipment only cools. Cooling-only equipment is usually paired with a warm-air furnace (gas, oil, or electric) that handles heating. Heat pumps do both heating and cooling.

By coverage. Central equipment serves the whole house or large parts of it. Room equipment serves one room or a few rooms. Room equipment includes window units, through-wall units, and a newer category called the mini-split.

By configuration. A split system has the evaporator coil inside and the condenser unit outside. A package system has everything in one cabinet.

By duct type. Most central equipment uses large low velocity ducts. A few systems use small high velocity ducts. High velocity systems are uncommon. Mini-split systems use no ducts at all.

By region. Two systems are found mostly in the arid west. The evaporative cooler, also called a swamp cooler, is found mostly in older houses in dry climates. It cools by pulling air through water-soaked pads, so it works best when humidity is low. Most have been replaced by central air conditioning. The radiant cooling system circulates cool water through pipes in the ceiling or floor. Heat moves into the water by conduction and radiation. Because radiant cooling only handles the sensible load (temperature) and does nothing about humidity, it is not recommended where humidity is high. It is uncommon, sometimes seen in adobe houses, and inspectors should disclaim it and recommend evaluation by a qualified contractor using the manufacturer's instructions.

By house size. Absorption coolers and absorption heat pumps are currently available only for larger houses. They burn natural gas, propane, or a renewable fuel, use ammonia as the refrigerant, and use an absorber in place of a compressor. They are mostly industrial and commercial equipment and are uncommon in houses, though one may be found as a refrigerator in an off-grid house where electricity is hard or costly to get. Home inspectors should disclaim absorption systems.

Uncommon combinations of common equipment. A dual fuel system combines two fuel types, most often an electric heat pump with a gas or oil furnace. Home inspectors should be able to inspect dual fuel systems. A hybrid heat pump combines two heat pump technologies, such as ground-source and air-source. Hybrids are very uncommon in houses.

Heat pump types: air, ground, and water source

Heat pumps are further classified by where they exchange heat with the environment.

The vast majority are air-source heat pumps. The outdoor condenser unit exchanges heat with the outside air.

Some are ground-source heat pumps. People commonly, and incorrectly, call these geothermal. The condenser and evaporator exchange heat with the ground through tubes buried in trenches or drilled into the earth. Water-source heat pumps exchange heat with water, either through tubes submerged in a lake or a well, or through a mechanical device such as a cooling tower that lets the water exchange heat with the air. In cooling mode, a ground-source or water-source heat pump dumps heat from the house into the ground or water. In heating mode it pulls heat out of the ground or water and moves it into the house.

Ground-source and water-source systems are also classified as closed-loop or open-loop. A closed-loop system circulates an exchange medium, usually water or a water and antifreeze mixture, through tubes that have no openings to the environment. Ground-source heat pumps are closed-loop. An open-loop system draws water from a lake or an aquifer (ground water) and returns the water to its source. Water-source heat pumps may be either closed-loop or open-loop.

The reason for the added expense is temperature stability. Ground and large bodies of water hold a nearly constant temperature a few feet below the surface. Air does not. An air-source heat pump loses ground at both extremes. It cannot shed heat easily into very hot air, and it cannot draw much heat out of very cold air. A heat pump working against those conditions is both inefficient and ineffective. Ground-source and water-source heat pumps avoid that problem and are reported to be up to 50 percent more energy efficient than air-source systems. In fact, air-source heat pumps become so ineffective much below 30 degrees F that supplemental electric heat strips are required in climates where the temperature regularly drops below 30 degrees F.

They remain a niche product because of the high installation cost and the five to ten years it takes to recover that cost through energy savings. Ground-source systems can need a lot of land, especially when the tubes are laid in trenches (a horizontal system). Tubes drilled into the ground (a vertical system) need less land. They are popular with buyers who want to shrink their carbon footprint.

All three heat pump types work on the same principles. The differences are in how ground-source and water-source systems are installed and connected to the ground or water, and most of those details are concealed. Home inspectors should operate every heat pump when conditions allow, and should report obvious visible defects such as leaks. Beyond that, inspection of ground-source and water-source heat pumps is out of scope, and home inspectors should recommend evaluation by a qualified HVAC contractor.

How air conditioners work

Home inspectors are not expected to repair cooling systems. A home inspector should understand how the system works well enough to recognize and interpret the defects that an inspection can reveal. This section covers the theory and the practical operation.

Basic theory

The manual gives four simplified rules about how the physical world behaves. They assume no outside mechanical force is acting on the system.

1. Heat energy flows from a hotter place to a colder place.
2. Water vapor in air diffuses from a place with more water vapor to a place with less.
3. Warm air can hold more water vapor than cool air. If air cools to its dew point, the water vapor in it may condense into liquid water.
4. Hotter air is lighter than cooler air and rises. This is the stack effect.

States of matter and latent heat

Air conditioners and heat pumps exist because of what happens when matter changes state. Matter has three common states: solid, liquid, and gas. Matter as a gas holds more heat energy than the same matter as a liquid, and a liquid holds more than the same matter as a solid.

Changing matter from one state to another takes far more energy than raising its temperature within a state. The energy involved in a change of state is called latent heat. Energy usually has to be removed to turn a gas into a liquid, or a liquid into a solid. Energy usually has to be added to turn a solid into a liquid, or a liquid into a gas. The refrigeration cycle depends on this fact. By forcing the refrigerant to boil and then condense over and over, the system moves a large amount of heat with a small amount of refrigerant.

The cooling cycle

An air-source air conditioner, and a heat pump in cooling mode, moves heat out of the house in five steps. The steps below follow the refrigerant around the loop.

1. Compression. The compressor receives cool, low-pressure refrigerant gas from the evaporator coil through the large suction line. It compresses that gas into a superheated, high-pressure gas.
2. Condensation. The superheated gas moves into the condenser coils in the outdoor unit. The fan pulls outside air across the coils, heat leaves the gas and goes into the outside air, and once enough heat is gone the gas condenses into a liquid. Less heat can be removed when the outside air is hotter, so the system gets less efficient as the outdoor temperature climbs. More coil surface lets more heat escape, which is why higher efficiency condensers usually have more coil surface area per ton of capacity.
3. Liquid line. The liquid refrigerant flows through the liquid line under high pressure toward the evaporator coil. The liquid line temperature at the condenser should measure a few degrees warmer than the outside air.
4. Metering. The high-pressure liquid enters the metering device at the evaporator coil. The metering device drops the liquid's pressure and temperature, and some of it flashes into gas. The liquid expands as it flows through the evaporator coil tubes, and as it expands it absorbs heat from the air the blower is pushing across the coil. That absorbed heat boils the liquid into a cool, low-pressure gas. This step is what actually cools the house.
5. Suction line. The cool gas flows through the larger, insulated suction line back to the compressor, and the cycle repeats. The difference between the suction line temperature and the liquid line temperature at the compressor is one way to estimate how much heat the system is pulling out of the house.

The type of metering device depends on the age of the evaporator coil.

• Unregulated orifice: found on older coils; a fixed opening that passes the same amount of refrigerant no matter what the pressure is in the evaporator coil; inefficient, and most coils that used one have been replaced.
• Thermostatic expansion valve (TXV or TEV): found in most modern standard-efficiency systems; a tube and a sensor bulb filled with CO2, together called an equalizer, sense pressure changes in the evaporator coil and open or close the valve to match.
• Electronic expansion valve (EEV): found in higher-efficiency units; gives the most precise control over how fast refrigerant enters the evaporator coil.

The heat pump heating cycle

A heat pump heats by running the same cycle backward. Two parts make that possible. The reversing valve changes the direction of refrigerant flow. The accumulator sits between the reversing valve and the compressor and feeds the compressor cool gas. Both are inside the condenser cabinet. Heating mode runs in seven steps.

1. The reversing valve inside the condenser cabinet is energized. This reverses the refrigerant flow for heating.
2. The compressor receives cool, low-pressure gas from the accumulator and turns it into superheated, high-pressure gas.
3. The superheated gas passes through the reversing valve to the coil inside the air handler. That coil is the evaporator coil in cooling mode, but now it is acting as a condenser coil. The air handler fan pulls heat out of the gas and pushes it into the house, and the gas condenses into a liquid. The gas leaving the air handler coil should be a few degrees warmer than the inside air.
4. The warm liquid flows through the liquid line under high pressure to the outdoor condenser.
5. The liquid enters a metering device in the condenser. Pressure and temperature drop, and the liquid expands through the outdoor coil, which is now acting as an evaporator. As it expands it absorbs heat from the outside air and boils into a cool, low-pressure gas. Less heat can be pulled from colder air, so the system loses efficiency as the outdoor temperature falls. Higher efficiency condensers have more coil surface per ton of heating capacity to compensate. At some point, usually somewhere in the 30s F depending on the system, there is not enough heat left in the outside air and the backup heat strips should turn on.
6. The cool gas flows through the reversing valve to the accumulator, and the cycle repeats.
7. When the outdoor temperature is near or below 32 degrees F, frost can form on the condenser fins. The heat pump senses this and starts a defrost cycle, which flips the system into air conditioning mode for a few minutes. Heat pulled from the house melts the frost. Water may be visible in and around the condenser during and after the defrost cycle, and cool air blows from the supply registers until the system returns to heat mode. This behavior is normal and is not a defect.

Determining air conditioning system capacity

Home inspectors are not required to determine or confirm whether an air conditioner's Btu capacity is correct for the house; however, they should understand how capacity is determined. That understanding is what allows a home inspector to interpret an observation and recognize a system that is significantly oversized or undersized.

Capacity should ideally be within about 1/2 ton of the calculated capacity, assuming accurate data went into the calculation. A system that is too small may not cool the house to the set temperature, especially in extreme weather, and comfort complaints follow. It may also run constantly, which can shorten its service life. A system that is too large turns on, satisfies the thermostat quickly, and shuts off. That behavior is called short-cycling. An air conditioner needs ten to twenty minutes to reach peak efficiency, so short-cycling wastes energy and raises operating cost. A short-cycling system may also fail to run long enough to dehumidify, which creates moisture problems that affect comfort and can contribute to fungal growth.

How capacity should be calculated

The cooling and heating loads for a house should be figured using the protocol in the Air Conditioning Contractors of America Manual J. The loads from Manual J are then used to pick the recommended equipment capacity from ACCA Manual S. These calculations should be done when the house is built and, in theory, whenever a new system goes in. Today they are done with computer software.

Manual J estimates both the latent and the sensible loads. The inputs that feed each one are listed below.

Rules of thumb

Home inspectors, and some HVAC contractors, use general rules to estimate capacity. These rules often produce significantly inaccurate results, because they cannot adjust for the characteristics of an individual house. In many cases they lead to equipment that is larger than required. Home inspectors should be wary of using a rule of thumb as the sole basis for reporting a system as improperly sized.

For reference, the general rule estimates run across a wide range. A poorly insulated, leaky house with single pane windows takes 800 or more Btu per 1,000 square feet. A well-insulated, air-tight house with multi-pane windows takes about 400 Btu per 1,000 square feet. Even those figures shift with geography. A figure that makes sense in Phoenix, Arizona does not make sense in Chicago, Illinois.

Numbers and Rules to Memorize

Every number in this table appears somewhere in this lesson.