Air Handler Units and Coils Flashcards

What is the primary function of an air handler unit (AHU) in an HVAC system?

Answer: Condition and circulate air throughout the building

An air handler unit (AHU) conditions and circulates air. It typically contains a blower, heating/cooling coils, filters, dampers, and controls to deliver conditioned air to occupied spaces.

What causes 'coil freeze-up' on an evaporator coil?

Answer: Insufficient airflow or low refrigerant charge causing suction pressure to drop below freezing

Coil freeze-up occurs when suction pressure drops low enough that coil surface temperature falls below 32°F. Causes include low airflow (dirty filter, failed blower) or low refrigerant charge, both of which reduce suction pressure.

What is the recommended face velocity for evaporator coils in air handling units?

Answer: 300-500 FPM

The recommended face velocity for evaporator coils is typically 300-500 FPM (feet per minute). Higher velocities can cause moisture carryover; lower velocities reduce efficiency and increase coil size requirements.

What type of coil is used in an air handler for the heating mode in a heat pump system?

Answer: The same evaporator coil, which acts as a condenser during heating mode

In a heat pump system, the indoor coil serves dual purposes — it acts as the evaporator in cooling mode and as the condenser in heating mode. The refrigerant flow is reversed by the reversing valve.

What does the 'fin spacing' on a coil affect?

Answer: Heat transfer surface area and airflow resistance

Fin spacing (fins per inch) affects the coil's heat transfer surface area and air-side pressure drop. Tighter fin spacing increases surface area for better heat transfer but also increases airflow resistance.

What is the purpose of the condensate drain pan in an air handler?

Answer: To collect water condensed from humid air on the evaporator coil

As warm, humid air passes over the cold evaporator coil, moisture condenses on the coil surface and drips into the drain pan, which routes it to a drain. Clogged drain pans cause water damage and microbial growth.

What is 'sensible heat ratio' (SHR) and why is it important for coil selection?

Answer: The ratio of sensible cooling capacity to total cooling capacity of a coil

SHR is the ratio of sensible cooling (temperature reduction) to total cooling (sensible + latent). A lower SHR means the coil removes more moisture. Matching SHR to the application's latent load requirements is critical for comfort and indoor air quality.

What material are most residential evaporator coil fins made from?

Answer: Aluminum

Most residential evaporator coil fins are made of aluminum because it is lightweight, inexpensive, and has good thermal conductivity. The tubing is typically copper, which provides excellent heat transfer and is easy to braze.

What is 'coil bypass factor' in HVAC?

Answer: The percentage of airflow that bypasses the coil without making contact with the coil surface

Coil bypass factor (CBF) is the fraction of air that passes through the coil without being fully conditioned — it 'bypasses' the coil surface. A lower bypass factor means more air contacts the coil, improving dehumidification and cooling.

What is the most common cause of a clogged condensate drain line?

Answer: Algae and biological growth in the drain line

Algae and biological growth (slime, mold, mildew) are the most common cause of clogged condensate drain lines. The warm, dark, moist conditions in the drain pan and line are ideal for microbial growth.

What is the correct refrigerant superheat at the evaporator outlet for a TXV-equipped system?

Answer: 8-12°F (at the outlet of the coil)

For TXV-controlled systems, the target superheat at the evaporator outlet is typically 8-12°F. The TXV modulates refrigerant flow to maintain this superheat, ensuring the coil is fully utilized without liquid refrigerant reaching the compressor.

What is 'dirty sock syndrome' in HVAC systems?

Answer: Foul odors caused by biological growth on the evaporator coil when the system cycles

Dirty sock syndrome is caused by biological growth (bacteria, mold) on the evaporator coil, typically on coating or brazing materials. The odor is most noticeable at startup when the coil is heating up during the off cycle.

What is the purpose of a drain pan float switch on an air handler?

Answer: To shut down the system if the drain pan fills with water to prevent overflow damage

A drain pan float switch is a safety device that shuts down the system when the condensate drain pan fills with water, typically due to a clogged drain line. This prevents water overflow and potential water damage.

What does 'coil circuiting' refer to in evaporator coil design?

Answer: How refrigerant tubes are arranged and connected to distribute refrigerant flow through the coil

Coil circuiting refers to how refrigerant flows through the multiple tube circuits of the coil. Proper circuiting ensures even refrigerant distribution, optimizes superheat across the coil face, and maximizes heat transfer efficiency.

What is an 'A-coil' configuration in residential HVAC?

Answer: An evaporator coil arranged in an inverted V or A shape with two slanted coil sections

An A-coil (or 'A-frame coil') has two coil sections arranged in an inverted V (A shape). This configuration increases the coil face area within a smaller footprint, fitting inside a furnace plenum and providing good airflow distribution.

What is the purpose of indoor coil subcooling measurement?

Answer: To determine if the refrigerant charge is correct when using the TXV charging method

Subcooling (measured at the liquid line or condenser outlet) indicates how much the liquid refrigerant has been cooled below its saturation temperature. The subcooling method is used with TXV systems to verify correct refrigerant charge.

What causes 'refrigerant flood-back' to the compressor from the evaporator coil?

Answer: Liquid refrigerant returning to the compressor due to low superheat or overcharge

Flood-back occurs when liquid refrigerant (not fully vaporized) returns to the compressor. Causes include a defective TXV (stuck open), overcharge, or very low load conditions. Liquid refrigerant damages the compressor through hydraulic slugging.

What is the function of a 'hard-start kit' (potential relay and start capacitor) on an air handler?

Answer: It is not used on air handlers — only on compressors

Hard-start kits (potential relay + start capacitor) are installed on compressors, not air handlers. They boost starting torque to help compressors start under high pressure differential or during low voltage conditions.

How should coil cleaning solution be applied on an evaporator coil?

Answer: Applied from the air outlet side, against the direction of airflow, to push debris out the inlet

Coil cleaner should be applied from the clean air (leaving air) side and rinsed from the dirty air (entering air) side — working against airflow direction. This pushes contaminants out the same way air brought them in.

What is 'enthalpy' in HVAC, and how is it relevant to coil performance?

Answer: Total heat content of air (sensible + latent), used to calculate total coil capacity

Enthalpy is the total heat content of air, combining both sensible heat (temperature) and latent heat (moisture content). Total coil cooling capacity is calculated from the enthalpy difference between entering and leaving air, multiplied by airflow.