Cooling Coil

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Cooling coil
Evaporator coils come in a variety of shapes and sizes, depending on the type of installation, the amount of cooling capacity needed, and the manufacturer. It is the source of cooling as air passes through the furnace or air handler. They are constructed of aluminum finned copper tubing. The copper tubing runs perpendicular to the aluminum fins, making U-turns back and forth until the desired coil size is achieved. Added cooling capacity without an increase in length and width is accomplished by adding more rows of copper tubing. Slant coils and horizontal coils have a slab appearance, similar to the radiator in an automobile. They can be installed in ductwork running horizontally or in an air handler. An A-coil is shaped like a capital A without the crossbar. It can be installed on top of a fuel burning furnace heat exchanger or in an air handler. The newest design is the multiflex coil which is a series of A-coils linked together at the base. The multi-flex coil can be installed in any position when encased in a special cabinet. All evaporator coils must have a drain pan to collect the water that condenses as the air flowing across the coil cools. The water can drain away by gravity or be pumped away. The cooling effect that takes place inside the coil requires a pressure drop in the refrigerant. This drop can be accomplished in a number of ways: capillary tube, piston or orifice, or thermostatic expansion valve. A capillary tube is a thin copper tube of predetermined length into which the compressed liquid refrigerant is pumped. The length of the tubing causes the pressure drop and subsequent cooling effect of the refrigerant. A piston or orifice blocks the flow of refrigerant and forces it through a tiny hole, creating the needed pressure drop. A thermostatic expansion valve meters the flow of refrigerant to meet the cooling demand of the coil. It determines this demand by way of a sensing bulb attached to the outlet tube on the coil. Because it can meter the flow to meet demand, the expansion valve can keep the coil at optimum cooling potential.

Here are some common defects to look for at the evaporator coil (cooling coil) in an air conditioner or heat

Dirt or debris blocking air flow through the coil (DIRTY COOLING COIL)

Damaged cooling / evaporator coil fins over more than 10% of the coil surface, blocking air flow (shown in our photo at left in this case, the damage is to a condensing coil, not an evaporator coil). Small areas of damaged cooling fins can be straightened and cleaned-up using a cooling coil comb. Cooling coils with extensive physical damage such as shown in our photograph need to be replaced. Evidence of refrigerant leaks (visual evidence may include stains from refrigerant oil left at the point of leakage) (REFRIGERANT LEAK DETECTION)  Evidence of mold growth on organic debris on the coil or elsewhere in the blower compartment (Mold Growth in Air Handlers)  Presence of unusual materials on the coil surface such as rodent debris, bird feathers and debris, fiberglass insulation, large trash fragments like paper or leaves confirming a duct or air filter problem. Some of these may indicate potentially serious health risks such as rodent or bird feces and debris which risk bacterial and viral hazards in building air. (Leaks, Rodents In Air Handlers)  Obvious coil-to-air-handler size mismatch of an add-on cooling coil onto an existing warm air system (ADDING A/C: RETROFIT SIZING)  Evaporator coil or cooling leaks or holes: if an evaporator coil is leaking (or also if the condensing coil is leaking) you'll find out pretty quickly as refrigerant will be lost and the cooling system will stop providing cool air. You'll need expert diagnosis by an HVAC service technician.


A lot depends on where the refrigerant leak has occurred and what caused the leak. If the cooling coil has a single point leak caused by some mechanical damage (one of our readers accidentally drilled a hole in his coil while trying to drill a drain hole in his air handler), it may be possible to find the hole and repair it using silver solder. If the refrigerant leak is in copper tubing anywhere in the cooling or heat pump system that is not too close to an evaporator coil or condensing coil, it should be possible to solder a repair, then evacuate and recharge the cooling system. If the refrigerant leak is in copper tubing in or close to the cooling coil (or in a condensing coil) a solder repair is hard to complete because the heat of the soldering process tends to de-solder other nearby connections. It might be possible if the technician is very expert and if s/he knows how to keep nearby surfaces cooled (we've used a wet rag). If the refrigerant leak is in an aluminum part, soldering aluminum is more tricky and may not be feasible. Ordinary procedures using a torch, for example, just melt the aluminum. Expert welders use inert gas welding methods. If the refrigerant leak is due to severe corrosion anywhere in an HVAC system we're not optimistic that a solder repair is possible. The conditions that caused a corrosion-related leak are likely to have thinned and weakened other parts. The cost of an attempted repair may be wasted. Replacement of the cooling coil (or condensing coil) is more often going to be recommended by your HVAC technician because of these difficulties.











How Air Conditioning & Heat Pump Evaporator Coils (Cooling coils) are Cleaned Evaporator coil cleaning often requires cutting refrigerant lines, removal of the coil
and other components for cleaning, and reinstallation, pulling a vacuum on the refrigerant lines, and recharge with refrigerant. Such service and repair may involve significant expense, although there are some "in place" cleaning methods using foams and sprays that are a simpler procedure.

FROST BUILD-UP - Frost Build-up on the Evaporator Coil in an Air Conditioner
The ice or frost formed on a cooling coil in an air conditioner air handler unit is usually caused by an improper refrigerant charge, possibly by inadequate air flow across the cooling coil, or by a thermostatic expansion valve (TEV) or other air conditioner or heat pump control defect. Ice blocks air flow through the coil, thus reducing air conditioner output; if the ice formation is extreme nearly all of the airflow across the coil is blocked and the air conditioner system runs but does not produce cool air flowing into the occupied space. Frost and ice can also form on refrigerant tubing at other locations, and frost and ice can form inside air conditioning duct work itself, leading to troublesome leaks into the building. Details of what causes frost on air conditioning equipment, what problems that creates, and how to diagnose and repair icing or frost on cooling coils or other air conditioner parts are provided at FROST BUILD-UP. There we discuss locations and causes of condensate, frost or ice formation in air conditioning systems, air handlers, compressor/condensers, refrigerant lines, and in air ducts.

BLOCKED COOLING COIL - Air Conditioner Evaporator Coil Blocked by Debris
Ice is not the only (nor even the most common) cause of blocked air flow in an air conditioner. This photograph shows how easily debris can stick to and clog the inlet side of the cooling coil in an air conditioning system. This evaporator coil was nearly totally blocked with dust and debris. How does this happen? There was no air filter installed in the system. Ordinary house dust is comprised largely of fabric fibers and skin cells. These and other debris in building dust such as soot and organic particles like pollen and mold spores all join to form a gray mat on the fins of the cooling coil in an air handler. Debris sticks particularly quickly to this surface because of the combination of close spacing of the cooling fins (about 1/16" apart) and the fact that condensate forming on the coil keeps the surface damp. Details about the detection and cleaning of dirt and debris which block an air conditioner cooling coil are at DIRTY COOLING COIL.

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