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11.2 Filter, Coil & Drain Maintenance for Alabama's Humid Climate
Alabama HVAC Contractor License Exam
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1 Alabama HACR Laws, Rules & Licensing
1.1 Alabama HACR Board Authority, Licensing Pathways & Experience Requirements 1.2 Contractor Bonding, Insurance, Fees & Annual Renewal 1.3 Permits, Local Jurisdictions, Inspections & Disciplinary Actions
2 Contractor Business, Contracts, Estimating & Alabama Lien Law
2.1 Business Organization, Risk Management & Project Administration 2.2 Construction Contracts, Change Orders & Warranties 2.3 Alabama Mechanics' Lien Law & Financial Management 2.4 Construction Estimating, Markup & Bid Preparation 2.5 Project Management, Scheduling & Field Supervision 2.6 Risk Management, Contractor Insurance & Surety Bonds
3 Safety Standards, OSHA & EPA Section 608
3.1 OSHA Construction Safety (29 CFR 1926) for HVAC Contractors 3.2 EPA Section 608 Refrigerant Regulations & Recovery Requirements 3.3 Refrigerant Management, Leak Repair Rules & A2L Transition
4 General Technical Knowledge & Thermodynamics
4.1 Principles of Heat Transfer, Temperature & Pressure 4.2 The Mechanical Refrigeration Cycle & Refrigerant Properties 4.3 Psychrometrics & Air Properties for Humid Climates 4.4 Basic Electricity, Ohm's Law & Electrical Power for HVAC
5 Piping Systems, Refrigerant Lines & Fuel Gas Piping
5.1 Refrigerant Piping Sizing, Materials & Installation 5.2 Fuel Gas Piping Sizing & Installation (IRC Chapter 24) 5.3 Condensate Drain Systems & Trapping
6 System Sizing & ACCA Manual J Load Calculations
6.1 ACCA Manual J Fundamentals & Design Conditions 6.2 Envelope Loads: Conduction, Radiation & Fenestration 6.3 Infiltration, Ventilation & Internal Heat Gains
7 Duct System Design & ACCA Manual D
7.1 Airflow Principles, Static Pressure & Fan Performance 7.2 ACCA Manual D Duct Sizing & Friction Rate 7.3 Duct Materials, Installation Standards & Air Distribution
8 Heating & Cooling Equipment Operation
8.1 Heat Pump Systems & Supplemental Heating 8.2 Gas Furnaces & Combustion Systems 8.3 Air Conditioning Units, Compressors & Metering Devices
9 Mechanical Codes, Electrical Applications & Maintenance Diagnostics
9.1 Combustion Air & Venting Systems (IRC Chapter 24) 9.2 Electrical Applications for HVAC (NFPA 70 / NEC Article 440) 9.3 Preventative Maintenance, Troubleshooting & Plan Reading
10 Employment Law, Payroll Taxes & Contractor Finance
10.1 Federal & Alabama Labor Laws for HVAC Employers 10.2 Payroll, Employment Taxes & Worker Classification 10.3 Job Costing, Break-Even Analysis & Cash Flow Forecasting
11 Preventative Maintenance, Service Procedures & Commissioning
11.1 Preventative Maintenance Programs & Service Agreements 11.2 Filter, Coil & Drain Maintenance for Alabama's Humid Climate 11.3 Evacuation, Dehydration & Refrigerant Charging Procedures 11.4 Airflow Verification, Static Pressure Testing & Commissioning
12 Electrical Applications & Construction Documents
12.1 Electric Motors, Compressors & Motor Protection 12.2 Transformers, Control Circuits & Low-Voltage Wiring 12.3 Branch Circuits, Conductors, Grounding & Disconnects 12.4 Construction Documents, Specifications & Submittals
11.2 Filter, Coil & Drain Maintenance for Alabama's Humid Climate
Key Takeaways Explain This Topic AI
MERV rates filtration efficiency on 0.3 to 10 micron particles, and raising MERV without checking static pressure can starve the blower and drop system capacity.
A one-inch pleated filter installed in a return grille sized for fiberglass media is the most common cause of high static pressure in Alabama residential systems.
A dirty condenser coil raises head pressure and compression ratio, cutting capacity and efficiency while driving discharge temperature toward compressor oil breakdown.
A negative-pressure evaporator drain must be trapped deep enough to overcome the cabinet negative pressure, or the drain will not flow no matter how clean the line is.
IRC M1411.3.1 requires secondary condensate protection whenever damage to a building component could result, and in Alabama attic installations this is normally an auxiliary pan with a float switch.
Last updated: September 2026
11.2 Filter, Coil & Drain Maintenance for Alabama's Humid Climate
Important
Why Alabama is different. Mobile and Montgomery see summer design conditions near 93 to 95 degrees dry bulb with coincident wet bulbs of 77 to 78 degrees. A residential evaporator in that climate runs wet for months, condensing 3 to 8 gallons of water per day. Everything in this section - drain hydraulics, biological growth, coil fouling - is a consequence of that continuous condensation.
Filters: MERV Versus Static Pressure
Minimum Efficiency Reporting Value (MERV) rates a filter's ability to capture particles between 0.3 and 10 microns, on a scale from 1 to 16 for HVAC media.
- MERV Captures Typical use
- 1-4 Lint, large dust, pollen Fiberglass throwaway; equipment protection only
- 5-8 Mold spores, dust mite debris, hair spray Standard pleated residential media
- 9-12 Legionella, humidifier dust, lead dust Better residential and light commercial
- 13-16 Bacteria, tobacco smoke, sneeze droplets Health-driven applications; needs a designed filter bank
Warning
Raising MERV without raising filter area is a capacity cut. Denser media has a higher pressure drop. A residential blower rated for 0.50 in. w.c. of total external static pressure loses airflow rapidly as filter drop climbs. The correct way to reach a higher MERV is to increase the face area - a four-inch or five-inch deep media cabinet, or a second return - so the higher-efficiency media is spread across more square feet and the velocity through it drops.
Field practice : measure the pressure drop across the filter with a manometer at installation and again when it is due. Replace on measured drop, not on a calendar guess. A clean one-inch pleat may start at 0.10 in. w.c. and reach 0.30 in. w.c. loaded - a 0.20 in. w.c. loss that comes straight out of the airflow budget.
Evaporator Coils
An evaporator coil in a humid climate is a continuously wet surface downstream of the filter. Anything that bypasses the filter sticks to it.
Symptoms of fouling : falling temperature split, falling suction pressure, rising superheat at a fixed metering device, ice forming on the suction line, and water blowing off the coil into the plenum.
Cleaning : for light fouling, brush and vacuum with the fins, then apply a no-rinse alkaline foaming cleaner and let condensate carry the residue to the drain. For heavy fouling, pull the coil and rinse it from the leaving-air side backward so debris exits the way it entered.
Chemistry caution : acidic cleaners attack aluminum fin stock and can pit copper. On coated or microchannel coils use only the manufacturer-approved chemistry. Never use a caustic condenser cleaner on an indoor coil that drains into a plastic pan.
Blower wheel : a loaded blower wheel loses airflow just as effectively as a plugged coil, and it is routinely missed. Look at the concave side of each blade; if the blades are packed with felt, the wheel must be pulled and washed.
Condenser Coils Compression Ratio = Absolute Discharge Pressure Absolute Suction Pressure \text{Compression Ratio} = \frac{\text{Absolute Discharge Pressure}}{\text{Absolute Suction Pressure}} Compression Ratio = Absolute Suction Pressure Absolute Discharge Pressure
A fouled condenser cannot reject heat, so condensing temperature and head pressure climb. The compression ratio climbs with it, volumetric efficiency falls, capacity falls, amperage rises, and discharge temperature rises toward the point where the oil begins to break down and form acid.
Worked illustration : an R-410A system with a 130 psig suction and a 350 psig discharge has a compression ratio of ( 350 + 14.7 ) / ( 130 + 14.7 ) = 364.7 / 144.7 = 2.52 (350 + 14.7)/(130 + 14.7) = 364.7 / 144.7 = 2.52 ( 350 + 14.7 ) / ( 130 + 14.7 ) = 364.7/144.7 = 2.52 . Let the condenser foul until discharge reaches 450 psig and the ratio becomes 464.7 / 144.7 = 3.21 464.7 / 144.7 = 3.21 464.7/144.7 = 3.21 - a 27 percent increase in the work the compressor must do for the same cooling.
Cleaning practice : shut off and lock out power. Remove the top and fan assembly on units that draw air through all four sides. Rinse from the inside outward , opposite the airflow, at moderate pressure. A pressure washer will fold fins flat and destroy the coil. Straighten bent fins with the correct fin comb.
Condensate: Hydraulics, Not Housekeeping
This is where most Alabama service calls in July actually come from.
Negative-Pressure Drains Need a Deep Enough Trap
A draw-through evaporator sits on the suction side of the blower, so the drain pan is under negative pressure . If the trap seal is shallower than the cabinet negative pressure, the blower simply holds the water in the pan and pulls air backward up the drain. Nothing flows, and the pan overflows even though the line is clear.
Rule of thumb : the trap seal depth must exceed the maximum cabinet negative pressure, with the outlet leg lower than the inlet leg by at least that same amount plus an allowance. A system operating at 1.0 in. w.c. of negative pressure at the pan needs roughly a 2-inch inlet leg and a 1-inch outlet differential; undersized traps are the classic cause of "the drain is clear but it still overflows."
Blow-Through Coils Trap Differently
A blow-through arrangement puts the pan under positive pressure, which pushes water out of the drain but also blows air out of it. Both arrangements require a trap; only the geometry differs.
Biological Growth
A wet, dark, 55-degree pan in a 78-degree attic grows biofilm continuously. Control it with condensate pan tablets rated for the pan material, an annual drain flush, and clean coils. Do not pour bleach into a pan connected to galvanized or copper components - sodium hypochlorite attacks both.
Secondary Protection Is Code, Not Courtesy
Under IRC Section M1411.3.1 , where damage to any building component could result from condensate overflow, one of four protections is required: an auxiliary drain pan with a separate drain, a separate overflow drain line from the equipment pan, an auxiliary pan with a water-level detection device that shuts the equipment down, or a water-level detection device in the primary pan or drain line that shuts the equipment down. In Alabama attic installations the auxiliary pan with a float switch is the practical standard, and the secondary drain must terminate in a conspicuous location so the homeowner sees it dripping.