CT HVAC cheat sheet

EPA Section 608 prohibits knowingly venting ozone-depleting refrigerants and their non-exempt substitute refrigerants to the atmosphere during service, maintenance, or disposal.

Evacuation levels before opening a system: appliances built after Nov 15, 1993 require 10 in. Hg vacuum (4 in. Hg if the compressor is inoperative); small appliances may use 80% recovery or 4 in. Hg.

Recovery cylinders must never be filled beyond 80% of capacity by weight, because liquid refrigerant expands as it warms and a hydraulically full cylinder can rupture.

Never mix two different refrigerants in one recovery cylinder; a mixed cylinder cannot be reclaimed and must be destroyed.

A2L refrigerants are mildly flammable (burning velocity no greater than 10 cm/s); charge limits and ignition-source control follow ASHRAE 15 and UL 60335-2-40.

OSHA's permissible exposure limit for carbon monoxide is 50 ppm as an 8-hour time-weighted average; NIOSH recommends 35 ppm as the ceiling for worker exposure.

Refrigerant vapor is heavier than air and displaces oxygen in pits, basements, and low spaces; atmosphere must be tested before entry.

Liquid refrigerant at atmospheric pressure boils far below 0°F, so safety goggles and insulating gloves are required to prevent frostbite on contact.

Lockout/tagout plus capacitor discharge is required before electrical service; stored charge can remain in run and start capacitors after power is removed.

Manifold gauge color code: blue hose and gauge = low side, red = high side, yellow = center port for recovery, evacuation, or charging.

Superheat (°F) = measured suction line temperature minus the saturation temperature at the measured suction pressure.

Subcooling (°F) = saturation temperature at the measured liquid-line pressure minus the measured liquid-line temperature.

Evacuation is confirmed with an electronic micron gauge, not a manifold gauge; a standing vacuum test holds below 500 microns for 10–15 minutes with the pump isolated.

1 in. w.c. = 249 pascals = 0.0361 psi; duct static pressures are read with a manometer, Magnehelic gauge, or digital differential pressure instrument.

Cfm = velocity in fpm × free area in square feet; velocity comes from an anemometer, velometer, or pitot tube traverse.

Clamp meters must be true RMS to read inverter-driven compressor and ECM blower current accurately; averaging meters under-report distorted waveforms.

Infrared thermometers misread shiny or reflective surfaces because of low emissivity; contact probes or thermocouples are used for refrigerant line temperatures.

Electronic leak detectors are refrigerant-family specific; a nitrogen pressure test with soap bubbles confirms the exact leak location.

The four core components are the compressor, condenser, metering device, and evaporator; the metering device separates the high side from the low side.

1 ton of refrigeration = 12,000 Btu/h = 3.517 kW = 200 Btu/min.

Saturation means a refrigerant boils and condenses at a single temperature for a given pressure (ignoring glide in blends).

Compression ratio = absolute discharge pressure ÷ absolute suction pressure, both in psia; high ratios raise discharge temperature and reduce volumetric efficiency.

Superheat is added after the evaporator; subcooling is produced in the bottom of the condenser as liquid backs up behind the metering device.

Latent heat of vaporization of water at atmospheric pressure is about 970 Btu/lb; most evaporator capacity is latent in humid climates.

R-410A is a near-azeotropic blend with about 0.2°F glide and operates at roughly 50–60% higher pressures than R-22.

Zeotropic blends fractionate: a vapor leak leaves a liquid charge with a different composition, so blends are charged as liquid.

Flash gas at the evaporator inlet consumes coil surface without absorbing room heat, so the metering device must pass liquid only.

Ohm's law: V = I × R; resistive power P = V × I; single-phase power P = V × I × power factor.

Balanced three-phase power: P = √3 × V × I × power factor.

Series circuits share one current and add voltages; parallel circuits share one voltage and add branch currents.

Capacitors must be discharged before testing; a capacitance reading more than about 10% below nameplate indicates a weak run capacitor.

Locked rotor amps are typically 5–6 times rated load amps; a compressor that stays at LRA without starting has a start-circuit or mechanical fault.

Residential 24 V control circuits are Class 2; R is 24 V hot, C is common, Y is cooling, W is heating, G is indoor fan.

ECM blowers are electronically commutated motors: line voltage powers them, a low-voltage signal sets speed, and airflow is programmed to stay constant.

A contactor coil is rated by voltage and VA; pitted or welded contacts cause continuous compressor operation or single-phasing.

Cooling-call sequence: thermostat closes Y and G, contactor coil energizes, compressor and condenser fan start, indoor blower starts.

Cfm = velocity (fpm) × duct cross-sectional area (sq ft); round duct area = π r².

Sensible Btu/h = 1.08 × cfm × dry-bulb temperature difference.

Total Btu/h = 4.5 × cfm × enthalpy difference; latent Btu/h = 0.68 × cfm × grains difference.

Residential blowers are rated at a total external static pressure, commonly 0.5 in. w.c.; readings above the fan table value indicate restrictive filters, coils, or ducts.

Equal-friction duct sizing commonly targets about 0.1 in. w.c. of friction loss per 100 ft of duct run.

Return grilles are sized for roughly 400–500 fpm face velocity and filters for about 300 fpm to limit pressure drop.

Duct leakage is measured with a duct blaster that pressurizes the system to 25 Pa and reports leakage in cfm25.

ASHRAE 62.2 residential ventilation rate: Qtot = 0.03 × conditioned floor area (ft²) + 7.5 × (number of bedrooms + 1), in cfm.

Indoor relative humidity should stay roughly 30–50%; sustained values above 60% support mold and dust mite growth.

Temperature rise = supply air temperature − return air temperature; it must fall inside the range printed on the rating plate at the correct airflow.

Natural gas manifold pressure is typically 3.5 in. w.c.; LP gas manifold pressure is typically 10–11 in. w.c.

Electric resistance heat delivers 3.413 Btu per watt-hour.

AFUE is annual fuel utilization efficiency; condensing furnaces exceed 90% AFUE by recovering latent heat in a secondary heat exchanger.

Condensing furnace flue pipe must slope back toward the furnace so acidic condensate drains to a trapped condensate line.

Flame rectification sensors typically read 2–6 microamps; the control module closes the gas valve when the signal falls near 1 microamp.

Natural-draft appliances depend on a draft hood and adequate combustion air; a spillage test at the draft hood verifies proper venting.

A heat pump balance point is the outdoor temperature where heat pump capacity equals building load; below it, auxiliary or emergency heat carries the load.

Defrost may terminate on coil temperature, pressure, or time, and the reversing valve shifts the system to cooling mode during defrost.

Common compressor types are reciprocating, scroll, and rotary; scroll compressors dominate residential split systems.

Low evaporator airflow lowers suction pressure and can produce coil icing, low superheat, and liquid floodback to the compressor.

Dirty or blocked condenser coils raise discharge pressure and temperature, increasing compression ratio and compressor amp draw.

A TXV modulates to hold constant evaporator superheat; a fixed-orifice piston's superheat varies with load and ambient conditions.

Typical target subcooling on a TXV system is about 10–12°F, matched to the manufacturer's charging chart.

Target superheat on fixed-orifice systems is read from a charging chart using indoor wet-bulb and outdoor dry-bulb temperature.

Condensate drains on a negative-pressure coil must include a trap and continuous slope, or the pan will not drain.

A crankcase heater keeps oil warm during the off cycle to prevent refrigerant migration and oil dilution; it must be energized before restart after long shutdowns.

A filter drier removes moisture and particulates; a flashing liquid line on a sight glass indicates a restriction or undercharge in many systems.

Commissioning order: verify airflow and static pressure first, confirm refrigerant charge second, then measure the air temperature split.

Cooling temperature split is typically 16–22°F; a low split suggests low load or excess airflow, a high split suggests low airflow.

New installations are charged by weight; field verification uses subcooling on TXV systems and superheat on fixed-orifice systems.

Compare measured compressor current to nameplate RLA; current well above RLA indicates high head pressure, low voltage, or a failing compressor.

Combustion analysis reports O2, CO, CO2, and stack temperature; air-free CO above about 100 ppm in flue gas indicates a combustion problem requiring correction.

High suction plus high head points to overcharge, excessive load, or poor condenser airflow; low suction plus low head points to undercharge or a restriction.

On a TXV system, high superheat with low subcooling indicates undercharge; low superheat with high subcooling indicates overcharge.

A blower door test pressurizes the house to 50 Pa and reports ACH50, quantifying envelope leakage for load and infiltration estimates.

Recording baseline pressures, temperatures, amps, and static pressures at start-up provides the reference for later diagnostics.