AC Installation & Repair

A modern air-conditioning system is a heat pump — it does not "make cold," it moves heat. Refrigerant changes phase between liquid and vapour, exploiting the latent heat of vaporisation to absorb energy at the evaporator and reject it at the condenser. Every fault in HVAC ultimately ties back to one of those four phase boundaries operating outside its design window.

Sizing in BTU/h or kW is the first commercial decision. Rule of thumb: 600–700 W of cooling per m² for typical East-African office at 25 °C indoor / 35 °C outdoor; 900–1,000 W/m² for solar-loaded rooms or kitchens; 1,500 W/m² for server-rooms. Manual J style heat-load calculation is more precise but rule-of-thumb survives in the field for one good reason — it works for 80% of cases and forces the engineer to spend time on the other 20%.

Refrigerant choice is regulated. R-22 is phased out under the Montreal Protocol; R-410A is being replaced by R-32 (lower GWP) and R-454B for new equipment in Europe and increasingly Kenya. Mixing refrigerants is a felony in some jurisdictions and always voids warranty. Recovery, recycling and reclamation are non-optional.

Pressure / temperature relationships are the diagnostic backbone. R-410A operating at 35 °C ambient typically presents 110–125 PSI suction (corresponding to ≈ 5–7 °C evaporator saturation) and 250–280 PSI discharge (≈ 40–45 °C condenser saturation). Suction below 100 PSI almost always means low charge or restriction; discharge above 320 PSI means dirty condenser or overcharge.

Superheat (suction-line temperature minus saturation temperature at suction pressure) tells whether the evaporator is fully active. Target 8–14 °F for fixed-orifice systems, 6–10 °F for TXV. Superheat too low — flooded compressor, liquid slugging, valve damage. Too high — starved evaporator, low capacity, oil return failure.

Subcooling (saturation temperature at discharge pressure minus actual liquid-line temperature) tells whether the condenser is rejecting heat fully. Target 8–14 °F. Subcooling too low — under-charged or over-loaded; too high — over-charged or restricted liquid line.

Ducting in central systems must respect static pressure. Total external static pressure ≤ blower spec; common error is 0.8 in.WC blower forced through 1.2 in.WC ducting → blower stalls, motor draws lock-rotor amps, breakers trip. Pressure-tested ducting and balancing dampers are not optional in commercial install.

Chilled-water systems shift the heat-rejection problem into a different topology. Primary (chiller) loop maintains 6–8 °C supply / 12–14 °C return; secondary loop distributes to AHUs / FCUs. Variable-primary, primary-secondary, and decoupled topologies each have specific pump-control and ΔT-stability requirements.

Refrigerant leaks are the silent killer. A 10% loss reduces capacity 20% and kicks compressor temperature up — the system runs longer trying to satisfy thermostat, eventually burning out. Annual electronic / nitrogen leak-test plus visual oil-stain inspection is mandatory.

Filter discipline finishes the cycle. A 1" pleated filter loaded to ΔP 0.4" reduces airflow 30%, drops coil temperature, ices the evaporator, and turns the world's best AC unit into a humidifier. Monthly filter change is the single highest-leverage maintenance task in residential HVAC.