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, and the units matter because the common rules of thumb are quoted in BTU/h. For a typical East-African office at 25 °C indoor and 35 °C outdoor, allow roughly 600–700 BTU/h per m², which is about 176–205 W/m². Solar-loaded rooms and kitchens run nearer 900–1,000 BTU/h per m² (about 264–293 W/m²), and server rooms start around 1,500 BTU/h per m² (about 440 W/m²) and climb steeply with rack density. Convert carefully — 1 kW is 3,412 BTU/h, and reading a BTU/h rule of thumb as though it were watts oversizes the plant by roughly three and a half times, which is exactly how a building ends up with equipment that short-cycles and never dehumidifies. 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. On a TXV system the valve actively holds superheat, so expect a tight band of roughly 8–12 °F at the evaporator outlet and treat a wandering value as a hunting or failed valve. On a fixed-orifice or piston system there is no such regulation: superheat floats with indoor wet-bulb and outdoor dry-bulb, so there is no single target figure and the correct value must be read off the manufacturer's superheat chart for the conditions on the day. Charging a fixed-orifice system to a TXV number is one of the most common ways a healthy system is wrongly diagnosed and then over- or under-charged. 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.

Sensible cooling and latent cooling are different jobs, and conflating them is why so many correctly-sized systems still feel clammy. Total capacity splits into a sensible part that lowers dry-bulb temperature and a latent part that condenses moisture out of the air. An oversized unit satisfies the thermostat quickly, short-cycles, and never runs long enough to dehumidify — the room reads 22 °C and feels unpleasant. Two things fix it: size to the actual load rather than the round number above it, and give the coil enough run time at a low enough surface temperature to condense. Ventilation air is where most of the latent load enters a commercial building, which is why ASHRAE 62.1 outdoor-air rates belong in the heat-load calculation from the start rather than being discovered at commissioning.

Kenyan installations fail in three predictable local ways. Coastal salt attacks the condenser first — bare aluminium fins in Mombasa, Malindi or Diani can corrode through in a few seasons, so coated coils or an epoxy-treated condenser are a specification item, not an upgrade. Inland dust does the same job more slowly by blanketing the coil and lifting head pressure, which is why condenser cleaning belongs on the quarterly list rather than the annual one. And supply quality finishes the story: inverter units are sensitive to sustained under-voltage and to the surges that follow a utility restoration, so a properly rated isolator, surge protection and, on critical rooms, a voltage stabiliser will outlast several rounds of PCB replacement. Rated capacity also assumes a rating ambient — a condenser sitting in a hot enclosed courtyard at 45 °C simply will not deliver its nameplate figure.