The core idea
The compressor's discharge arrives as hot, high-pressure vapour — well above saturation. The condenser therefore works through three zones: de-superheating (the gas cools down to saturation), condensing (vapour turns to liquid at constant temperature — most of the coil's area does this), and subcooling (the liquid cools a few kelvin below saturation — the 4–8 K margin that keeps flash gas out of the expansion valve, Lesson 14's insurance). Through the whole condensing zone the refrigerant obeys the P–T rulebook (Lesson 12): the pipe wall sits at exactly the saturation temperature, readable with any thermometer.
What sets the condensing temperature? The rejected heat must climb from refrigerant to outdoor air:
Q = U·A·(T_cond − T_air)
where U·A is the coil's heat-transfer ability (area × fin condition × airflow) and the margin T_cond − T_air is the "condenser split" — typically 10–15 K for a clean, correctly sized coil. This one line explains head pressure: read the gauge, convert to T_cond via the P–T chart, subtract ambient, and you have the split. If dirt, a failing fan, or recirculated hot air shrinks U·A, the only way to reject the same heat is a bigger split — and head pressure climbs. The compressor does not choose the condensing pressure; the condenser does, given the weather.
Why it's money: Lesson 15 showed compressor work (h₂ − h₁) grows faster than proportionally with pressure ratio. Rule of thumb: every extra 1 K of condensing temperature costs roughly 2–3 % more compressor power — while discharge temperature climbs too, cooking the oil and inviting high-pressure cutouts.
Real-world example
A restaurant's rooftop R-410A condensing unit on a 30 °C day. Healthy: head gauge ≈ 27 bar abs → the chart says 45 °C condensing; split = 15 K, right on spec. Against the 5 °C evaporator (≈ 9 bar abs) the pressure ratio is 27/9 ≈ 2.9. Over one greasy summer the coil mats with kitchen dust; on a similar day the gauge reads ≈ 34 bar abs → 55 °C condensing, split 25 K — the coil is confessing. Ratio 34/9 ≈ 3.7, about a quarter higher, so compressor power is up roughly 25 % and the high-pressure switch now trips on the first heat wave. The diagnosis took a gauge, a P–T chart, and the outdoor thermometer — and the fix was a coil bath, not a compressor.
Common pitfall
Reading high head pressure as "a compressor problem". The compressor sits downstream of the condenser's decision; its discharge pressure is simply whatever heat rejection requires. The honest first suspects are the coil: dirt, flattened fins, a sluggish condenser fan, hot exhaust air recirculating off a wall. Corollary: cleaning badly makes things worse — a pressure-washer jet fired straight at aluminium fins folds them shut and removes area. Wash from the inside face outward, gently, and comb bent fins straight.