Head pressure shoots past 500–700 psig (R-410A) and low and high rise TOGETHER

Tools: Gauges, two clamp thermometers, air thermometer, the SH/SC, Expected and Standing tabs

1. Logic. A liquid-line/TXV restriction raises head but CRASHES suction (starved evaporator, high superheat). If low and high rise together, there's too much of “something” in the system (refrigerant or air) or the condenser isn't rejecting heat. Then tell them apart by the numbers.
2. Number 1 — subcooling (SH/SC tab, liquid/bubble). 18–30 °F and up with high head = liquid stacking in the condenser = OVERCHARGE (or air, see number 4). Normal/low (0–10) with high head = the condenser isn't rejecting heat: dirt, fan, recirculation, heat wave.
3. Number 2 — superheat. Around 0–3 °F, suction line icy down to the compressor = overcharge (liquid to the compressor). Normal 8–14 = more likely condenser/air. High 20+ with high head and LOW suction = restriction, a different symptom.
4. Number 3 — condensing over ambient (CTOA): high-side saturation minus outdoor temperature. Normal 15–30 °F by SEER (Expected tab). 35–50 °F = the condenser can't keep up, or overcharge/air. Touch: liquid line too hot to hold, discharge at the compressor > 225 °F — poor heat rejection.
5. Number 4 — standing pressure (Standing tab): shut off, let it equalize 20–30 minutes, compare with the chart at the coldest place's temperature. 10+ psi ABOVE the chart on a cooled unit = air/nitrogen in the system. Matches = no air, so overcharge or condenser.
6. Quick test overcharge vs air: recover 4–6 oz into a cylinder while running. Overcharge — head and subcooling drop noticeably right away. Air — head barely changes, subcooling “floats”, standing pressure still above the chart. Never vent to the air.
7. Quick test for the condenser: wash the coil inside-out and check fan speed/amps, clearances around the unit. Head dropped 50–100 psi — that was it. A fan turning slowly / blowing the wrong way (after a motor swap) is a common cause of “700 psig”.
8. Don't confuse with load: 105 °F outside + 85 °F house + windows open give both pressures above normal with normal SH/SC. Run 20–30 minutes and compare with Expected for that outdoor temp.
9. 700 psig on R-410A: the high-pressure switch must cut the compressor at ~610–670 (exact value per the unit's manual). If it reached 700 — the switch is jumped, failed or missing. Find and restore it BEFORE any charging: this is a burst line and an injury.

Tools: Gauges, thermometer, recovery machine, vacuum pump, scale

1. Where it comes from: poor evacuation after a repair, hoses not purged, nitrogen not evacuated after a pressure test, system left open, a suction-side leak while the circuit was in vacuum (R-600a, low-temp refrigeration).
2. Running signs: head high and “jumpy”, subcooling per chart looks high, liquid-line temperature LOWER than it should be at that pressure (because part of the pressure isn't refrigerant), compressor hot and noisy, capacity mediocre.
3. Confirmation — standing pressure: shut off, equalize 30 minutes (overnight better), compare with the chart at the coldest place's temperature. 10 psi or more above the chart on a cooled unit = non-condensables. Air doesn't condense and adds its own partial pressure.
4. Second way: a refrigerant identifier shows “air %” / low purity.
5. What does NOT work: “bleeding air off the top of the condenser” on a residential split — refrigerant leaves with the air, the blend composition drifts, and the air is distributed through the system anyway.
6. Right way: recover the whole charge, new filter drier, nitrogen leak test (plate/manual test pressure, often 150 psig, never above design low) (if you suspect a leak), evacuate to 500 µ with a decay test, charge BY WEIGHT with fresh refrigerant (the recovered gas contains air — don't put it back).
7. Prevention: purge hoses, evacuate to 500 µ every time, use plenty of nitrogen while brazing but pump it out before charging.

Tools: Water hose, coil cleaner, fin comb, clamp meter

1. Shine a light through the fins from outside: if you can't see light through — plugged. Cottonwood, grass, pet hair, construction dust.
2. Touch the small (liquid) line: too hot to hold — head pressure is high, the condenser isn't rejecting heat.
3. Kill power. Remove the top grille with the fan (mind the wire). Wash from INSIDE OUT with a water stream to push dirt out. Chemical if grease/soot: apply, 5–10 min, rinse heavily.
4. Straighten bent fins with a fin comb.
5. Fan: spinning, speed, bearing noise, blades intact. Motor amps vs plate. Fan capacitor (usually 5–7.5 µF) — check.
6. At least 18″ clear around the unit, 5 ft above, hot air must not recirculate (fence, wall, second unit).

Tools: Gauges, clamp thermometer, hand

1. Signs: suction low, superheat high, subcooling HIGH (liquid backs up in the condenser) — unlike undercharge, where subcooling is low.
2. Touch the filter drier on the liquid line: cold or frosted, more than 3 °F drop across it → plugged, replace.
3. At the TXV: frost on the valve or right after it, hissing. Bulb poorly clamped/lost charge — valve “starved”. Warm the bulb in your hand: suction should rise. No reaction — valve dead.
4. Piston (fixed orifice): pull it, look at the screen — debris, copper shavings from brazing. Flush/replace.
5. After TXV/drier replacement — new filter drier, evacuate, weigh in the charge.

Tools: Multimeter, eyes

1. Find the switch on the small (liquid) line at the compressor or on the discharge: a small body with two wires, sometimes with a reset button. R-410A setting usually around 610–670 psig (opens — check the unit's manual for the exact value), reset around 420–470.
2. Inspect the wires: a splice around the switch, a jumper on its terminals, switch removed with the wires joined — all of that is “jumped”. Sometimes that's how a summer shutdown got “fixed” — and left.
3. Ohm it with the unit off: closed (0 Ω). Replace if it's open at rest at normal pressure, or doesn't open when the gauge already shows the setting. Never push the pressure past the setting on purpose.
4. Check the action: as pressure climbs toward the setting the compressor must cut out (board shows a high-pressure code). Didn't cut out — switch/board/jumper.
5. Never jumper it “to finish the season”: 700+ psig is a burst hose/line, a burn, an injury. The switch costs $20–40.