3.5-TON ROOFTOP UNIT: RUNNING BUT NOT COOLING, SUCTION 148 PSIG

#1 TXV stuck open or sensing bulb lost contact (refrigerant overfeed)

For: An overfeeding metering device dumps liquid back to the compressor: suction rises toward standing-pressure values, head sags because the compression ratio collapses, the coil floods and delta-T drops to the ~15°F seen here. The most common mechanical reason is a bulb strap that worked loose or a bulb that was never moved off the install position.

Against: No superheat number exists yet — without it this cause is a pattern match, not a confirmation. Low delta-T alone is also consistent with causes #2 and #3.

#2 Compressor inefficiency (worn scroll, failing to build pressure)

For: A tired compressor shows exactly this gauge pair: high suction, low head, warm-ish supply air. Head at 252 psig on a 95°F day is lower than this unit should make even before anything else is wrong.

Against: Scroll units rarely degrade gradually — they usually fail loudly or not at all. And an overfed compressor (cause #1) produces the same low compression ratio, so the gauges alone cannot separate them.

#3 Low evaporator load (wet coil, high indoor humidity, high return temp)

For: A coil carrying a heavy latent load runs high suction and cool-but-not-cold supply air. On a muggy 95°F day the return-air wet-bulb — never measured here — decides how the gauges should be read.

Against: It explains the high suction, but it fights the delta-T: a load-driven high suction normally comes with a LOW delta-T of 8–12°F, not the 15°F measured here. That conflict is why it ranks below the overfeed.

#4 Overcharge from a previous service visit

For: Extra refrigerant raises suction and floods the compressor; the assumption that nobody has touched the charge since install is unverified.

Against: An overcharge pushes head pressure UP, and 252 psig is soft, not high. If the charge were the story, the head would tell it.

#5 The readings themselves are wrong (bad gauge core, thermometer in the wrong spot)

For: Schrader depressors and tired gauge sets skew both numbers, and a register thermometer reads mixed air if it sits near a diffuser vane.

Against: The customer complaint (space not cooling) matches the gauge story independently. Two errors agreeing by accident is less likely than either single cause.

? What should the delta-T be on an R-410A rooftop unit?

Most healthy systems run a supply-to-return split of roughly 16–22°F, and high indoor humidity narrows it — so 15°F on a muggy day is borderline on its own, not a verdict. It becomes a red flag when it travels with suction pressure that is too high, which is the pattern in this case: the coil is being fed more refrigerant than it can boil off, or the compressor is not lifting what it should.

? High suction pressure on R-410A — is that a bad compressor?

Not by itself. High suction with soft head pressure means the compression ratio is low, and two different failures produce that: a metering device overfeeding liquid back to the compressor (TXV stuck open, bulb strap loose) or a compressor too worn to build pressure. Superheat, subcooling and amp draw separate them — which is why they are measured before any compressor is condemned.

? How do you charge an R-410A package unit correctly?

A package unit with a TXV is charged to subcooling, typically in the 8–12°F range, with superheat around 10–15°F as the cross-check — never by the old add-until-it-looks-right method. On a system showing high suction and soft head, charging is the expensive wrong move: if the valve is overfeeding, more refrigerant floods the compressor further and can take it out.

? Can a stuck economizer make an RTU run but not cool?

Yes, and it is the first free check on a no-cool call. A damper stuck open on a 95°F day feeds the unit hot outside air, the space climbs, and the gauges can look like a refrigerant fault because the load is artificial. Verify damper position against the minimum-position setting and feel whether the mixed air is hot before touching a gauge to the unit.