Before the Truck Leaves, Make Them Prove Your New AC Actually Cools

Your brand-new system can run and still be wrong

A unit that's low on refrigerant runs. A blower wired to the wrong speed runs. A return that's too small for the new equipment runs, and it sounds impressive doing it. You'd never catch any of that from the driveway, and you won't catch it in the hallway either, because air out of a vent feels cold compared to a Florida afternoon no matter how the job went.

It's the shower test. You turn the handle, warm water shows up, so you decide the water heater is fine. Something is happening. How much is happening is a completely separate question, and you can't answer it with your hand.

The split answers it. Put a thermometer where the air enters the system, put one where the air leaves, subtract. That's the whole test. Any crew that commissioned your install properly already did it — they just did it while you were in the other room, and there's no reason you shouldn't watch.

Where they put the thermometer decides the answer

Most of the arguing about temperature split is about the probe, not the math.

The reading that counts is taken at the equipment: one probe in the return air going in, one in the supply plenum right off the coil. That's the system grading itself. Take the same reading at a bedroom vent way down the line and you're measuring your attic too, because that duct spent the whole trip crossing a space that hits oven temperatures in August. Cold air loses an argument with an attic every time.

The instrument matters as much as the spot. You want a probe sitting in the moving airstream, giving it a few seconds to settle. An infrared gun pointed at a vent is measuring the vent — a rectangle of painted metal that's been quietly absorbing room heat since the house was built. Home inspectors do this constantly and get numbers that mean nothing.

And give it time. Doors shut, windows shut, fan set to auto, and let the system run a solid fifteen minutes before anybody writes anything down. A coil ninety seconds after startup hasn't settled into what it's going to do.

The 20-degree rule is the wrong target in a Florida house

Ask around and somebody will tell you the answer is twenty degrees. It's the most repeated number in the trade, and in a muggy house it can send you chasing a problem that isn't there.

Here's what actually moves it: water. When the air coming back to the coil is loaded with humidity, the system spends a big chunk of its effort condensing that moisture instead of dropping temperature. You can literally watch that work leave the house through the drain line. So the thermometer shows a narrower gap while the equipment is doing more, not less. Published targets drop closer to fifteen degrees when return air is genuinely humid, and widen out when the house is already dry. On install day in Tampa, with the front door propped open half the afternoon, humid is exactly what you've got.

Which flips the worry most people walk in with. A big fat split isn't automatically the good outcome — an oversized number usually means air isn't moving fast enough across the coil, so the little bit that gets through gets overcooled on the way by. A restricted filter does that. So does duct that was sized for the system you just replaced.

So the question to ask isn't whether it hit twenty. It's this: what's the target split for this equipment at today's return conditions, and where did we land? A crew that measured has an answer, usually off the manufacturer's chart. A crew that eyeballed it will hand you the folk number.

One fair limit, so you're not asking for something the test can't give. The split proves the coil is working and air is moving through it. It doesn't by itself prove the refrigerant charge is right — you can find a respectable-looking split on a system that's off. Charge gets verified with gauges, as subcooling and superheat compared against the spec for that model. Ask for those numbers too, and ask for all of it written on the paperwork before anybody signs anything. Written down is the part that turns a measurement into a record.

Most of this was decided before anyone showed up

By the time you're standing in a hallway holding a thermometer, the quality of the job is largely already locked. Was the equipment sized for your house, or for whatever was on the truck? Did the crew have enough hours in the day to do a real startup, or was the price so thin that the last twenty minutes of the job quietly became optional? A rushed install has a way of showing up in the split, and the split is just where you finally get to see it.

Which means the worst possible moment to figure out what a system should cost is while the old one is dead and the house is climbing a degree an hour. That's how people end up signing whatever's in front of them and hoping the crew was thorough.

That's the part COLD AC GUY built the price tool for. Tell us about your home and get a real replacement number now, while you're comfortable and nobody's waiting on you — so when the day comes, you already know what the job is worth and you can spend your attention on how it's done instead of what it costs.

Then, when they're loading the truck, you've got one small thing left to ask: so what was the split?

Common questions

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