The HVAC and boiler service call, translated

"It's running, but it's not blowing cold." That is what the customer says on the phone. The note the tech leaves on the ticket an hour later reads more like code: "Split 9. SH 28, SC 2. Leak at evap coil. Bubbled, recovered, repaired, pulled to 400 microns, weighed in." Both describe the same afternoon. Heating and cooling techs work in a compressed vocabulary built for diagnosing machines nobody can see inside, and once it is translated, every word in that note turns out to be a measurement or a decision.

The no-cool call

The first thing the tech checks is the split: the temperature drop between the return air going into the indoor coil and the supply air coming out. A healthy system shows about 16 to 22 degrees F, a little lower on a muggy day when the coil spends more of its effort pulling water out of the air. A split of 9 means the fan is moving air but the system is not moving much heat.

Next comes a hand on the copper. The suction line, the fat insulated one in the line set, should feel beer can cold: cold and sweating like a can straight out of the fridge. Lukewarm is a bad sign. So is the opposite extreme, a coil iced up solid because of a filthy filter, a dead blower or not enough refrigerant.

If the outdoor unit hums but the fan and compressor will not spin, the first suspect is the cap, the capacitor that gives the compressor and fan motors the boost they need to start and run. A weak or bulged cap is one of the most common fixes in the trade, and the meter, not the looks, condemns it. Its neighbor, the contactor, gets a look too, because burned contacts can keep the compressor from getting power. A compressor that hums and trips on startup may get a hard start kit to give it extra torque.

Two numbers that tell the story

Then the gauges go on. The manifold reads pressures, but techs think in temperatures, because refrigerant at saturation has its temperature locked to its pressure. A PT chart, or the app built into a digital gauge, does the conversion.

Two numbers do most of the diagnostic work. Superheat is how many degrees the suction vapor has warmed above its boiling point. Subcooling is how many degrees the liquid has cooled below its condensing point. High superheat with low subcooling, like the SH 28 and SC 2 on the ticket, says the evaporator is starved: not enough refrigerant is reaching it, and a leak is the likely reason. High superheat with high subcooling points instead to a restriction, often a plugged filter-drier or a stuck TXV. A TXV that keeps swinging between overfeeding and starving the coil is said to be hunting.

Head pressure rounds out the picture. Too high usually means a dirty condenser coil outside, choked with cottonwood fluff and grass clippings.

Overfeed the coil instead, with too much charge, a TXV stuck open or too little airflow across the coil, and the danger moves to the compressor. Floodback is liquid refrigerant returning steadily through the suction line. Slugging is liquid or oil actually drawn into the cylinders, where it cannot be compressed and something has to give: a valve, a rod or a scroll.

Finding and fixing the leak

The shortcut everyone in the trade looks down on is to top off: add refrigerant and drive away without finding the leak. The proper fix starts with finding it. To bubble it is to brush soap solution on a suspect joint and watch for foam.

Once the leak turns up, the remaining charge comes out by recovery into a cylinder, never into the air. Copper gets brazed with flowing nitrogen inside so no scale forms in the pipe. Then the tech will pull a vacuum to boil off moisture and clear out non-condensables, watching a micron gauge until it drops below 500 microns and holds there. Last, the charge goes back in by weight. To weigh in is to put the nameplate amount on a scale, adjusted for the length of the line set. That is the whole ticket note, decoded.

The no-heat call

Boiler rooms have their own vocabulary. The most common opening line is a lockout: a safety tripped, most often a burner that tried to light and failed, and the control is holding it off until someone presses reset. The flame safeguard runs the start sequence and watches the flame through a flame scanner, so a dirty scanner can cause a nuisance trip even when the flame itself is fine.

Water problems come next. On a steam boiler, the low-water cutoff kills the burner when the level drops, and the pressuretrol cycles it to hold pressure. On a hot water boiler, the aquastat handles temperature. A pop-off dripping on the floor usually means the expansion tank has lost its air cushion, so pressure climbs every time the water heats. A boiler that rumbles and crackles like a teapot is kettling: water flashing to steam on hot spots, from scale on the heat exchanger or too little flow through it. In a steam boiler room, the tech also watches the gauge glass. A level that jumps up and down instead of holding steady is bouncing glass, and it usually means dirty water or surging.

Back on a hot water system, cold radiators upstairs get a bleed: open the little vent on the rad until water comes out instead of air. Then the tech checks delta T, the temperature difference between supply and return water, to confirm the water is actually delivering heat.

Dialing in the fire

The last step is combustion. Techs push an analyzer probe into the flue and dial in the burner, adjusting the excess air until oxygen, carbon monoxide and stack temperature land where they belong. Too much air carries heat up the chimney. Too little, and the burner is running rich, making carbon monoxide and soot, and left alone long enough, the boiler ends up sooted up on the fire side. On oil, the worst case is a puffback, a small explosion of oil vapor that sends soot drifting through the house.

Five words to learn first

1. Split: the temperature drop across the indoor coil, the quickest health check there is.
2. Superheat: how far the suction vapor sits above its boiling point.
3. Subcooling: how far the liquid sits below its condensing point.
4. Lockout: a safety shutdown that waits for a human to press reset.
5. Top off: adding refrigerant without fixing the leak, the phrase no customer should see on a ticket.