An HVAC service tech in February is the trade with the worst temperature swing of any job site. You start the day at 22 degrees in someone's driveway. You go down to a 60-degree basement. You climb to a 38-degree windy roof. Then back in the truck. Then a 75-degree retail space the customer wants warm right now. Twelve times.
The wrong move is one big jacket. The right move is three thin layers you can shed and stuff in the truck.
Base layer: merino wool, full stop. Smartwool Merino 250 crew if you can spend $90, Minus33 Chocorua midweight if you want the same wool at half the price. Polyester base layers stink by Wednesday, and you're going to be inside a customer's house. We push merino hard.
Mid layer: a 1/4-zip fleece with a chest pocket for your phone. Carhartt Force Fleece is what most guys grab. Some prefer the lighter Carhartt Rugged Flex Rigby midweight. Either way, the chest pocket is the move because you're going to lose a phone out of a hip pocket sliding under a furnace.
Outer layer in the cold: a Carhartt Yukon Extremes parka for the brutal days, but for a normal February the Carhartt Detroit Jacket J97 with a hood liner is the daily driver. Some guys like the Carhartt Super Dux Bonded jacket — lighter, more flexible, easier when you're wrenching on a condenser. The Dickies Eisenhower jacket has a smaller cult following but it works.
Pants: a Carhartt Duck or Washed Duck Double-Front in winter weight, with a Carhartt Force base layer pant under for sub-30 days. The base layer pant is the move people skip — it's the difference between functioning and being miserable on a roof at 9 AM.
Boots: this is the one most HVAC guys get wrong. You need waterproof, you need warm but not so warm you're sweating in the truck (because then your feet are wet from sweat, which is worse than cold), and you need a sole that grips on metal roof flashing AND on a snowy driveway. The Irish Setter 83607 Ramsey Pro is what I sell most to HVAC. The Thorogood 804-4910 GEN-flex2 8-inch waterproof composite-toe is the other big seller. 400g of Thinsulate is plenty — 1000g is overkill for a guy who's walking and climbing all day.
Gloves are tricky. You can't wrench in mittens, and a thick ski glove makes you fumble copper fittings. A Mechanix Original Insulated for general work, a Carhartt A511 insulated leather palm for outside, and a thin nitrile or mechanic's glove for the actual fitting work — keep all three in the truck. Guys who only carry one pair are guys with cold hands by 10 AM.
Hat: a beanie under a hard hat for roof work, a stretch beanie like the Carhartt A205 for everything else. Skip the lined trapper hat — too warm, too bulky, and it takes a chunk of your hearing.
Truck setup matters too. A change of socks (Darn Tough or wool, not cotton, ever) and a backup mid-layer. You will fall in slush, and the guy who has dry socks at 11 AM finishes the day; the guy who doesn't quits at 2.
The HVAC kit is the trade with the most layering math. Buy the system, not the jacket.