How to Change a Thermostat (and 5 More Winter Shop Equipment Mistakes I Made)

If you're the one responsible for keeping a shop running when the temperature drops, this checklist is for you. It's for anyone who has ever been asked whether the heat will hold through a deadline week and had to answer with I think so. I've been that person too many times.

For six years, I've handled equipment repairs and replacements for a 12-person fabrication shop. Since 2019, I've personally made (and documented) 11 significant mistakes, totaling roughly $8,000 in wasted budget. I now maintain the team's winter checklist so the next person doesn't repeat them. These are the six items on it.

1. Size the compressor for the tools you actually run

My first real compressor mistake happened in late 2021. I bought new pneumatic tools to speed up a job that kept getting delayed. The tools worked great. The problem was the air supply. Our Hitachi 2hp air compressor had been fine for years because the old tools never ran at the same time. The new cutoff tool plus the sander pushed it past its rated output at 90 psi. Nothing exploded. The line just crept along while we waited for the tank to refill.

I should have done the math first. Here's what I do now:

• List every air tool that could run at the same time on a worst-case day.
• Write down each tool's CFM requirement at the pressure you actually use, usually 90 psi.
• Add them together and multiply by 1.3 for a safety margin.
• Compare that with the compressor's CFM rating at that pressure, not its max tank pressure.

If your number is close to or above the compressor rating, you need a bigger unit or a posted rule that certain tools never run simultaneously.

2. Buy the AC for the heat load, not just the square footage

Two summers ago, we turned an old storage room into a control room. The window unit couldn't keep up, so I ordered a Hitachi inverter AC 1.5 ton, which every size guide said was right for a room that size. The unit was fine. My assumption wasn't.

People assume square footage tells you which AC to buy. The reality is that people, computers, monitors, sunlight, and poor insulation all add heat. The room had three workstations, a switch stack, and a west-facing window that baked for half the day. The inverter AC ran almost continuously in the afternoon.

What I check now before any AC purchase:

• Count the heat sources in the room, not just the floor area.
• Read the itemized install quote for line-set length, electrical work, and drain routing. The lowest bid is often the one that quietly leaves those out.
• Ask what happens if the install timeline slips. When you're cooling a room full of electronics, a vague answer is a risk.

3. How to change a thermostat without turning it into a two-day project

The shop heater kept short-cycling last winter, so I decided to replace the thermostat. It looked like a 20-minute job. I managed to turn it into two days of guessing.

I skipped the photo of the old wiring, trusted the wire colors, and left the hole in the wall open behind the new thermostat. Cold air leaking from inside the wall made the new thermostat read low, so the heat ran longer than it should have.

Here's the short version of how to change a thermostat:

1. Turn off power at the furnace and at the breaker. Confirm it with a voltage tester.
2. Take a photo of the wiring before you remove the old faceplate.
3. Label each wire with tape as you disconnect it. In old buildings, wire colors are a suggestion, not a law.
4. Check for a C wire. Many programmable and smart thermostats need it. If your old wiring doesn't have one, check compatibility before you buy.
5. Seal the hole in the wall behind the new thermostat with caulk, foam, or electrical putty.
6. Mount it on an interior wall, about four to five feet off the floor, away from doors, vents, direct sun, and fans.

Step five is the one most people skip, and it's the one that cost me the most confusion. A draft inside the wall is enough to fool the sensor and make the system run longer than it should.

4. Point the fan at the cold spots, not at the thermostat

After the thermostat was fixed, we still had uneven heat. The far end of the shop stayed cold no matter how long the furnace ran. I brought in a Vornado fan because it moves air in a straight line, which is exactly what I needed. It was also exactly the problem.

I set the Vornado fan on a bench near the warm end, aiming it toward the cold end. Unfortunately, the air stream passed right over the thermostat. The thermostat felt a steady warm breeze and decided the whole shop was warm. The furnace shut off while the far end was still cold.

The surprise wasn't the fan. It was that a $100 fan could make a heating system lie to itself for two weeks before I noticed.

If you use a circulation fan, check where the air is actually going. You want it hitting the cold zone, not the thermostat. If the room layout makes that impossible, a smart thermostat with a remote sensor in the cold zone solves the problem permanently.

5. Treat a kerosene heater like emergency equipment, not a furnace

In January 2024, our main furnace died in the middle of a deadline week. The replacement control board was backordered, so we brought in a kerosene heater to keep the shop warm enough to finish the order.

People assume a kerosene heater works like a small furnace: set the wick, adjust the knob, and walk away. The reality is that it's basically an open flame with a fuel tank, and it needs attention. On day three, I found a scorch mark on a cardboard box about two feet from the heater. I thought I'd left enough clearance. The box had shifted.

If you ever need a kerosene heater in a shop or warehouse:

• Keep at least three feet of clearance in every direction. Cardboard and packaging creep closer over time.
• Open a window or door a crack. Kerosene heaters consume oxygen and produce carbon monoxide.
• Put a carbon monoxide alarm in the same area, with fresh batteries.
• Refuel only after the unit has cooled, and do it outside using 1-K kerosene stored in an approved container.
• If the flame smokes or the smell gets strong, turn it off. A properly adjusted wick burns clean. One that isn't is a warning.

Per the FTC's Green Guides (ftc.gov), an environmental claim like clean burning has to be substantiated with evidence. If nobody can show you real emissions or efficiency data, assume the label is doing the work.

6. When the deadline is real, pay for certainty

This one is more of a purchasing rule than a maintenance task, but it has saved us more downtime than any tool. In March 2024, our dust collector motor died on a Monday. We had a customer job due that Friday. An online retailer had the motor for $380, with estimated delivery in three to five business days. The local supplier had one in stock for $780 and guaranteed it would be there Tuesday morning.

I hesitated over the extra $400. Our shop manager asked one question: what happens if the motor doesn't arrive by Thursday? The answer was a missed $15,000 event. We paid the $780.

The motor arrived at 7:45 the next morning. The extra $400 didn't buy speed. It bought certainty, a promise with a name, a phone number, and a physical location behind it. When a supplier quotes an estimated delivery, ask what happens on the day after that estimate. If the answer is basically a shrug, and you can't afford a shrug, pay for the guarantee.

Honestly, I'm not sure why some suppliers consistently beat their quoted timelines while others consistently miss. My best guess is it comes down to inventory buffers and whether they actually stock what they claim. What I know for sure is that the cheapest option is only cheap when the arrival date doesn't matter.

Run this checklist before you need it

If you take nothing else from this, remember the pattern: size the compressor for peak demand, match the AC to real heat load, point fans away from the thermostat, and treat a kerosene heater as the temporary risk it is. The meta-mistake is waiting until January to think about any of it.

The best part of finally getting this list written down is that I don't spend winter mornings wondering what is about to break. Equipment still fails, because it's mechanical. But now failures are events, not emergencies.

If this checklist feels like overkill, good. Overkill is cheaper than the alternative. Take it from someone who paid for these lessons with real money and one very cold January.