The $25 Wire That Nearly Wasted My $17,000 HVAC Investment

"A $25 oversight had made my $5,000 upgrade completely useless."

Let me tell you a quick story.

I recently upgraded the HVAC system in a historic home I rent as an Airbnb. It’s a simple setup: a dual-stage gas furnace with two stages of cooling. I paid an extra $5,000 for that second cooling stage and larger ductwork. Why? Because I wanted guests to stay comfortable even during peak summer heat. But I also didn’t want the system to short-cycle on mild days. I needed it to run efficiently year-round.

Total cost: $17,000.

My contractor—a friend I trust—installed everything, wired up the thermostat, turned it on in heating and cooling modes to show it ran, and sent the invoice.

That should’ve been the end of the story.

But I’m an HVAC engineer. So I did what any curious engineer would: I tested it myself.

When I tried to kick the system into second-stage cooling, nothing happened. A quick inspection revealed the issue: the Y2 wire hadn’t been connected. That’s the wire that tells the system to engage the second cooling stage.

A $25 oversight had made my $5,000 upgrade completely useless.

And unless you're someone like me—someone who knows what to look for—you’d never know. Most people would assume everything's working just fine.

What Is Commissioning Really?

Commissioning sounds technical, but it's really just a structured checklist—a final walkthrough for your HVAC system. It's how you confirm that what you paid for actually does what it's supposed to.

Visually identical. Functionally very different. Commissioning is how you tell.

And for a dual-stage system, it's surprisingly simple.

Here's an example:

❄️ Cooling Functional Test (example)

Commissioning isn’t complicated. Sometimes it's just a checklist—and it can save you thousands.

Item	Technician Initials	3rd Party Witness Initials
Set cooling setpoint to 60°F
Stage 1 cooling (Y1) engaged
Stage 2 cooling (Y2) engaged
Outdoor unit staged up (compressor behavior verified)
Indoor airflow increase observed
No error codes on the outdoor unit or air handler

Time to complete: 45–60 minutes.

Cost to do it right: minimal.

Cost to skip it: possibly thousands—and you may never realize it.

I've seen homeowners and building owners spend far more fixing problems caused by untested HVAC installs—things like damaged finishes, ruined floors, failed adhesives, and mold caused by poor moisture control—all because no one checked the system before signing off.

It's expensive to be cheap.

One side didn’t test the system. The other did. Guess which one paid for drywall repair.

Why This Matters—Big Time

You'd be surprised how many commercial systems get installed and never even start.

We once designed a basic 12.5-ton rooftop unit for a brewery’s upstairs seating area. For two summers, the space was hot and humid. Guests complained, and the owner nearly spent $50,000 on a supplemental cooling system, convinced our design was to blame.

The real problem?

The thermostat was never connected.

They were ready to spend $50,000 to fix a system that had never even been turned on.

No testing. No checklist. No verification. Just assumptions—and unnecessary frustration.

They were ready to spend $50,000… because one wire wasn't connected.

Final Thought

Whether you spent $15,000 or $750,000 on HVAC, wouldn’t you want to know it’s actually working before you sign off?

Commissioning isn't expensive. It isn't complicated. It just takes intention. A basic checklist—like the one above—can catch the things no one else is looking for.

And it protects everyone:

✅ The owner gets what they paid for

✅ The contractor avoids callbacks and liability

✅ The engineer avoids unfair blame

✅ The building stays protected from moisture damage and comfort issues