What the Thermostat Test Is Actually Doing
The inspector raises the heat setpoint above the current room temperature, usually by 5 to 10 degrees, then waits. The goal is to confirm the heating system receives the signal, responds to it, and operates. If the furnace fires up, the blower runs, and air comes out of the registers at a reasonable temperature, the system passes that part of the test.
For cooling, the inspector drops the setpoint below ambient and does the same thing: confirm the compressor kicks on, the air handler responds, and cool air moves through the ducts. This typically happens during visits when outdoor temperatures allow safe compressor testing. Running an air conditioning compressor when it's below about 60 degrees outside can damage the system, so inspectors sometimes have to note that cooling wasn't tested due to ambient temperature.
The whole sequence takes a few minutes per system. Multiple systems in the house means multiple cycles of this. If I was going back to the thermostat more than once, I probably had two HVAC zones or a separate system in the basement or attic. That's what Denise was seeing, not me troubleshooting a problem.
The Big Limitation: Operating vs. Functional
The thermostat test confirms the system operates, not that it's functioning efficiently, safely, or for how much longer. This distinction trips up a lot of buyers.
"It came on" tells you the system responds to a control signal. It doesn't tell you anything about refrigerant charge, heat exchanger condition, duct leakage, filter restriction, or whether the system is distributing conditioned air to every room evenly. A 22-year-old furnace that fires up immediately and runs for three minutes passes the operational test. It might also be near the end of its useful life.
According to the InterNACHI Standards of Practice, home inspectors are required to describe the type of heating and cooling systems, operate the systems using normal controls, and describe any deficiencies observed. They are not required to test every component within the system, verify refrigerant charge, or evaluate efficiency ratings. That's the job of an HVAC technician doing a service inspection.
If the inspector recommends further HVAC evaluation by a qualified technician, that recommendation doesn't mean the system failed. It often means the system is older, showed signs of heavy use, or had something the inspector couldn't fully evaluate within the scope of a general inspection.
Common HVAC Control Findings
Some things do show up during the thermostat test that end up in the report.
Delayed Ignition
Gas furnaces should ignite relatively quickly when called for heat. A long pause before ignition, repeated clicking before the burners catch, or a small puff or thump when they do ignite are signs of delayed ignition. This can be a dirty burner assembly, a failing igniter, or a gas pressure issue. It's a finding worth having an HVAC tech evaluate before closing.
Short Cycling
If the system fires up and then shuts off after a minute or two without reaching setpoint, it's short cycling. Causes include an oversized unit, a tripped limit switch, airflow restrictions from a clogged filter, or refrigerant problems on the cooling side. Short cycling causes accelerated wear and usually means something is wrong with the system or how it was sized for the house.
Thermostat Doesn't Respond Correctly
Occasionally the thermostat itself is the issue: wrong wiring, a bad connection, a dead battery in a wireless unit, or a failed component. If the system won't come on at all during the test, the inspector notes it. This could be a $30 thermostat replacement or it could point to something deeper in the system requiring further diagnosis.
Unusual Odors on Startup
Musty smells when the heat or AC first comes on can indicate mold in the air handler or ductwork. A brief burning smell on first startup of the heating season is sometimes just dust burning off the heat exchanger, which clears quickly. A persistent burning smell is more concerning. A sulfur or rotten-egg odor is a gas issue that should stop everything immediately.
When the Inspector Couldn't Test the System
Utilities off, breakers tripped, system in seasonal lockout, outdoor temperature too low for A/C testing, or equipment that simply won't respond during the inspection window: all of these result in a note that the system wasn't tested, often with a recommendation to have it evaluated before closing.
This is not a finding about the condition of the system. It's a gap in the inspection. You need to fill that gap before you close. The standard approach is to ask the seller to demonstrate the system operates or to bring in an HVAC company for an independent inspection. A basic HVAC service call runs $100 to $200 in most markets. A more thorough evaluation including refrigerant charge check and heat exchanger assessment typically runs $200 to $400.
The U.S. Department of Energy recommends annual HVAC maintenance regardless of apparent system condition. If the system in a house you're buying hasn't been serviced in years, factoring in a service appointment and any resulting repairs is reasonable even when the operational test goes smoothly.
Denise's inspection turned out fine, for what it's worth. The furnace was nine years old and working normally. The central air tested fine on a follow-up visit when the weather allowed it. She closed on the house two weeks later. The repeated thermostat visits were just me being thorough.