5 Mistakes Your HVAC Tech Always Finds (And How You Can Prevent Them)

It doesn’t matter if the furnace is brand-new or twenty years old—when I open it up, I almost always find the same handful of mistakes. Not because homeowners don’t care, but because no one ever explained what actually matters.

This article isn’t about blaming homeowners or installers.
It’s about showing you what technicians see every day, why those issues cause breakdowns, and—most importantly—how to stop them before they cost you money or heat.

Let’s pull the curtain back.

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❌ Mistake #1: Ignoring Air Filters Until Something Breaks

If I had to pick one mistake responsible for more furnace problems than anything else, this would be it.

What Techs Find

• Filters so clogged they bow inward

• Filters installed backwards

• Filters too restrictive for the system

• Filters that haven’t been changed in a year (or longer)

Why It Causes Problems

A furnace is designed to move a specific amount of air. When airflow drops:

• Heat builds up too fast

• High-limit switches trip

• Burners shut down early

• Short cycling starts

• Motors overheat

The furnace isn’t “weak”—it’s overheating.

How to Prevent It

• Change filters every 1–3 months

• Use the correct size

• Avoid ultra-high MERV filters unless your system is designed for them

• Write the change date on the filter frame

📎 Filter basics:

https://www.energy.gov/energysaver/air-conditioner-maintenance

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❌ Mistake #2: Closed Vents & Blocked Returns

This one surprises people.

Homeowners close vents to “push heat elsewhere.” Furniture gets moved. Rugs cover returns. Suddenly airflow is choked—and the furnace pays the price.

What Techs Find

• Bedrooms with vents fully closed

• Returns blocked by couches or shelving

• Finished basements starving the system for air

Why It Causes Problems

Closing vents doesn’t redirect heat—it restricts airflow. That leads to:

• Overheating

• Limit switch shutdowns

• Uneven comfort

• Noisy operation

How to Prevent It

• Keep all supply vents open

• Never block return grills

• Use balancing dampers (if installed) instead of vent covers

• Address comfort issues at the duct level—not the vent face

📎 Airflow fundamentals:
👉 https://www.energy.gov/energysaver/furnaces-and-boilers

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❌ Mistake #3: Thermostat Settings That Don’t Match the Furnace

Thermostats cause more confusion than any other component.

What Techs Find

• Thermostat set for heat pump on a gas furnace

• Wrong cycle rate or anticipator settings

• Cheap thermostats incompatible with furnace logic

• Smart thermostats installed without configuration

Why It Causes Problems

When the thermostat and furnace don’t “speak the same language”:

• Heat calls don’t register correctly

• Furnaces short cycle

• Systems won’t start at all

• Comfort becomes inconsistent

How to Prevent It

• Confirm thermostat is configured for gas/oil furnace

• Use a thermostat recommended for your system type

• Avoid DIY wiring changes without documentation

• If you upgrade thermostats, reprogram from scratch

📎 Thermostat basics:
👉 https://www.energy.gov/energysaver/programmable-thermostats

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❌ Mistake #4: Skipping Annual Inspections (Until Winter Forces the Issue)

Most furnace failures don’t happen suddenly—they build quietly.

What Techs Find During Emergency Calls

• Flame sensors never cleaned

• Burners partially clogged

• Loose electrical connections

• Venting issues that developed over time

• Minor problems that became major ones

Why It Costs More Later

• Emergency service costs more

• Parts fail under stress

• Secondary damage occurs

• Downtime is longer (and colder)

How to Prevent It

• Schedule a pre-season inspection

• Catch wear items before peak demand

• Address venting and combustion issues early

📎 Consumer furnace safety

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❌ Mistake #5: Ignoring Small Warning Signs

This is the quietest—and most expensive—mistake.

What Techs Hear from Homeowners

• “It’s been noisy for a while…”

• “It smelled funny once, but it stopped”

• “It shuts off sometimes, but comes back on”

Why This Backfires

Furnaces almost always warn you:

• New noises

• Short cycling

• Delayed ignition

• Smells

• Error codes

Ignoring those signs lets minor issues turn into:

• Motor failures

• Heat exchanger damage

• Control board replacements

• Emergency shutdowns

How to Prevent It

• Treat changes as important—even if heat still works

• Address noises, smells, and behavior shifts early

• Read furnace error codes when they appear

📎 Manufacturer learning center:
👉 https://www.goodmanmfg.com/resources/hvac-learning-center

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🧠 Bonus: The Mistake Techs Can’t Fix

Here’s the hard truth:
Some problems start at installation.

Common Installation Issues Techs Discover

• Undersized or oversized furnaces

• Poor vent pitch

• Marginal return air design

• Incorrect gas pressure

• Duct systems never designed for the equipment

These don’t show up on day one—but they show up eventually.

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🛠️ Jake’s “Prevent It” Checklist

Use this once a season:

• ✅ Change air filter

• ✅ Walk the house—open vents, clear returns

• ✅ Check thermostat batteries & settings

• ✅ Listen for new sounds

• ✅ Look outside—clear vent terminations

• ✅ Test CO detectors

📎 CO safety reference:
👉 https://www.cdc.gov/carbon-monoxide/prevention/index.html

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✅ Jake’s Final Take

Most HVAC failures aren’t bad luck—they’re patterns.

The same five mistakes show up over and over:

1. Ignored airflow

2. Misused thermostats

3. Skipped maintenance

4. Blocked vents

5. Early warnings brushed off

Fix those, and your furnace:

• Runs longer

• Breaks less

• Costs less

• Keeps you comfortable when it matters most