Why Your HVAC System Runs But Doesn’t Heat or Cool Properly

You adjust the thermostat. The system kicks on. You hear it running. But your home stays uncomfortably warm in summer or cold in winter. This frustrating problem is surprisingly common, and the cause isn’t always what homeowners expect.

The Mystery of the Running-But-Not-Working System

When your HVAC system runs constantly without achieving the desired temperature, most people assume the equipment is broken. While that’s sometimes true, the actual problem often lies elsewhere in your home’s heating and cooling system, or even in the home itself.

Understanding these hidden causes can save you from unnecessary equipment replacement and help you solve the real problem.

The Hidden Culprits

1. The Undersized or Oversized System Problem

Many homes have HVAC systems that were never properly sized for the space. This happens when:

• Original contractors estimated rather than calculated
• Additions were built without upgrading the system
• Insulation improvements changed heating/cooling needs
• Room configurations changed after installation

The undersized system problem: Your AC or furnace simply doesn’t have enough capacity to heat or cool your space. It runs continuously trying to reach the thermostat setting but never quite gets there. On moderate days it might keep up, but during temperature extremes it falls behind.

The oversized system problem: Counterintuitively, an oversized system also causes problems. It reaches the thermostat setting too quickly and shuts off before completing a full heating or cooling cycle. This prevents proper dehumidification in summer and creates uneven temperatures throughout your home.

2. Air Distribution Issues Nobody Talks About

Your HVAC system might work perfectly, but if the heated or cooled air doesn’t reach your rooms effectively, you’ll stay uncomfortable.

Blocked or closed vents: Furniture, curtains, or deliberately closed vents in “unused” rooms disrupt system balance. Your HVAC is designed to move a specific volume of air. When you block vents, pressure increases and airflow decreases to other areas.

Duct leaks you can’t see: Up to 30% of heated or cooled air escapes through duct leaks in attics, crawlspaces, and walls. Your system runs constantly trying to compensate for this loss. The air never reaches your living spaces, it’s heating or cooling your attic instead.

Poor return air flow: Most homes don’t have enough return air vents. This creates negative pressure that restricts airflow throughout the system. Think of it like trying to drink through a straw with a hole in it, the system works harder but delivers less.

3. The Insulation and Air Sealing Gap

Your HVAC system can only work with what your home gives it. Poor insulation and air leaks are like trying to fill a bucket with holes.

Insufficient attic insulation: Heat rises in winter and radiates down from your roof in summer. Without adequate attic insulation (R-38 to R-60 in Northwest Indiana), your HVAC system fights a losing battle against temperature transfer.

Air infiltration: The average home has air leaks equivalent to a 2-foot by 2-foot hole in the wall. These gaps around windows, doors, electrical outlets, and penetrations let outdoor air in and conditioned air out. Your system runs constantly trying to overcome this air exchange.

Missing wall insulation: Many older homes have little or no wall insulation. While this is expensive to fix, it’s worth knowing whether you’re asking your HVAC system to do the impossible.

4. Thermostat Location and Programming Issues

Where your thermostat sits dramatically affects how your system performs.

Poor thermostat placement: Thermostats in direct sunlight, near drafty doors, above heat-generating appliances, or in rarely-used rooms give false readings. Your system responds to these false readings rather than actual home temperatures.

Anticipator settings: Older thermostats have “anticipator” settings that control how long the system runs. If set incorrectly, your system may run too briefly or too long, never achieving comfort.

Incorrect programming: Programmable thermostats set with unrealistic temperature swings (like 60°F overnight and 72°F at wake-up) ask your system to do too much too fast. The system runs constantly trying to make up an impossible temperature difference.

5. The Refrigerant Charge Mystery (For AC and Heat Pumps)

Low refrigerant charge is obvious, your AC doesn’t cool. But overcharged refrigerant causes a sneaky problem that’s often missed.

Overcharged symptoms: The system runs constantly, feels like it’s working hard, but doesn’t cool effectively. High head pressure reduces efficiency and can damage the compressor. Many homeowners and even some technicians miss this because they assume “not enough cooling” always means “not enough refrigerant.”

Proper refrigerant charge requires precise measurement of subcooling and superheat, not just adding refrigerant until “it feels cold.”

How to Diagnose Your Specific Problem

Start with these DIY checks:

Temperature test: Use a thermometer to measure supply air temperature. AC supply air should be 15-20°F cooler than return air. Furnace supply air should be 40-70°F warmer than return air. If your temperature difference is correct but you’re still uncomfortable, the problem isn’t the equipment.

Airflow check: Hold tissue paper near each supply vent. Weak airflow suggests duct problems, dirty filters, or blower issues. Uneven airflow between rooms indicates distribution problems.

Pressure test at vents: Feel the air pressure from supply vents. Strong, forceful air indicates good duct design. Weak air suggests restrictions or leaks.

Visual insulation check: If you can access your attic, measure insulation depth. Less than 10-12 inches in Northwest Indiana means you need more.

Solutions That Actually Work

For sizing problems: A load calculation (Manual J) determines your home’s actual heating and cooling needs. This $200-400 investment prevents buying the wrong equipment.

For duct problems: Professional duct sealing and insulation typically costs $1,000-2,500 but can reduce energy bills by 20% while dramatically improving comfort.

For insulation issues: Attic insulation upgrades deliver the best return on investment of any home improvement, often paying for themselves in 2-3 years through energy savings.

For distribution problems: Adding return air vents, balancing dampers, or redesigning ductwork solves uneven temperatures without replacing equipment.

For thermostat issues: Relocating a thermostat costs $150-300 and can solve problems that seem like they require new equipment.

The Bottom Line

Before replacing your HVAC system because it “doesn’t work,” investigate these hidden causes. Many comfort problems that seem like equipment failures are actually distribution, insulation, or sizing issues. Fixing the real problem costs less and delivers better results than replacing equipment that’s actually working correctly.

Need help diagnosing why your system runs but doesn’t keep you comfortable? Call AA Heating and Cooling at (219) 258-7313 for a comprehensive system evaluation.