Heat Transfer
Conduction, convection and radiation, and why every HVAC system is really a heat moving machine.
What it is
Heat transfer is the movement of thermal energy from a warmer place to a cooler one. It only ever moves in that direction on its own. Heating and cooling equipment does not create cold, it moves heat, and every piece of HVAC equipment you will work on is built around one or more of the three transfer methods.
Conduction
Heat moving through a solid material or between two materials in direct contact, such as heat passing through the wall of a copper tube.
Convection
Heat carried by a moving fluid, such as air across a coil or water through a boiler loop. Forced convection uses a blower or pump, natural convection relies on warm fluid rising.
Radiation
Heat travelling as infrared energy between surfaces with no contact and no fluid in between, such as a hot flue pipe warming nearby framing.
Why it matters
Almost every measurement you take in the field is really a heat transfer measurement. Temperature split across a coil tells you how much heat the air gave up. Superheat tells you whether the evaporator finished absorbing heat. Flue gas temperature tells you how much heat went up the vent instead of into the house.
- A dirty coil reduces conduction and convection, so capacity falls even though the compressor is fine.
- Low airflow reduces convection, which lowers suction pressure and can flood the compressor.
- Missing insulation on a line set or duct wastes capacity before it reaches the space.
How it works
The rate of heat transfer depends on the temperature difference between the two bodies, the surface area available, and how easily the materials or fluids carry heat. Increase any of those and heat moves faster.
Heat transferred = temperature difference x surface area x transfer rate of the materialConceptual relationship. Manufacturer capacity tables do this arithmetic for you.
This is why coils use thin fins packed close together, why blowers push air across them, and why refrigerant is used at all. A fluid that boils and condenses at useful temperatures can absorb and release large amounts of heat without a large temperature change.
What can go wrong
| Condition | Effect on heat transfer | What you observe |
|---|---|---|
| Dirty evaporator coil | Reduced airflow and surface contact | Low suction pressure, low superheat, possible icing |
| Dirty condenser coil | Heat cannot leave the refrigerant | High head pressure, high subcooling, high amp draw |
| Plugged filter | Reduced convection on the indoor side | Low airflow, high static pressure, small temperature split in cooling |
| Soot on a heat exchanger | Insulating layer blocks conduction | High flue temperature, low efficiency, possible limit trips |
How to diagnose it
Measure supply and return air temperature at the equipment
The split shows how much heat the air actually exchanged
Measure total external static pressure
High static means the air is not moving well enough to carry heat
Inspect both coils and the filter in good light
Visible fouling is the most common cause of poor heat transfer
Compare liquid line temperature to outdoor ambient
A large gap points to a condenser that cannot reject heat
Tools involved
- Digital thermometer or clamp probes
- Manometer for static pressure
- Anemometer or airflow hood
- Digital manifold with temperature clamps
Practice & Mastery
- Questions3
- Quiz1
- Heat Transfer Troubleshooting Scenarios2
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