Table of Contents
ToggleViews in the last 30 days: 31
Estimated read time: 5 minute(s)
Convection in HVAC — The Basics + Quick Calculator
Change the air speed, surface area, or temperatures and watch heat transfer respond in real time. Namespaced, clean, and WordPress‑friendly.
🔍 Let’s Start With the Basics: What Is Convection?
Imagine this: you’re sipping a hot cup of tea ☕. As the steam rises, the air above the cup feels warmer. Why? Because the heat from the tea is transferring into the surrounding air. That’s convection happening right in front of you.
Now picture this in a room with an air conditioner or heater on — cool or warm air is moving throughout the space. That’s forced convection helping regulate temperature.
In HVAC systems (Heating, Ventilation, and Air Conditioning), convection is everywhere — from ductwork to heat exchangers. It’s the silent, invisible courier that delivers or removes heat where we need it.
Natural convection happens without fans or pumps — density differences do the work. Forced convection uses fans, blowers, or pumps to move air or liquids and dramatically boost heat transfer.
✌️ Two Types of Convection You Need to Know
1) 🌡️ Natural Convection (free convection)
Happens without any fans or pumps. It’s all about temperature and density differences. Hot air rises, cool air sinks because warmer air is less dense. A radiator warming a room without a blower is a classic example.
2) 🌀 Forced Convection
Uses fans, blowers, or pumps to move air or fluids across surfaces — massively increasing heat-transfer rate. You see it in AC blowers, AHU heating coils, chillers circulating water across tube bundles, and more.
🔢 Convection Heat Transfer Calculator — q = h · A · (ts − t∞)
📊 Typical Heat‑Transfer Coefficients (guide values)
Source: ASHRAE Handbook — typical ranges| Convection Type | Typical h (W/m²·K) |
|---|---|
| Free convection — gases | 2 to 25 |
| Free convection — liquids | 10 to 1,000 |
| Forced convection — gases | 25 to 250 |
| Forced convection — liquids | 50 to 20,000 |
| Boiling / condensation | 2,500 to 100,000 |
🧰 Practical HVAC Design Tips to Improve Convection
🧪 60‑Second Quiz
🏠 Where Convection Shows Up in HVAC
- Coils (evaporators, condensers, heating coils): air ↔ refrigerant/water via convection.
- Ductwork: forced convection moves heat from A to B.
- Fan‑coils & radiators: moving air across hot/cold surfaces conditions the room.
📚 Go Deeper (Recommended next reads)
Bonus: Radiation and Convection Often Work Together
In real systems, heat doesn’t pick a single lane. Warm coils both convect and radiate heat. Use the live calculator below to see each contribution and the combined effect.
How engineers combine them
When a surface exchanges heat with its surroundings, engineers often linearize radiation as an equivalent coefficient hr and add it to the convective coefficient h:
q = A · (h + hr) · (ts − t∞)
Here, hr comes from radiation physics. A common linearization around the two temperatures is:
hr = ε · σ · (Ts2 + T∞2) · (Ts + T∞)
Use absolute temperature in Kelvin (K): T = t(°C) + 273.15. σ = 5.670374419×10⁻⁸ W/m²·K⁴. We assume view factor F = 1 unless set otherwise.
qrad = ε · σ · F · A · (Ts4 − T∞4). The linearized form above lets you treat radiation like an additional h and simply add it to convection for quick sizing checks.Why convection is king (in HVAC)
- It dictates how fast rooms heat/cool and how big the coils need to be.
- It drives fan/blower sizing and power draw.
- It responds immediately to air‑side changes — tweak air speed, watch capacity jump.
Master convection and you’re a giant leap closer to mastering HVAC design.
🌡️ Convection + Radiation Calculator
📊 Typical Convection Coefficients (quick guide)
| Convection Type | Typical h (W/m²·K) |
|---|---|
| Free convection — gases | 2 to 25 |
| Free convection — liquids | 10 to 1,000 |
| Forced convection — gases | 25 to 250 |
| Forced convection — liquids | 50 to 20,000 |
| Boiling / condensation | 2,500 to 100,000 |
Rule‑of‑thumb values — consult detailed tables and correlations for design.
