Hot Water Coil Calculator
Step-by-step guide for sizing hot water heating coils, including glycol mixtures, flow rates, and pressure drop analysis.
1. Overview
The Hot Water Coil Calculator designs finned-tube heat exchangers for air heating applications. Hot water from a boiler, heat pump, or solar thermal system circulates through the coil tubes while air passes over the finned surface. The engine uses the LMTD (Log Mean Temperature Difference) method with corrections for cross-flow arrangement and fin efficiency.
Hot water coils are always 'dry' coils — there is no condensation since the coil surface is warmer than the air. This simplifies the analysis compared to chilled water coils.
2. Input Parameters
Air Side
| Parameter | Unit | Typical Range |
|---|---|---|
| Entering Air Temperature | °C | -20 to 25°C |
| Air Flow Rate | m³/h | 500–100,000 |
| Altitude | m | 0–3,000 |
Water Side
| Parameter | Unit | Description |
|---|---|---|
| Inlet Temperature | °C | Hot water supply (typically 60–90°C) |
| Outlet Temperature | °C | Return water (typically 40–70°C) |
| Flow Rate | L/s | Water volumetric flow rate |
| Glycol Type & % | — | For freeze protection in outdoor or preheat applications |
3. Coil Geometry
Hot water coils typically use fewer rows (1–4) compared to cooling coils because the temperature difference between the hot water and air is larger. Standard configurations:
- 1–2 rows: Reheat coils, mild heating
- 2–4 rows: Primary heating coils in AHUs
- 4–6 rows: Preheat coils with very cold outdoor air
4. Understanding Results
The calculator returns heating capacity (kW), leaving air temperature, water pressure drop, and air pressure drop. Since hot water coils are always dry, there is no latent capacity or condensate rate.
5. Glycol Correction
When glycol is added for freeze protection, the fluid properties change significantly:
- Viscosity increases (reduces heat transfer coefficient by 10–30%)
- Specific heat decreases (requires higher flow rate for same capacity)
- Pressure drop increases (higher pumping power required)
Warning: A 30% propylene glycol solution can reduce coil capacity by 15–20% compared to pure water. Always account for this in your design.
6. Freeze Protection Design
For preheat coils exposed to sub-zero outdoor air, freeze protection is critical:
- Use glycol concentration sufficient for the minimum expected outdoor temperature plus 5°C safety margin
- Maintain minimum water velocity > 0.5 m/s to prevent stagnation and freezing
- Consider face and bypass damper control for capacity modulation
- Use steam preheat for extreme cold conditions (-30°C and below)
7. Applications
- AHU Heating Coil: Primary heating in air handling units, typically 2–4 rows with 80/60°C water
- Reheat Coil: Zone reheat after cooling, typically 1–2 rows with lower water temperatures
- Preheat Coil: Outdoor air preheating, requires freeze protection with glycol
- Heat Pump Coil: Lower water temperatures (35–50°C), requires more rows for equivalent capacity
8. Warnings & Best Practices
- Keep water velocity between 0.5–3.0 m/s
- For heat pump applications with low water temperatures, increase coil surface area (more rows or larger face)
- Always verify that the leaving air temperature meets the design requirement
- Consider the pump head required for the coil water pressure drop