Coil Selection Calculator

These examples show typical input ranges and the kind of outputs you can expect.

• Sensible airflow estimate: CFM approx Qsens / (1.08 * rhoRatio * deltaT_F), where Qsens = SHF * Qtotal.
• Face area: A_face (ft2) = CFM / V_face (fpm).
• Coil dimensions (rule-of-thumb): choose W/H ratio, then H = sqrt(A/ratio) and W = A/H; round to nearest inch.
• Water coil load: Q (Btu/h) approx 500 * GPM * deltaT_F for quick flow estimation.
• Steam flow estimate: lb/h approx Q (Btu/h) / 970 for latent heat approximation.
• Air pressure drop trend: dP approx K * (V/500)^2 * (Rows/4) * (FPI/12), used for screening only.

This calculator provides a shortlist estimate; final selection must use manufacturer performance data and project specifications.

1. Enter the total design load and pick the correct unit.
2. Provide airflow if known; otherwise, set a realistic SHF.
3. Set entering and leaving dry-bulb temperatures for your design case.
4. Choose target and maximum face velocity limits to control noise and carryover risk.
5. Adjust FPI and tube size to match the coil family you expect to procure.
6. If using water coils, enter water in/out temperatures or override the flow.
7. Press Calculate, review the notes, then export CSV/PDF for documentation.

Coil selection purpose in building systems

Coils are selected to meet required cooling or heating while fitting the air handler, fan static, and service clearances. This calculator screens options from capacity, entering and leaving air temperatures, and either known airflow or a sensible airflow estimate.

Typical load and airflow relationships

Loads are entered in kW, tons, or kBtu/h. When airflow is unknown, SHF typically ranges 0.70–0.85 for mixed-air comfort systems. A 30 C entering and 14 C leaving case creates 16 C delta-T, driving airflow and face area.

Face velocity targets and practical limits

Face velocity affects carryover, noise, and coil footprint. Cooling coils often target 450–550 fpm; heating or reheat coils commonly run 600–750 fpm. If velocity is high, increase face area or revisit airflow and casing constraints.

Rows, fin density, and tube diameter effects

Rows, fin density, and tube diameter control surface area. Typical ranges include 4–8 rows for cooling and 2–6 rows for heating. FPI around 10–14 is common; higher fin density can raise fouling sensitivity. Tube availability and pressure rating still govern choice.

Airside pressure drop screening values

Airside pressure drop must stay within fan allowance. Packaged units often budget 0.30–0.80 in. w.g., while custom air handlers may allow 0.80–1.20 in. w.g. The calculator uses a trend model scaling with velocity squared, rows, and fin density to flag risk.

Water coil flow and temperature rise checks

For water coils, a fast check uses Q = 500 × GPM × delta-T (degF). Chilled water delta-T is often 10–16 F; hot water can be 20–40 F. Use estimated GPM to verify valve size and pump head.

Steam and electric coil sizing notes

For steam coils, divide load by roughly 970 Btu/lb to estimate lb/h, then confirm traps and return piping. Electric coils are constrained by kW stages, breaker sizing, and airflow safeties. Confirm controls, high-limit cutouts, and proving switches.

Commissioning and documentation best practices

After screening, confirm selection with manufacturer ratings at the actual entering conditions, altitude, and coatings. Record final coil size, rows, fin density, and air pressure drop in submittals. Include filter condition, coil casing bypass factors, and drain pan details during review to reduce rework and ensure condensate management. Export results for RFQs, then verify leaving temperatures and differential pressure at startup.

What coil type should I choose first?

Start with the project standard. Use chilled water when a hydronic plant exists, DX for packaged systems, hot water for reheat or heating, steam for makeup air, and electric for small loads or fast staging. Always confirm availability and controls.

How accurate is the suggested row count?

It is a screening estimate based on face area and surface trends. Use it to narrow options, then finalize with manufacturer ratings at your exact entering air, fluid temperatures, and altitude. Coil coatings and fin spacing can shift performance.

Why does face velocity matter so much?

Higher velocity reduces face area but increases moisture carryover risk, noise, and air pressure drop. Many comfort cooling designs aim near 450–550 fpm, while heating can run higher. If limits are exceeded, increase coil size or revise airflow.

What should I enter for SHF if I only know total load?

Use 0.70–0.85 for typical comfort cooling with ventilation and mixed air. Drier climates and high sensible spaces trend higher; humid applications trend lower. If dehumidification is critical, use a full psychrometric analysis and manufacturer data.

How do I use the water flow result?

Treat it as a quick check for valve sizing and pump head. Compare the estimated GPM against design pipe velocities and coil connection sizes. Confirm the selected delta-T matches the plant strategy and coil schedule.

What does the air pressure drop warning mean?

It indicates the estimated coil air dP may exceed your available static budget. Consider increasing face area, reducing rows or fin density, or selecting a different coil series. Always verify the final dP from manufacturer submittals.

Can I use this for final equipment approval?

No. It supports early sizing and documentation, but final approval requires manufacturer performance data, code compliance, and project review. Validate selections with commissioning measurements, including leaving air temperature, water flow, and differential pressure.