Heating coils must deliver full capacity at design conditions, and the control valve must modulate water flow without hunting or cavitation.
Sizing the Coil
Coil capacity follows the same heat-transfer logic as cooling: rows, fin density, and face velocity set how much heat the coil can pass from water to air at the design temperature difference, and the same pressure-drop cautions apply on the air side.
A heating coil adds heat to the supply air using hot water, and its sizing is a balance between the water temperature available and the face area the duct can give. The same heat must be delivered with the lowest practical water temperature so the coil is gentle on the air and efficient with the plant's heating source.
The entering water temperature and the flow set the coil's leaving air temperature; a higher water temperature or a lower face velocity lets a smaller coil do the job, but pushes the surface hotter, which can feel like a hot spot in the airstream. The selection keeps the coil surface below the dust-ignition concern while meeting the load.
The control valve is where the capacity is actually managed. A modulating valve with the right Cv holds the water flow to the demand so the coil tracks the room load instead of flooding and short-cycling, and the valve authority, the ratio of its pressure drop to the circuit drop, must be high enough that the modulation is real and not mushy.
Face velocity and fin density decide the static pressure the fan pays; a coil crammed into a small frame needs high velocity and costs fan energy, while a generously sized coil runs quietly and efficiently. The trade is footprint against operating cost, weighed over the heating season rather than at first price.
The company's 80,000 m² production site builds heating and cooling coils under ISO 9001:2015 with CCC certification, and with more than 30 years of manufacturing experience its engineers respond to selection enquiries within 24 hours to size the coil and the control valve together for the available water conditions.
Freeze protection is a real duty for heating coils fed from a system that can fall to outdoor temperature: a coil with trapped water and no flow can burst, so low-water-temperature designs use glycol or a flow-proofed circuit, and the control holds a minimum flow during cold setbacks. The valve and the plant sequence are set so the coil is never left full of stagnant water at risk of freezing when the heating call stops.
Low-temperature hot water, around 60-80°C supply, is common from condensing boilers and heat pumps, but it carries less exergy than high-temperature water, so the coil needs more surface to deliver the same kW and must be selected accordingly.
Entering air temperature in winter sets the load; a coil sized only on mild weather will fall short during a design cold snap, so the duty must use the worst-case outdoor condition published for the location, not an average that hides the peak.
Face velocity affects both capacity and the risk of throwing warm air unevenly; staying in the usual 1.5-2.5 m/s band keeps the coil effective and the airflow stable, while very low velocity can let stratification develop in the discharge plenum.
Selecting the Valve
The control valve should be sized to the coil flow at the design ΔT, not to the pipe size, because an oversized valve loses authority and oscillates around the setpoint, causing the space temperature to swing and the pump to see varying demand.
Equal-percentage trim suits heating coils, giving fine control near the closed position where most modulation happens, while a linear trim can be acceptable on stable loads but tends to be coarse at low openings where precision matters most.
Pressure drop across the valve must be enough for authority but not so high that cavitation occurs on hot water, especially as the differential pressure across the circuit varies with other coils opening and closing on the same pump header.
A two-way valve with a bypass or a three-way valve maintains pump flow when the coil throttles, protecting variable-speed pumps from low-flow shutdown and avoiding water hammer on shut-off that could disturb the whole hydronic system.
| Item | Specification | Note |
|---|---|---|
| LTHW supply | 60-80°C | Condensing source |
| Face velocity | 1.5-2.5 m/s | Stable airflow |
| Valve trim | equal-% | Good authority |
| Coil ΔT | design | Size to flow |
| Bypass | 2-way + bypass | Protects pump |
| Cavitation | avoid | Hot water risk |
Selection Checklist
- Size the coil to the winter design outdoor condition, not mild weather.
- Allow extra surface for low-temperature hot water duties.
- Size the valve to coil flow, not to the pipe diameter.
- Use equal-percentage trim for stable modulation.
- Provide bypass or three-way action to protect the pump.
- Check for cavitation across the expected pressure range.
About the Manufacturer
Keruite Air Conditioning Group, headquartered in Dezhou, Shandong, has more than 30 years of HVAC manufacturing experience and operates an 80,000 m² production site with its own performance testing laboratory. Products are built under ISO 9001:2015 quality management and carry CCC certification, and are supplied to customers in over 50 countries.
Our engineers respond to selection enquiries within 24 hours and can provide project-specific sizing, performance curves, and documentation to support specification and procurement.