Design a radiant cooling system that puts cold panels on the ceiling (chilled-ceiling cooling). Adjust the room temperature, panel surface temperature and relative humidity to see the cooling capacity, the load coverage, and — most important of all — whether the panel will condense, all in real time.
Parameters
Room temperature T_room
°C
Room air temperature to be maintained
Panel surface temperature T_panel
°C
Ceiling panel surface temperature. Colder means more capacity but higher condensation risk
Room relative humidity RH
%
Sets the dew-point temperature. Higher humidity condenses more easily
Panel area A_panel
m²
Total area of the radiant panels installed on the ceiling
Room cooling load Load
W
Sensible heat load to remove (occupants, equipment, solar gain, etc.)
The chilled ceiling panel radiates cooling downward and absorbs radiant heat from the occupant and floor. The dew-point indicator at the top-left is green when safe and red when the panel condenses.
Cooling capacity vs panel temperature
Load coverage vs panel area
Theory & Key Formulas
$$q=h\,(T_{room}-T_{panel}),\qquad Q=q\,A$$
Cooling capacity per unit area q [W/m²] and total panel capacity Q [W]. h: combined heat transfer coefficient, A: panel area. h ≈ 11 W/m²K lumps radiation (≈ 5.5) and natural convection (≈ 5.5) and is treated as a constant in line with the order of magnitude of EN 14240.
Saturation vapour pressure p_sat [Pa] and the actual vapour partial pressure p_v from the Magnus equation. RH: relative humidity. Inverting this equation gives the dew-point temperature T_dp.
A radiant cooling panel must always be kept above the dew-point temperature T_dp of the room air. Below it, water vapour condenses on the panel surface, causing condensation, dripping and mould. A sound design must satisfy both the cooling capacity (sensible) and the dew-point constraint (no condensation) at the same time.
What is radiant cooling?
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How is "radiant cooling" different from a normal air conditioner? I heard it puts a cold panel on the ceiling.
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Good question. A normal air conditioner blows cold air and stirs the room air to cool it — that is convective cooling. Radiant cooling instead fixes cold panels to the ceiling or walls, and those panels absorb radiant heat directly from occupants, the floor and furniture. There is almost no airflow. Think of it as the cooling version of a heated floor in winter. When you sit near a cold surface, the panel quietly draws away the infrared your body emits — that is the coolness you feel.
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I see. So if I make the panel really cold, I can cool a lot — that sounds like a win, right? When I lower the "panel surface temperature" on the left, the cooling capacity q keeps rising.
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That is exactly the trap of radiant cooling. The capacity is q = h·(T_room − T_panel), so yes, a colder panel gives more capacity. But cool the panel too far and the water vapour in the air condenses on its surface. Fill a glass with cold water and droplets form on the outside — same thing. If droplets form on a ceiling panel, they drip down and mould grows. So there is an absolute rule: you must never go below the dew-point temperature.
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The dew point... when I raise the "relative humidity" on the left, the T_dp result also goes up. Does high humidity mean it condenses more easily?
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Exactly. The dew point is "the temperature at which water vapour starts to turn into liquid as you cool that air." More humid air holds plenty of water vapour, so it only needs a little cooling to start condensing — that means a high dew point. At the default 27 °C room and 55% humidity, the dew point is about 17.2 °C. With the panel set to 18 °C, the condensation margin is only 0.8 °C. It is right on the edge. Try raising the humidity to 70%. The dew point jumps up, and an 18 °C panel will be flagged as condensing.
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If I raise the panel temperature to avoid condensation, then the cooling capacity will fall short. Is "load coverage" the number that shows that?
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That is precisely it. Load coverage = total panel capacity ÷ room cooling load. At 100%, radiant cooling alone is enough. But within the no-condensation range the panel temperature can only go down to about 17-18 °C, so the per-area capacity caps out around 60-100 W/m². In a room with a large load, even covering the whole ceiling with panels often will not reach 100%. That is why, in practice, the standard answer is a "hybrid" system that pairs radiant cooling with a ventilation unit that removes moisture.
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So if the ventilation side removes moisture, the room dew point drops and you can run the panel colder — a nice virtuous circle.
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Perfectly understood. The ventilation side (primary air) handles the moisture of the outdoor air — the latent load — and lowers the room humidity, which lowers the dew point. Then the radiant panel can run colder and its sensible capacity rises. The division of labour locks together neatly. In this tool, lower the humidity first, then lower the panel temperature, and you can see that relationship in the numbers. Designing radiant cooling is a two-front campaign — keep an eye on both "capacity" and "condensation" at once.
Frequently Asked Questions
The cooling capacity per unit area of a chilled ceiling is q = h·(T_room − T_panel), where h is the combined heat transfer coefficient that lumps radiation and natural convection. This tool uses a constant h ≈ 11 W/m²K (radiative ≈ 5.5 + convective ≈ 5.5), in line with the order of magnitude of EN 14240. T_room is the room temperature and T_panel is the panel surface temperature. The total panel capacity is Q = q·A, where A is the panel area. For a 27 °C room with an 18 °C panel, q = 99 W/m² and a 20 m² panel gives Q = 1980 W.
When the panel surface temperature drops below the dew-point temperature of the room air, water vapour condenses on the panel. The dew point is found from the Magnus equation, and the higher the room relative humidity, the higher the dew point. The single most important constraint in radiant cooling is to always keep the panel temperature above the dew point. Three ways to prevent condensation: (1) do not over-cool the panel (keep the temperature difference to the room small), (2) lower the room relative humidity by ventilation or dehumidification to drop the dew point, and (3) control the panel supply-water temperature automatically with a dew-point sensor. This tool shows the condensation margin (panel temperature minus dew point) and flags condensation when it is at or below 0 °C.
Because the panel cannot be cooled below the dew point, the capacity of radiant cooling per unit area tops out at roughly 60-100 W/m². If the room cooling load is large or the panel area is small, the load coverage falls below 100%. The required panel area is A_req = Load / q; when this exceeds the ceiling area available, the usual answer is a hybrid "radiant + ventilation" system that combines radiant panels with a dedicated outdoor air system. The ventilation side handles the latent load (moisture) and fresh air, while the radiant side handles the sensible load (temperature).
A normal air conditioner blows cold air and circulates the room air — convective cooling. Radiant cooling instead uses cold panel surfaces that absorb radiant heat directly from occupants, furniture and the floor, with almost no airflow. The advantages are no draught discomfort, quiet operation with small temperature gradients, and a feeling of coolness even at a higher set-point room temperature because the mean radiant temperature drops. The drawbacks are a slow response and the need for a separate system for condensation control and humidity handling. This tool focuses on the sensible capacity of the radiant side and the condensation risk.
Real-World Applications
Offices and public buildings: Chilled-ceiling radiant cooling is widely used, especially in Europe, in office buildings, libraries, airports and museums. It is quiet, draught-free, does not blow papers or exhibits around, and creates a uniform, comfortable environment with small temperature gradients. Designs often build chilled beams (combined radiant and induction panels) or capillary mats into the ceiling and pair them with a dedicated outdoor air system (DOAS) to split the sensible and latent loads.
Residential radiant heating and cooling systems: Running pipes through floors, ceilings or walls — chilled water in summer, hot water in winter — is increasingly popular in highly insulated homes. In cooling mode, condensation is the biggest challenge, exactly as this tool shows: the room humidity must be controlled by ventilation and dehumidification, and the supply-water temperature must be raised and lowered automatically according to the dew point.
Local cooling for data centres and server rooms: Placing radiant or contact-cooling panels above high-heat-density racks cools the equipment while cutting the power and noise of supply fans. The dew-point control is far more demanding, however, and cold panels are only allowed in dedicated spaces where the humidity is kept tightly low.
Early HVAC design and CAE: Before solving the room temperature, airflow and radiation field with detailed CFD, a heat-balance estimate like this tool gives a first read on "is the panel area enough" and "how much condensation margin is there." If the estimate shows a negative condensation margin, that is a sign to revisit the humidity design before running CFD.
Common Misconceptions and Pitfalls
The biggest misconception is "as long as the cooling capacity is enough, the design is fine." Radiant cooling can only handle the sensible load (temperature); it removes none of the latent load — the moisture. If you try to raise the capacity q by lowering the panel temperature, a humid room will push the panel below the dew point and it condenses. This is exactly why this tool shows the condensation margin as a separate, independent number. Cooling capacity and condensation prevention are two distinct constraints, and a design only works if it satisfies both at once. Always handle the moisture with a separate ventilation or dehumidification system.
Next, "looking only at the steady-state capacity and missing the humidity swings." This tool deals with a steady heat balance, but real room humidity changes minute by minute with people coming and going, doors opening, outdoor air leaking in and rainy weather. Even if you run with a 0.8 °C margin on a clear day, an evening shower that brings in humid outdoor air pushes the dew point up sharply, and the panel condenses at that instant. Real systems always include a dew-point sensor and feedback control that keeps the panel supply-water temperature at least 1-2 °C above the dew point. Running at a fixed temperature is dangerous.
Finally, "treating the combined heat transfer coefficient h as a single fixed value." This tool uses a constant h ≈ 11 W/m²K, but that is only a representative order of magnitude for a chilled ceiling. The real h varies with whether the panel is on the ceiling, a wall or the floor (the convective component changes a lot), the panel surface emissivity, the mean radiant temperature of the room, and how furniture blocks the view factor. Floor radiant cooling tends to trap cold air below it and weakens convection, so it delivers less capacity than a ceiling system for the same temperature difference. For the final design, use standard test values such as EN 14240 or manufacturers' capacity curves, and treat this tool as an estimate for study purposes.
How to Use
Enter room air temperature (18–28°C) and set the target panel surface temperature (2–16°C) to establish the radiant temperature difference.
Input relative humidity (30–80%) and occupied area (10–500 m²) to define condensation risk and cooling demand.
Click Calculate to obtain specific cooling capacity (W/m²), total panel capacity (W), load coverage percentage, dew-point temperature, condensation safety margin, and required panel area for your chilled-ceiling system.
Worked Example
A 120 m² office requires cooling load of 8.5 kW. Set room air temperature to 24°C, panel surface temperature to 14°C, relative humidity to 55%, and occupied area to 120 m². The simulator returns: specific cooling capacity q = 48 W/m², total panel capacity Q = 5,760 W, load coverage = 67.6%, dew-point temperature T_dp = 13.9°C, condensation margin = 0.1°C (critical), and required panel area = 119 m² (nearly full ceiling). Reducing humidity to 45% increases T_dp to 11.2°C, margin to 2.8°C, ensuring safe operation with 2°C clearance above dew point.
Practical Notes
Maintain minimum 2–3°C margin between panel surface temperature and dew-point; condensation at 13.9°C panel temperature with 55% RH in 24°C air causes mold and system failure.
Chilled-ceiling systems deliver 30–60 W/m² in typical office conditions; high humidity zones require oversized panels or supplemental dehumidification.
Panel temperature below 12°C in high-humidity spaces (>60% RH) demands active moisture control; use control algorithms to modulate chilled-water flow (6–12°C inlet) and prevent condensation events.
Cooling coverage below 70% indicates undersized panels; combine with perimeter convective units or reduce internal gains (lighting density, occupancy) to close the gap.