Radiant heating and cooling systems

Radiant systems heat or cool a space by changing the temperature of large surfaces, usually floors or ceilings, rather than by blowing conditioned air. People exchange heat with those surfaces directly by radiation, and the surrounding air is warmed or cooled by convection off the same surfaces. The result can be very even comfort, quiet operation and low energy use.

Radiant heating has a long history in Canada, from in-floor heating in garages and warehouses to radiant ceiling panels at perimeter windows. Radiant cooling is less common but growing, especially in high-performance and net-zero-ready buildings paired with dedicated outdoor air systems. This guide covers the main system types, typical design values and the practical issues that matter most.

Why radiant works

Thermal comfort depends on air temperature and on mean radiant temperature, the average temperature of the surfaces around a person. ASHRAE Standard 55 combines them into operative temperature. A warm floor or ceiling raises mean radiant temperature, so occupants can feel comfortable at a slightly lower air temperature in winter. A cool ceiling does the opposite in summer. See thermal comfort and ASHRAE Standard 55.

Water carries heat far more compactly than air. Moving heat with pumps and pipes instead of fans and ducts reduces transport energy and allows smaller ductwork, since air systems only need to handle ventilation and latent loads.

System types

Embedded slab systems have large thermal mass. They are well suited to steady loads and can shift energy use to off-peak hours, but they cannot respond quickly to changes like a room filling with people or sun on a window. Panel systems respond quickly and suit spaces with variable loads.

Typical design values

Heating

• Floor surface temperature is limited for comfort. ASHRAE Standard 55 gives a recommended floor surface range for occupied spaces with people wearing typical footwear, roughly 19 to 29 C (66 to 84 F). Perimeter edge zones near cold walls can run warmer.
• That limit caps floor heating output, typically to around 100 W/m2 (about 30 Btu/h per sq ft) in occupied areas, depending on room temperature.
• Supply water temperatures are low, often in the range of 30 to 45 C (86 to 113 F) for floors. This makes radiant floors a natural match for condensing boilers and air-to-water heat pumps.
• Ceiling panels can run hotter water and deliver higher output per area, but warm ceilings over occupants should be designed to avoid radiant asymmetry discomfort.

Cooling

• Supply water is typically in the range of about 14 to 18 C (57 to 64 F), always kept above the room dew point.
• Floor cooling capacity is modest because cool floors become uncomfortable; ceilings achieve higher cooling output per area.
• Radiant cooling handles only sensible load. Latent load (humidity) must be removed by the ventilation system, usually a dedicated outdoor air system. See dedicated outdoor air systems.

Detailed design methods are given in ISO 11855 and in ASHRAE Handbook chapters on panel heating and cooling. In Canada, installation of hydronic heating systems is covered by CSA B214, which provinces adopt to varying degrees.

Condensation control

The defining risk of radiant cooling is condensation. If any surface falls below the dew point of the room air, moisture forms and can drip, stain finishes or support mould. Controls must prevent this:

1. Keep space dew point low by dehumidifying ventilation air with a DOAS.
2. Measure dew point in each zone or representative zones.
3. Reset chilled water supply temperature to stay a margin above the highest measured dew point.
4. Close zone valves if local dew point rises, for example when a door or window is opened.
5. Avoid radiant cooling in spaces with uncontrolled humidity sources or operable windows without interlocks.

Canadian climates help here: summer dew points in much of the country are moderate, and many hours have dry outdoor air. Coastal and southern Ontario summers are more humid and need more care. The guide on condensation and mould prevention covers dew point principles.

Design considerations

Floor coverings

Carpet, wood and rubber all add thermal resistance and reduce output. Designers must know the finished flooring early. Changes during fit-out, such as adding thick carpet, can leave a zone short of capacity.

Zoning and control

Slab systems are usually zoned by exposure and controlled on water temperature reset with outdoor temperature, plus slab or operative temperature feedback. Trying to control a high-mass slab with a simple room thermostat leads to overshoot. Panel systems can be controlled like any other terminal unit.

Combining with air systems

Most commercial radiant designs pair radiant surfaces with a DOAS providing ventilation and dehumidification. Some use supplementary fan coils or chilled beams in high-load areas.

Acoustic and architectural coordination

Exposed concrete ceilings for slab cooling conflict with acoustic ceilings. Panels and capillary systems need coordination with lighting, sprinklers and diffusers.

Commissioning

Pressure test tubing before concrete pours and keep it pressurized during placement. Balance loop flows using manifold flow meters. Verify dew point controls and water temperature resets before cooling season.

Advantages and limitations

Frequently asked questions

Can radiant floors be used in warehouses and garages?

Yes. Radiant slab heating is common in Canadian warehouses, fire halls and maintenance garages. Heat stays near the floor where people work, and the slab recovers quickly after doors open.

Does radiant cooling work in humid climates?

It works when space humidity is controlled by a dedicated ventilation system and dew point controls are in place. Without humidity control, it is risky.

Key takeaways

• Radiant systems condition people through surface temperatures, using water rather than air to move heat.
• Floor surface temperature limits heating output; dew point limits cooling output.
• Low supply water temperatures make radiant an excellent partner for heat pumps and condensing boilers.
• Radiant cooling needs a DOAS and robust dew point controls.
• Choose slab or panel types based on load variability, and test the design with energy modeling early.