In recognition of the major benefits associated with radiant systems, building projects worldwide are adopting radiant methods of heating/cooling.
Most notably, radiant systems have been used to great effect in large commercial and industrial buildings, such as the Bangkok International Airport.
However, the extension of radiant systems to residential buildings has been tentative at best.
The initial hesitation to install radiant cooling systems in residential buildings was in some ways warranted. In older residential buildings, it was difficult to control the latent load within the space.
As such, humid climates experienced high relative humidity inside. This raised legitimate concern over radiant cooling systems, which could provide a cool surface for condensation. If an HVAC system causes condensation it is a big problem, as moisture can cause structural damage, and drastically reduce indoor air quality (IAQ).
However, modern advances in radiant cooling technology have made radiant cooling systems not only appropriate, but ideal for many residential applications. In this post, I will discuss what type of radiant cooling systems are appropriate for residential buildings, and what sort of controls should go with those systems.
The type of radiant system to use in residential buildings depends largely on the size of the building, and whether it is a retrofit or a new building. For small residential buildings low mass radiant systems are best. Low mass radiant cooling systems offer small scale residential buildings quick response times to optimally cool different zones according to the intermittent occupancy and low occupancy levels. Low mass systems are also generally best for retrofit, as they are easy to install without having to change or affect the structure of the building.
For larger residential building projects, the options for a radiant cooling system are practically the same as commercial buildings:
- Thermally active building systems – this is where the PEX tubing is installed directly into the structural concrete of the building, to activate the thermal mass of the building. This can be installed with insulation between layers (uni-directional) or without insulation (bi-directional, typical of multiple-story applications). The most common forms of TABS are floor slab on grade and slab on steel deck.
- Radiant floor cooling – either topping slab on slab, or slab on wood subfloor. In slab on slab, PEX tubing is embedded in a shallow (2-3 inches) layer of concrete, separate to the structural concrete of the building. This is installed over an existing structural slab, with insulation in between them. The layer with the tubing embedded does not have any structural weight bearing role. For slab on wood subfloor, installation can be similar to topping slab on slab, or tubing can be installed in Uponor Quik Trak. This is a premanufactured 1⁄2-inch thick plywood panel solution with aluminium heat transfer sheets and pre-cut grooves 7 inches on centre. The panels are screwed directly onto the wood deck and the tubing is snapped in place prior to installation of the flooring.
- Radiant ceiling cooling – PEX tubing is added to the ceiling either via prefabricated ceiling panels with tubing embedded, or by being attached to the ceiling and then plastered over. Radiant cooled ceilings are installed on existing ceiling structures, and are not weight bearing.
- Radiant wall cooling – PEX tubing is embedded in the walls, similar to low mass floor installations. The tubing can run continuously from the floor into the walls if needed. For wall installations special note should be made for future modifications such as hanging pictures.
Which of these systems to use will depend on a range of factors, and a radiant cooling professional should be consulted for the best system for the desired project.
In residential buildings, the control of radiant systems really is critical. Often, each zone will need independent monitoring by a range of sensors, including air temperature, mean radiant temperature (MRT), and relative humidity (dew point temperature).
The water temperature circulating to each zone can be controlled either by keeping a constant source temperature and varying the flow, or varying the temperature and keeping a constant flow rate. In residential applications where different zones frequently require different levels of radiant cooling, variable flow is typically employed. Variable flow systems allow for the most accurate control of different zones.
Regardless of which of these control strategies is used, the most important concern is controlling for condensation risk. To mitigate the risk of condensation the circulating water used for cooling needs to remain above the dew point temperature. For small scale residential building that ventilate using natural sources (open doors and windows), control of the radiant panel temperatures will have to quickly respond to relative humidity changes.
Fortunately, the temperature of the water needs to typically only be 2-4 °C below the air temperature to provide sufficient cooling. For larger residential building projects where the building envelope is tight, positive pressure can be maintained in internal spaces, and the internal latent load is managed by an independent dehumidification system, radiant cooling present no problems in terms of condensation risk.