Hopefully, you are already aware of the benefits radiant systems offer compared to conventional HVAC systems. These range from higher profitability, to improved energy efficiency, through to superior indoor environmental quality (IEQ).
However, in order to realise all these advantages, you have to face the design, construction, and installation of radiant systems. This is a complex process, with a lot of considerations and nuances. In fact, it is simply the familiarity with simpler, conventional HVAC systems that often leads people not to go with radiant systems. In modern society, this simply is not good enough! Contemporary technology offers a great deal, and in the building industry, radiant systems are the way forward. It is best to embrace the change, and join the community at the forefront of design and technology.
To help you do so, I will highlight four ways that radiant cooling floors will impact the design and construction of industrial buildings. This information will hopefully clarify what installing a radiant cooling system will mean for your next building project, and give you the basic understanding you need to tackle the challenge and reap the major rewards.
High mass radiant systems
The first thing to understand is that high mass radiant cooling floor systems are high mass systems. The tubing that circulates water to provide the cooling is embedded in the structural concrete during construction. These are referred to as thermo-active building systems (TABS).
Designers will have to understand the final use of the building – where heavy equipment will move or be bolted to the floor, expected occupancy levels in different zones, and how the building’s orientation will affect solar radiation exposure differences between zones among other things. Essentially, the cooling capacity and layout of the tubing in each zone will depend on all these factors. However, because the piping is embedded in structural concrete, it must be carefully planned, zoned, and sized prior to concrete being poured.
The layout of the tubing can take various forms:
- Full coverage – used when the major heat load is evenly distributed, such as when internal heat loads and/or unheated air exchanges are excessively high or when high resistance floor coverings are used. The full coverage option features tubing installed 12 inches on centre throughout the entire floor.
- Perimeter-only coverage – tubing is installed inside and along the perimeter walls of the building, not in the interior areas of the room. Industrial projects usually install minimal floor coverings, resulting in diminished upward resistance to heat transfer. This improves the effectiveness of the radiant floor system so that perimeter-only designs are both effective and efficient.
- Varied coverage – used when the major heat load is at the perimeter, but a small load is anticipated in the interior of the building. Install the tubing 12 inches on centre near the perimeter and at increased distances (18 to 24 inches on centre) in the interior areas.
- Reduced coverage – used when the heat loss is minimal and evenly distributed throughout the building. Install the tubing throughout the floor at distances greater than 12 inches on centre.
The fact that high mass, integrated radiant floor systems are built into the structural concrete means that they affect construction in a number of ways.
Firstly, there is a choice of two main construction methods for high mass integrated systems: floor slab on grade and floor slab over steel deck.
Figure 1 – Schematic of the floor slab on grade method of construction. Note that the tubing is secured to the Rebar so that structural integrity is not compromised. For multi-story buildings, the construction can be done without insulation to enable bi-directional radiant cooling.
Figure 2 – Schematic of the slab over steel deck method of construction.
Secondly, the tubing itself has to be fixed in place prior to the concrete being poured. This can be done using three methods:
- Plastic staples
- Fixing wire
- PEX rails
Thirdly, a type of concrete will have to be selected. This will affect how the radiant system is installed. When concrete is laid with mesh reinforcement (steel-reinforced concrete, pre-stressed concrete with mesh reinforcement), the tubing is attached to the lowest level of the mesh. When concrete is laid without mesh reinforcement (steel-fibre reinforced concrete, pre-stressed concrete without mesh reinforcement, non-reinforced concrete), the tubing must be attached to a raised support structure that is laid onto the concrete base.
This method is a patented Uponor system, which allows the tubing, or radiant plane, to be positioned in the centre of the concrete slab. The tubing thus sits between the lower and upper levels of the mesh reinforcement. The raised pipe supports are attached using special spacers, which are attached to the upper reinforcement. Once the concrete is poured, it needs to be compacted. Radiant systems are compatible with this process, so no special considerations are required here.
Fourthly, some industrial buildings require heavy-duty vehicles to travel across the surface within which the radiant system is installed. This means that the radiant cooling floor system will have to bear very high weight loads and traffic from vehicles without suffering damage or reduced function.
In some cases, it is possible to use the steel reinforcement framework within the concrete as a support structure the tubing, which means that the radiant cooling system will never compromise the structural integrity and strength of the building construction. It is worth mentioning here as well that Uponor PEX-a tubing is capable of bending, contracting and expanding if necessary without fracturing. This is not true of more rigid piping alternatives such as copper or PVC. In addition, make sure to consider the weight bearing properties of the insulation layer if one needs to be installed. Insulation does not reduce weight bearing capacity if properly installed and considered.
Finally, the joints in the concrete require particular attention. Connecting pipes that cross over expansion joints must be protected against the anticipated mechanical stresses in the area around the joint using protective pipe sleeves of 1 m in length. Pipes that cross a construction joint (day joint) must be sheathed for a distance of 1 m using protective pipe sleeves in cases where the pipe is subject to mechanical stress before pouring the concrete, for example, due to the positioning of formwork over the pipe. Dummy joints do not require the use of protective pipe sleeves.
Radiant systems require parallel ventilation systems
Radiant cooling floor systems handle the whole or the majority of the sensible load. However, they will require a parallel system to handle the latent energy load, as well as for ventilating the space. The best option for this is generally considered to be a dedicated outdoor air system (DOAS), which can handle the latent load, any residual sensible load (if needed), ventilation, and energy recovery.
Figure 3 – a schematic of a radiant cooling system working parallel with a DOAS to condition the space. The radiant cooling system handles the sensible load and regulates thermal comfort, while the DOAS deals with air temperature, ventilation, and energy recovery.
Having the sensible load wholly, or mostly handled by the radiant cooling floor system will mean that the entire ventilation system and air handling unit (AHU) can be downsized. This means that the design of the building will have up to 0.6 meters of extra space per floor that would have been required for a suspended ceiling for the ducting and plenum space of a conventional all-air system.
A parallel DOAS system will mean that fewer materials are required for construction of the building (the space saved per floor translates into saved building costs). The space and size of the AHUs and chillers are significantly reduced as well.
Condensation risk management
Contemporary radiant cooling systems do not have problems with condensation. However, this is because of good design and construction protocols.
The risk of condensation is mitigated even before construction in designing good control strategies, and an efficient dehumidification system. Condensation occurs on surfaces that are cooler than the dew point temperature of the air in the space. Therefore, the radiant cooling floor system will have to be designed and controlled such that its surface temperature never drops below the dew point temperature. This is easily achieved with two concurrent methods:
- The parallel AHU deals with the latent energy load, controlling the indoor relative humidity within a desired range (25 – 60 % relative humidity)
- The water temperature circulating in the radiant system piping does not go below ASHRAE recommended levels (lower limit of 62°F (16.7°C) and 66°F (18.9°C) for walls or ceilings and floors, respectively; ASHRAE Standard 55-2010). While it may seem intuitive that the water would have to be much colder to provide adequate cooling, this is not the case. The water temperature needs to only be 2 – 4 °C below room air temperature to provide sufficient cooling power. A range of sensors (dew point, mean radiant temperature, and air temperature) may have to be installed to monitor the operative temperature of each zone, as well as the dew point, so that control strategies can be adjusted accordingly.
With careful design, condensation should be dealt with even before construction. However, there are still considerations during construction that are vital to ensuring that the risk of condensation is completely removed. To begin, the building envelope must be tight, such that water cannot penetrate. Depending on the degree of exposure of the substrate to ground moisture, non-pressing and pressing water, appropriate waterproofing measures must be provided in accordance with local standards. Usually, waterproofing takes the form of rolls of material (e.g. bitumen sheets, PVC sheets). For buildings where the AHU regulates the latent load (humid climates), the construction of windows and doors will have to be carefully considered. Too many leaks in the building envelope will cause improper control of the latent load, and lead to a risk of the dew point increasing above the surface temperature of the radiant slabs.
As I mentioned above, radiant cooling floor systems can save a large amount of space each floor of a multi-story building by removing the need for the suspended ceiling typically required by conventional all-air HVAC systems. However, the exposed concrete may also create acoustical challenges due to the high reflectivity of the hard surface. In this case, additional solutions to regulate the acoustic component of indoor environmental quality will have to be taken.
Designers will have to incorporate a strategy to deal with the acoustic challenges afforded by radiant slabs. This may include installing an ‘acoustical cloud’ on the ceiling, to buffer sound and regulate the acoustic component of IEQ. Modifications to deal with the acoustic problems caused by exposed hard concrete may diminish the cooling capacity of the radiant slabs. This makes it imperative that designers take into account how acoustic solutions will impact the cooling load, and cooling capacity of the system.
Acoustic solutions may require extra space per floor, which will affect construction and the amount of materials needed. For the radiant cooling system construction, the acoustic requirements may affect zoning and sizing of the system, which will impact construction as well. Good coordination is always required between all teams involved in a construction project to ensure these considerations are met.
Installing a high mass radiant cooling floor system clearly impacts the design and construction of a building in a variety of ways. However, the advantages conferred by doing so are remarkable. The installation of radiant systems has helped a large number of modern buildings move up to a Platinum rating on the LEED ranking system by becoming energy neutral.
For example, the LEED Platinum rated Department of Energy’s National Renewable Energy Laboratory (NREL) used Uponor’s Radiant Rollout Mat to install a radiant system as part of the $64 million complex. Other notable examples of building projects which used Uponor radiant heating/cooling systems to help achieve LEED Platinum rating include the David Brower Center, Berkeley, California, and Manitoba Hydro Place, Winnipeg, Manitoba. The Manitoba Hydro Place was also awarded AIA’s 2010 COTE Top Ten Green Project. Clearly, although installing radiant systems presents new challenges, it is not only worthwhile, it is the future of commercial and industrial HVAC.