Whether you are retrofitting or designing a new commercial building, the best way to achieve optimal thermal comfort is to install radiant systems to manage the sensible energy load.
One of the best systems to manage the sensible load in a commercial space is a high mass, integrated radiant cooling system. The thermal mass refers to the energy retention properties of the system (high mass can store a large amount of energy and release it slowly over a long period). Integrated radiant systems refers to the fact that the radiant cooling is delivered via piping that is embedded in the structural concrete (although not necessarily in the load bearing layer). The system therefore forms an integrated part of the building, rather than being installed post-construction, or separately from the building structure.
When you choose to use this system for radiant cooling, there are 6 important factors to consider. Each of these is very detailed, and could take a blog or more on their own. However, here I will just cover some basics for each.
The best place to start is the design of these systems. This begins with zoning. Essentially, this is understanding the potential energy transfer rates of different spaces, and calculating the cooling load needed within each space. Different rooms within a commercial space will heat and cool naturally at different rates, so will have to be designated different zones, for independent control.
Energy flow will have to factor in the building envelope load, and the intended use of the space (occupancy schedule). In
commercial buildings, spaces that will become kitchens, for instance, might produce a lot of extra heat that must be factored into calculations.
In many cases the use of the space will change over time, so the zoning has to be done with potential for flexibility in use over time. It is important to note that integrated radiant systems allow for the handling of sensible and latent energy to be decoupled.
The radiant cooling system can be controlled separately to regulate thermal comfort, while an independent ventilation system (such as a dedicated outdoor air system, DOAS) can regulate the air flow and latent energy into the space. This has a huge impact on design of the building, because this air-and-water system removes the need for large plenum space required by all-air systems, saving considerable height on each floor.
Moreover, it means that air does not need to be recirculated (improving indoor air quality) and that air handling units (AHUs) do not need to deal with the sensible load. Therefore, the whole ventilation system can be downsized, saving more space and material on ducting and plenum space. This affords new opportunities for design. With all this said, bear in mind that the Uponor Radiant Cooling Design Manual is 230 pages long. So, while these are some thoughts on the basics, make sure you consult a professional at Uponor for the technical details.
Radiant cooling systems have a number of components:
Insulation: Insulation is one of the most important components of properly functioning radiant systems. Insulation is usually installed between the slab that has the piping embedded, and the lower foundation layer. This will increase the efficiency of the radiant system, through only facilitating the transfer of energy in the desired direction (in and out of the occupied space).
Proper tubing: Not all tubing or plastics are made equal. The type of tubing used is important because it needs to be leak proof, in some cases resistant to freezing damage, flexible in design and application, and robust (durable and long-lived). In this regard, Uponor PEX-a tubing is a premier product. In short, PEX-a is highly durable, inexpensive (relative to copper), and perfectly suited to underfloor radiant cooling systems. Its flexibility in particular gives it the ability to be fitted and moulded to a wide range of spaces and applications, ideal for a range of industrial projects.
Pipe fittings: Given that PEX-a tubing is highly flexible, it can be shaped to reach all areas of the space, and provide even heating across the floor surface. However, as it is shaped and placed where it needs to go, it will have to be fixed.
Depending on the type of installation, this can be done using three methods:
1. Plastic staples, which can fix the tubing directly to the insulation layer
2. Fixing wire, which can fix the tubing to non-structural wire mesh
3. PEX rails which are fastened to the subfloor.
The water: Unlike traditional HVAC, the water temperatures in radiant systems can often be within 20°F (11°C) of space temperature. These low temperatures for heating and high temperatures for cooling enable higher efficiencies in traditional components such as chillers, boilers, solar and heat pumps, and enable the use of non-traditional sources such as ground water.
The circulators: Standard circulators with or without speed control can be used for radiant systems.Moreover, due to the necessarily higher cooling fluid temperatures, there is less concern for condensation on circulator motors in comparison to traditional systems.
The manifolds: The central hub for distributing flow to a slab is the manifold and cabinet which can be furnished with a number of flow control and service options. To account for system hydraulics and maintenance, manifold locations must be carefully coordinated with the architect. You will need to install the Industrial Manifold before the concrete is poured. The manifold can be fixed either to the existing wall where it will stay once construction is finished, or to a temporary structure that can be modified to fit to the final structure once it is built. The tubing is then fed out and connected to the manifold, prior to concrete being laid.
The conditioned slab: The slab is warmed or cooled by water flowing through tubing
embedded in the slab, either the structural slab or a topping slab. The tubing material of choice for radiant heating and cooling applications is Wirsbo hePEXTM, which is a PEX-a material that has an oxygen diffusion barrier to protect the system’s ferrous components.
The control system: The HVAC control system is a critical component of any building system; and will manage indoor temperature, fluid and surface temperatures and relative humidity. In addition to being a key component in facilitating plant efficiency, it will also provide safety dew point monitoring in case of failure in the latent control systems.
In a previous blog I covered various considerations for the installation of a high mass radiant cooling system. I will not go over them again here, but suffice it to say that installation of radiant systems is a multifaceted process. In short, adding radiant cooling systems means the inclusion of a number of new
materials to the building process (insulation, piping, manifolds, etc.), as well as a few extra steps (laying piping before concrete pouring, system testing, etc.). Particular attention should be given to the type of joints used, as expansion joints and day joints require the radiant piping to be protected by pipe sleeves to withstand mechanical stress. The installation process requires attention to detail and rigorous application of the correct procedure. However, using new technology developed by Uponor (e.g. Radiant Rollout Mat; figure 1), the installation time and effort can be greatly reduced.
Figure 1: Uponor Radiant Rollout Mats are delivered to the job site in 5-foot or 10-foot wide sections that are rolled out onto the prepared grade, significantly reducing installation time and labour.
The construction aspect of integrated radiant cooling systems overlaps strongly with the installation process, and has many similar considerations. To begin with, it is important to point out that installing integrated radiant cooling systems in commercial spaces does not compromise structural integrity. The inclusion of a radiant system only changes the construction process slightly, it does not create differences in the structural functionality of the final product. Integrated radiant cooling systems are embedded in a concrete layer, which means that you will have to choose from a few types of concrete; reinforced concrete, pre-stressed concrete, roller compacted concrete, steel fibre concrete, and vacuum concrete. These affect how the tubing for the radiant system will be installed. Once the concrete is poured, it needs to be compacted. This is usually achieved using high-frequency internal vibrators. In most cases, the vibrators are drawn slowly through the freshly poured concrete at the same time it is levelled. This process is compatible with an integrated radiant cooling system, which means that the pouring and compaction of the concrete is not affected by the presence of the radiant system. The four most common construction methods for radiant cooling systems in commercial spaces are:
1. Floor slab on grade
2. Floor slab over steel deck
3. Topping slab on slab
4. Floor slab on wood subfloor.
Only floor slab on grade and floor slab over steel deck are high mass integrated systems (figure 2), so I will discuss those two further.
Figure 2: Floor slab on grade (left) and floor slab over steel deck (right) methods of construction when installing a high mass integrated radiant cooling system.
The most common method for commercial applications is floor slab on grade. This is where PEX-a tubing is embedded directly in the structural slab, generally with a vapour barrier, such as high density polyethylene sheeting, between the radiant slab and supporting layers. The structural slab on metal deck method is very similar to the floor slab on grade method, and is common for the upper levels of multi-story buildings. The main difference is the insulation, which is typically polyurethane spray foam applied to the underside of the deck. In some cases, the contractor may wish to lay rigid foam board insulation on top of the metal deck under the structural slab.
A series of sensors (space temperature, operative temperature, relative humidity and slab temperature) is used to evaluate space conditions to determine the optimum target supply water temperature for the zone.
The control strategy depends on the design characteristics, such as building envelope, thermal inertia, the system response times and others. In modern systems, the control will be automatically operated, using complex algorithms to ensure proper regulation. Even so, these strategies will need to be modified for each new application. Generally, control can either be constant flow with variable temperature, or variable flow with constant temperature (figure 3).
Figure 3: Water temperature and flow rate differences between different control strategies: constant flow, variable temperature (above), or variable flow, constant temperature (below).
The control of high mass radiant cooling systems allows for them to shift the burden of the energy input to outside of peak hours. Basically, the high energy retention of the radiant slabs means that they can be cooled overnight, and provide active cooling throughout the day with minimal energy input (figure 4).
Figure 4: Energy input profile over a 24h period. Peak energy input is overnight, thus providing cooling during occupied hours with minimal energy expenditure.
Be aware that a poorly designed or managed control system can lead to sub-par performance, poor response times, energy inefficiencies, inconsistent operation and condensation. As a side note here, the myth that radiant systems struggle with condensation issues persists. It is thus worth pointing out that maintaining the surface temperature of radiant slabs above the dew point temperature eliminates the possibility of condensation.
An HVAC system that adequately controls the latent load will achieve a low dew point temperature, which will allow the surface temperature of the radiant slabs to operate at a temperature well above one that would incur any risk of condensation. In fact, radiant cooling systems do not have to maintain very low temperatures to begin with to deal with a high cooling load (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)). Therefore, even in spaces where the latent load is not closely controlled (such as where natural ventilation is used), radiant cooling can still be effectively employed without the risk of condensation damage.
While integrated radiant systems allow for the removal of suspended ceilings, thus saving space and construction material, 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. Fortunately, options are available that do not significantly compromise the radiant cooling system’s functionality.
A recent study (Karmann et al. 2017) studied the effect of an ‘acoustical cloud’ system suspended from the ceiling, and found that, “the acoustical results showed that if the cloud covered 30% of the ceiling in a private office or 50% in an open plan office, acceptable sound absorption at the ceiling was achieved. We showed that good acoustic quality can be achieved with only a minor reduction of cooling capacity.” This suggests that integrated radiant cooling systems are compatible with good acoustic environmental quality with minor modifications. However, if these steps have to be taken to address the acoustic environment, it will be important to consider the extra space needed for the acoustic modifications for design and construction, as well as the small decrease in cooling capacity for load calculations.