Selecting radiant ceiling cooling and heating systems (part 3)
This three-part series provides an overview of the most commonly available and applied commercial building radiant cooling systems. Part 3 describes concrete core conditioning (radiant slab ceilings).
Editor’s note: This three-part series provides an overview of the most commonly available and applied commercial building radiant cooling systems. There are also a wide variety of radiant “heating-only” devices that are available, such as low- and high-intensity fuel-fired infrared heaters and electric radiant heating panels and sheets, but the scope of this series is to examine active radiant cooling applications for commercial buildings.
Concrete core conditioning (radiant slab ceilings) consists of plastic tubing (usually oxygen barrier PEX) piping cast directly into the structural slab of a concrete building system. A variation on this is Tarmac’s Termodeck system —hollow core precast concrete slabs that uses cool nighttime air moved through the hollow cores to pre-cool the concrete for the next day of cooling, or conversely, warm air to pre-heat the slabs.
The PEX tubing is generally 5/8-in. or 3/4-in. diameter, depending on the slab loop areas/lengths, and water flow rates required for the heating and cooling loads in the building/space. Spacing of the tubing can be anywhere from 6 to 12 in., depending on the desired average surface temperature of the ceiling. This system makes use of the extremely high thermal mass of the concrete structure as a space temperature stabilizing system. The exposed underside of the structural slab is intended to be the finished radiant ceiling surface in the occupied space.
This system is like a radiant floor slab, but used as an overhead ceiling radiant slab, with the tubing kept as close to the underside of the slab as possible (usually on top of the bottom layer of reinforcing bars).
Pioneered in Switzerland more than two decades ago, concrete core conditioning systems (thermoaktive decke/Batiso Buildings) are becoming more popular in Europe. Because the whole ceiling area is the radiant surface, the operating temperature differences relative to the occupied space can be kept very small for heating or cooling. Obviously this system is meant to be used where the building structure is cast-in-place concrete slab floors, and an exposed concrete ceiling can be aesthetically integrated into the interior space design.
An innovative design team, could, however, use this system in a steel-framed building using the concrete floor topping, designing the underside of the floor deck and exposed steel structure as an articulated exposed ceiling. The exposed underside of the Q-Deck fluting would provide increased radiant surface area leading to a slight overall efficiency gain in terms of the required radiant fluid operating temperatures in the floor tubing. Using indirect lighting in this case would be a design challenge, however.
These radiant slab systems are extremely energy-efficient due to the huge thermal mass of the structure creating very stable interior building temperatures, and its capability to very slowly absorb temperature changes. The key design issue is that the huge thermal mass also takes a very long time to change temperature, so it cannot be controlled by conventional modulating control valves, or easily divided into smaller discrete different thermal control zones. This also means that the radiant slab systems cannot respond quickly to high variations in transient heating and cooling loads that commonly occur at perimeter areas of conventionally designed buildings.
The building glazing system and solar control systems must be carefully designed and selected to minimize the perimeter peak transient loads to be within the radiant slabs’ capabilities. Transient loads from rooms being occupied or unoccupied by people are “small” transients, and well within the radiant slabs’ operating capability. The designer must be aware of the time lag/time response/thermal storage factors associated with the radiant slab system and how it responds to short-term “high peak” transient thermal loads versus long-term “low level” transient thermal loads.
Radiant slab systems, whether for heating or cooling or both, are normally operated by using a constant water flow with a variable temperature reset schedule. The slab system water temperature is reset up or down depending on the seasons and long-term trends (weekly/monthly) of the outdoor ambient temperature. Typically, the maximum winter peak temperature of a radiant ceiling slab surface is around 76 F, while the minimum summer peak cooling temperature is around 63.5 F, which can provide comfortable space “resultant” temperatures.
In summertime, because of the time lag of the slab absorbing heat gains during the day, the slab can be cooled off at night with an evaporative closed-circuit fluid cooler type cooling tower, to be ready for the next day of cooling, in climates where cooler nighttime temperatures are available. No large cooling plant equipment is needed, and in most temperate climates, there is no need for any refrigerant-based water cooling equipment. The slab essentially acts like a giant static heat pump since any “spot thermal loads” are absorbed and spread out over the whole slab by the constantly circulating water system. Imbedded slab temperature sensors coupled with space temperature sensors will allow the system to be monitored and fine-tuned as necessary.
The total building costs for radiant slab systems with high-performance glazing systems have been proven to be equal to, or less than, a conventional building approach with conventional all-air HVAC systems, despite the premium for high-performance glazing systems. Further savings can be achieved by reduced floor-to-floor heights possible with the exposed slab ceiling and a raised floor (access floor) system, saving a great deal of building envelope costs for multi-storey buildings.
This also can result in adding additional floors to a multi-storey building within the same height restrictions as a conventional building, providing more rentable area for a very small incremental construction cost. One of the key design issues is to “design in” future flexibility to the slabs by providing cast-in-place inserts and empty “cans” for future services installations without having to drill into the slab. Adaptive re-use of the building over the long-term life of the concrete building becomes easily accomplished--a very sustainable building approach.
Radiant floor heating systems are reasonably well understood, and many designers attempt to use radiant floor slabs or concrete topping systems for both heating and cooling applications, thinking that radiant floor cooling will also work. While some radiant cooling capability may be found via the cooled floor, it is nowhere near as effective as a ceiling radiant emitter in terms of human comfort. A warm floor certainly provides comfort, but a cool floor does not.
Floors are a poor choice for a radiant cooling emitter for the following reasons:
--Floor coverings, furniture, and finishes can affect and mask the output and floor surface temperature, limiting future room finish revisions
--Radiant cooling heat exchange from the floor relative to human comfort is poor compared to an overhead radiant surface emitter
--A raised-floor air-distribution system cannot be used
--Future anchoring and drilling into the floors becomes problematical if the radiant tubing may be close to the floor surface of the slab
--Installing the radiant tubing in the floor slab will require chairs or other labor consumptive methods to keep the tubing near the middle to top of the slab surface.
Radiant slab systems advantages include:
--Extremely good energy efficiency, very low energy use
--Integrated into building structure and almost completely invisible
--Very low maintenance requirements--passive hydronic system with very few controls, minimal moving parts
--No system work required for tenant renovations, other than air supply points
--Can be installed to provide very large radiant surface areas at low capital costs
--Silent operation and speed of light self-compensating operation
--Very long operational life--it lasts as long as the building structure
--Provides extremely stable space comfort conditions
--Silent operation, at the speed of light.
--Not well understood by North American building and HVAC designers. Thermal mass storage and radiant temperature control cannot be modeled accurately by conventional HVAC systems computer modeling software.
--Requires detailed coordination between architect, mechanical engineer, and structural engineer (requires an integrated design team).
--Severe fire or seismic event will render the system inoperable, with expensive repairs or alternate retrofit HVAC systems required (applied capillary tubes would be an easy solution).
--Aesthetics of an exposed slab as a finished ceiling can be difficult to deal with, some exposed services will have to be designed cleanly: sprinklers, air exhaust systems, lighting and electrical distribution, etc.
--Hard surfaces in the occupied space require acoustic design and review to insure acceptable noise levels.
The radiant slab heating and cooling systems offer the best energy and capital cost efficiencies, provided the building cooling, heating, and transient loads and time response/thermal storage aspects are controlled properly. Radiant slab ceiling cooling systems in Europe have a track record of more than 25 years, and the energy performance has typically been in the 60% to 70% below even the best conventional “all-air” HVAC systems. [ reference 6 and 7 ]
Despite of the perceived premium costs of the high-performance glazing systems required for these radiant building systems, the extra capital costs of the glazing systems and solar control systems are easily offset by the savings in the installed costs for the PEX tubing in the slab and the dedicated outdoor air system, along with the much smaller connected mechanical loads for the electrical system, and the smaller main mechanical plant equipment requirements.
North American applications of large non-residential radiant heating/cooling slab systems are few, but growing, and the track record indicates that a properly designed thermo-active slab building can be built at the same or less cost than a conventional building approach, while consuming 60% to 70% less energy than conventional “all air” HVAC systems.
Radiant panels are easily available for creating a cost-effective energy-efficient HVAC system in North America, but the capital costs are more difficult to resolve. The costs of the radiant panels are such that there is a desire to minimize their area, which results in trying to operate them right at the extreme limits of the surface temperatures for cooling. Although there are some successful examples of operating radiant heating/cooling panels systems in North America, there is generally a cost premium to create effective radiant surface panel areas to perform adequately. [ reference 8 ]
Fabricated PEX tubing systems are a good application for wood frame residential and small wood framed commercial buildings. The radiant floor heating systems can be easily adapted to work as wall or ceiling surfaces to perform combined radiant heating and cooling. The costs of these systems are well known and the installations can be carried out by anyone who has already been working with floor heating systems.
Capillary tube systems are supported in North America by only a couple of sales agents, and their cost is high because the capillary tube systems are imported from the manufacturers in Europe. While these capillary tube systems are seeing widespread use in Europe, they are not yet something that the North American HVAC industry can rely on for a radiant system option. If the economics of capillary tubes can be addressed, they do offer an excellent way of providing large radiant surface areas, which can be applied in building renovation projects, new construction, and other applications.
The main design issue to consider when deciding on a radiant cooling system is that radiant cooling from the ceiling, at a radiant surface temperature of approximately 64 F-65 F, will provide about 25 Btuh/sq. ft of effective cooling capacity. But a radiant floor cooling system at the same surface temperature will only provide an effective cooling capacity of 10 to 14 Btuh/sq. ft due to the location relative to the human occupants, and masking from furniture.
6. Lawrence Berkeley National Laboratories, Paper: www-epb.lbl.gov/thermal/dissertation.html
8. Personal Files Tender and Construction Costs results for radiant slab ceiling structures in Canada. Article: “Economic Considerations DOAS/Radiant vs VAV”, by Dr. S. Mumma, Pennsylvania State University, Weblink: doas-radiant.psu.edu/econ.html