Just Radiant

Radiant heating and cooling experts explain the ins and outs of this growing technology and home in on common specification mistakes. CONSULTING-SPECIFYING ENGINEER: Why use radiant heating/cooling vs. conventional forced air? FIEFHAUS: First, it's compatible with sustainable energy sources, such as geothermal and solar systems.

By Barbara Horwitz-Bennett, Editor August 1, 2004

Radiant heating and cooling experts explain the ins and outs of this growing technology and home in on common specification mistakes.

CONSULTING-SPECIFYING ENGINEER: Why use radiant heating/cooling vs. conventional forced air?

FIEFHAUS: First, it’s compatible with sustainable energy sources, such as geothermal and solar systems. But from an energy-efficiency standpoint, radiant heating systems use large areas to emit heat, allowing the use of low system water temperatures. Lower temperatures, even when conventional heat sources are used, provide significant energy savings. A final advantage is outdoor reset water temperature control, which allows one to supply only the temperature required to meet building needs at any given time.

NALL: Another advantage is that it separates the process of sensible cooling from those of ventilation and dehumidification, allowing them to be controlled separately. This increases comfort and decreases excessive ventilation or inadequate dehumidification caused by lack of direct control of these parameters.

Efficiency is also increased because heating or cooling conveyed by water requires transport energy that is a small fraction of that required when heating or cooling is conveyed by air. In other words, the radiant system basically creates a mean radiant temperature environment that is closer to ideal comfort conditions. It also tends to mitigate extreme radiant sources and sinks in the space.

MCDONELL: Andy noted radiant systems use large areas to emit heat, allowing the use of low system water temperatures. But further, radiant slab floors use less energy than other types of radiant floors, enabling the thermal mass storage of concrete to provide stable temperatures and save more energy with less heating cycles.

Other real economic benefits include low life-cycle cost; there is very little maintenance or long-term operating cost associated with running a radiant floor system, provided the proper mechanical heating plant is installed and maintained regularly. Less fossil fuel is consumed due to lower operating temperatures, and pollution can be reduced by using condensing boilers.

CSE: On the subject of sustainability, how many points can the technology potentially earn in the U.S. Green Building Council’s LEED accreditation program?

MCDONELL: The system can earn two to 10 points for Energy and Atmosphere credit #1 for optimizing energy efficiency by saving energy and allowing the ventilation air system to be downsized. Radiant systems also allow a wider swing in air temperatures while maintaining a comfortable “resultant space temperature.” The technology can also earn two points for indoor environmental quality.

Taking it a step further, a radiant heating and cooling ceiling approach, coupled with a dedicated outdoor air system, creates a “complete” radiant system that may yield many more LEED points. (For more information provided by ASHRAE Fellow and Penn State Professor Stanley Mumma, visit https://doas-radiant.psu.edu/leed.html .)

CSE: What design and installation challenges are associated with radiant systems?

FIEFHAUS: Because radiant systems cover large areas, they must consider having the heating system integrated into the floor structure. This means that structural designers must allow for the heating system components, and similarly, the HVAC designer must fit the system to the building. Essentially, a successful radiant floor requires early, close communication between all designers.

NALL: Air temperature distribution is not uniform, so prediction of stratification effects is extremely important. In most cases, we use computational fluid dynamics (CFD) to simulate the conditioned space in design heating, cooling and off-peak conditions to observe temperature distributions, and to ensure comfort.

MCDONELL: There’s also the client’s perception of comfort and air temperature being the relative measure of indoor comfort. Mean radiant temperature comfort is a term not well known by most folks, and trying to tell them that they will be comfortable in lower and higher air temperatures associated with a radiant heating/cooling system can be a learning curve.

NALL: Dehumidification and avoidance of condensation is also a major consideration. Issues include concentrating dehumidification at entrances or other humidity sources; avoidance of locally intense infiltration; and maintenance of dehumidification during all operating conditions. Avoiding condensation on the floor requires much effort during the design process. Tools include three-dimensional heat transfer analysis of the floor construction, extensive psychrometric calculations for various operating conditions and CFD analysis incorporating humidity sources.

MCDONELL: That’s another good point. On the design side, specialized computer modeling software is required to accurately predict what the indoor conditions and radiant terminals sizing criteria will be when using a radiant heating/cooling system. Most common HVAC calculation software is based on air temperature calculations and conventional all-air HVAC systems, whereas radiant systems require solutions based on the operative or resultant temperature in a space. This resultant temperature is a combination of the mean radiant temperature of the surfaces in a space, combined with the ambient air temperature. This is especially critical in the perimeter areas of a building where conventional windows and glass become radiant cooling panels in the winter and radiant heating panels in the summer. There are very few radiant system computer modeling tools for buildings out there right now, and they are expensive and require a learning curve to use properly.

CSE: How do you control the system?

MCDONELL: Controlling radiant slab floors and radiant slab ceilings is poorly understood, particularly by HVAC contractors, controls contractors and owners. While lightweight radiant ceiling panels and lightweight staple-up radiant floor heating systems can be controlled with conventional, fast-acting controls, a radiant slab of any kind cannot be controlled in the same way. Concrete has thermal mass, and as a result, has a very slow response to temperature changes. One cannot use night setback at all with radiant slab thicknesses over two in., as it takes more energy to try to reset the slab temperature than it would if the slab was just kept at a steady state temperature. The other issue is that the mean radiant temperature cannot be measured by conventional air temperature sensors and thermostats. This often results in much “fudging” of control algorithms to achieve some semblance of space temperature control.

NALL: We always control the floor temperature to a setpoint measured at an embedded sensor, rather than controlling the heating or cooling rate by a room thermostat or other device. This makes it possible to deal with differential solar heat gain across the floor, and prevents dealing with the massive floor area based on a stimulus from the air temperature, which can be highly variable locally. The key is utilizing an algorithm by which the floor temperature setpoint is reset and how the heating/cooling changeover is controlled. This algorithm varies from project to project, based upon the occupancy and the architecture.

CSE: What does the future hold for radiant heating and cooling?

FIEFHAUS: For heating, growth will come from increased acceptance. The situation for radiant cooling, however, is different. A lack of design knowledge and experience is restraining the technology’s popularity. As Dan noted, humidity monitoring and control is important for radiant cooling success, and improvements in controls that integrate temperature and humidity monitoring will simplify these systems.

MCDONELL: Radiant systems are becoming better understood by architects and engineers as part of the human comfort equation, and hopefully, this will lead to better building design. But the key is to first recognize the significance of mean radiant temperature on human comfort, and then recognize the building physics requirements to allow radiant temperature control systems to work properly. Following this, better computer modeling tools and knowledge diffusion to the installing building contractors will help radiant systems become better understood and applied.

Finally, there is a need for economical commercial mean radiant temperature sensors to allow these systems to be controlled effectively as they cannot be controlled properly by conventional air sensors/thermostats.

CSE: What are the issues in the field?

MDONELL: Installation issues are mainly related to contractors’ unfamiliarity with the radiant slab installation details and the labor costs to install it, which results in inaccurate costing. During construction, there is more coordination required between the concrete trade, the reinforcing steel trade and formwork trades to add a level of care while the in-slab tubing is installed.

Common problems on the jobsite are:

  • The formwork carpenters laying down their circular saws onto the tubes while the blade is still spinning, nicking the radiant tubing.

  • Reinforcing bars being cut with grinders and hot sparks damaging the tubing.

  • Other trades walking on the tubing and damaging it.

  • Too tight a bend at loop ends resulting in kinks in the tubing.

  • Inadequate tie-downs allowing the tubing to float during the concrete pour.

  • Lack of coordination with the electrical trade conduits and boxes in the slab.

NALL: From a construction point of view, preventing damage to the radiant floor tubing after the topping slab has been poured is also a big challenge. It is crucial for the design to incorporate routing for the tubing, details for partition construction that mitigate post pour fastener insertion into the topping slab, specifications that alert the contractor to the problem and contractor education regarding radiant floor tubing.

CSE: Does first cost continue to be an obstacle? How do you address this point with building owners?

NALL: For many applications using radiant heating/cooling floors, first cost is equal to or less than conventional systems. And when the cost of architectural accommodations for additional ductwork and larger air handling units is taken into account, the cost is almost always less.

FIEFHAUS: Compared with conventional heating systems, radiant floor heating systems have a number of advantages over conventional heating systems which justify the investment.

  • Radiant systems provide better thermal comfort with warm surface temperatures and milder temperatures at head level.

  • A radiant floor heating system is hidden within or underneath the floor, giving decorating freedom without the constraints of vents, baseboards or panel units.

  • The technology is more energy efficient than traditional forced air heating systems.

  • Radiant systems are virtually noiseless, with no blowing dust or allergens, and provide a safer and more comfortable environment for children and elderly.

  • Systems are durable. Because the system is built into the structure of the building, it is well protected from physical damage.

MCDONELL: A properly integrated radiant heating/cooling system should result in lower HVAC system first costs and allow for better performing building envelope components, primarily glass, to be used without impacting the overall building costs. Radiant floor heating systems and concrete core conditioning systems, or thermoactive slabs, are normally not a first cost challenge since they are simple to install and relatively well known to most installing contractors. On the other hand, suspended radiant ceiling panels are a first cost challenge since they can be somewhat more expensive for the required surface areas required by an effective radiant ceiling cooling/heating system (For more information on radiant ceiling panels, see “The Scoop on Radiant Ceiling Panels,” at www.csemag.com .)

Generally, radiant panel or slab systems are not a premium cost to the whole project. I believe the perception that radiant systems are a higher cost system stems from the residential market where the comparison is being made to other common commodity-type HVAC systems like warm air furnaces and hot water baseboard heaters. A radiant floor heating system requires a whole building design approach, so the perceived higher installation costs, i.e., slabs and staple-ups, are properly accounted for and can be equalized by savings in other areas of the project such as smaller mechanical rooms and less duct space requirements.

(For more on the field challenges of radiant heating, visit the plumbing community at www.csemag.com .)


Geoff McDonell P.Eng.

Senior Mechanical Engineer

Omicron Ltd.

Vancouver, B.C., Canada

Dan Nall

Senior Vice President

Flack + Kurtz

New York

Andy Fiefhaus

General Manager

Viega North America

Bedford, Mass.

Standard Procedure

In addition to the usual codes and standards related to the design and installation of the heating and cooling plant equipment, there are a few other codes and standards to be aware of:

RHWHA Hydronic Guidelines for Design & Installation manual, available from Residential Hot Water Heating Assn.

Standard Guidelines for the Design and Installation of Residential Radiant Panel Heating Systems, available from the Radiant Panel Assn.

CSA International B214-01 Installation Code for Hydronic Heating Systems.

For PEX tubing, the standards to look for are: ASTM F876-93, ASTM F877-93 and CSA B137.5 (in Canada). The German DIN 16892 is also a good reference standard, but may not be accepted locally due to dimensions which do not match North American standards.