This article was published in ASHRAE Journal, March 2013. Copyright 2013 ASHRAE. Posted at www.ashrae.org. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org.
54 AS HRAE Jou rna l ash rae .o rg M a r c h 2 0 1 3
After the physical design of a radiant slab system has been com-
pleted, the project requires the appropriate control strategies and
the use of tested construction techniques to be successful. Control strategies
for radiant slab systems differ significantly from those of standard HVAC
components in that they must not only respond to current space conditions
but also recognize the thermal lag of the components they are control-
ling. The control sequences must avoid either “driving” the slab or causing
the slab system to conflict with the ventilation/dehumidification system.
the thermal conditioning requirements of the space. Deviation of a setpoint by a degree or two may result in a similar space temperature deviation, but that range is well within the operating be-havior of a conventional HVAC system. The fact that the thermally active floor influences comfort both by moderating the air temperature and the mean radi-ant temperature makes the system very forgiving for minor temperature excur-sions.1
In general, the floor control system is handled by a local panel and individual slab temperatures are not reported back to the building management system (BMS). The BMS typically will recal-culate floor temperature setpoints and
About the AuthorDaniel H. Nall, P.E., FAIA, is senior vice president at Thornton Tomasetti Group in New York. He is an ASHRAE certified Building Energy Modeling Pro-fessional and High-Performance Building Design Professional.
Thermally Active FloorsBy Daniel H. Nall, P.E., FAIA, Member ASHRAE
Part Three: Making it Work
Radiant Heating and CoolingControl Sequences
The entire operating range of tem-perature setpoints for the radiant floor is limited by condensation and com-fort considerations to a minimum of 68°F (20°C) and a maximum of 84°F
(28.9°C). Floors colder than the mini-mum easily can result in temperature contact discomfort or condensation. Temperatures higher than the maximum also result in contact discomfort. Con-trol is not necessarily precise to tenths of a degree, but must logically track
communicate them to the local floor control panels. The floor control panels will then activate the local two-position valves to maintain the setpoint. Heating and cooling changeover and control of floor loop supply temperature are also typically handled by the BMS.
Control of the thermally active floor is also very different from control of other faster responding systems.2 A basic rule of controls theory is to avoid using a quickly varying stimulus to control a slowly responding system because doing so can result in “thrashing” the system by sending conflicting inputs within the time frame of the system’s response. The thermally active floor is a relatively slow-acting system, so controlling this system with a thermostat sensing a potentially variable medium, such as the room air, likely will have less than opti-mal results, especially responding to moving patches of solar radiation.3 By the time an air thermostat reacts to rising air temperature from a solar heated floor, the floor will have risen to a temperature that may take a significant time to correct. The best control strategy would seek a more stable stimulus and count upon the long time constant of the radiant floor to dampen the oscillations of the more volatile room air tempera-ture. The most important consideration is control of heating/cooling changeover.
Heating cooling changeover should be a rare event and should be controlled to avoid driving the floor from one mode to another. Control sequences that recognize the thermal mass of the floor and exploit reasonable temperature deadbands can avoid such “thermal thrashing” of the floor.4 Systems de-signed by the author have used two general control schemes for thermally active floors, both of which use embedded tem-perature sensors in the floor to enable it to be controlled to a setpoint, but with two different strategies for determining the floor temperature setpoint.
The first of these strategies is applicable to buildings or spaces that incorporate a high thermal mass exterior envelope. In this strategy, the setpoint for the floor temperature is reset based upon the inside surface temperature of an exterior wall. This strategy was pursued in the implementation of a thermal-ly active floor system for the renovation of the Saint Meinrad Archabbey Church in St. Meinrad, Ind., a late 19th century Gothic Revival cathedral church (Photo 1). The church has
the building are glass under extensive overhangs. This project features a variable air volume system serving all closed areas, such as classrooms, offices, conference room, and store. The exhibit/circulation/break-out area that connects these closed spaces is conditioned with a thermally active floor (Photo 2). Thermostats in the closed rooms control the temperature and airflow to those rooms and the return air flows from these rooms through the circulation areas back to the air-handling unit. The return air from the closed spaces provides dehumidi-fication and ventilation to the circulation spaces. Adequate
Photo 1: Saint Meinrad Archabbey Church, thermally active floor.
Exterior Sol-Air (°F)
Interior Air(°F)
Wall Surface(°F)
Wall Interior(°F)
Floor Setpoint(°F)
16.0 70.0 65.9 35.0 80.0
31.0 70.0 67.0 45.0 77.0
47.0 70.0 68.2 55.0 74.0
62.0 70.0 69.4 65.0 Off
71.0 75.0 74.7 72.5 Off
79.0 75.0 75.3 77.5 72.0
83.0 75.0 75.6 80.0 70.0
87.0 75.0 75.9 82.5 68.0
an air system for ventilation, dehumidifica-tion and some sensible cooling and a ther-mally active floor system. In this project, the floor setpoint temperature is reset based on the temperature of the 2.5 ft (0.75 m) thick sandstone exterior wall, outside of the 0.75 in. (19 mm) insulation and finish drywall ap-plied to the inside of the wall. The actual set-points were tuned in the field, but the initial ramps were as shown in Table 1.
On a later project, the Virginia Hand Call-away Discovery Center, near Columbus, Ga., the structure is not nearly as massive as the Saint Meinrad Church. Most of the walls of
Table 1: Radiant floor setpoint temperature as a function of wall interior tem-perature with coincident driving temperatures.
ventilation airflow is ensured by controlling outside air frac-tion using differential CO2 sensor readings between the cir-culation space and outside air. The thermally active floor in this area is controlled to a setpoint that is reset according to the temperature of the underside of the tongue and groove plank roof above this space. When the ceiling is cold, the floor is warm; when the ceiling is warm, the floor is cold. The to-tal temperature range for the floor is 68°F to 80°F (20°C to 26.7°C). The circulation space has no air thermostat.
On both projects, the time constant of the instrumented structure is longer than that of the floor. The potential time lag in a heating/cooling changeover event is sufficiently long so the system is never called to “undo” any thermal “effort” it previously expended on the slab. Similarly, control setpoints are slowly changed so that the thermal mass of the slab does not cause large heating or cooling spikes to conform to large setpoint revisions. The control sequences of the thermal floor and the ventilation/dehumidification system should be inter-locked to prevent these two systems from fighting each other. Sometimes, the upper limit on supply dew-point temperature for the ventilation system may conflict with the space sensible comfort condition, resulting in a floor that is heating and a ventilation system that is cooling.
A second strategy toward controlling a thermally active floor also uses floor temperature sensors, but subordinates the floor to a conventional air system, providing ventilation, de-humidification and sensible conditioning, under the control of a standard air thermostat. Typically, the air systems in these projects are both heating and cooling, so the control sequence is very straightforward. The controller is set up to define a per-cent of heating or cooling capacity for the air handler at any moment as a function of the fraction of design airflow (or VFD frequency) and of the fraction of maximum heating or cooling temperature differential across the AHU.
The typical sequence would establish a deadband (be-tween 40% heating and 40% cooling) in which the floor is inactive. When the heating percentage of the AHU reach-es 40%, the floor becomes active with a setpoint of 74°F (23.3°C). As the percentage of heating for the AHU rises to 100%, the floor setpoint uniformly rises to 80°F (26.7°C). Similarly, when the AHU reaches 40% cooling, the floor is activated with an initial setpoint of 74°F (23.3°C). As the cooling output of the AHU rises to 100%, the floor setpoint temperature uniformly drops to 68°F (20°C). This strategy has been implemented on a number of buildings, including the atrium of the Syracuse University School of Manage-ment, the lobby of the Hearst Headquarters in New York City, the atrium of the SAP Headquarters in new Town
Photo 2: Virginia Hand Callaway Discovery Center radiant heating cooling floor with bamboo plywood floor finish and ceiling/roof with tongue and groove plank above glulam beams. Temperature sensor is on plank.
www.info.hotims.com/44630-40March 2013 ASHRAE Jou rna l 57
Square, Pa., and the William Jefferson Clinton Center in Little Rock, Ark.
This strategy is also very effective at avoiding rapid heating/cooling changeover and large spikes in floor thermal demand. For the space to change from 40% heating or cooling to 40% of the alternate thermal input would require significant change in exterior or interior conditions and some amount of time to overcome the thermal inertia of the floor. Furthermore, the initial setpoints for floor heating and cooling are identical. If system heating increases above 40% and the heating thermo-stat setting is not more than 74°F (23.3°C), one can be assured that the floor temperature at that point will be less than the initial setpoint.
The increase in heating load is almost certainly generated by envelope heat losses, resulting in thermal downdrafts that flow across the floor, cooling it well below the initial setpoint. Similarly, increase of the space cooling load above 40% al-most certainly would be a result of solar heat gain, which would also certainly, fall on the floor, heating it well over the initial setpoint. The logic of this system is that the change in environmental conditions that cause heating/cooling change-over would always cause floor temperature excursions well be-yond the initial setpoint for the thermally active floor, preclud-ing “time-lapse” fighting between heating and cooling modes.
The latter system can be applied to multizone air-handling systems also, with percentage heating/cooling calculated for the type of terminal unit used to create the zoning, and differ-ent setpoints for the areas of floor covered by each terminal. Implementation of a multizone floor control strategy should respect the same design considerations to prevent thermal “fighting” of adjacent terminals.
Control sequences for thermally active structures can be refined to provide peak load mitigation either through pre-conditioning or demand response.5 Preconditioning may be a
challenge for cooling systems because the floor typically is operated with a minimum temperature at the lower limit of the comfort range, limiting pre-cooling before a demand re-sponse event. Resetting that lower temperature limit upward in response to increasing electrical demand will help the con-trolled thermal mass to mitigate the load. A further refinement of this approach would be to continue the operation of the floor circulation system during the period of demand response while limiting or eliminating cooling input to the system. In that way, the heat from local high temperature areas caused by solar gain on the floor slab can be spread across the entire mass of the thermally active floor. Areas already cooled to the minimum can rise in temperature and contribute their stored cooling, helping to maintaining space comfort without addi-tional cooling input. This technique also can be used to obtain maximum comfort with limited peak cooling capacity.
Construction and Coordination CaveatsObservation of a few rules and guidelines for construction
can significantly ease the construction and start-up of a ther-mally active floor and avoid long-term problems with the sys-tem. One issue with high density polyethylene tubing is that it transpires oxygen. The tubing generally is available with an oxygen barrier, but as a further safety measure, the author usu-ally avoids all ferrous materials in the radiant loop. Equipment that might contain ferrous materials includes pumps, heat ex-changer, air eliminator, valves, strainers, and all piping acces-sories. Non-ferrous air eliminators are limited in size. Con-sider using two in parallel, and be cautious of ferrous nipples on expansion tanks. While these measures may seem extreme, the embedded tubing system must last the life of the building because it is an integral part of the building. The small tubing is almost impossible to access for cleaning. Fouling within the tubing will be a permanent debilitation of the system capacity.
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58 AS HRAE Jou rna l M a r c h 2 0 1 3
Another issue of the intricate small tubing network is that elimination of air bubbles after construction can be difficult if air eliminators are located below the tubing field. Meeting this goal places architectural limitations on the location of the manifolds.
The engineer must review submittals for some of the archi-tectural products to ensure that they are consistent with the system design. The two most important products are the floor finishes and the topping slab product. Ensure the actual floor finish furnished is consistent with the thermal performance as-sumed during the design process. In some cases, it may be nec-essary to contact the manufacturer to get the required thermal information. Ensure the topping slab has a minimum density of 120 lb/ft3 (1925 kg/m3) concrete to ensure good heat transfer.
Topping slab detailing under some floor finishes, especially terrazzo, should be coordinated with the architect and struc-tural engineers to avoid uncontrolled cracking. Cracking gen-erally is not a function of excessive thermal variation of the thermally active floor. In many cases, the experienced temper-ature range of an active floor system is less than that of a pas-sive floor system, especially if the floor experiences direct so-lar gain. Active floors vary between 68°F and 80°F (20°C and 26.7°C). Passive floors can go over 100°F (37.8°C) in bright sunlight and under 65°F (18.3°C) on cold days. Glass fiber reinforcement in the topping slab is especially effective in
limiting cracking. Tubing emplacement in structural concrete should meet the requirements of the American Concrete Insti-tute Standard 318, Building Code Requirements for Structural Concrete, and should be thoroughly analyzed by the structural engineer for its impact on structural capacity.
Explicitly locate tubing on design documents. Show the individual control zones, and the location of the temperature sensors. Show the double serpentine routing for at least one loop so that the contractor can understand how to lay out the tubing. Review shop drawings and have them resubmitted until the tubing layout is exactly as the design requires. Tie down tubing to ensure it stays in place. Use lightly tightened nylon or plastic-coated metal wire ties to wire mesh or barbed staples into slab insulation. Uncoated wire ties can damage the polythene tubing and un-barbed staples can pull out of the insulation during the topping slab pour, allowing the tubing to drift from its initial location.
Configure tubing layout in a double serpentine pattern to mini-mize temperature differences across the floor (Photo 3). Although a single serpentine layout may work for a high temperature dif-ference heating-only system, cooling systems with limited water temperature range require the double serpentine layout.
The preferred subtopping slab insulation, to minimize floor finish deflection during loading, is 100 psi polystyrene foam,
available in a minimum depth of 1.5 in. (38 mm). Floor sec-tion design should address this minimum depth.
The design team should plan for partition construction and relocation if likely to occur. Partition bottom channels will likely be secured to concrete slab with shot fasteners. Fas-teners can penetrate tubing embedded in the topping slab. If partitions will not move, plan tubing routing to enter rooms through doors rather than crossing walls. If partitions may be moved or installed later, consider increasing the depth of the topping slab, or specifying short fasteners used with mastic to secure bottom channel to slab.
The engineer should coordinate location of slab sensors and conduit connected to them with radiant tubing layout. Proper location of the sensor may involve crossing the floor tubing. The crossing may have implications for the required coverage depth of the topping slab over the tubing. The sensor installa-tion should accommodate removal and replacement of sensors should they fail.
Finally, while radiant floor tubing may cross contraction control joints created to control random cracking and con-struction joints caused by phased construction, they should never cross expansion joints that can experience far greater independent vertical and horizontal movement of the slab sec-tions.
ConclusionThe technology of thermally active slabs has become increas-
ingly popular over the last few years and is being applied in many challenging climates. This system can be very effective for a number of applications, ranging from highly glazed sunspaces, to super-insulated residential buildings. One of the consummate examples of the application of this technology is the main floor
Photo 3: Syracuse University School of Management atrium, radiant heating and cooling layout.
and the bridges of the Newseum in Washington, D.C. (Photo 4). Although the technology seems very robust and operates well (even when the implementation is a less than an optimal configu-ration), reliable long-term operation of the system requires atten-tion to many details. The exact solutions suggested by this paper have proved effective; however, other solutions may be equally effective. At a minimum, the system designer should consider all of the issues raised by these solutions for the design of each new project.
AcknowledgmentsThe author would like to thank Robert Ellington, Charles
Kryksman, Frank Cuomo and Carmine Carannante for their assistance on the projects described in these articles. He would also like to thank the owners and architects of the projects for their willingness to pursue these innovative design solutions.
2. Olesen, B.W., K. Sommer, B. Düchting. 2002. “Control of slab heating and cooling systems studied by dynamic computer simula-tions.” ASHRAE Transactions 108(2).
4. Nall, D., R. Ellington. 2000. “Design and operation of radiant heating/cooling systems for assembly occupancies.” Proceedings of the American Solar Energy Society Conference.
5. Gayeski, N.T. 2010. “Predictive pre-cooling control for low lift radiant cooling using building thermal mass.” Unpublished doctoral dissertation, Massachusetts Institute of Technology.
Figure 4: The Newseum in Washington, D.C. showing thermally active main floor and skybridges.
