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Radiant Floor Heating

Visit our comprehensive bibliography on radiant cooling and heating.

This study was financed by the Netherlands Agency for Energy and the Environment NOVEM, section EV&B. Thanks is given to Arjen Raue (BBA) for reviewing draft versions.

For Further Study read the transcripts from the Healthy Buildings Workshops.

see: What makes a good architect?

This section under development...

Radiant Conditioned Architecture (Photo credits attributed to original source)

The following projects are just a small sample of buildings conditioned with radiant cooling or heating or radiant heat/cool systems (aka "reversible systems").

Bangkok Airport1

Mountain Equipment Co-op2


Hilton Hotel4
Dolce Vita Tejo Mall5
California Academy of Science6
Urban Outfitters HQ7
Normand Maurice Building8
Uponor Distribution Center9

credit/source: csa presentation on manitoba hydro building, kpmb architects, transsolar,, earthtec, omicron et al
Manitoba Hydro Building10
Independence Station11

credit/source: coop himmelb(l)au, wolf d. prix/ w. dreibholz & partner zt gmbh, ibe consulting engineers
Akron Art Museum12

j. urban, tower sir norman foster, flack & kurtz , copyright 2003 the new york times company
Hearst Tower13

credit/source: Geoff Mcdonell,p.eng.,leed ap, omicron engineering
Simon Fraser University14

credit/source: dwight mckinnon
BCIT Aerospace Hangar15

photo credit ©Jonathan Hillyer /Atlanta
Blue Ridge Parkway Center16

credit/source: transsolar
Deutsche Post Headquarters17

BMW World18

Photo credits
2.High Performance Buildings
3.Wal-Mart Canada
4.Hilton Hotels
7.Urban Outfitters
8.High Performance Buildings
10.KPMB Architects
12.IBE Consulting Engineers
13.The New York Times
14.Geoff McDonell
15. Dwight Mckinnon
16.Jonathan Hillyer /Atlanta
18.Coop Himmelblau


Suggested Reading

Workshop 24: Alvar Aalto as a Healthy Building Architect, Healthy Buildings Conference, August 2000

Suggested Reading

Cortical, thalamic, and hypothalamic responses to
cooling and warming the skin in awake humans

Egan, Johnson, Farrell et al



The Health, Safety and Comfort Advantages of Low Temperature Heating Systems - A Literature Review
Healthy Buildings Conference, August 2000

Atze Boerstra1, Peter Op'Veld2, Herman Eijdems3
BBA Indoor Environmental Consultancy, the Netherlands (
2 Novem Netherlands Agency for Energy and the Environment, the Netherlands
3 Cauberg Huygen Consulting Engineers, the Netherlands


A literature review was conducted that aimed at mapping the impact of Low Temperature Heating (LTH) systems on Thermal Comfort, Indoor Air Quality, and Safety. With LTH systems defined as systems that use "low exergy" heat sources, with heat supply elements like floor heating, wall heating, enlarged radiators/convectors, or enlarged heating coils for air heating. Low Temperature supply elements turned out to have many advantages over their "high temperature counterparts".

  • Thermal Comfort: diminished vertical temperature gradients, less radiant heat asymmetry, more comfortable floor temperatures, less temperature fluctuations, and reduced draft risk;

  • Indoor Air Quality: less mites, fewer airborne particles, lower average air temperature thus improving the Perceived Air Quality;

  • Safety: minimized burning risk, less changes on injuries when falling.

Note that a couple of the aspects named only apply for floor and wall heating systems.

KEYWORDS Heating, Thermal comfort, Air Quality, Safety.


Due to better thermal insulation of new and retrofitted buildings and thanks to new techniques for reducing ventilation losses the heating demand of modern buildings is decreasing. A further reduction of the energy use of buildings (needed to meet future global targets for emission reduction) implies that we look at ways to improve the heat generation process.

The heating system of the "average" Dutch dwelling consists of a high efficiency natural gas boiler, a water pipe system, and radiators or convectors as heat supply elements. The system operates at a water temperature of around 70-90 deg. C under "winter conditions" (outside temperature between around -5 and +10 deg. C), therefore we speak of high temperature or HT systems.

Note that also a large amount of smaller utility buildings use comparable systems.

On the long run it will be inevitable to shift towards so-called "low valued energy" or heat sources at "low exergy levels". It is available from residual heat, ambient heat and renewable sources. It can be used for Low Temperature Heating (LTH) systems in residential and utility buildings. For this purpose todays new buildings and installations should be designed for (future use of) Low Temperature distribution systems (NOVEM definition, see table 1). The generally used (water based) heat distribution systems (pipes) in buildings have a life cycle of 40 to 50 years. In order to allow for a broad introduction of low exergy heating systems in the next half century it is necessary to start changing our heat distribution systems into systems suitable for Low Temperature water distribution soon.

Aware of this need the NOVEM initiated a program of feasibility and theoretical studies, field experiments, and demonstration projects to stimulate the introduction of Low Temperature heating systems in general and Low Temperature distribution systems in particular.
The overall goal of this study was to identify the (proven) advantages and disadvantages of Low Temperature Heating systems (heat supply elements) in comparison with their "high temperature counterparts" (e.g. wall and floor heating were compared with conventional HT radiators, enlarged LT radiators with HT radiators, LT air heating with HT air heating). These "qualitative aspects" of LTH systems were identified on the basis of a review of national (Dutch) and international (scientific) literature.

In the context of this study Low Temperature Heating systems were defined as systems that use "low exergy" heat sources, with heat supply elements like floor heating, wall heating, enlarged radiators or convectors, or enlarged heating coils for air heating (see also table 1, "Low Temperature" and "very Low Temperature" combined).


Water temperature    

Supply flow

Return flow

High temperatures (HT)

90 deg. C

70 deg. C

Medium temperatures (MT)

55 deg. C

35-40 deg. C

Low Temperatures (LT)

45 deg. C

25-35 deg. C

Very Low Temperatures (VLT)

35 deg. C

25 deg. C

  Table 1. NOVEM Definition of heating systems based on operative water temperature under "winter conditions" (outside temperature below + 10 deg. C)


Radiant heat transmission

The radiant heat transmission component of LT-systems is higher than for High Temperature systems. For example, due to large surfaces and Low Temperatures the radiant component of floor and wall heating is about 50 to 70%. For conventional HT-radiators this is only 30 to 50% and even less for conventional HT-convectors [3]. Therefore the amount of heat "transferred by air" (by conductivity) is reduced with LT heat supply elements, which implies that air temperatures can be 12 deg. C lower at the same comfort level (with the same operative temperature). Field study results indicate a high appreciation by building occupants for heating systems that work primary on radiant heat [4]. It is assumed by some authors (e.g. [4]) that a relative large contribution of radiant heat transfer (e.g. in a room with warm surfaces but relatively cold air) better fits comfort needs of human beings because it is more "natural" (compare this with being outside on a sunny day in winter). Note that the advantage named especially applies for wall and floor heating.

Vertical temperature gradient

Laboratory and field experiments alike show a clear difference between the vertical temperature gradients (temperature difference in a room dependant upon height above floor level) between LT floor heating and HT-radiator heating. With floor heating practically no gradients are found, assuming well-insulated spaces (e.g. [2], [5], [6]). For radiators, convectors and comparable heating systems the extent in which gradients appear is much more sensitive to the quality of the heating design. Normally, with HT heating, gradients range from 2-3 deg. C between floor and ceiling; but "less well" designed HT systems show gradients up to 7 deg. C. (Compare - NEN-ISO 7730 demands that the vertical temperature difference does not exceed 3 deg. C per meter; preferably it stays below 2 deg. C). Even though no data where found on LT-heating systems other than floor heating, the general assumption is made that for LT-heating in general gradients are considerably lower than for HT-systems.

Window temperature asymmetry

Cold window surfaces sometimes cause discomfort through radiant heat losses, which are not in balance with radiant heat flows in other directions. Complaints occur when differences exceed 20-25 W/m2 or when a temperature asymmetry of 10 deg. C occurs [7].
Conventionally compensation was provided by placing hot radiators/convectors under windows. Removing HT radiators/convectors from facades and replacing them with for example wall or floor heating is often thought to introduce extra thermal discomfort. Due to the development of high insulation glass however, this aspect has lost importance. Given Dutch average outdoor temperatures in winter, at U-values under 1.5 W/m2.K no significant differences in discomfort between heating systems occur near windows, under the condition that windows are less than 1,5 to 2 meters high[8].

Surface temperature of heated floors

Floor heating offers specific users a comfort level that is not achievable with regular facade mounted heating systems (with or without textile floor coverings). Where people walk bare foot a lot (e.g. in swimming pools, but also in dwellings), or where some sit on the floor a lot (e.g. in nurseries) floor heating is the obvious choice. Research shows that the optimum floor temperature lies between 20 and 28 deg. C with shoes and between 23 and 30 deg. C bare foots depending on the material the floor is covered with [5]. Without floor heating, floor temperatures normally lie between 15 and 20 deg. C in winter, which is out of the "comfort" range described (especially when walking bare foot and sitting). A study for increased bacteria growth on feet (in shoes) due to exposure to heated floors showed no significant impact [9].

Air velocities and turbulence intensity

Too fast temperature fluctuations around a constant mean air velocity (a too high turbulence intensity) might result in draught complaints. LTH-systems in general are more "inert" than HT systems. Which is explained by the fact that the driving forces are smaller due to relative large surfaces and lower temperature differences between heat the supply element and it"s surroundings. Both laboratory and field results show that LT systems and especially floor heating systems generally have a lower turbulence intensity "about 20% - and therefore lower risk for draught complaints [5] [11] [14]. Note by the way that the mean air velocities for floor heating are in the same order as those for conventional HT radiators within the living zones assuming limited influence from ventilation through facades on indoor air flows". When a building is ventilated "naturally" (through vents or windows) the design of the facade openings in combination with the location and type of heat supply systems need special attention. Especially with LT systems like floor and wall heating systems. Otherwise LT systems might in some situations (vents too wide open, high wind pressure, low outside temperatures) indeed result in more draught complaints near windows.

Warming up period & general inertia

A disadvantage of LT systems, and especially of wall and floor heating systems, that one often hears is the relative long warming up period needed.

The warming up period first and for all is related to the amount of "thermally active" building mass. So in cases where (like in many new example buildings) the actual floor and wall heating systems (upper package) are well insulated from the rest of the building mass, and the heating package itself is relatively light (the case with some of the newer floor heating systems), the warming up time of LT systems too can be rather short.

Moreover, heating up is often associated with increasing air temperatures. Looking at overall thermal comfort or the operative temperature the so called differences between LT and HT-heating systems become less pronunciated.

Also, for high-insulated buildings the energy gain from a night setback is small due to the fairly limited reduction at night of the building structure temperature. Therefore, for LT systems in general only a minor night setback (around 2-3 deg. C) is recommended [1] [12]. So warming up time becomes less of an issue.

Another aspect often talked about with wall and floor heating systems is their inertia with incoming solar heat or with sudden changes in internal heat loads, resulting in an unwanted (uncontrollable) increase in temperatures. This effect however in practice is much less spectacular than often thought, LTH-systems and wall and floor heating in particular take profit of the so called "self-regulating" effect. Due to the small temperature differences (delta T) between heat supply elements (e.g. a heated wall) and the inside air (often only a couple of degrees), a sudden increase in heat load (e.g. solar input in one room) instantly results in a considerable reduction of the delta T; because this is the main driving force for the heat exchange, the declined delta T in it"s turn instantly decreases the amount of heat supplied through the LT system (in that room) therefore "self-regulating" the amount of heat that is being delivered to that specific space [5] [10]

Cooling abilities

Increasing the insulation of buildings together with reduction of ventilation losses and increased utilization of solar gains introduces the risk of overheating during summer time [13]. In that context LTH systems offer an interesting advantage namely opportunities for cooling (both in commercial buildings and in dwellings). Especially when combined with a ground collector (and heat pumps) a limited capacity of (high temperature) cooling can be accomplished with a rather limited energy use [5]. Other expensive measures against overheating thus can be avoided.


Many studies show a positive effect from floor heating systems on reduction of mite populations in dwellings resulting considerable health improvements of allergic building occupants (e.g. [17]). The underlying mechanism is a lowered moisture content in the upper boundary layer of the floor (of the floor covering) due to the heating of the floor.
The mite survival threshold is a relative humidity level RH under 45 % during a prolonged period (at least several weeks). The influence of floor heating system on the RH in the boundary layer is calculated to be in the order of 10 % points. Given a Moderate Western European See Climate, this reduction is just enough to bring the RH in the boundary layer under 45% for most of the winter, thus decimating mite populations [21]. Note that with floor and wall heating systems the change for mould growth on interior surfaces is also decreased. A Finnish field study concluded that heating elements of the "low surface temperature type" result in less eye-irritation and throat and other mucous diseases (e.g. they found most symptoms with the "hottest" heating elements, electrical heaters) [15]. Also a correlation was found between the temperature of the heating surface and the amount of particle deposition. It is assumed that the lower grade of air fluctuations from LTH systems causes a lower quantity of suspended particles in spaces [16].

It is well known that inhaling dust can cause allergic reactions [20]. The sensitivity of humans for inhaled particles is not only dependent on the amount of particles inhaled, more important is the quantity of suspended matter [18]. At temperatures exceeding 55 deg. C the process of dust singe starts. Some suggest that particles get more reactive and irritating after contact with the relatively hot surfaces of HT heating elements [15].

Conclusion, indications exist that the application of LTH systems not only results in less suspended particles, but also leads to less reactive (less irritating) particles due to the absence of dust singe.

Temperature effect on SBS symptoms & PAQ

As a result of the high contribution of radiant heat the room air temperature can be 1-2 deg. C lower in spaces with LT systems. Studies show that the amount of complaints about stuffiness increase and the perceived air quality (PAQ) decreases at higher air temperatures (e.g. [18]). Also the amount of mucous irritation complaints and general SBS symptoms increases significantly with higher air temperatures (indoor air temperatures > 22-23 deg. C) [19]. Because LT systems allow for slightly lower air temperatures the risk for SBS symptoms is lower and the expected Perceived Air Quality better.


Hand Burning

Field experiments (e.g. [5]) show that heat supply elements introduce a risk for hand burning when surface temperatures exceed 40 to 45 deg. C. The surface temperature of supply elements with water based piping systems is approximately the same as the mean between water supply temperature and return temperature. So with conventional HT radiators and convectors the surface temperature of the heating supply element can be as high as 60 to 80 deg. C under winter conditions. Only when the outdoor temperature is above 10 deg. C - not the case during about 5 months a year - the HT supply element will be at or below 40 deg. C [1]. With LT radiators water temperatures lie substantially lower (see table 1), but still with outdoor temperatures under about -3 deg. C the surface temperature will come below 40 deg. C. With wall and floor heating systems surface temperatures of heating elements seldom go over 30 deg. C, so hand burning is not an issue (also the case with air heating systems in general).

Physical Injury

Traditional radiators and convectors often form substantial obstacles in living and working spaces. These are a potential cause for injuries. No specific statistics were found on the yearly amount of people injured after falling against radiators and convectors, but anecdotal evidence exists that the amount of accidents with especially the elderly and children is substantial. This explains why e.g. in design guidelines for buildings for the elderly one is often stimulated to avoid the "conventional" radiator or convector. Wall and floor heating systems and also air heating systems (HT or LT) have the advantage over (HT or LT) radiators that they are embedded in the building structure (system) so possibly dangerous obstacles are avoided [1].


The literature review showed that Low Temperature Heating (LTH) systems have many advantages over High Temperature systems in terms of safety, health and comfort;

With LTH systems Thermal Comfort increases on many aspects (more radiant heat transfer, less temperature gradients, more comfortable floor temperature, less draught and air turbulence);

The Indoor Air Quality is positively influenced (less dust mites, better Perceived Air Quality through lower air temperatures, and less SBS symptoms due to less suspended particles and decreased dust singe);

Safety is improved - Less risk for hand burning and for physical injury when falling.

The disadvantages found (e.g. warming up time, radiant asymmetry near windows) in most cases only apply in case of improper design. The original study also mapped energy advantages. This was not dealt with in this article due to space limitations, We narrow ourselves with just a short statement about energy savings. Given the Dutch climate (moderate Western European see climate) the energy reduction of LT heat supply elements on their own is about 2 to 10% compared to "conventional systems" depending upon the type of system used (highest with wall and floor heating). It can be up to 30% or more when the LT supply element is combined with alternative heat sources selected for optimum combination with LT supply elements e.g. heat pumps [1]. The energy advantage of Low Temperature Heating systems generally are well known by parties involved in building projects. The literature review resulted in a broad range of new arguments that can be used to convince clients, consultants and architects to chose for Low Temperature Heating.


This study was financed by the Netherlands Agency for Energy and the Environment NOVEM, section EV&B. We thank Arjen Raue (BBA) for reviewing draft versions.


1. Eijdems, H H E W, and Boerstra, A C. 1999. Kwalitatieve aspecten van lage temperatuur warmte-afgiftesystemen, Cauberg-Huygen / BBA, Amsterdam NL
2. Dijk H A L van, Bruchem, L van, and Wolveren, J van. 1998. Voorstudie naar de effecten en het gedrag van Laag Temperatuur Systemen. Delft, NL.
3. Zvllner, G, et al. 1985. Wdrme-abgabe; wdrmetechnische pr|fung und auslegung von warmwasser-fussbodenheizungen, Proceedings Clima 2000, Summaries and author index, Vol. 7, pp. 341.
4. Dongen, J E F van. 1985. Ervaringen en gedrag van bewoners in woningen met verschillende verwarmingssystemendeg. Onderzoek in het demonstratieproject Westenholte te Zwolle. Leiden, NL
5. Olesen, B W. 1997. Fldchenheizung und K|hlung; Einsatzbereiche fur Fussboden-, Wand- und Deckensysteme, Proceedings Velta Congress "97, pp. 35. Norderstedt, Germany.
6. Cox, C W J, Oldengarm, J, and Koppers, J M. 1993. Binnenklimaatmetingen in een zestal Ecolonia woningen in de winterperiode. Delft, NL.
7. Erhorn, H, and Szerman, M. 1988. Arrangement of space heating surfaces and resulting effects on thermal comfort and heat losses, Proceedings Healthy Buildings "88, Vol. 2, pp. 403.
8. Olesen, B W, Mortensen, E, Thorshauge, J, and Berg-Munch, B. 1980. Thermal Comfort in a room heated by different methods, ASHRAE Transactions. Vol. 86 (l), Nr 18.
9. Theuss, Th, Bischof, W, Banhidi, L, and Csoknai, I. 1994. Impact of floor temperature on feet rnicroflora; a pilot study, Proceedings Healthy Buildings "94, Vol. 2, pp. 787.
10. Fort, K, 1995. Dynamisches Verhalten von Fussbodenheizsystemen, Proceedings Velta Congress "95, Tirol.
11. Olesen, B W. 1998. Heizsysteme - Komfort und Energieverbrauch, Proceedings Velta Congress "98, pp. 93. Norderstedt, Germany.
12. ISS0.1985. publicatie 10, 2e druk. Vloerverwarming. Rotterdam, NL.
13. Poel, A, Eijdems, H H E W. 1991. Transparante isolatiematerialen en oververhitting. Bouwfysica,Vol 2 (l).
14. Peng, S. 1996. Investigation of draught due to cold window in a climate chamber, Proceedings Indoor Air "96, Vol 1, pp. 245.
15. Sammaljarvi, E. 1998. Heating, indoor dusts, stuffiness and room space electricity as health and well-being risks, Proceedings Healthy Buildings "88, Vol 3, pp. 697.
16. Lengweiler, P, Nielsen, P V, Moser, A, et al. 1997. Deposition and resuspension of Particles, Proceedings Healthy Buildings "97, Vol 1, pp. 501.
17. Schata, M, Elixman, J H, and Jorde, W. 1990. Evidence of heating systems in controlling house-dust mites and moulds in the indoor environment, Proceedings Indoor Air "90, Vol 4, pp. 577.
18. Fang, L, and Fanger, P O. 1997. Impact of temperature and humidity on acceptability of indoor air quality during immediate and longer whole-body exposure, Proceedings Healthy Buildings "97, Vol 2, pp. 231.
19. Skov, P, and Valbjorn, O. 1990. The Danish Town Hall Studydeg. a l-year follow up, Proceedings Indoor air "90, pp. 787.
20. Mxlhave, L. 1996. Huisstof gaat in je lichaam zitten. In Janus nr. 24 (Originele publicatiedeg. Huisstof en binnenhuisklimaat). Aarhus, Denmark
21. Korsgaard J. 1983. Mite astma and residencydeg. A case control study on the impact of exposure to house dust mites in dwellings. Amer.Rev.Resp.Dis. 128, pp. 231-235, 1983.


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