Illustration by Diego Marmolejo.
Combining the old local heating practices with modern radiant and conductive heating systems could lower energy consumption, improve health, and increase thermal comfort. This is especially true for uninsulated buildings, where air heating is particularly disadvantageous. Local heating sources can be applied on their own, but can also be used in combination with air heating. This may mean, however, that thermal comfort standards need to be redefined.
Heating is a huge source of fossil energy use in cooler climates. In the Netherlands, for instance, heating accounts for 20 to 25% of total primary energy use, despite relatively mild winters. This means heat supply guzzles as much fuel as transportation.  According to many, the solution to the high energy use of heating systems is to be found in better strategies for thermal insulation.
A well-insulated building can indeed lower energy use spectacularly, to the point that there’s no need for a heating system: the heat produced by people, electric devices and the sun can ensure thermal comfort. Orientating a building (or a whole city) around the sun is another important design element that can render heating redundant. For new buildings, the design and the orientation are much more important factors for energy efficiency than the choice of the heating system, if that’s needed at all.
When we talk about existing buildings, however, things look very different. There are several methods for insulating older buildings, but their effect on energy use is usually limited in comparison to what a new building can achieve. What’s more, insulating existing buildings can be expensive and some of the easier-to-apply methods can cause problems, such as crack formation, frost damage, mould and rot.  And, of course, it’s not easy to re-orientate an existing building toward the sun.
If we rely solely on insulation, solar energy and sustainable architecture, it would take too much time to address the high energy use of buildings. Based on the current yearly number of new construction in the Netherlands, it would take 88 years before the Dutch building stock would meet today’s strict insulation standards, for example.  And that doesn’t take into account the energy required to demolish old buildings and build new ones.  If we are serious about reducing our dependence on fossil fuels, we’ll also need to find affordable short-term solutions that can lower energy use in existing buildings.
Local Heating as an Alternative to Insulation
One previously discussed solution is clothing. Insulating the body is more efficient than insulating a building, and thermal underclothing is particularly effective. In this article, we discuss another solution, which can be applied on its own or in combination with better clothing: local heating.
Contrary to air heating, which distributes warmth throughout a space, radiant and conductive heating systems act much more locally; they can make people comfortable without having to warm the whole space. Radiant heating systems transfer energy through electromagnetic waves (in many ways similar to the energy coming from the sun), which are converted to heat when absorbed by the skin. Conductive heating systems warm the body through direct physical contact.
Infrared thermal image showing the heat loss of a building. Source.
While local heating systems could improve thermal comfort and energy efficiency in most types of buildings, they are extra advantageous in older, uninsulated buildings. This is because they provide comfort at colder air temperatures, decreasing the heat loss from the building and thus making thermal insulation relatively less important.
If we are looking for quick and substantial energy savings for existing buildings, then local heating systems deserve our closest attention
Insulation can further improve the energy efficiency and thermal comfort of radiant and conductive heating systems, so this is certainly not a plea against insulation. But if we are looking for quick and substantial energy savings for the large share of uninsulated buildings, then local heating systems deserve our closest attention. Switching from air heating to radiant and conductive heating, or combining them in a hybrid system, can bring energy savings that are at least as large as insulating an existing building.
Hybrid Systems: The Best of Both Worlds?
In a report for Historic Scotland titled "Keeping Warm in a Cool House" (PDF), researcher Michael Humphreys advocates a return to the old-fashioned way of heating, combined with modern heating devices, in historic buildings throughout Scotland (some 20% of the total housing stock). Humphreys argues that this approach should be considered more often at the expense of thermal insulation, which is more expensive and changes the character of a building. 
He proposes a hybrid system in which an air heating system delivers a "background temperature" of about 16ºC (61ºF), a sufficiently high temperature for household activities. For sedentary activities like reading, studying or watching television, local heating systems provide thermal micro-climates of 21-23ºC (70-73ºF) using radiant heat sources.
A hybrid system has interesting advantages. Because air heating is so inefficient — the whole volume of air in a space has to be warmed — large energy savings can be obtained even if the thermostat is turned down just a few degrees. At the same time, the background temperature delivered by the air heating system improves thermal comfort because the difference in climate between local hot spots and the rest of the room (the "radiant temperature assymetry") becomes smaller.
A hooded chair protects against radiant temperature assymetry. "Ahrend Kaigan Chair", Marijn van der Pol
Local insulation, in the form of hooded chairs and folding screens, can further protect the body from the colder parts of the space, increasing comfort in an uninsulated building. Finally, in a hybrid system, the local heating sources should not be dimensioned for exceptionally cold periods, and the air heating can be of a lower capacity. 
For every 1ºC (1.8ºF) that the thermostat is lowered, 7-10% of heating energy can be saved.  If the temperature in the space is lowered from 21 to 16ºC (70 to 61ºF), the energy savings can be as high as 35-50%. The heating sources that produce warmer microclimates introduce extra energy use that, of course, should also be taken into account.
According to Humphreys, local heating can save 30-40% of energy compared to air-heating alone, taking into account the (primary) energy use of the local heating sources. In an old, uninsulated building in Scotland, he calculated, a vertically positioned radiant heat source needs to provide 425 watts per person to achieve the desired microclimate at a background temperature of 16ºC (61ºF). 
Using local insulation — an antique hooded chair — this comes down to 340 watts per person, which can be provided by a radiant heating panel of only 60×60 cm. In his experiments, Humphreys makes use of outdated radiant heating systems from the 1970s, so that his results may be on the conservative side. 
Local heating can save 30-40% of energy compared to air-heating alone, taking into account the energy use of the local heating sources
Conductive heating systems can be even more energy efficient. According to a recent study, a heated office chair can keep 92% of subjects (with clothing insulation of 0.8 clo) comfortable at an operative temperature of 18ºC (64ºF), while 74% of subjects are still comfortable at only 16ºC (61ºF). The desk chair itself uses only 16 watts (around 30-40 watts primary energy if electricity is generated by fossil fuels), demonstrating the effectiveness of heat transfer through conduction. 
The numbers Humphreys provides are in line with research investigating summer comfort in offices, which showed that ventilating fans and other personal cooling devices are preferred over air-conditioning, while using much less energy.
Few People, Lots of Space
While local heating systems have the potential to be more sustainable and save large amounts of energy, this effect is not guaranteed. There are situations in which local heating will use more energy than air heating. Likewise, exactly how much energy can be saved by local heating depends on many factors: the interior volume of a space, the amount of people in it, how frequently the space is used, the insulation level of the building, the ventilation requirements, the efficiency of the local heating system, and the efficiency of the air heating system.
The most important factors are the volume of the space and the amount of people who use it. Obviously, the larger the space and the less people are inside, the more interesting local heating will become compared to an air heating system. Local heating systems also become comparatively more efficient as ceilings become higher. Hot air rises, and so the energy efficiency of air heating further deteriorates in spaces with high ceilings. It’s no coincidence that churches in northern countries have been heated by giant tile stoves for ages.
Illustration by Diego Marmolejo.
Another factor is the local heating system. Radiant heating is not tied to a specific primary energy source. For instance, hot water for a hydronic heating panel can be delivered by a solar collector, electricity, a heat pump or a gas, coal, or wood-fired boiler. Naturally, the choice of the primary energy source will affect the energy efficiency of the heating system. In particular, the use of electric radiant panels may raise eyebrows, because electric heating isn’t considered sustainable: burning fossil fuels to make electricity and then convert it back to heat has large energy conversion losses, which can be avoided if you heat a space directly with fossil fuels.
However, things aren’t as simple as they might seem. Electric radiant heating panels can offer energy savings even if the electricity is generated by fossil fuels, because they are able to heat locally and quickly. Since it takes less than five minutes for an electric radiant heating panel to produce maximum output, it can be used only when and where it is needed. Air heating systems, tile stoves, or radiant building surfaces need considerably more time to bring a space to a comfortable temperature, and therefore they have to work continuously throughout the day (or they have to be oversized) in order to provide instant comfort.
How much energy can be saved depends on — among other things — the interior volume of a space, the amount of people in it, and how frequently the space is used
Do the advantages of electric heating panels outweigh the disadvantages? This depends mainly on how frequently the space is used. Their efficiency advantage is largest for rooms that are less frequently used. Many spaces are only used intermittently, and it’s these spaces that could benefit the most from electric radiant heating panels. On the other hand, if electric heating panels are used continuously throughout the day, their quick heating capacity brings no efficiency advantage and they might end up using more energy than an air heating system. 
Open the Windows
Local heating can provide a healthier indoor climate compared to air heating. Indoor air pollution is a growing problem for two reasons. Firstly, people spend more and more time indoors: up to 90% of their lives in the western world. Secondly, building materials and household items have become increasingly polluted. Harmful chemicals can be expelled from building materials, furniture, and household cleaning products, while additional pollution is generated by human activities (mainly cooking and smoking), and by the intrusion of outdoor pollutants. 
Local heating is better combined with natural ventilation than air heating. When we heat a space by air, the medium for heat storage is also the medium for ventilation. Measures that improve the efficiency and comfort of air heating, such as making a building air-tight, have a negative impact on the health of the indoor environment, while measurs that promote a healthier indoor climate, such as regularly opening the windows, are detrimental to the efficiency and comfort of the heating system.
Illustration by Diego Marmolejo.
With local heating, the air is not the medium for heat storage. Heat is directly transferred to people. Every radiant or conductive heating source also heats the air, so it will still cost energy to open the windows and bring in fresh air. However, since local heating provides thermal comfort at cooler air temperatures, it will cost less energy to bring in more fresh air. It’s an alternative to complex and costly mechanical ventilation systems, which work good if they are built, used and maintained as they should be, but can actually worsen the indoor climate if that’s not the case.
Local heating also minimizes the continuous air circulation that is typical in air heating systems: warm air rises, cools and comes down again, is warmed up and rises, and so on. This turbulence causes a circulation of dust particles that can cause or aggravate allergies or mucous membrane infections. If the air temperature is reduced, these effects are minimized. Cooler indoor temperatures also reduce the prevalence of house dust mites. 
Improving Thermal Comfort
The obvious downside of local heating is that you are tied to a certain space when you want to be comfortable. The great advantage of air heating — at the expense of very high energy consumption — is that the warmth is distributed uniformly across the space, at least on the horizontal plane, so that thermal comfort is independent of your location. However, the fact that local heating fixes you at a certain point in space is not as disadvantageous as it might seem, and it actually brings an important and unexpected benefit: more comfort, at least in shared spaces.
The uniform comfort temperature prescribed by international comfort standards — 23.3ºC (74ºF) with a clothing insulation of 1 clo — is actually aimed at people in rest (activity level of 1 "met", which corresponds to "seated, quiet"). Using the CBE Thermal Comfort Tool, we can see that an increase in activity has a profound effect on comfort. If the metabolism increases from 1 to 2.2 met ("seated, heavy limb movement") or 2.7 met ("house cleaning"), the ideal comfort temperature decreases to 13ºC (55ºF) and 9ºC (48ºF), respectively. Even a slight increase from 1 to 1.1 met ("typing") already lowers the comfort temperature from 23.3 to 22.4ºC (74 to 72ºF).
People are different, wear different clothes, and perform different activities, while air heating creates a thermal environment that’s for everyone the same
In an air-heated space with a uniform temperature of 23.3ºC, the person sitting in the couch watching TV could be comfortable, but the person typing might be slightly hot and the person cleaning the room or performing an animated conversation could be sweating. In a space that is heated by radiant and conductive heating sources, everybody can find the thermal comfort that suits their needs best.
While this still implies that you are tied to a certain spot in order to be comfortable if you are inactive or performing light activity, it’s very common even with air heating to be in a specific location for extended periods: on the couch, at a desk, at the kitchen table. There are many places in a room where we are never at rest, and so there is no need to heat them to the same temperature.
A modern "kotatsu", a heated table from Japan, using an electric heater instead of glowing fuel. Source: Rakuten.
People not only differ in their activities, but also in their clothing and personality. When performing similar activities and wearing similar clothes, the difference in neutral temperature between individuals can still be as high as 5ºC.  In an air-heated space, these people are condemned to a thermal climate that’s the same for everyone — a compromise. This fact is recognised by international comfort standards, which state that even at a "perfect" temperature a maximum of 80% of users will be comfortable. In other words, using modern heating systems, one in five will be too warm or too cold in the best case scenario. 
In a space that’s warmed by local heating sources, occupants that are more active or better dressed can find a cooler spot, while those at rest, dressed lighty or extra sensitive to cold can find a warmer micro-climate. 100% of occupants would be able to find their ideal environment. Personal control of the thermal environment can be organised in two ways: everyone regulates their individual comfort by means of a personal radiant and/or conductive heating source, or everyone "migrates" through a space that is heated by a central radiant heating source. Both methods can also be combined, as it was in the old days.
Comfort Studies in Office Buildings
The performance of local radiant and conductive heating systems, combined with a lower background temperature provided by air heating, has been most extensively researched in office environments. Most of these studies have concluded that personalised heating systems can lower energy use and simultaneously improve thermal comfort and overall performance.  In offices, a multitude of people share the same space for an extended period of time, without much or any personal control over their thermal environment. Research has shown that about one in two office workers is — year-round — unhappy with the thermal climate. 
In offices, personalized heating systems can lower energy use and simultaneously improve thermal comfort and working performance.
By providing each office worker with personal heating sources, everyone can decide the thermal environment they prefer. The systems under study are usually electric or hydronic radiant panels, which can be built into the walls of privacy cubicles, hung at the ceiling above office workers, or attached below the desk surface.
These can be combined with conductive heating elements embedded into furniture. Systems that warm the hands and the feet usually work best, because these body parts are most sensitive to cold. Because personal heating systems can produce heat very quickly, they can be turned off automatically when the office worker leaves the desk, using energy only when it’s necessary.
Illustration by Diego Marmolejo.
Of course, to be advantageous, the energy use of the personal heating sources should be smaller than the energy saved by turning down the thermostat a few degrees. Otherwise, thermal comfort might improve comfort but energy savings won’t materialise. This can happen when the air heating system has too little capacity (in which case extra energy use is expected), but it’s also possible that people start dressing more lightly because of personal energy sources, which can lead to more energy consumption.
Adaptive Thermal Comfort
Restoring the old concept of "heating people, not places" requires a new definition of thermal comfort. For all its advantages, the use of local heating systems doesn’t comply with international comfort standards, because the average temperature in the room will not obtain the minimum recommended values. As discussed in a previous article, the same goes for cooling: a space that is cooled by local cooling systems (such as fans) exceeds the maximal temperature values for summer comfort.
Modern comfort standards don’t recognise the freedom to actively move throughout a space in search of thermal comfort, although this could have profound consequences for energy use while maintaining thermal comfort, write Humphreys and two of his colleagues in "Adaptive Thermal Comfort: Principles and Practice".  We have been conditioned by ideas that comfort implies a steady temperature throughout a space, but this is an intrinsic feature of modern air heating and cooling systems, not a condition for feeling comfortable.
A steady temperature throughout a space is an intrinsic feature of modern air heating and cooling systems, not a condition for feeling comfortable.
In reality, we constantly adapt ourselves to the thermal environment, not only by moving between different thermal environments, but also by changing clothes or our activities, by opening or closing windows or curtains, by consuming hot or cold drinks, by changing posture, and so on. Field studies have demonstrated that people can be comfortable in much wider temperature ranges than those prescribed by comfort standards if they have the freedom to react to changing conditions. This "Adaptive Thermal Comfort" model, which leans heavily on local heating/cooling sources and clothing insulation, is at odds with the established comfort standards, which are based on research in climate chambers. 
Climate chambers are special laboratories in which the temperature, humidity and air speed are precisely controlled, while the subjects’ thermal comfort is measured. All subjects are made to perform the same task, wearing the same clothes, and sitting in a fixed location. They can’t change clothes or activity or move closer to a heating or cooling source, while these actions could have large consequences for their thermal comfort. Comfort standards — which are the guidelines for most architects and engineers — treat us as if we are passive beings living in climate chambers. We have come to believe that we are.
 Stralingsverwarming: Gezonde Warmte met Minder Energie, Kris De Decker, 2015
 Keeping Warm in a Cooler House. Creating Comfort with Background Heating and Local Supplementary Warmth (PDF). Historical Scotland Technical Paper 14, Michael Humphreys, Historic Scotland, 2011
 Adaptive Thermal Comfort: Principles and Practice, Fergus Nicol, Michael Humphreys & Susan Roaf, 2012
 Energy-efficient comfort with a heated/cooled chair, Center for the Built Environment, UC Berkeley, Wilmer Pasut, 2014
 Beispielhafte Vergleichsmessung zwischen Infrarothstrahlungsheizung und Gasheizung im Altbaubereich, Peter Kosack, TU Kaiserslautern, 2009
 Indoor Pollutants, Committee on Indoor Pollutants, National Research Council, 1981
 Individual control at each workplace: the means and the potential benefits, David Wyon, in "Creating the productive workplace", Derek Croome, 2000
 Persoonlijke beïnvloeding als sleutel tot een A+ klimaat (PDF), Atze Boerstra, in TVVL Magazine, 04, 2010
 Comfort, perceived air quality, and work performance in a low-power task-ambient conditioning system (PDF), Hui Zhang et al., Center for the Built Environment, 2008
 Air quality and thermal comfort in office buildings: results of a large indoor environmental quality survey (PDF), in "Proceedings of healthy buildings 2006, Lisbon, Vol.III, 393-397