CPD 7: Radiant heating and cooling panels
This module, sponsored by Zehnder, examines how these ceiling-mounted systems can be an effective and efficient means of heating and cooling buildings
How to take this module
To take this module read the technical article below and click through to a multiple-choice questionnaire, once taken you will receive your results and if you successfully pass you will be issued automatically with a certificate to print for your records.
Building’s free continuing professional development distance learning programme is open to everyone who wants to develop and improve their professional knowledge. These modules can contribute to your annual CPD activity and help you maintain membership of professional institutions and bodies.
Each of these modules will contribute 1 hour towards your CPD obligations. If successfully completed, certificates will be distributed two weeks after the deadline.
Introduction to radiant heat
Radiant heat is a form of radiation, falling between ultra-violet and infra-red on the electromagnetic spectrum. Though it is invisible to the human eye, it shares the characteristics of light. As such it penetrates the air without energy loss or deterioration, exchanging heat with any object of lower temperature by shining on it. The way that the sun heats the earth is an example of radiant heating.
As humans, our comfort level is determined by a combination of three variables:
- radiant temperature
- ambient air temperature
- air movement.
All three variables combine to produce the resulting or perceived temperature. The following example illustrates how this may happen on a warm, sunny day. The temperature drops as low as 6ºC overnight as the Earth is starved of radiant heat from the sun. As the sun rises, it begins to warm the Earth’s surface, raising the temperature of the solid or liquid objects that it meets by conduction. As air moves over these objects, it too is slowly warmed. By midday, the air temperature may have reached 23°C.
A man can now comfortably sit and sunbathe on the beach, even though the air temperature is well below his core body temperature, due to the high level of radiant heat offered by the sun. If a large cloud blocks out the sun, the man instantly feels colder. However, as he can see there is only one cloud, he waits for it to pass rather than putting on warmer clothing or going inside. When it is gone, he feels comfortable again. In this situation, the air temperature remained the same, but the man’s comfort level had changed due to the varying levels of radiant heat.
On the other hand, if the wind changes direction so that it is no longer coming over the land but over the colder sea surface, the man will again feel colder. But this time, it will be because the air temperature has changed even though the radiant heat level has remained the same.
High levels of radiant heat can compensate for low air temperatures in some situations. An extreme example is skiers sunbathing in the Alps in sub-zero temperatures, though they will still need warm clothing to protect them. Such an extreme temperature difference is not a viable solution for use within a building. We must therefore achieve a temperature that is not too uncomfortable for humans to work in while wearing normal everyday clothing.
Benefits of radiant heating
There are three key benefits of using radiant heating within a building: the optimisation of wall space, lower air temperatures and less convection. These can help save energy and money, while creating a comfortable environment for occupants.
- Optimisation of wall space With the installation of ceiling-mounted radiant heating panels, perimeter walls and floor space can be left free for equipment or furniture, ensuring the largest possible lettable area. In addition, eliminating the need for surface temperature radiators at ground level creates a safer environment, there is no need to hide or cover pipework, and the possibility of accidental or deliberate damage to radiators is removed. There are no radiators to paint or maintain, and cleaning costs are reduced as less air is circulated. Installation costs are also reduced as services can be run through the ceiling void and do not need to be dropped down to a lower level.
- Lower air temperatures High radiant values contribute to a higher temperature in a room. The air temperature within a radiant heated room can therefore be up to 3°C lower than in an alternatively heated room. This enables quicker heating response times, reduced infiltration losses, more even temperature distribution and a more comfortable working environment. In addition a lower air temperature means direct energy savings, which in turn reduces a company’s carbon footprint.
- Less convection The combination of higher radiant values and lower air temperatures also results in less convection, or air movement, leading to more efficient operation. It also means lower dust levels - beneficial for allergy sufferers - and helps save on cleaning and maintenance costs. Radiant heating can also be quieter than other forms of heating.
What is a radiant panel?
Radiant panels should not be confused with radiators, which are actually primarily convectors. Radiators generally distribute 80% of their output via convection and only 20%
via radiant heating. If you take a radiator, remove the convectors from the back, insulate it and install it in a ceiling, you would be converting it into a radiant panel, and the
output ratios would change to 70% radiant heat and 30% convection.
Putting these figures in context, for a radiator to heat a room on a cold winter’s day, it must first warm itself and then the air around it. This air will move around the room by convection until the room is at a homogenous temperature and the desired comfort level is achieved. By standing close to the radiator, we can take comfort from the 20% radiant heat available once the panel has reached its operational temperature. So if the radiator has an output of 1kW, 200W provides instant comfort, while 800W is being used to increase the air temperature. With a 1kW radiant panel, 700W would be available as soon as the panel reached its operational temperature. Moreover, if the panel was sited correctly in the ceiling, the benefit could be felt throughout the room.
Specifying radiant panels
There are many different radiant heat panels available and different manufacturers will use subtly different methods. However, the basic concept in all cases is the same. Essentially, water is distributed throughout the metal panel via a tube, providing the maximum surface area and thermal contact. The water heats the tube, and the tube heats the panel.
Performance will vary according to the size of the tube, how it is spaced and the quality of thermal contact. It is impossible to achieve a surface temperature equal to the mean water temperature because there will always be some element of loss. In practice, a drop of 8-10°C is the norm. A panel’s output may vary from 400W/m2 to 600 W/m2 at UK standard temperatures.
There is only one specification applicable to testing the output of a radiant panel: BS EN 14037. There is only one test chamber in Europe certified to conduct this testing, at the University of Stuttgart in Germany. The test is conducted on a panel that is 3m long and 600mm wide, in a free-hanging/open ceiling position. The test chamber offers repeatability by controlling the fabric temperatures.
Any manufacturer who has tested their product will be able to supply you with a BS EN 14037 certificate from the University of Stuttgart stating the output of a 3m by 600mm panel. This requirement should form part of any specification to ensure the correct heat outputs are obtained. Conversely, manufacturers using the BS EN 442 radiator test output will be reporting an incorrect output figure.
Because BS EN 14037 is an open ceiling test, some convection will still apply. There is currently no certification for radiant outputs within a closed ceiling. A revision to BS EN 14037 to show outputs within closed ceilings is planned, and outputs will reduce as a result.
How does radiant heating and cooling work?
Radiant panels distribute heat into the room, shining on the objects below. As these are at a lower temperature, they absorb heat and conduct it, increasing in temperature. These objects in turn become low temperature radiators, giving off lower levels of radiant heat and slowly warming the air within the room by convection.
Radiant cooling panels are passive chilled beams, which work by convection. The radiant panel itself is the coolest item in the room and as the warmer items create convection, the warmer, lighter air rises. This air then passes over the cool surface of the panel and cools down. It becomes heavier and drops back to the lower level, thus aiding natural convection and enhancing the cooling effect.
Positioning radiant panels within a room
When designing a radiant heating system, the positioning of panels is a key consideration for ensuring that everyone in the room will benefit from the heat produced.
The field of spread from a radiant panel is approximately a ratio of 2:1 to its mounting height. For example, a panel mounted at 3m will provide a radiant field of 6m. A mounting height of 5m will provide a radiant field of 10m. Therefore as the mounting height is increased, the intensity of the heat is reduced and the output is spread over a greater area (see diagram 1, above).
The larger the output area, the greater the heat-load requirement. For installations up to 10m, there is no need to increase the panel output as radiant heat passes through the air without any loss. For installations between 10-15m, BS EN 12831 on heating systems in buildings recommends a 15% increase in output to compensate for lower intensity levels.
Within the radiant field itself, there are also varying levels of intensity. Diagram 2 (above) shows that the area directly under the centre of the panel (E1) receives an output level of 90-100%, while the outer edge (E3) is 50% of the intensity of E1. These calculations are particularly important when positioning panels in specific buildings, such as directly over a patient’s bed in a hospital.
When using more than one panel within a room, it is good practice to provide an even spread and an overlap of intensity above head height (see diagram 3, below). In order
to achieve this, the distance between the panels should not be greater than the mounting height.
When installing radiant panels within rooms with low ceilings, careful design is required to avoid uncomfortable extremes of temperature. One large output panel will create areas of high temperatures and an uneven distribution of heat. In this module, we have compared radiant heat to light. A lighting engineer would not light a small room with a low ceiling with a few, high output lights. Rather, they would use many small intensity lights spread evenly around the room to achieve a constant lux value. We should consider radiant heating in the same way. The lower the intensity and the more even the distribution, the better the resulting environment will be.
Using resultant temperature
As previously discussed, the resultant temperature can be calculated as the average of the radiant and ambient temperatures.
Using this calculation, it is possible to achieve the same level of comfort by increasing the radiant component and decreasing the ambient component. For example, a
radiator is installed in a room and provides an air temperature of 21°C and a radiant value of 17°C, thus giving a resultant temperature of 19°C.
If the radiator is replaced with a radiant panel, the air temperature can be lowered to 16°C and the radiant value increased to 22°C to achieve the same resultant temperature of 19°C. With this configuration, the increased radiant value and lower air temperature mean the response time is quicker, the system has a lower inertia and direct energy savings can be made.
Opportunities for radiant cooling
Radiant panels resting idle in the summer could be used to provide cost-effective cooling or partial cooling. For example, the radiant panels for a typical sports hall, providing 40kW of heating, could also provide 16kW of cooling. Radiant cooling is clean, passive, silent and responds quickly, and offers savings on installation, maintenance and running costs compared to other systems. It also offers good sound absorption, as perforated panels tend to be used.
When considering the use of radiant panels for heating and cooling, it is important to consider their use from the outset as part of the building’s indoor climate strategy.
To take this module read the technical article and click through to a multiple-choice questionnaire, once taken you will receive your results and if you successfully pass you will be issued automatically with a certificate to print for your records.
Module Closing Date : 3 August 2012
Company name: Zehnder