In recent years, underfloor heating has become increasingly popular. This is due to the fact that underfloor heating provides a range of opportunities not offered by traditional heating methods, both in residential and commercial buildings. However, underfloor heating also offers some challenges and, in order to make the very most of the opportunities, it is important to choose the right solutions.
Why choose underfloor heating? — There are many areas in the commercial sector where underfloor heating can be incorporated to benefit potential clients, both in terms of comfort and energy reduction. Money matters now more than ever before, and if an engineer can effectively demonstrate to the client that there is the potential to reduce operating costs and provide increased levels of comfort by incorporating lower energy sources, then underfloor heating has clear benefits.
Some of these benefits include:
– Underfloor heating provides comfortable heating as your feet are kept warm while your head is kept slightly cooler;
– There is much focus today on the indoor climate where increasing numbers of people are plagued by asthma and allergic problems. Due to the large heating surface associated with underfloor heating, less air is mixed and therefore less dust occurs in the room compared to a conventional radiator system with its convection currents of air;
– The room temperature can be lowered 1-2°C which can result in energy savings of 6-12%;
– Low surface temperatures which eliminate contact hazards;
– Long life span.
There are some circumstances where underfloor heating is not suitable and a more conventional form of heating may be more appropriate. This can be due to client’s requirements or perceptions, structural constraints or the flexibility of the space.
Some of the disadvantages associated with underfloor heating are:
– Generally it has a slower response time (in solid floor applications only) compared to radiators;
– Incompatible with certain floor coverings;
– Floor penetrations should be avoided or carefully planned;
– Difficult to change pipe routes once installed.
Applications and design parameters
The principal characteristics of underfloor heating include energy efficiency, economy and excellent thermal comfort. It is an ideal choice for most, but not all applications.
Buildings or areas that are used very used continually or frequently; Buildings ore areas with relatively low heat loss; All ceiling heights including atria; Use with all heat sources.
Buildings or areas that are used very intermittently or infrequently; Buildings or areas with high heat losses, particularly when due to high ventilation rates; Buildings where unpredictable re-zoning may occur; Areas where the floor is largely obscured by permanent fixtures and fittings.
The temperature experienced in a room is the result of two different factors, namely the air temperature and the ambient radiation, i.e. from the heated elements in the room. It can be an advantage in many ways that heat radiation constitutes a relatively high part of the “overall” temperature, or the operative temperature as it is also called.
Underfloor heating is not purely a radiant form of heating, typical emitted proportions are 70% radiation and 30% convection. If a large part of the operative temperature is made up of the air temperature, it means that there will be a high convection or mixing of air in the room. If there is high mixing, air is whirled around which leads to higher dust content in the air and therefore a poorer air quality.
Where convection is a high proportion of the operative temperature, the frequent opening of doors or windows can have a negative impact on occupant comfort. For this reason underfloor is particularly suited to foyers or atriums where the proportion of the operative temperature is primarily radiation and is less effected by the opening of doors and windows.
The way convection/radiation occurs with radiators and floor heating respectively can be seen in Figure A. As can be seen, with radiator systems the air temperature or convection makes up approximately 70% of the operative temperature. This is also logical if you think about how a radiator has quite a small surface from which to transfer heat to the whole room.
Figure A. Left – Radiators: 70% convection/ 30% radiation. Right – Uf Heating: 30% Convetion/70% radiation
Conversely, floor heating supplies heat through a very large surface evenly distributed in the room, which means that the ratio is just the reverse with 70 % of the operative temperature being added by radiation.
The temperature of the floor surface must be sufficient to provide the required heat transfer into the space, but not be so high as to cause discomfort to occupants. Generally in areas where occupants are seated there should be a maximum of 9°C between floor surface temperature and room temperature. This equates to a maximum surface temperature of 29°C in normal occupied areas and 33°C in changing/shower areas with a room temperature of 24°C. In areas where people would not normally be seated such as atria/foyers or areas with high glazing levels, the maximum floor surface temperature can be increased to 15°C above room conditions.
In practice, it is not possible to maintain the same temperature everywhere in a room. It is recommended that a difference of approximately 2°C between floor and head height should be maintained. This is because most people want to have warm feet while keeping “a cool head”. But the difference in temperature should not exceed about 3°C as the body will become “confused” and comfort is reduced.
Characteristics and key elements
The rate of heat output from underfloor heating is determined by the following factors:
— Mean water temperature circulating through the floor piping;
— Temperature of the space;
— Spacing and diameter of pipework;
— Thermal resistance of floor coverings;
— Thermal resistance of insulation/flooring under the pipework.
The mean water temperature and pipework spacing can be varied to provide the required design output. This can be used to overcome issues such as high heat loss, floor covering with high thermal resistance or restricted emitter area. The heat output is a function of the surface area and the mean water temperature. Given that the size of the floor area, is so large the mean water temperature can be much lower than that commonly-used in radiators while still providing the same output.
The following describes the major elements of underfloor heating systems:
Underfloor heating can utilise a wide range of energy sources. Unlike conventional heating systems that require hot water at temperatures up to 80°C, underfloor heating is effective at flow temperatures of 45°C. It is particularly suited to condensing boilers due to the low return water temperatures. The boiler should be fully condensing under all operating conditions, with the exception of when domestic hot water generation is required. Some of the sources of providing low grade heat and low carbon emissions that are suited to underfloor heating include condensing boilers; biomass boilers; various forms of heat pumps and solar panels.
Heat pumps can take various forms, of which the most common types are, ground-source, water-source and airsource heat pumps. These typically have a Coefficient of Performance (CoP) of 3.5 to 4.0.
Solar panels may also be used as themain heat source, but they are be unlikely to be cost effective. They can, however, be used in conjunction with a conventional heating system or heat pump with the aim of reducing the load imposed on the main heat generator.
Ideally, a single pump should be used to circulate water to the manifolds and through the pipework, though some systems may incorporate a secondary pump and mixing valve attached to the manifold. Pump energy consumption should be minimised by avoiding excessive pressure drops in the distribution system and employing variable speed pumps. All pumps selected must comply with the European ErP Directive EC 641 for glandless pumps, which comes into effect in 2013.
The manifolds take the low temperature hot water from the heat source and distribute it through the underfloor heating circuits. There are separate manifolds for both supply and return and they incorporate electric control valves and balancing valves. Depending on the size of the project and the number of circuits, it is not uncommon to have a number of manifolds located throughout the building.
Underfloor heating pipe
There are generally three main types of material associated with underfloor heating:
(1) Cross-linked high density polyethylene (PE-X) to BS EN 15875-2 : 2003;
(2) Polybutylene (PB) to BS EN ISO 15876-2 : 2003;
(3) Aluminium/plastic multilayer composite pipe to DIN 4726 : 2000.
All plastic heating pipes should incorporate an oxygen barrier, and in the case of composite pipes, an aluminium foil incorporated into the pipe provides the oxygen barrier.
Generally, two pipe sizes are available f or underfloor heating, and these can vary slightly depending on the manufacturer, but are commonly 16mm and 21mm in diameter. Increasing the diameter will reduce the flow velocity and pressure drop but also influence the heat output per unit area of the floor (assuming the spacing remains the same).
Insulation is a key factor in maximising the output of underfloor heating and particular attention should be applied to this element. BS EN 1264 Part 4 states that the maximum limit to the downward heat loss should not be more than 10% of the heat supplied. This applies even to where the underfloor heating system is installed above another occupied space (see Figure B).
It is very important to provide edge insulation around the floor perimeter where underfloor heating is installed as this prevents thermal bridging. Edge insulation also allows expansion of the floor slab. Factors to be considered when selecting insulation include compression strength, thermal conductivity and moisture resistance. This should be discussed between the building services engineer, the structural engineer and the architect.
The design of an underfloor heating system is no different to any conventional heating design in that the designer must agree the scope of design. All the necessary information on the design requirements has to be gathered from the client/architect etc, and this information is generally greater than with traditional systems, as details of the floor make-up and floor covering need to be considered in the design.
All underfloor heating systems should be designed in conjunction with BS EN 1264 parts 1 to 5, as well as other industry norms including, but not limited to, Irish building regulations and CIBSE guides.
The required surface temperature, and consequently heat output, are achieved by varying the pipe spacing, circulation rates and flow temperature. The depth of the pipe and the thermal response of the structure and covering are usually dictated by other factors.
To achieve the required heat output for each individual area, designers will generally base the required pipe spacing on pre-determined required flow and return water temperatures. Flow temperatures of 45°C and return temperatures of 35°C are commonly used.
Depending on the area in which underfloor heating is to be installed, the spacing requirements can take two approaches. The spacing can be installed to match the required heat output or the pipe can be spaced closer together to increase comfort and/or response time. The latter approach may be beneficial if the floor coverings are unknown or undecided. Obliviously, the closer the pipe spacing the more material will be required and hence there will be an increase in capital cost. The manufacturers of underfloor heating pipework produce a range of tables that cover the selection of pipework and required spacing to achieve the desired output.
Generally in commercial installations the underfloor heating circuit is arranged as a secondary circuit from a primary header. The primary header usually feeds various circuits such as AHU’s, radiators and a calorifier, as illustrated in Figure C.
There is an inherent disadvantage with using this arrangement with underfloor heating in that the boiler flow temperature is matched to the highest requirement, which is usually the AHU’s or calorifier. It may be beneficial to consider separating the lower temperature underfloor heating loads by the use of condensing boilers or heat pumps dedicated to the underfloor heating load.
The layout of the pipework is also vital to ensure a cost-effective and efficient design. Too many manifolds can increase the capital cost of the installation, and too few can lead to pipework congestion, local overheating and reduced control.
Generally as a guide the length of a pipework circuit should not be more than 110 metres. Attention to the location of the manifolds needs to be considered, as particular areas of concern are where pipes travel through a corridor to a number of heated zones. This can lead to pipework congestion resulting in uncontrolled and local overheating.
It is also important to consult with the structural engineer with regard to location of all expansion joints. If underfloor heating pipes pass through an expansion joint in a structural slab, then these pipes must be sleeved at least 200mm either side of the expansion joint.
Unlike with conventional heating systems, an underfloor heating specialist is generally employed to install and commission the heating system. This specialist is generally a sub-contractor of the mechanical contractor.
One of the key areas during installation that needs to be strictly enforced, and is not always complied with, is the quarantine of the work areas during the pipe-laying and covering. This is a vital area of concern that can sometimes be overlooked and needs close co-ordination with the project schedule, main contractor and associated sub-contractors. Damage to the underfloor system from personnel or equipment can easily occur where pipes are left exposed.
There are many ways of controlling underfloor heating systems, from small scale residential to large commercial building management systems. However, the basic components remain the same. The following components are commonly used – electronic room temperature sensors for each zone; outside temperature sensor; electronic mixing valves; two-port zone valves; and master controller.
It is highly recommended that a weather-compensator be incorporated into the underfloor heating design. This raises the floor temperature as the outside air temperature gets colder. This will be beneficial in terms of comfort and also energy efficiency.
There is a preconception that underfloor heating is inevitably more expensive to install than conventional heating systems. This is not always the case and projects involving over 300sq m of underfloor heating can be cheaper in “install costs” to a comparable conventional heating system. As with all projects, an accurate cost comparison should be conducted which takes account of the whole life costs of the system.
The running costs of underfloor heating can also be very beneficial since, as we discussed earlier, the room air temperature can be reduced by approximately 2°C. This translates into a significant reduction in building heat losses and energy consumption. The type and height of a zone also has big impact on the running cost – rooms with high ceiling or atria can see significant reductions in energy consumption where underfloor heating is incorporated.