Underfloor heating

The floor heating is a surface heating , the spaces by means of pipes laid in the floor heating .

history

Roman underfloor heating (hypocaust)

The first underfloor heating was used by the Romans ( hypocaust ), later, around 700 AD, also by the Koreans , the underfloor heating Ondol .

The big breakthrough in underfloor heating has happened rapidly since the 1970s.

Hot water heaters

In or under the heating screed , pipes made mainly of plastic or, less often, copper are laid. The most commonly used plastic is cross-linked, oxygen-tight (otherwise there is a risk of corrosion on iron parts) polyethylene ( PE-X ), often with an additional aluminum intermediate layer. The laying is either meandering (same pipe spacing), modulating (different pipe spacing depending on the position in the room, flow on the outer wall) or bifilar (spiral shape , flow and return are side by side). Which laying should be given priority is disputed, as different objectives (uniform room temperature, uniform floor surface temperature) and technical possibilities depending on the pipe material, fastening and laying technology must be taken into account.

Plastic capillary tube mats are also known , the PP tubes arranged in parallel (diameter, for example, 4.3 mm wall thickness 0.8 mm) being traversed in the same direction.

Pipe distances of 5 to 30 cm cause a slight temperature ripple on the screed surface - the distances can be adapted to the heat demand. However, temperature differences> 5 K within screed fields that are not divided should be avoided.

The use of capillary tube mats for concrete core activation leads to a very homogeneous component temperature distribution, which increases the heat storage capacity required with alternative energies compared to larger tube spacings.

In underfloor heating, a distinction is made between wet systems ( cement screed or anhydrite screed , very often due to the better heat transfer in flowing screed, including mastic asphalt and rolled asphalt ) and dry systems (dry screed panels or steel tiles). With the wet system, the pipes are installed in the screed.

Wet system (type A / C)

Pipes for underfloor heating (wet system)

Controller for underfloor heating (wet system)

With the wet system , the pipe is completely enclosed by the screed and there are various ways of fixing the pipes before pouring the screed:

with clips on steel support mats
on clamping rails made of steel or plastic
on a knobbed plate made of plastic
with brackets on the load-bearing insulation (actually not permissible due to the damage to the protective layer for insulation and the insulation)
thread between a steel honeycomb panel
the pipes are provided with Velcro tape and pressed onto carrier mats coated with fleece

Dry systems (type B)

In the dry system , the pipes are located below the floor covering in the insulation layer. It is fastened there on the carrier insulation, which can be equipped with grooves and heat-conducting lamellas. The lamellas are intended to improve heat distribution. The dry system is suitable for low floor structures and is used in old buildings or in building modernization. Dry systems can also be designed with directly laid top floors (screed tiles , tiles , floating parquet and laminate ) and thus lead to a further reduction in the flow temperature and to a faster heating and cooling phase.

Another variant of the drying system consists of dry screed panels with a prefabricated milling that fixes the heating pipes. This system brings together the previously separate components - dry screed panels and underfloor heating - from earlier systems. The short assembly time is particularly beneficial for architects who only have vacation times available for installation in public facilities such as schools and kindergartens. In addition, this simplified system can also be installed by private individuals, so that the building owner only needs a heating engineer for the pipe connections.

Newer underfloor heating systems are increasingly being designed for building renovation without interfering with the existing floor structures. This results in very low installation heights from approx. 8 mm. A special leveling compound is the basis for the floor covering.

Hot water distribution

A heating circuit distributor is required for both systems for heat distribution . All heating circuits are connected to a heating circuit distributor with the flow and return. Each individual heating circuit can be hydraulically balanced on the heating circuit distributor using a valve. By built-in heating circuit flow meter which may make volume flow can be observed. The hydraulic balancing is necessary because the individual components of the floor heating (eg. B. heating circuit, pipe circles, etc.) have different high flow resistances. Even heat distribution is only possible with the same high flow rates in all heating circuits. However, an uneven distribution of heat may also be sought in order to compensate for the stronger cooling in rooms with large external wall surfaces compared to internal rooms . Since underfloor heating as opposed to radiators very much slower responding (see dead ), the flow temperature often from an outside temperature - probe derived. The control unit sends an electrical signal to the servomotor , which then further opens or closes the four-way valve. In the case of higher-quality systems, the energy supply can be regulated with room temperature controllers whose temperature sensors are installed in the heating area (e.g. living room). In well-insulated residential buildings, the heating output is designed for around 50 to 100 W / m². In addition, the underfloor heating can be connected directly to the existing heating circuit up to a certain area (depending on the flow resistance of the underfloor heating used). The regulation takes place via a RTL valve (return temperature limiter, German: return temperature ), which is mounted in the return pipe of the floor heating and stops the flow when the set floor temperature is reached.

There are numerous ways of laying the pipes. In order to achieve a largely uniform heat distribution in the room, pipes should be laid with the hot water flowing in the opposite direction. This is achieved by arranging the forward and reverse lines next to each other.

Electric heaters

Electric underfloor heating (50 cm × 200 cm) before installation

In addition to hot water-based heating systems, electrically operated heating systems are also used. Resistance cables or heating foils with integrated heating conductors are laid under, in or on the screed. They are suitable for all types of laying that are also common for hot water systems. Because of the low overall height, they are particularly recommended for direct installation under floor coverings. Heating cables with a diameter of 3 mm or more can even be laid under laminate in the adhesive bed of tiles and foils. For underfloor heating (not full heating) there are mats from a height of approx. 2 mm. Cables with a grounded screen are used for bathing, showering and other damp rooms to guarantee safety against electrical accidents. The applicable standards for electrical surface heating elements are DIN EN 60335-1 and DIN EN 60335-2-96.

Applications in commercial and municipal buildings

In addition to the underfloor heating, which is also used in residential construction, industrial surface heating or sprung floor heating (sports halls) are used here. Air heaters heat the air in the room, which escapes immediately when the hall doors are opened. It takes a long time and requires a lot of energy to raise the hall temperature again. At a soil temperature of z. B. 10 ° C and an air temperature of 20 ° C arithmetically a perceived temperature of only 15 ° C. Therefore, floor temperature control is recommended for an acceptable thermal ambient temperature. With underfloor heating, the heating of a hall consists of radiant heat. This is still available to its users while the hall gates are open. After closing the hall gates, the user will feel this cozy radiant heat again in a very short time.

Room air conditioning with hot water systems

Underfloor heating systems are also used for underfloor cooling. In connection z. B. with a heat pump heating and geothermal energy , this variant is ideal. The surface temperature of the finished floor should not fall below 20 ° C and not exceed 29 to 35 ° C - depending on the location - (see section on boundary conditions). In addition, the dew point should be monitored with an appropriate humidity sensor and the flow temperature should be regulated accordingly. The flow temperature of the cold water is usually 16 ° C with a spread of 2 to 3 K ( Kelvin ).

In addition to classic heat pump heating, underfloor heating can also be used in conjunction with cold local heating networks for cooling. Depending on the temperature of the heating network, the use of the heat pump for cooling can also be dispensed with.

Advantages and disadvantages

One reason for underfloor heating is comfort. For example, underfloor heating makes it possible to go barefoot in the house even in winter. Another advantage is the architectural freedom of room design, because no radiators or pipes can be seen. There are also the hygienic aspects of underfloor heating. The dust is not raised by radiators. The dry heat on the floor prevents the growth of house dust mites and the formation of mold - even when carpeting is laid.

Another advantage is the low flow temperature, which is a great advantage for solar heat use and heat pumps. In particular, underfloor heating installed in screed can serve as a heat store and thus increase the effectiveness of the boiler.

boundary conditions

The following standard applies to underfloor heating:

DIN EN 1264 (old DIN 4725).

Other standards that interface with underfloor heating:

EnEV: Energy Saving Ordinance
DIN 18560 : screed standard
DIN 1055: traffic loads
DIN 18202: Tolerances in building construction
DIN 4109: Sound insulation in building construction
DIN 4726

The flow temperature of the heating water is 35 ° C (with older systems up to 55 ° C) with a spread of 5 ° C (difference between flow and return temperature). The surface temperatures of the finished floor must not exceed 29 ° C in the living area, 33 ° C in the bathroom and 35 ° C in the edge zones. These temperatures have been determined in long-term studies. The physiology of humans was taken into account and it was found that these temperatures were classified as harmless for the majority of people. They have thus found their way into the corresponding DIN standards and now also into the European standard for underfloor heating, which also means that a uniform standard has been found for underfloor heating. Correctly set underfloor heating systems only reach these maximum values at the limit of the design conditions, i.e. the coldest days; Usually it should be about 10 ° C less. Previously, the poor properties of underfloor heating were known to be thick feet, varicose veins, etc., when surface temperatures above these limits were sometimes used and surface temperatures of around 27 ° C were considered normal.

The thermal resistance in the floor covering should not exceed 0.15 m²K / W. Most textile coverings are marked with the underfloor heating symbol and are therefore approved. ${\ displaystyle R _ {\ lambda, B}}$ ${\ displaystyle \ mathrm {\ frac {m ^ {2} \, K} {W}}}$

Edge insulation strips are to be arranged at the edges of the screed. They should allow the screed to expand and also ensure sound insulation. Expansion joints must also be provided for larger areas. The screed standard DIN 18560 applies here .

The routing of the pipes is determined by the laying distance, which is determined from a calculation. The deviation from the determined pipe spacing must not be exceeded within a certain tolerance because this creates the risk of excessive waviness in the screed. The ripple is the temperature difference between the pipes.

calculation

The calculation of the underfloor heating is based on DIN EN 1264 parts 2 and 3 .

The basic characteristic is an important parameter. This is the relationship between the mean floor temperature and the heat flow density as an empirical function:

{\ displaystyle {\ dot {q}} = 8 {,} 92 (\ Theta _ {F, m} - \ Theta _ {i}) ^ {1,1}}

With

{\ displaystyle {\ dot {q}}}

- heat flux density in W / m²

{\ displaystyle \ Theta _ {F, m}}

- mean floor surface temperature in ° C

{\ displaystyle \ Theta _ {i}}

- Room air temperature in ° C

Example:

{\ displaystyle {\ dot {q}} = 8 {,} 92 \, \ mathrm {\ frac {W} {m ^ {2} \ cdot \, ^ {\ circ} \ mathrm {C}}} (29 \, ^ {\ circ} \ mathrm {C} -20 \, ^ {\ circ} \ mathrm {C}) ^ {1 {,} 1} = 100 \, \ mathrm {\ frac {W} {m ^ {2}}}}

Here you can clearly see that every increase in the internal temperature results in a lower heat flux density.

The heat transfer coefficient can be calculated from the heat flow density : ${\ displaystyle \ alpha}$

{\ displaystyle \ alpha = {\ frac {\ dot {q}} {\ Delta \ Theta}} = {\ frac {100 \, \ mathrm {\ frac {W} {m ^ {2}}}} {9 \ mathrm {K}}} = 11 {,} 11 \, \ mathrm {\ frac {W} {m ^ {2} \ cdot \ mathrm {K}}}}

In order to calculate the heat flow density in the design case, the so-called logarithmic heating medium excess temperature is also required:

{\ displaystyle \ Delta \ Theta _ {H} = {\ frac {\ Theta _ {V} - \ Theta _ {R}} {ln {\ frac {\ Theta _ {V} - \ Theta _ {i}} {\ Theta _ {R} - \ Theta _ {i}}}}}}

With

{\ displaystyle \ Theta _ {V}}

- Flow temperature in ° C

{\ displaystyle \ Theta _ {R}}

- Return temperature in ° C

{\ displaystyle \ Theta _ {i}}

- Room air temperature in ° C

The heat flow density can now be calculated

{\ displaystyle {\ dot {q}} = \ mathrm {B} \ cdot \ Pi _ {i} \ cdot \ Delta \ Theta _ {H}}

With

{\ displaystyle \ mathrm {B}}

- System-dependent coefficient, which is derived from the pipe properties

{\ displaystyle \ Pi}

- Factor for specific properties of the floor structure depending on the selected pipe division

The manufacturers of underfloor heating systems supply performance diagrams for their systems for various floor coverings and pipe spacings, from which the heat flow density in W / m² can be graphically determined using the heating medium excess temperature mentioned above. ${\ displaystyle {\ dot {q}}}$

There is also a general calculation method for wet systems based on the FAXEN algorithm that can be used for all pipe dimensions and pipe spacings. The calculation method is verified in DIN EN 1264-2. The method presented and described in detail in has been extended to dry systems and wet systems with dynamic operation, including materials with phase change effects (so-called PCM). PCM use a phase transition for additional heat storage ( latency heat storage ).