Wet underfloor heating is a radiant heating system, although the radiant share of the total heat transfer from the heating surface is only slightly higher than the heat flow by convection (55%:45%). This has a positive effect on both modes of heat transfer into the interior.
You will find the complete range in the underfloor heating category – system insulation boards, pipe, manifolds, controls and electric heating mats.
Wet underfloor heating is also a large-surface system, i.e. the heating pipes form part of the floor construction. Because of the hygiene requirements that limit the surface temperature of the floor, and therefore the relatively low specific heat output of a heated floor, the heating pipes are almost always laid under the entire floor area. This has an exceptionally positive effect on the evenness of heat transfer in the interior and helps to create a thermally homogeneous, uniform environment in both the vertical and the horizontal direction.
The temperature of the heating water is usually below 50 °C, so wet underfloor heating can also be described as low-temperature heating, which has several advantages. One is the substantial saving of heat energy in operation; another is the possibility of using non-traditional low-grade energy sources such as solar radiation, geothermal water or heat pumps that draw on ambient heat.
It is precisely these facts that place wet underfloor heating among the progressive heating systems that deliver on all three Es, i.e. the energy, environmental and economic aspects of heating buildings. Although this method of heating existed more than 2 000 years ago, it is only today that it is enjoying its greatest boom and renaissance.
The historical development of heating
The oldest method of heating among all peoples was local heating, in the form of burning wood in an open hearth. The greatest shortcoming of the hearth was the excessive amount of smoke. To prevent it, the ancient Romans invented charcoal, which was burnt without smoke in flat metal pans. In antiquity this was the most widespread method of heating.
From about the 10th century AD the open hearth was replaced by a partly enclosed hearth, from which the smoke was led through a vent into the roof space, and later by a fully enclosed hearth with a chimney rising above the roof of the building. These hearths – fireplaces - were at first made of stone and clay, later of tiles. They have survived to this day, while in the 17th century iron stoves were used for heating.
The first central heating was the ancient Roman hypocaust (designed by Sergius Oratus in 80 BC). It was in fact a hearth without a grate, for burning wood or charcoal, located outside the heated rooms, beneath the house. The hot flue gases flowed into cavities under the whole house and so warmed the floor (Greek hypo = from below, kaiein = to burn), which in turn warmed the air in the rooms of the house. The flue gases themselves were carried away through one or more shafts in the external walls, which opened to the outside through side openings. In the first centuries AD many houses and buildings heated by hypocaust were built in Rome and across the Roman Empire, for example the ancient Baths of Caracalla (AD 211 to 217), the Baths of Diocletian, and other baths in what is now Trier. Remains have even been found in Pompeii.
An improvement on this system was channel heating, in which there was no cavity under the floor of the house; instead the flue gases flowed through a branching system of channels beneath the floor. This can be regarded as the first underfloor heating, although the heat transfer medium was still air.
Hot-water heating first appeared in France in the 18th century, but found its widest use in Germany. At the beginning of the 20th century hot-water heating with forced circulation became very widespread and turned into the usual way of heating flats and public buildings. The terminal units of such heating systems were heat emitters – radiators - standing freely in the rooms. The originator of radiant heating systems with heating elements cast in concrete slabs fixed to the surface of the masonry was the Englishman Barker, in 1907. In 1926, together with the English firm Crittal, he laid heating pipes directly in the structure of the building (most often in the ceiling, sometimes in the floor). In 1935 the Dutchman van Dooren patented the use of radiant heating pipes in the ceiling as a reinforcing part of the building structure. From this point we can speak of radiant heating, specifically large-surface hot-water heating.
OPERATION OF WET UNDERFLOOR HEATING
In the operation of wet underfloor heating we look at:
- the methods of preparing the heating water,
- the influences on the operation of the heating system,
- the options for saving energy.
METHODS OF PREPARING THE HEATING WATER
The heating water for wet underfloor heating can be prepared either directly, straight from the heat source, or indirectly, via a heat exchanger.
DIRECT HEATING OF THE HEATING WATER
Direct heating of the heating water is provided by traditional or alternative heat sources which, depending on the mode of operation, can work:
- in monovalent mode,
- in bivalent mode,
- in combined mode.
MONOVALENT OPERATION
Monovalent operation means operation with a single heat source that covers the entire heat loss of the heated space. Ordinary boilers for various types of fuel can be used as the heat source.
Connecting underfloor heating to a solid fuel boiler is the least suitable option, because the output of the boiler is difficult to control. In this case the heat source must without fail be supplemented by a buffer tank, which stores the surplus heat not being drawn off into the heating system.
BIVALENT OPERATION
This method of preparing the heating water is based on the use of two different heat sources - a primary and an alternative one. The primary source covers the heat demand on its own down to a set outdoor temperature; at lower temperatures it is shut down. The alternative source then takes over the whole of the remaining heating demand.
COMBINED OPERATION
In this case two different heat sources work at the same time. One source works alone down to a set outdoor temperature. At lower outdoor temperatures the second source covers the peak load in cooperation with the first.
The low heating water temperature of underfloor heating allows more efficient use of low-grade sources of non-conventional energy, and from an operating point of view heat pumps appear to be the most suitable for this purpose. The outlet water temperature from the heat pump condenser is at most 55 °C and at least 35 °C. Given these outlet temperatures of the working fluid, heat pumps appear to be the optimum heat source for low-temperature underfloor heating.
In principle, regardless of the specific application, heat pumps can be incorporated into the circuit as a monovalent source, but more often they are used in bivalent or combined operation.
INDIRECT HEATING OF THE HEATING WATER
Indirect heating - by means of a heat exchanger - is advantageous above all when underfloor heating is combined with convective heat emitters. The heat emitters are supplied with water directly from the boiler, while the heated floor is warmed indirectly through the heat exchanger. With correctly chosen exchanger materials, this method of heating the floor is particularly suitable for limiting the diffusion of oxygen through the walls of plastic pipes.
INFLUENCES ON THE OPERATION OF THE HEATING SYSTEM
The operation of wet underfloor heating is influenced by :
- the setting of the heating curve,
- the self-regulating effect,
- the mode of operation,
- the thermal properties of the floor,
- the control strategy.
INFLUENCE OF THE HEATING CURVE SETTING
The heating curve should always be set optimally, i.e. for the room with the smallest heat gains, e.g. a bedroom. A wet underfloor heating system reacts very sensitively to an inaccurately set heating curve.
It is clear that an inaccurate heating curve setting has a substantial effect on energy consumption in operation. An optimally set heating curve was taken as the reference case. First the heating curve was shifted upwards by 1 K, and then it was set with an even more pronounced slope, i.e. by 2 K. It turned out that shifting the heating curve upwards increases energy consumption not only because of the higher required indoor temperature but also because of the self-regulating effect.
INFLUENCE OF THE SELF-REGULATING EFFECT
The self-regulating effect, which results from the large heat storage capacity of the underfloor heating surface and the consequent thermal inertia of several hours , is a significant factor , affecting the thermal regime of interiors heated in this way. At the same time, however, it does not depend on raising or lowering the heating water temperature, as can be derived from a steady-state analysis.
When designing the controls we must bear in mind that with underfloor heating the storage of heat in the floor construction plays an important role. The heating surface has a great deal of mass, and so the thermal inertia of the heated floor is also very large. Depending on the composition of the floor, we can expect a value of 3 to 8 hours. This is why, with large-surface underfloor heating, rapid changes in heat input are not possible, and why control interventions (e.g. a change in the heating water temperature) show up in the heated room only gradually; the warm-up and cool-down times are likewise long. For this reason we recommend using underfloor heating only in continuous, uninterrupted operation.
The large thermal inertia of underfloor heating is particularly unsuitable for modern lightweight buildings with large glazed areas, where it leads to sluggish and uneconomical heating operation.
INFLUENCE OF THE MODE OF OPERATION
The self-regulating effect described above also explains the following facts, which arise from the way the heating system is operated. Heating with a night-time switch-off (22,00 h to 5,00 h) was compared.
It turned out that for buildings with good external thermal insulation, without rapid morning warm-up, switching off at night results in a loss. The heating curve has to be raised during the warm-up period, which dampens the self-regulating effect. With rapid warm-up the influence of this stored heat is considerably suppressed. For poorly insulated buildings (where underfloor heating is not very common), night-time heating may on the contrary be advantageous.
However, the following principles of economical operation must be observed:
- avoid uncontrolled ventilation
- monitor the heat losses in the heating system
- check the electricity consumption of the circulation pumps
- ensure rapid warm-up.
These influences slightly reduce the difference between the two modes of heating, or even tip the balance in favour of switching the heating system off at night.
Since a building with underfloor heating must have external structures with good thermal and heat storage properties, we can assume that the phase shift of the temperature fluctuations in the building envelope has approximately the same value as the thermal inertia of the heated floor.
At low outdoor temperatures the relatively large thermal inertia of the heated floor has a favourable effect on the scope for weather-compensated control of the underfloor heating. Even so, the large thermal inertia of the heated floor can have an unfavourable effect in the transitional seasons, spring and autumn , when relatively rapid changes in outdoor temperature combined with solar radiation can lead to overheating of the interior.
This can, however, be limited by suitable shading of the glazed elements or by laying several heating circuits in parallel alongside one another. With this laying pattern one or more heating circuits can be taken out of operation when necessary without the floor construction cooling down completely. The floor is kept warm by the remaining heating loops and responds more flexibly to changes in the required heat input.
INFLUENCE OF THE THERMAL RESISTANCE OF THE FLOOR
Thermal resistance, whether that of the floor covering on the surface alone or that of the entire construction of the underfloor heating surface, has a major influence on the operating temperature of the heating medium inside the heating pipes. A room whose floor construction has a higher thermal resistance also needs a higher operating temperature of the medium in the pipes. That in turn has a negative effect on the self-regulating effect and so on heat consumption. Increasing the thermal resistance of the floor covering from 0,02 to 0,2 m.K.W-1 (e.g. with a thick carpet) raises heat consumption by 10 to 15 %.
INFLUENCE OF THE CONTROL STRATEGY
So far we have analysed the influence of the individual thermal properties of the building structures and of the building as a whole. But how great is the influence of the control strategy on energy consumption?
In practically all cases, the combination of weather compensation with a room thermostat and a solar radiation sensor is the best control strategy in terms of heat consumption.
If we assume that today's building structures are sufficiently massive, we can expect a saving of 5 to 12 % of heat energy. This energy saving is achieved with an optimally set heating curve. The worse the setting, the greater the corrective action of the room thermostat. The thermostat can, however, reduce the extra consumption caused by a heating curve set too high only to a certain extent (by two thirds at most).
In exceptional cases the room thermostat can even increase energy consumption. Reducing the flow of heating water at the inlet to the system when the room temperature is higher, with a simultaneously lower water temperature, then leads to "cooling" of the room and so reduces the distribution of internal and external heat gains throughout the building.
Controlling the heating water temperature according to the indoor temperature is used only where the underfloor heating is intended merely to provide background heating of the interior. By combining control based on both outdoor and indoor temperature, it is possible to allow for uneven heat loads in the individual heated rooms or to achieve different temperatures in different rooms. It is usually enough to use a thermostatic radiator valve with a remote indoor temperature sensor located in the heated room. The thermostatic valve is fitted on the flow or return of each heating circuit.
REQUIREMENTS FOR THE OPERATION OF WET UNDERFLOOR HEATING
The operation of low-temperature, large-surface wet underfloor heating must meet the following requirements :
a) allow the flow water temperature to the heating circuits to be controlled within the range 20 to 60 °C,
b) prevent the temperature of the flow heating water from rising spontaneously above 60 °C,
c) rule out low-temperature corrosion of the heat source,
d) ensure permanent venting of the heating circuits
e) provide thermal comfort in the heated interiors.
GENERAL REQUIREMENTS FOR THE USE OF UNDERFLOOR HEATING
The method of heating must be chosen in the initial phase of designing the heating system. The following criteria need to be taken into account:
a) Underfloor heating is particularly suitable in combination with low-temperature heat sources (solar collectors, heat pumps etc.), because the self-regulating effect is very favourable here.
b) For ordinary residential buildings underfloor heating is suitable thanks to its favourable properties in terms of room comfort. In buildings with strong solar gains the following measures must be taken:
- shading or active solar blinds,
- use of a solar radiation sensor incorporated into the control loop, - a low inlet temperature of the heating medium (low thermal resistance of the floor).
c) For the design and installation of wet underfloor heating it is recommended to follow CEN TC 130. This standard contains detailed instructions for installing this type of heating, but does not deal with its control.
d) In buildings with very variable use (variable internal load or rapid changes in the set room temperature), underfloor heating on its own is not recommended. Here underfloor heating can be used as the main heating together with faster-responding conventional heat emitters.
REQUIREMENTS FOR DESIGN AND INSTALLATION
a) The thermal resistance of the upper part of the floor should always be low wherever possible. With a higher thermal resistance the inlet temperature of the heating medium has to be raised accordingly, which in turn dampens the self-regulating effect.
b) Where this is permissible for the purposes of separate billing of heating costs, rooms with very different use and orientation should, if possible, be divided into separate circuits with their own flow temperature of the heat transfer medium. Where this is not possible, additional thermostatic radiator valves should be installed.
REQUIREMENTS RELATING TO CONTROL
a) On housing estates it should be possible to set the heating curve separately for individual heating circuits, for houses facing different points of the compass and for houses with differing levels of use.
b) Weather-compensated control based on outdoor temperature should always be used as the basic form of control.
c) Thermostatic radiator valves are suitable for north-facing rooms, rooms with little sun or rooms with internal heat sources that vary little, where individual setting of the required room temperature is needed. The decision to fit them must be taken at the design stage, because installing them later on an underfloor heating system means an enormous increase in costs.
Weather-compensated control based on outdoor temperature should always be used. For a small family house in which the use of the individual rooms is known and fairly constant, an additional solar radiation sensor should be used. If individual room-by-room control is specified, thermostatic radiator valves should be installed. For terraced houses for several families, with differing use, internal heat sources and orientation, a combination of a solar radiation sensor and a thermostatic valve is recommended. In buildings with small external heat gains the use of thermostatic valves is justified.
REQUIREMENTS FOR COMMISSIONING
Because the heating curve has a great influence on room comfort and heat consumption, it should be set with care, while observing the following conditions:
a) The room with the smallest heat gains is usually chosen as the reference room. Care must be taken, however, that room comfort in this room lies at the lower limit of the permissible tolerance band, so that all the other rooms have a set temperature that is higher or at least the same.
b) During the actual setting, typical conditions should prevail in the room wherever possible (internal sources, ventilation etc.).
OPERATING REQUIREMENTS
In heating systems without rapid morning warm-up, the heating curve must be raised accordingly when the heating is switched off at night. This, however, suppresses the self-regulating effect. In well-insulated buildings (with little air exchange through leaks), switching off at night can actually result in additional energy consumption of up to 23 %. In less well insulated buildings (or those with greater heat loss through air leakage) this drawback becomes even greater, together with a further drop in room temperature.
In well thermally insulated buildings, underfloor heating usually operates without problems. The favourable results deteriorate somewhat once the electricity consumed by the pumps is taken into account, along with a certain loss of heat downstream of the manifold. The electricity consumption of the pump and the heat losses in distribution depend on the type of boiler used and on the type of building. While distribution losses in new buildings are about 3 to 4%, in older buildings this share can rise considerably and should therefore always be estimated in advance.
Switching the heating off at night in well-insulated buildings does not, in most cases, lead to a saving; quite the opposite. Warming of the room after a night-time switch-off improves only with rapid warm-up. The heating curve then shifts downwards and so strengthens the self-regulating effect. Built-in thermostatic valves work in the same direction.
In older houses with poorer thermal insulation and greater infiltration, switching the heating off at night can lower the room temperature and so save heating energy. But here too, rapid warm-up should be used, if possible in combination with a small (upward) shift of the heating curve.
If the heating regime changes after the heating curve has been set (e.g. from night-time background heating to normal daytime operation), the heating curve must be set again.
OPTIONS FOR SAVING ENERGY
Starting from a comparison of energy consumption, we set out some of the ways of saving energy.
COMPARISON OF ENERGY CONSUMPTION
The fact that underfloor heating saves energy is documented by research from the Stuttgart Institute for Building Physics. Over 10 years, in three high-rise buildings of the same type with different heating systems, the institute measured on average 28 % higher energy consumption in flats heated by convective emitters than in flats with underfloor heating.
WAYS OF SAVING ENERGY
With underfloor heating the following ways of saving energy are considered:
a) Lowering the room air temperature
Unlike with convective heating methods, the air temperature in a room heated by an underfloor system can be lowered by 2 to 3 °C without reducing thermal comfort in the interior. Every 1 °C by which the air temperature is lowered corresponds to a fuel saving of 6 %.With underfloor heating this means saving 12 to 18 % of energy. In high rooms (sports halls, garages) this figure rises to as much as 25 %.
In terms of control, the savings for underfloor heating also include managing the heating water temperature so that the required temperature of the heated rooms is achieved without overheating and only while they are in use. This presupposes automatic control of the operation of the underfloor heating surface.
b) Improving the thermal properties of the building structure
This way of saving heating energy can of course also be applied to buildings with other heating systems, but with underfloor heating the building structure must be adequately insulated. The reason is the limited heat output of the floor resulting from the maximum permissible floor surface temperature. The one-off investment in the building structure is repaid many times over through lower heating costs.
c) Using low-grade heat sources
In principle any heating system can be connected to sources that use low-grade energy. In our country, hot-water heating systems with a flow water temperature of 90 °C are still the ones mostly being connected. However, because low-grade heat sources are more efficient at lower working fluid temperatures, preference must be given to low-temperature heating systems, in which heat pumps connected to an underfloor heating system cover up to 80 % of the annual heat consumption for heating, i.e. down to an outdoor temperature of ±O °C. Only 20 % of the energy has to be covered by traditional sources and fuels. As a result, the use of low-temperature underfloor heating systems reduces the demand for traditional fuels. This not only achieves better management of fuels but also improves the heat balance in the biosphere.
References:
Dušan Petráš: Podlahové teplovodné vykurovanie (Wet Underfloor Heating)
