Missing or incorrectly made expansion joint between the heating panel and the building structure
The next step, right after the preparation of the load-bearing base, is to distribute the wall tape and thermal insulation over the entire surface intended for underfloor heating. The purpose of the wall tape is to separate the target heating panel from any vertical building partitions, both external and internal ones. This will allow the heating panel to operate freely and safely in the future by compensating for thermal expansion. The wall tape must be strictly laid on the entire perimeter of the heating panel and must not be stretched at corners as it may cause cracks during screed floor pouring. The lack of wall expansion joint is a potential risk of the heating panel cracking at the point of contact with the vertical partition. A further consequence will be damage to the lining, e.g. ceramic tiles.
Dividing and expansion joints of heating panels
Expansion joints of panels in doors, between two separate rooms are also often forgotten. The result is the creation of a large, non-dilated heating panel. Another problematic issue is the appropriate expansion joint height. The correctly made expansion joint starts with the thermal insulation of the heating panel and reaches above the thickness of the screed including the lining. In short, the expansion joint must fully separate the two adjacent panels together with the floor lining.
Separated heating panels will be subjected to the phenomenon of thermal expansion, which will occur individually for each heating panel. The passage of the pipes between these panels, made rigidly, can lead to the "cutting" the transit pipes due to the individual thermal movements of two different panels. This phenomenon can be eliminated by using special expansion joints for transit pipes. This consists in guiding the pipes, at the place where the joints of the panels are present, in special protective covers of about 50 cm in length.
Choice of power of floor radiators
Increasingly more often underfloor heating is the only basic heating system in a building and must cover 100% of its heat demand. A very common mistake or even a plague in the approach to underfloor heating systems is the lack of good design and optimal pipe spacing. Underestimating the underfloor heating performance against the building's actual heat demand will result in permanent room underheating and thermal discomfort. In such situations, the contractors use an increase in the underfloor heating supply temperature, which results in overheating of the floor. This situation has a very negative impact on the usefulness of the installation and the wellness of the users. It also happens that several construction mistakes are combined in one installation and their elimination by increasing the supply temperature of the installation or correction of flow on the manifold is impossible. Always remember that a poorly constructed floor heater cannot be corrected without having to replace the floor.
Planning the system and length of heating loops
In order to properly plan the layout of the underfloor heating loop, it is necessary to consult the investor in scope of the planned furniture layout or permanent installations in rooms. This avoids the possibility of covering heating loops and limiting the output of the heating system. In case of underfloor heating in rooms such as bathrooms, heating loops under the showers or bathtubs should be avoided. Such a floor installation may expose the room to unpleasant sewer smell. However, the most serious mistake in the installation of underfloor heating systems is making too long heating loops. Too long circuits cause very high pressure losses, which can be hard to compensate by a circulating pump.
Fixing pipes to thermal insulation
Very often, in order to save on the components of the underfloor heating system, unprofessional, self-made "fastening" systems of the pipe for thermal insulation are used. The frequent consequence of this approach is that the pipe is not properly and securely fixed to the thermal insulation and is "rising" during the screeding process. The result is an uneven thickness of the screed above the heating pipe, which can lead to excessive local cracks in the heating panel and lining of for example ceramic tiles.
Selection of manifolds
Another quite frequent mistake, when selecting and completing the underfloor heating system, is the wrong choice of manifolds. For proper and even operation of the underfloor heating system, the so-called hydraulic regulation of all heating loops is required. We can do this with special flowmeters or control valves, which are equipped with manifolds for underfloor heating. Another issue is the possibility to control underfloor heating on the basis of room temperature measurement. It is also possible thanks to manifolds, which beside the flowmeters or control valves, have special inserts to install control actuators. Selecting a manifold which will be devoid of these elements will deprive us of the possibility to perform these key operations, which in consequence will prevent proper operation of the entire underfloor heating system.
Pouring out the screed floor
A very important issue during concrete pouring is increased caution when using tools that can damage the folded out pipe in any way. It is therefore absolutely recommended that the pipes should be filled with pressurised water when pouring out the screed floor. This will allow for quick identification of the damage location of the pipe, still at the stage of concrete pouring. In order to obtain a durable concrete screed, it is essential to provide at least its minimum thickness and keep it at this level when laying it. In order to reduce batched water and achieve a suitable consistency of concrete, it is recommended to add so-called plasticizers. The lack of a plasticizer in the concrete is an obvious construction mistake and causes a significant increase in porosity of the screed floor and consequently leads to a significant decrease in the thermal conductivity of the floor and its mechanical properties. Do not cut off the wall tapes and expansion joints right after pouring out the screed floor. They should protrude above the floor until the lining is laid, thus providing a guideline for the finishing crews where the expansion joint should be made of for example ceramic tiles.
Treatment of screed
The bond time of the screed floor is from 21 to 28 days. Only after this time can you start the process of slowly heating up the screed floor. It is essential to avoid starting the underfloor heating system immediately after the screed floor has been poured for example in order to protect the filled system from freezing. Heating the fresh screed will cause excessive premature evaporation of the water necessary for the concrete binding process. As a result, the flooring strength will be much lower.
MSc Eng Mariusz Choroszucha, Head of KAN-therm Technical Guidance Department
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