KAN-therm: Underfloor Heating System: How to avoid mistakes? (2)

Underfloor Heating System: How to avoid mistakes? (2)

Where to pay attention and what construction mistakes are most often made - Part 2

In part 1, we have presented you with the material concerning the most frequently made mistakes in the selection, installation and initial start-up of KAN-therm Underfloor Heating System. We discussed issues related to damp insulation, preparation of the base for installation, expansion joints and distribution of heating panels, as well as appropriate power selection of floor heaters.

In this issue, we present further design and construction requirements whose lack or unreliable completion may result in incorrect operation of the underfloor heating system and thus thermal discomfort during the operation of the building. 


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 will allow for optimal layout of the underfloor heating system. 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 cause excessive drying of the siphons and thus 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. In this particular case, it is best to follow the manufacturer's recommendations for underfloor heating systems and to use the lengths of the loops appropriately selected and recommended for a specific pipe diameter.

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. They consist of purchasing regular Styrofoam and placing on it a separate layer of aluminium foil or regular PE foil, and then fastening the pipe with pins. 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. This situation, in extreme cases, may lead to the pipes detaching completely from the insulation during leakage tests and surfacing during screed floor pouring. 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. As it turns out, not every installer knows that the main fastening element of a pin is not the insulation (Styrofoam) itself, but the film layer permanently glued to the thermal insulation. It is there that the pin will be anchored when it is pulled out, thanks to specially profiled cuts.

So it is not worth saving these so-called few cents on the structure which is to be flooded with concrete for many years. Apparent savings due to the use of a slightly cheaper elements of dubious quality can turn into incomparable additional costs necessary to repair the resulting failures.

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. This means that for each heating loop, both long and short, we have to set specific flow rates. 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 thermostatic inserts to install control actuators. Selecting a manifold which will be devoid of these elements, in particular flowmeters or control valves, 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 good, and indeed 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. A very common case that adversely affects the service life of the floor is the reduction of the thickness of the screed above the pipe. This leads to a significant weakening of the floor and a tendency to cracking. 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.  The situation may also look the other way around, which means that it is unnecessary and not recommended to increase the thickness of the screed. This increases the thermal inertia of the underfloor radiators and the weight of the screed, which is particularly important when it loads the ceiling. If it is too high for the structural components, it must be ensured beforehand that the reserve is filled with Styrofoam and remains in the thickness of the 65-70 mm screed. 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. When pouring out the screed floor we should check again the condition of wall tapes and expansion joints separating the heating panels. 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. During this time, the screed must be protected against sudden changes in environmental conditions (draft, sunlight) and heavy objects. Only after this time can you start the process of slowly heating up the screed floor. Both processes require that the building is already equipped with windows and doors to ensure optimal conditions for screed floor treatment. 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. It is even recommended that during periods of severe heat the fresh screed should be covered with foil to prevent excessive drying.

Preheating of the screed floor

The process of preheating of the screed floor consists in removing the moisture remaining in the screed after the treatment period of the screed. In order for this process to proceed evenly, the underfloor heating system must at least be hydraulically preadjusted - a lack of such regulation will lead to uneven operation of the system and thus uneven heating of the floor. The heating should be performed relatively slowly and start with an initial feed water temperature of 25°C. This water temperature should be maintained for at least 3 days and the entire process of screed underfloor heating should last not less than 8 days. If the preheating process is omitted, the panels are deformed due to different humidity levels in the upper and lower screed floor areas. During the operation of the installation, water accumulates in the upper part of the panel, while the lower part, where the pipes heat, is kept dry. This distribution of moisture leads to the panel edges bending downwards. Because the corners are bent the most, the whole surface of the panel, lifting upwards, may lose the support surface, which should be the horizontal insulation.

Getting to know that, you are safe to go.

Head of KAN Technical Guidance Department

MSc Eng Mariusz Choroszucha