Installing wet (hot water) underfloor heating consists of the following steps:
You will find the complete range in the underfloor heating category – system insulation boards, pipe, manifolds, controls and electric heating mats.
- building up the heated floor,
- the installation work itself,
- testing the heated floor.
BUILDING UP THE HEATED FLOOR
A heated floor gives the designer several options for how it is constructed. The individual alternatives can be divided into the following basic types:
By the method of construction we distinguish:
- a heated floor built using the dry method (dry underfloor heating installation)
- a heated floor built using the wet method (wet underfloor heating installation)
- a heated floor built from modular climate floor panels.
By the layout of the heating circuit we distinguish:
- a serpentine (meander) layout of the heating coil,
- a parallel layout of the heating coil.
The choice of a suitable option in terms of floor construction and circuit layout depends on several factors. First and foremost, however, you must allow for the effect of cold surfaces by creating perimeter zones, or minimise uneven floor surface temperature by laying the pipe in a bifilar pattern.
METHODS OF CONSTRUCTING THE HEATED FLOOR
By the technology used to build the heated floor, there are three basic methods of underfloor heating installation: dry, wet and modular climate floors.
a) Dry method
The pipes are laid in an insulation layer placed beneath the concrete slab. They are separated from the cement screed by a special layer, either a plastic or a metal foil. The metal plate increases the strength of the floor and spreads the heat evenly.
With the dry method the floor works with higher heating water temperatures; the flow temperature can range from tpr = 40 to 70 °C.
This method is used where lower heat flux densities are needed, up to 50 W.m-2, for example for background floor warming or supplementary heating, and wherever a lower build-up height of the heated floor is required (for example in renovations).
It is a light and quick method of underfloor heating installation.
b) Wet method
The pipes are cast directly into the cement bed above the thermal and acoustic insulation. The pipes are fully surrounded by concrete. The expected flow temperature of the heating water is tpr 35 to 55 °C, and the floor works with a specific output above 50 W.m-2.
The wet method can also be used in our conditions, because it does not require the individual elements of the heated floor to be prefabricated and the components are essentially available from the domestic product range.
c) Modular climate floors
Climate floors are hollow profiles manufactured specifically for this purpose. They are laid on the thermal insulation as a continuous surface, formed by hydraulically connecting the individual heating cushions. Their advantages are a more responsive heating system, a low build-up height and an even distribution of the surface temperature of the heated floor.
A heated floor built in this way works with low heating water temperatures (tpr = 25 to 35 °C). No such element is manufactured commercially in our country yet, but it exists as the subject of invention AO 258 622.
LAYOUTS OF THE HEATING COIL
By layout we distinguish two basic ways of forming the heating circuit: serpentine and parallel.
a) Serpentine laying of the heating circuit. With this arrangement the floor surface temperature is even across the whole area. The drawback is a drop in room temperature in the horizontal direction from the internal to the external structure. This effect can be partly eliminated by creating a perimeter zone.
Pipes with a larger diameter (e.g. 18 x 2 mm, 20 x 2 mm) are specified for the serpentine layout, as this layout allows smaller bend radii, formed at an angle of lp= 90 °C.
b) Parallel layout of the heating circuit is shown in Fig. 5.6. With this method of laying, the heating water temperature falls from the external wall towards the internal one, which gives a more even temperature distribution in the heated room. The bends are formed at an angle of lp= 180 °C, which requires pipes with smaller diameters (e.g. 16 x 2 mm, 17 x 2 mm).
Both methods allow laying with a perimeter zone, which eliminates the negative effect of cold building structures; such zones are most often placed under windows or in the corners of the building, to a depth of 0,5 to 1,0 m.
By laying the flow and return pipes side by side in a bifilar pattern, you can achieve the same average heating water temperature at every point of the heating circuit and therefore a more even surface temperature of the heated floor. This effect is more pronounced with the serpentine layout.
TECHNOLOGY AND UNDERFLOOR HEATING INSTALLATION
Next we will go through the procedure for building a heated floor using the wet method. The proposed approach reflects both our own and international experience.
We point out at the very start that, according to STN 06 0312, central heating with pipes cast in concrete may only be installed by a specialist installation company, i.e. a central heating contractor.
The procedure for the wet-method installation of underfloor heating is as follows:
1. preparing the building,
2. preparing the floor,
3. laying the perimeter insulation strips,
4. laying the thermal and acoustic insulation,
5. laying the damp-proof membrane,
6. fixing the pipes,
7. laying the heating pipes,
8. forming the expansion joints,
9. laying the concrete screed,
10. laying the floor covering.
The first prerequisite is that the flow and return manifolds have already been made and installed in their positions according to the design.
CONNECTING THE PIPES
How the ends of the heating pipes are connected during underfloor heating installation depends mainly on the type of pipe material used:
1. If the pipes are made of a softer polyolefin, brass couplings are suitable. Together with the shut-off valves, these couplings give a reliable connection only where it is certain that the joint will not be subjected to any mechanical stress during operation. These couplings cannot be dismantled.
2. One method that scores well for availability and reliability is connecting the pipes to the valves using d/G adaptors. There are two types:
- an adaptor without a metal insert,
- an adaptor with a brass insert.
Both types require radiator union nuts of the appropriate size as an intermediate piece, so that the pipe ends can be quickly connected to and disconnected from the valves. This also ensures that the joint can be dismantled. They are made with male and female threads.
The joint between the adaptor and the free end of the pipe is made by hot polyfusion welding using a special fitting. This operation requires sufficient experience so that the joint is reliably tight and, at the same time, an unwanted narrowing of the pipe mouth at the weld does not restrict circulation.
If the pipes are very stiff, bending them to a small radius where they rise to the manifold can be a problem. In that case we recommend using polyolefin 90° elbows of the required size. The exit of the pipes from the concrete floor must be handled professionally, using flexible electrical conduit.
FILLING THE SYSTEM
Once all circuits have been connected to the flow and return manifolds, the pipes are filled with treated water containing an inhibitor. The most practical way is to fill the heating circuits by circulation. Using a pump (e.g. a submersible one), water is pushed from a tank (a polyethylene container) of treated water placed close to the return manifold through the filling valve of the return manifold into the system. A hose is connected to the filling valve of the flow manifold, with its other end led back into the tank. This creates a closed circulation loop. The pump is switched on and the system fills without any need for venting.
Make sure there is always enough water in the tank while doing this. If uncoloured pipes are used, the progress of filling can be checked visually. You can also fill the circuits one at a time by closing and opening the relevant valves.
PRESSURE TEST
We fill the pipe system with water and carry out a pressure test, which consists of two parts:
a) a pressure test of the conventional boiler section and part of the pipework,
b) a pressure test of the pipework in the floor.
The pressure test verifies that the system is tight at the overpressure specified by the pipe manufacturer. It is one of the most important operations in underfloor heating installation. The pressure test of the conventional boiler section and part of the pipework is familiar from convective heating, so it is not covered in this chapter.
The pressure test of the pipework in the floor is carried out immediately after the system has been filled with heating water. Before pressurising, therefore, check that the shut-off valves on the main supplies to the flow and return manifolds are properly closed. We also recommend disconnecting these supplies, because if there is a leak the pressure can escape through the main shut-off valve into the distribution system and the boiler circuit. Such a leak would be very hard to identify.
A special pressure test pump is connected via the filling valve of the flow manifold and used to raise the water pressure to five times the operating pressure. Once the specified overpressure has been reached, the filling valve is closed. The tightness of the joints on all valves, the joints on the pipework and the entire pipe system on the floor are checked.
The overpressure in the pipework must hold for at least 24 hours. During this time the pressure may fall only to 2/3 of the original test pressure. This drop can partly be attributed to the elasticity of plastic pipes (the so-called cold flow of plastics). If any leak is found during the pressure test, the fault must be rectified and the test repeated.
While the pressure test is in progress, the whole pipe system laid on the floor is photographed. Take care to capture every detail. This photographic record is then archived by the system supplier. It can be used later when repairing faults caused by human error (cutting or drilling through a pipe in the floor).
Concreting is carried out with the system under pressure. The overpressure must be left in the pipes for 7 days after the concreting work has finished. The pressure may fall naturally during this time without being topped up. The result of the pressure test is recorded in the installation log.
HEATING TEST
The first heating test may only be carried out 28 days after concreting, i.e. in one of the last phases of underfloor heating installation. During the first firing the heating water temperature must not be raised sharply. We recommend warming the concrete floor at a rate of 5 °C per day (this refers to the floor temperature). This roughly corresponds to a rise in heating water temperature of 0,5 °C per hour. A record of the heating test is made in the site log.
The heating test is carried out by gradually raising the temperature by 3°C per day until the planned temperature of the heating medium is reached. In doing so, make sure that:
- all threaded joints and seals are checked after the heating test,
- the floor covering is not laid sooner than 14 days after the heating test,
- floor coverings are bonded to a cold floor with the appropriate adhesive and the temperature is only raised from the third day after bonding; the temperature should be increased gradually over three days until the operating temperature of the heating medium is reached.
BALANCING THE SYSTEM
After a successful heating test, the weather-compensated controller is set to the values that match the design calculations (the relationship between the mean heating water temperature and the outdoor air temperature). The settings of the protection system against exceeding the maximum permitted heating water temperature are checked, and all the regulating elements on the manifolds are set according to the results of the hydraulic calculation. The heating is then switched on. Once operating temperatures have been reached, temperature setbacks or increases can be set on the electric controller as needed or as the client requires. The individual circuits can be fine-tuned separately.
When programming the control by time switch, allow for a temperature phase lag of roughly three hours (the thermal inertia of the floor).
After the underfloor heating pipes have been cast into the floor structure, the contractor carries out a functional test. This lasts no more than 15 days, and if no fault occurs in the heating system during it, the floor covering can be laid on the cooled surface of the heated floor, which successfully completes the underfloor heating installation.
References:
Dušan Petráš: Podlahové teplovodné vykurovanie
