Accumulation heating in the foundation slab - 10 or 20 W wire?

Electric heating of the foundation slab is currently gaining a lot of popularity, mainly due to the unstable electricity prices in recent times. A properly installed foundation slab heating system, equipped with a suitable floor thermostat, can be run during the hours when electricity is cheapest. The stored energy is then sufficient to heat the house around the clock.

Unfortunately, the heating installation is not always done correctly. In our practice, we have noticed many mistakes made by professionals and installation companies that can reduce the efficiency and durability of the system.

The most common mistakes of electric storage heating

We encounter many erroneous installations of electric baseboard heating. They mainly result in low efficiency. Sometimes they also fail to provide adequate comfort. However, it is worth remembering that a poorly designed layout of heating cables can also negatively affect their durability. What mistakes are made most often?

• It happens that the heating cable is installed too deep. The large layer of concrete above it does not act as an effective heat accumulator. Instead, the floor is cold and underheated.
• Low nominal power of heating cables, which can not cope with heating the entire floor slab. This results in improper distribution of heat on the floor - there are alternating warmer and cold zones.
• Too little power of the installation compared to the heat needs of the room. It naturally results in underheating of the house.
• Installing more wires too close together. While this seems like a good solution due to better temperature distribution, it could result in damage to the installation. It would not make much sense to repair it, because without fixing the problem, another failure would only be a matter of time.
• Excessive gaps between heating cables will, in turn, result in cooler zones on the floor, which will significantly reduce the comfort of residents.

To avoid making such and similar mistakes, it is worth hiring an experienced and proven installation team. It is also worth reading the following simulation to understand the essence of underfloor storage heating.

Simulation of accumulation heating in the foundation slab

The general principle of an accumulation heating system is not complicated. Taking advantage of the large mass of concrete and its ability to store heat, it is possible to run the heating cable at the time when the price of electricity is most profitable. However, the way the cable is placed has a huge impact on the temperature distribution. To further illustrate this, we decided to simulate the operation of three sample installations. This makes it easy to see how errors affect the malfunction of the entire heating system.

Some of the conditions are common to all three cases. On the other hand, some parameters will vary. In this way, the impact of variables will be easier to assess. As general assumptions can be mentioned:

• The thickness of the concrete slab, which will be 25 cm each time.
• Thermal insulation made around the perimeter of the slab with a thickness of 1 cm to ensure adequate insulation of the slab.
• Air temperature at 20°C.
• Foundation slab 18°C.
• No wind during the measurements taken.
• Lack of floor finish. This has a big impact on the temperature distribution. The simulation results are easiest to show directly on the foundation slab.

Using the same assumptions in all cases, we wanted to reflect the same boundary conditions (installation performed in the same house, but in different ways). This will allow us to compare the described installation methods and evaluate their performance.

1. simulation of storage heating, 10 W/m every 16 cm

In the first case, T2Blue-10 heating cable was used, laid in the slab at 16 cm intervals. This yielded a total installation power of 62.5 W/m². The cable is located at a depth of 15 mm. Below is a graphic with the surface temperature distribution shown.

At a glance, you can see that the temperatures obtained after booting the system are not high. The maximum reading did not exceed 26°C. In contrast, a temperature of 29°C is required for living spaces. This means that there would be an uncomfortable temperature in the apartment above such a foundation slab, and an additional heat source would be needed.

However, it is worth noting that the bottom of the slab still has a temperature of 18°C, so it has not changed during the operation of the heating duct. All the heat generated is directed upward and heats the rooms in the house.

2. simulation of storage heating, 10 W/m every 8 cm

In the second case, we again used a 10 W/m T2Blue-10 heating cable, but the spacing was reduced to 8 cm. The depth of the cable installation remained unchanged - it is still 15 cm below the top layer of the slab. However, the total power of the entire installation has doubled and now reaches 125 W/m². What effect have we managed to achieve?

You will notice a very even temperature distribution - the difference between the maximum and minimum values on the surface does not exceed 1°C. At the same time, the amount of heat achieved is enough to heat the living quarters. However, the increase in surface temperature did not affect the underside of the foundation slab.

This is a perfectly executed installation, ensuring optimal temperature distribution on the surface. This means the highest comfort. At the same time, the amount of heat gained allows for ideal conditions in the house. On the other hand, the cold bottom of the slab means no heat loss.

3. simulation of storage heating, 20 W/m every 16 cm.

You may be wondering, instead of laying more heating cables, isn't it better to simply increase their power? We decided to check it out! Again, we went back to 16 cm spacing between successive cables, but (in order to make the total power higher) we chose a T2Blue-20 heating cable with 20 W/m. This gave us 125 W/m², exactly the same as in simulation number two. However, the higher wattage of the wires means that they have to be installed deeper - at a depth of 30 mm below the top layer of the slab. How did the temperature distribution look like?

The overall heating output is similar to the previous example. However, the temperature distribution shows greater differences (from 2.6 to 2.8°C), which may cause some people to feel a slight discomfort, especially those walking without shoes and socks.

The maximum temperature of 31°C is completely sufficient to heat a living room. The result is certainly better than in simulation one. However, it is worth noting that many problems have not been solved, which will reduce the user experience.

Our conclusions

The simulations carried out confirmed our experience in performing electric storage heating. They concerned the distribution of heating wires and their power.

It turns out that the optimal distance between successive wires is 8 - 12 cm. This ensures an even distribution of temperature over the entire floor area. This means great comfort for residents.

The temperature of the foundation slab should reach 29°C to provide enough heat for a residential building. In our second simulation, we were able to achieve these conditions using a 10 W/m cable. In contrast, using a larger spacing and more powerful heating cables did not produce the desired result. Although the amount of heat was also sufficient (similar power of the entire installation), there were warmer and cooler areas on the floor.

However, it is worth remembering that the simulation was for a room with certain boundary conditions: among other things, it was an assumption that there would be a living room, hallway or room above the slab. If there were a bathroom, the electric storage heating would have to be more powerful and produce temperatures above 34°C to ensure comfortable conditions. Therefore, each project should be approached individually, preferably with the help of specialists. Feel free to contact us.

• Also read: 8cm to 16cm heating cable spacing.