Why a heated floor can be cold?


The most frequent question is: “Why has the heated floor become cold or not warm enough?” This question is usually asked at the beginning of the heating season.

Many users try to replace the thermostat, to connect the heating cable or mat directly, and call a service technician only when this has not yielded the desired results. Let us see what the causes for a lack of heating may be.

1. Thermostat failure

Any electrician can easily identify this problem. In this case, the thermostat cannot supply power to the heating cable or thin mat under any circumstances. This may be caused by failure of the thermostat itself, or, for example, by floor sensor failure, or simply by a tripped circuit breaker in the power panel.

Users can most likely identify and correct this problem themselves, without calling a technician.

For instance, if the thermostat displays nothing and its LEDs do not light up, check whether there is power to the thermostat. Moreover, floor sensor failure can be recognised by the following:

- Green LED flashing (for DEVIreg™ 530);

- Message Err5 Err6 is displayed (for DEVIreg™ 535); 

- Messages “sensor fault” or “sensor short circuit” are displayed (for DEVIreg™ Touch).

What if you have checked everything and the thermostat shows 220V output, but there is still no heating? Let's investigate further.

2. Heating cable fault

This failure can be detected using a resistance-measuring instrument (Avometer, Multimeter or simply a tester).

What to do then: Disconnect the heating mat or cable from the thermostat terminals and perform two measurements:

- Measurement 1: between the conductors; these are normally black (or brown) and blue wires. The resistance depends on the cable length (mat area), and should correspond to the value shown in the catalogue (with +5 to -10% tolerance).

Please bear in mind the effective range of the instrument you are using. Most instruments have the first effective range limit up to 200Ω, then up to 2kΩ, then up to 20kΩ, and so on up to 2-20MΩ. Measurements are normally performed at the limit of 200Ω, while heating cables/mats with short length or small area have resistance over 200Ω.

For example, this applies to DEVIflex™ 18T cables with a length of 7, 10, and 13m, or DEVIcomfort™ 150T heating mats with an area of 0.5, 1 or 1.5m². In this case, switch the instrument to the measurement effective range limit up to 2kΩ.

- Measurement 2: between the cable shield (yellow/green conductor) and the brown and blue conductors joined together. In this case, the measurement effective range limit should be maximum. The instrument should display a value over 20MΩ. In most cases, it is impossible to measure such high resistance, and the instrument will show infinity (failure). This is normal.

Have you taken the measurements? Is everything OK? You can safely connect and install the thermostat, since your heating system is in good condition.

What then? The thermostat supplies 220V to the heating cable, the cable has normal resistance, the current is present and almost all electric power is converted into heat.

However, there is still no floor heating. Then we will try to sort it out further on, after studying some theory first.

3. Brief course in theory

Most people feel warmth of a ceramic tile surface at a temperature of about 26-27°С, while a temperature of 24-25°С is perceived as ‘comfortable’ (neither warm nor cold).

Surfaces at lower temperatures are perceived as ‘cold’.

The heating system power of 130W/m2 enables it to heat the floor surface to a temperature roughly 14°С higher than the ambient temperature. This does not include the heat loss downwards. Taking into account this loss, the difference between the air and floor temperatures will be 10-11°С.

Operation of a floor heating system requires a normally operating heating system in a room. In other words, the air temperature in a room should be maintained at least at 17°С. Only in this case will the floor surface be heated to a temperature that is perceived as warm. However, if floor heating is the only source of room heating, it should be considered as a heating system which operates somewhat differently and is the subject of a separate article.

The temperature to which floor heating can warm the surface depends on the installed power, air temperature (!) in the room and floor insulation. A thermostat (with floor sensor) can only maintain (limit) the temperature at a preset level. 

For instance, let's look at an apartment in a building where the heating system is not yet operating:

- Outdoor air temperature is about -5 to -7°С; 

- The temperature in the adjacent apartment is about 0°С; 

- The temperature in the room with floor heating is 10°С. 

Under these conditions, the floor is perceived as cold. In fact, the floor is only cold in our perception, while the floor heating is working and heat is emitted, otherwise how could the temperature in the room be +10°С?

Floor heating will gradually warm the air, thus functioning as a heating system, and only when the air temperature has reached 17-20°С will the floor surface warm up to a temperature perceived as warm.

Here is another example: An apartment building is not yet connected to the district heating system. The outdoor air temperature is +5 to +7°С, and the temperature in the room has dropped to +15°С. Let us assume that floor heating has been switched on, but the floor does not warm up even after two hours.

In such a case, the lack of heat on the surface has two reasons: the need for additional heating to warm the air by a couple of degrees, and the inertia of the system. To put it simply, the floor slabs, screeds, tiles, walls and furniture have cooled down. It will take a long time, up to several dozen hours or days, to heat such a great amount of concrete, stone and ceramics. The fact is that heat inside solid objects does not rise up; it tends to spread towards a colder side (unlike in the air, where convection is present).

Moreover, there is no infrared radiation within a solid floor, so it is impossible to speed up the heating process by using reflective films and equivalent materials to thermally insulate the floor. Using such materials for floor insulation is ineffective.

Similar processes occur in a floor on the ground. If the thermal insulation in this floor is insufficient, the downward losses may reach 50% of all the heat emitted by the heating system. In order to reduce heat losses through the ground, thermal insulation must be thick enough.

As for balconies, the situation may be much worse: a balcony slab is normally made from reinforced concrete, which conducts heat very well. If there is outdoor air under such a slab, downward heat losses may reach 80%. In other words, this system is absolutely incapable of providing a comfortable temperature on the floor surface. There is only one solution: to install thermal insulation of sufficient thickness (at least 50mm) plus a heating cable with smaller spacing (8-10cm), or a thin high-power (about 200W/m2) heating mat.

If a system has already been installed and connected, but there are doubts regarding its operability, it is quite easy to check this: cover a small area of heated floor with a piece of foam plastic, cardboard or rug. Wait about 15-30 minutes and, if the heating system is working properly, you will feel warmth in this area.

Another important comment: The power of a heating cable or thin DEVI mat intended for floor heating systems does not change over time, so they cannot provide worse heating after they have been operated for some time.

If a heating system that has worked properly for many years starts heating poorly, first of all, external causes should be sought, such as supply voltage reduction, insufficient temperature of room air, or deteriorated thermal insulation. Or, for example, a balcony below used to be insulated and even heated, and now it has no heating and the window is always open there.

After apartment renovation is completed, nobody wants to redo anything, and sometimes it is impossible. Small savings made during construction or renovation can result in an inoperable heating system in the future. What should be done?

You should observe recommendations on cable system installation and thermal insulation; be careful with very thin thermal insulation with ‘fairy tale’ characteristics; never use water-absorbing materials for floor insulation (especially with ground floors), or materials not designed for floor structures; do not try to save money by laying the cable with large spacing; and always choose a reliable and qualified manufacturer of heating systems.

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