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This article is about the difference between underfloor heating and floor warming in the series “Hydronic Balancing DIY“. As I suspected underfloor heating in the kitchen of our example building and couldn’t find any documentation, I asked the manufacturer Oventrop to be on the safe side. They explained to me the function of the “RTL box” (more on this later).
It turned out that it is not underfloor heating, but floor warming with a return temperature limiter. This article therefore explains the difference between underfloor heating and floor warming.
Table of Contents
Difference between underfloor heating and floor warming
The difference between underfloor heating and floor warming lies in the fact that underfloor heating must be able to heat a room on its own. Floor Warming on the other hand, is usually only installed in the floor as an addition to the actual room radiator. With floor warming, the room temperature is still controlled via the radiator. Both systems are surface heating systems.
Note on differentiation:
Underfloor heating and floor warming systems are surface heating systems. However, underfloor heating has its own heating circuit, is controllable and must cover the heat load of the area to be heated alone in accordance with DIN EN 1264.
Floor warming is not individually controllable, but is usually an additional heating system to the radiator and in no case covers the heating load.
Many thanks to Alexandra Bartsch from the Federal Association for Surface Heating and Cooling Germany (Bundesverband Flächenheizungen und Flächenkühlungen e. V.) for the information.
Below you will find a more detailed explanation of the terms surface heating, underfloor heating and floor warming.
Surface heating
Surface heating systems are heating systems that are integrated into the surrounding surfaces of rooms, such as walls, floors and ceilings, and are therefore invisible to the occupants of a house – in contrast to radiators. Surface heating systems mainly emit their heat to a room by radiation, which results in a pleasant temperature distribution, a comfortable room climate and hardly any air movement due to convection. As surface heating systems require low flow temperatures, heat pumps can be operated particularly energy-efficiently with surface heating systems. The disadvantages of surface heating systems are their inertia, slow controllability and higher purchase and repair costs compared to radiators.
Underfloor heating
You can usually recognise underfloor heating systems by the fact that they have an underfloor heating manifold and their own control system. Figure 1 shows an example of two heating circuit manifolds for underfloor heating systems. The heating water flows into this manifold and is distributed from there to the various rooms via heating pipes in the floor. Underfloor heating systems are operated with maximum flow temperatures of 35 °C and return temperatures of up to 28 °C. The spread is typically 5 Kelvin. An important characteristic is that underfloor heating must be able to heat a room on its own.
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Floor Warming
Floor warming is often found in kitchens or bathrooms and is a supplementary heating system to the radiator. Floor warming is usually branched off at the flow and return from a nearby room radiator and laid in the floor (see Figure 2).
The room temperature is still controlled via the radiator. A return temperature limiter (RTL Box with RTL thermostat) can also be used to control the floor temperature. The RTL thermostat does not measure the room air temperature like conventional thermostats, but the return temperature at the heating pipe via a copper transfer pin, as shown in Figure 2 on the right.
In our example building, the temperature of the floor warming is controlled via an RTL return temperature limiter from Oventrop.
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The RTL thermostatic head has 4 stages and regulates the return temperature of the floor warming in the respective stages to the specified temperature ranges. According to the Oventrop data sheet, the individual stages have the following temperatures:
Stage 1 = approx. 10 °C
Stage 2 = approx. 20 °C
Level 3 = approx. 30 °C
Level 4 = approx. 40 °C
The RTL thermostatic head ensures that the return temperature of the underfloor heating does not exceed approx. 40 °C at level 4. The graduation marks between the digits each correspond to a return temperature change of approx. 5 Kelvin.
Floor warming can be operated at higher system temperatures and is predominantly located in the same heating circuit as the nearby radiators (see figure 2). It is not able to heat a room on its own.
Conclusion
It was very interesting for me to find out how floor warming works. Asking the manufacturer was therefore absolutely right. As there is no other radiator in the kitchen, I assume that the floor warming in the kitchen is intended as supplementary heating for the nearby radiator in the hallway (radiator no. 7).
The radiator in the hallway could therefore be counted as part of the kitchen. The corridor should be supplied with sufficient heat via the heat losses from the neighbouring rooms to the corridor. However, we will only be able to see whether this assumption is confirmed once the hydronic balancing has been completed.
Below you will find an overview of the “Hydronic Balancing DIY” series:
Overview of the series:
- Hydronic Balancing DIY – Example for a detached house
- Hydronic Balancing DIY – Step 1: Fundamentals
- Hydronic Balancing DIY – Step 2: Heating Load Calculation
- Hydronic Balancing DIY – Step 3: Data Recording
- Hydronic Balancing DIY – Step 4: Calculate Radiator Output
- Hydronic Balancing DIY – Underfloor Heating and Floor heating?
- Hydronic Balancing DIY – Step 5: Calculate volumetric flow rate
- Hydronic Balancing DIY – Step 6: Presetting the radiator valves
- Hydronic Balancing DIY – Step 7: Calculate heating pump
Related articles outside the series:
- What is hydronic balancing?
- How do Thermostatic Radiator Valves work?
- Calculation of old radiators in stock
- What does a hydraulic balance cost?
Important: Before you start with the instructions for hydronic balancing, I would like to point out that the procedures described here are based on my personal experience and my personal train of thought. Trying out and implementing the procedures described is entirely at your own risk and responsibility. I do not accept any responsibility. Furthermore, I recommend that you always have the calculated values checked by a specialised company or engineering firm. Because even if the method described here seems simple, calculation errors can always creep in.
Best regards! Martin
Further links and sources:
Oventrop data sheet: “Unibox” Individual room temperature control and limitation of return temperature in surface heating systems
DeltaQ – Heating surface types (Recknagel Sprenger) – german
Guide for the rough hydraulic balancing of existing underfloor heating systems – german
