Underfloor Heating as a Main or Supplementary Heat Source

When deciding on underfloor heating, one of the first questions you should ask is: should underfloor heating be the sole heat source in the room, or just a pleasant supplement to a radiator? The answer is not a cosmetic detail that can be decided later - it directly affects the density of the pipework in the floor, the required heat-source output, the heating-water temperatures, and even which floor covering you will ultimately be able to afford. And since pipe embedded in concrete practically cannot be densified or rerouted after the building is handed over, a mistake in this decision only fully shows up years later - typically in the first really frosty winter, when you find that your underfloor heating simply cannot keep up with the room's heat loss.

In practice we commonly encounter both approaches, and both have their place. Someone building a family house "on a green field" plans from the start that the whole building will be heated exclusively by an underfloor system, without a single radiator - in that case underfloor heating is the main and sole heat source and must be designed accordingly. Someone else is only dealing with a bathroom during a flat renovation and mainly wants a warm tiled floor underfoot after a morning shower, while the room's actual heat loss continues to be covered by the original radiator - in that case it is a supplementary solution with completely different demands on the design and the budget. Both approaches work; what matters is knowing which category your particular project falls into, and setting everything else accordingly: heat-source output, pipe density, the type and size of the manifold, and the choice of floor covering itself.

In this article we go step by step through when it makes sense to design underfloor heating as the main heat source and when a supplementary solution is entirely sufficient, how underfloor heating can be combined with radiators in a single heating system, what influence the choice of a wet or dry system has on the whole decision, along with good insulation under the pipe, the type of manifold, and the way the pipe itself is laid in the floor. At the end you will find two real-world examples - one for a main heat source, the other for a supplementary solution - and answers to the most frequently asked questions we receive on this topic.

What underfloor heating is and how it works

Underfloor heating heats a room over a large area through pipes embedded in the floor (in a wet system), or laid in a dry system under the flooring, through which hot water flows from a boiler or heat pump - this is referred to as a hydronic system. The other option is electric underfloor heating, where instead of water, heat is generated directly by electric heating cables or mats laid in the same layer. The principle of transferring heat into the room is the same in both variants: heat spreads evenly across the whole floor area and rises upward from there.

It is precisely this even "bottom-up" spread of heat that is the main difference compared to classic radiator heating. A radiator heats the air at one point, where it is mounted, and the air then circulates around the room by convection - the result is usually warmer air near the ceiling and a cooler floor, or cold corners far from the radiator. Underfloor heating, by contrast, creates a pleasant feeling of warmth right at foot level, evenly across the whole room area, without the cold corners typical of radiator heating. For many people this feeling of warmth "from below" is the main reason they choose underfloor heating - and in some rooms (bathroom, children's room) it has, besides more economical operation, a purely comfort-related significance as well.

Before you get down to choosing specific components - the manifold, the pipe, the insulation - it is therefore necessary to clarify precisely the question from the introduction of this article: will underfloor heating be the sole heat source in that room or in the whole house, or just a supplement to another heating appliance? Literally everything that follows depends on this answer. We also discuss the detailed technical principle of how the whole system works, including water circulation and control, in the separate article How underfloor heating works.

The choice between a main and a supplementary heat source determines all the further steps of designing underfloor heating.

Main heat source vs. supplementary comfort - a fundamental decision at the start of the project

Underfloor heating designed as the main heat source must cover the room's entire heat loss - that is, fully replace the radiator or other heating appliance. This requires a thorough calculation of output and pipe density already at the design stage, because the density of the pipework in the floor cannot be changed afterwards. If the designer or installation company underestimates the output and the loop spacing is too wide, underfloor heating simply will not be able to deliver enough heat on frosty days, and the room will be permanently cold no matter how high you set the heating-water temperature. Conversely, too dense a pipe network needlessly increases the cost of the installation. An accurate calculation of pipe density based on the room's heat loss is therefore an absolutely key step for the "main heat source" variant, one that should not be underestimated or rushed.

By contrast, underfloor heating as a supplementary heat source - typically in a bathroom alongside a radiator, or in one room of a flat where the rest of the space is heated by another system - mainly addresses the comfort of a warm floor underfoot. In this case the room's main heat loss continues to be covered by another source, most often a radiator. The advantage of a supplementary solution is that the demands on the precision of the output calculation are lower - even somewhat more sparsely spaced pipe will still fulfil its purpose, since the actual heating of the room is provided mainly by the radiator. This makes the supplementary solution a more accessible choice for renovations as well, where there is no scope for a complex recalculation of the whole house's heating system.

The difference between the two approaches is therefore not that underfloor heating "works worse" or "better" in one case - in both cases the same technical principle applies. The difference lies in the demands on the design and in what you expect from the system. If you want to forget about radiators in the room entirely, underfloor heating must be designed as the main heat source with the corresponding calculation. If you are primarily after the pleasant feeling of a warm tiled floor, a supplementary solution is simpler, cheaper to install, and in the vast majority of cases entirely sufficient.

The fundamental difference lies in the calculation demands and in what you expect from underfloor heating.

Combining underfloor heating and radiators in one system

Combining underfloor heating with radiators in a single heating system is common and works reliably - a typical example is a house where living rooms (living room, bedrooms, children's rooms) are heated by underfloor heating as the main heat source, while the bathroom, alongside underfloor heating, also has a heated towel rail, or a utility room or hallway has a classic radiator instead of underfloor heating.

This combination, however, requires correct hydraulic balancing of the whole system, because underfloor heating operates at a significantly lower heating-water temperature than radiators. While a radiator circuit needs a higher water temperature to cover the same heat loss, underfloor heating, thanks to its large heating surface, manages with a lower supply temperature. If both circuits are connected to a shared heat source without proper balancing (for example via a mixing valve for the underfloor-heating circuit), either underfloor heating will be needlessly overheated, or the radiators will not receive sufficiently hot water. This is exactly why systems combining underfloor heating and radiators usually have a separate, partly independently controlled circuit for each type of heating appliance, connected to a shared heat source (boiler or heat pump).

The good news is that this hydraulic balancing is a normal part of designing a heating system, and installation companies encounter it in practically every project that combines both types of heating appliances. When choosing a boiler or heat pump for a house with a combined system, it is therefore a good idea to tell the installation company in advance that part of the house will be on underfloor heating and part on radiators, so that correct connection and control of both circuits is already accounted for at the design stage.

Wet or dry system - which to choose

The question of main vs. supplementary heat source is also closely related to the choice between a wet and a dry underfloor heating system. The wet system lays the pipe in a layer of concrete screed - it is embedded directly in concrete, which means the screed itself acts as a large thermal storage mass. Thanks to this, the wet system achieves the best heat storage and the most even heat distribution across the floor area of all variants. The price for this is a greater floor build-up height - typically an extra 7 to 10 centimetres of screed - and a longer drying time before commissioning, measured in weeks. This is exactly why the wet system is most often chosen for new builds, where this build-up height and the drying schedule are already accounted for in the house design.

The dry system, by contrast, lays the pipe into system boards or between mounting rails without a wet screed, with a considerably thinner load-distribution layer - for example made of gypsum fibreboard. This is a choice that pays off especially in renovations where the floor cannot be raised by many centimetres (for example because of existing door frames or continuity with adjacent rooms), or in situations where the system needs to be commissioned quickly, without weeks of waiting for the concrete to dry.

For a project where underfloor heating is meant to be the main heat source, new builds generally favour the wet system - the greater heat storage helps to even out fluctuations, and the system maintains a more stable room temperature with smaller control interventions. For a supplementary solution, typically a bathroom renovation in a flat, where it is more about comfort than covering the entire heat loss, a dry system is practically the only realistic option, since the floor build-up height in a panel building or older flat tends to be significantly limited. A detailed comparison of both variants, including recommendations on when to choose which, can be found in the article Wet vs. dry underfloor heating system.

A wet system suits new builds best, a dry system suits renovations with limited floor height.

Insulation under the pipe - why it is essential

Regardless of whether it is a main or a supplementary heat source, a thermal insulation layer is always installed under underfloor heating pipes. Most often this is a studded system polystyrene board for clipping in the pipe (exactly like the STIROTERMAL DUO 20 board above), or plain polystyrene, to which the pipe is fastened with clips or mounting rails. The task of this layer is clear - to direct the heat the pipe gives off upward into the room, while preventing it from escaping downward, into the neighbour's ceiling below you or directly into the ground at ground-level floors.

The quality and thickness of the insulation has a direct effect on how well underfloor heating will perform in practice. Thicker and better-quality insulation reduces downward heat loss and at the same time improves the response time of the whole system - that is, how quickly the floor reacts to a change in the set temperature. It is precisely in the variant where underfloor heating is the main heat source that it is worth investing in better and thicker insulation, since here the aim is to maximise output into the room without unnecessary losses. In a supplementary solution in one smaller room, the impact is somewhat less critical, but the rule "never lay the pipe directly on the ceiling below it" always applies without exception.

You can find more about the various types of insulation boards, their thicknesses, and how to lay insulation correctly in the separate article Insulation under underfloor heating - polystyrene and system boards.

Manifolds - controlling individual circuits and zones

The manifold is the central element of the whole underfloor heating system - it distributes water from the boiler or heat pump to the individual circuits (loops) and back. Each room or zone usually has its own loop connected to a shared manifold, which makes it possible to regulate the temperature in each room independently. This is exactly where the difference between a main and a supplementary solution shows up again: for a whole house heated by underfloor heating as the main heat source, you need a manifold with enough circuits to cover all rooms and zones, while for a supplementary solution in a single bathroom it may be just one or two circuits connected to the existing pipework.

The flow rate for each circuit is set on the manifold separately, according to the length of the given loop and the area of that room, and actuators controlled by the room thermostats of the individual zones are fitted to it - thanks to this, for example, a bedroom can have a lower temperature than the living room, even though both rooms are supplied by the same manifold. Manifolds are made of brass or stainless steel and are housed in a manifold cabinet, which can be flush-mounted (recessed into the wall) or surface-mounted.

The choice between a stainless steel and a brass version is mainly a matter of budget and personal preference - both materials are common and reliable in practice, with the brass manifold usually working out somewhat cheaper. A detailed guide on how to set the manifold, how to balance the flows in individual circuits, and what to watch out for when placing the manifold cabinet can be found in the article Underfloor heating manifolds - how they work and how to set them.

A supplementary solution in a bathroom is fine with 1-2 circuits, a whole house on underfloor heating needs a manifold with more circuits.

Pipe-laying patterns - serpentine or spiral

The way the pipe is laid in the floor also differs depending on whether it is a main or a supplementary heat source. The serpentine (also called the meander) is the simplest laying pattern - the pipe runs in parallel rows across the whole room. It is faster to install, but creates a slight temperature gradient: the part of the floor near the hot-water supply is warmer, while near the end of the loop, where the water has already given off part of its heat, the floor is somewhat cooler.

The spiral, sometimes also called a double meander, runs the supply and return pipe alternately next to each other. This averages out the temperature differences between the warmer and cooler part of the loop, and the floor surface is overall more evenly warm. This laying pattern is used especially along perimeter walls and in rooms with higher heat losses - that is, exactly where uneven heat distribution is felt the most and where you want to get the maximum, even output from underfloor heating.

In a project where underfloor heating is the main heat source, the choice between serpentine and spiral is part of the overall output calculation and should be based on the heat losses of individual parts of the room - typically the spiral is used along a perimeter wall with windows, while a simpler serpentine is sufficient in the centre of the room. In a supplementary solution in a small bathroom, the difference between the two methods is less pronounced, since the main heat loss is covered by the radiator anyway, but even here it is worth choosing a spiral near the wall where the bathroom is most exposed to cold from outside.

We describe the actual installation procedure - from laying the insulation, through fixing the pipe and the pressure test, to pouring the screed in a wet system - in detail in the article Underfloor heating installation - step-by-step procedure.

Which floor covering is suitable

The choice of floor covering is closely related to whether underfloor heating is the main or a supplementary heat source, because this determines what maximum surface temperature the floor will typically reach. The most suitable covering for underfloor heating is tile and stone - they have high thermal conductivity, so heat passes quickly into the room without unnecessary losses as it passes through the covering itself. This is exactly why tiles are so often combined with underfloor heating in bathrooms, hallways, and entire houses heated exclusively by an underfloor system.

Laminate and wooden floors are also possible, but must be specially certified for underfloor heating - that is, they must have a declared lower thermal resistance and a stated maximum surface temperature with which they can safely operate. For such floors, the manufacturer usually limits the maximum floor surface temperature, typically to 27-29 °C, so the wood does not dry out and warp. In a project where underfloor heating is the main heat source and therefore has to deliver higher output, it is more important to take this limit into account already at the calculation stage - if you are planning a wooden floor in a room with high heat loss, you need to allow for denser pipe spacing so that the same output is achieved at a lower surface temperature.

Carpet coverings significantly impair heat transfer and are generally not recommended for underfloor heating, or only with a declared low thermal resistance. This limitation applies equally to both main and supplementary solutions - carpet simply acts as an insulating layer between the heat source and the room, which is the exact opposite of what you expect from underfloor heating. Details on individual covering types, their certifications and recommendations can be found in the article Which floor covering is suitable for underfloor heating.

Real-world examples

Example 1: A family house with underfloor heating as the main heat source. A new-build family house with good thermal insulation, where the investor planned from the start that there would not be a single radiator in the house. In all living rooms - the living room, kitchen, bedrooms and children's rooms - a wet underfloor heating system was chosen, embedded in a concrete screed, with emphasis on an accurate calculation of pipe density according to the heat loss of each room. Studded system insulation boards were laid under the pipe to minimise heat loss into the ground under the house. The house was divided into several zones according to orientation to the points of the compass and by room type, each zone connected to a separate circuit on the central manifold with its own actuator and room thermostat. Along perimeter walls with larger glazed areas the pipe was laid in a spiral pattern so that temperature differences near the windows would be averaged out evenly, while a simpler serpentine was sufficient in the centre of larger rooms. Large-format tiles were chosen for most of the house precisely because of their high thermal conductivity and rapid heat transfer into the room; only in one bedroom did the investor insist on a wooden floor - there a specially certified floor for underfloor heating was used, with a declared maximum surface temperature and denser pipe spacing, so that the same output would be achieved at a lower surface temperature.

Example 2: A flat in a panel building, bathroom renovation with supplementary underfloor heating. During the renovation of a bathroom in an older panel-building flat, the original radiator (a heated towel rail) stayed in place and continues to cover the room's main heat loss. The owner additionally wanted a warm tiled floor underfoot, so underfloor heating was added under the tiles purely as a supplementary heat source. Since it was not possible to raise the floor in the panel building by the several centimetres needed for a wet screed (the door and the threshold to the hallway would otherwise no longer be level), a dry system with a thinner load-distribution layer was chosen, which could be commissioned practically immediately after installation without weeks of waiting for the concrete to dry. In this small room the pipe was laid in a simple serpentine, connected to just one separate circuit of the flat's existing manifold, since there was no need to deal with multiple zones. Tiles remained the covering, as originally planned for the bathroom anyway - an ideal choice precisely because of their fast heat transfer. The result is a pleasantly warm floor throughout the day, while the actual heating of the room on frosty days continues to be provided by the radiator.

Frequently asked questions about underfloor heating as a main or supplementary heat source

Do I have to decide between a main and a supplementary solution already at the start of the project?
Yes. The pipe density and output of underfloor heating are designed precisely according to whether the system is meant to cover the room's entire heat loss (main source) or just supplement an existing heating appliance (supplementary source). Since pipe embedded in the floor cannot be densified afterwards, this decision needs to be made before installation, not after it.

Can underfloor heating and radiators be combined in one house?
Yes, this combination is common and works reliably - for example, living rooms on underfloor heating and a bathroom with a supplementary heated towel rail. It only requires correct hydraulic balancing of both circuits, since underfloor heating operates at a lower heating-water temperature than radiators.

Is a wet or a dry system better for underfloor heating as the main heat source?
In a new build, where underfloor heating is meant to cover the whole house's heat loss, the wet system is generally preferred thanks to better heat storage and more even distribution. The dry system is chosen mainly when the floor build-up height or the time available for the screed to dry does not allow for the wet variant.

Which floor covering is most suitable if underfloor heating is the sole heat source?
Tile and stone, thanks to their high thermal conductivity and fast heat transfer. Wooden and laminate floors are only possible if specially certified for underfloor heating, with a declared maximum surface temperature (usually up to 27-29 °C).

Why is carpet not recommended for underfloor heating?
Carpet significantly impairs heat transfer from the floor into the room, which is the exact opposite of what you expect from underfloor heating. If it cannot be avoided, it should at least have a declared low thermal resistance.

Why is there always insulation under the pipe, even when underfloor heating is only a supplementary solution?
Insulation directs heat upward into the room and prevents it from escaping downward, into a neighbour's ceiling or into the ground. Without it, part of the heat you would otherwise feel underfoot would escape where you do not need it - so it is always installed, regardless of whether it is a main or a supplementary heat source.

How do I know how many circuits I will need on the manifold?
The number of circuits corresponds to the number of independently regulated zones or rooms connected to the given manifold - each pipe loop connects to one circuit with its own flow setting and actuator. A main heat source for a whole house generally needs more circuits than a supplementary solution for a single room.

What is the difference between laying pipe in a serpentine and a spiral, and when should each be used?
The serpentine is simpler to install, but creates a slight temperature gradient between the start and end of the loop. The spiral runs the supply and return alternately next to each other, so the floor surface is more evenly warm - it is used mainly along perimeter walls and in rooms with higher heat losses.

Related topics

• How underfloor heating works
• Wet vs. dry underfloor heating system
• Which floor covering is suitable for underfloor heating
• Insulation under underfloor heating - polystyrene and system boards
• Underfloor heating manifolds - how they work and how to set them
• Pipe-laying patterns - serpentine vs. spiral
• Underfloor heating installation - step-by-step procedure
• Servicing, bleeding and common faults in underfloor heating
• Frequently asked questions about underfloor heating

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