The short answer: Usual design values are 28 to 35 °C in a well-insulated new building, 35 to 40 °C in a renovated existing building and up to 45 °C in an uninsulated old building. In a KfW-40 house 25 to 30 °C are often enough. The real limit, however, is not the flow temperature but the surface temperature of the floor: to DIN EN 1264 a maximum of 29 °C in the occupied zone, 33 °C in edge zones and in the bathroom. Every kelvin less flow saves around 2.5 percent electricity with a heat pump – that is why lowering it is worthwhile, but only step by step and with patience.
What the flow temperature is
The flow temperature is the temperature of the heating water that flows from the heat generator to the underfloor heating. The return temperature is the temperature of the water that comes back after it has given up its heat to the screed. The difference between them is called the spread and with underfloor heating is typically 5 to 8 kelvin.
The difference from radiators is fundamental. A radiator has perhaps one square metre of area and therefore needs 50 to 70 °C to give off enough heat. An underfloor heating has the entire floor area at its disposal – with a room of twenty square metres that is twenty times as much. The temperature may be correspondingly lower.
That is precisely what makes it the ideal partner for a heat pump: its efficiency depends directly on how far it has to heat the water. As a rule of thumb every additional kelvin of flow costs around 2.5 percent more electricity. Lowering from 40 to 32 °C therefore means roughly 20 percent less consumption – with the same comfort.
Guide values by building type
| Building | Typical design flow | Note |
|---|---|---|
| KfW 40, passive house level | 25–30 °C | basic tempering is often enough |
| New building to GEG | 30–35 °C | the normal case in new buildings |
| Renovated existing building with insulation | 35–40 °C | depends on windows and envelope |
| Old building, partly renovated | 40–45 °C | pipe spacing usually too large |
| Old building without insulation | up to 50 °C | underfloor heating reaches its limits |
These values apply to the design case – that is, the coldest day of the year. On a mild November day with 8 °C outside, the actual flow is considerably lower, because the heating curve lowers it. Anyone who reads 32 °C in autumn and is surprised at the figure "35 °C design" is seeing exactly this interplay.
The real limit: the surface temperature
The question about the "maximum flow temperature" is misleading, because the standard limits something else: the temperature of the floor surface. To DIN EN 1264 the following apply:
| Zone | Maximum surface temperature |
|---|---|
| Occupied zone | 29 °C |
| Edge zones (window area) | 35 °C |
| Bathrooms and wet rooms | 33 °C |
These limits are not technical but physiologically based. From about 29 °C floor temperature most people find the floor unpleasantly warm; in the long term it also strains the venous circulation. That is why an underfloor heating that you clearly feel is, as a rule, set wrongly – a correctly designed system stays neutral to the feel.
How high the flow may be for these limits to be kept depends on the pipe spacing, the screed thickness and the floor covering. A close pipe spacing transfers more output at a lower flow – which is why it is standard in new buildings and often the limiting factor in old buildings.
The floor covering has a say too
Between pipe and room lies the floor covering, and every material slows the heat to a different degree. This slowing effect is called thermal resistance and is stated in m²K/W. The standard limits it in total to 0.15 m²K/W – if that is exceeded, the flow has to rise to reach the same room temperature.
| Covering | Thermal resistance | Suitability |
|---|---|---|
| Tiles, porcelain stoneware | approx. 0.01 m²K/W | ideal |
| Natural stone | approx. 0.01–0.03 m²K/W | very good |
| Vinyl, design flooring | approx. 0.02–0.05 m²K/W | good, observe manufacturer approval |
| Laminate | approx. 0.05–0.10 m²K/W | usable, watch for the insulating underlay |
| Parquet, 10–14 mm | approx. 0.05–0.12 m²K/W | possible, glued better than floating |
| Carpet | approx. 0.10–0.20 m²K/W | often too much, choose thin goods |
Tiles are the best covering for an underfloor heating, because they let heat through practically unhindered. That is why a tiled surface manages with the lowest flow temperature – and why bathrooms and open living areas with tiles have an energy advantage.
With parquet and wooden floors a material limit also applies: wood moves with temperature and moisture. Manufacturers as a rule specify a maximum surface temperature of 27 to 29 °C; if it is exceeded, the wood can shrink and form gaps. Glued parquet transfers heat considerably better than floating parquet.
The most frequent subsequent mistake is a thick carpet on the underfloor heating. It acts like an insulating layer, the room does not get warm, and the reaction is usually to raise the flow – as a result of which the rest of the free floor becomes too warm.
Setting the heating curve correctly
The flow temperature is not set at a fixed value but coupled to the outside temperature via the heating curve: the colder it is outside, the higher the flow. Two parameters control it.
Steepness (slope)
Determines how strongly the flow rises when it gets colder outside. For underfloor heating it is typically between 0.3 and 0.6 – considerably flatter than for radiators. If it gets too cold in frost but is fine in autumn: increase the steepness.
Parallel shift (level)
Raises or lowers the entire curve evenly. If it is evenly too cold or too warm in all weathers: adjust the parallel shift, not the steepness.
How to lower it step by step
The procedure is simple, but demands patience – and that is the real point.
1. Open all individual room controllers completely. They are limiters, not a control; half-closed valves falsify any assessment.
2. Lower the heating curve by 2 kelvin.
3. Wait two to three days. An underfloor heating reacts over hours, a screed needs up to a whole day to reach a new temperature level. Anyone who readjusts after two hours is regulating against the inertia of the system.
4. Check the room temperature. If it is still pleasant, repeat. If it gets too cold, take back the last step – and stay there.
5. Observe the whole thing over a heating season, including the cold days. The design case shows itself only in frost.
The target state is reached when the rooms hold the desired temperature and the individual room controllers do not constantly have to throttle down. If they constantly throttle down, the flow is still too high.
Night setback: usually counterproductive
What works with radiators rarely works out with underfloor heating. The screed is a large heat store: it hardly cools down overnight, but in the morning has to be reheated with an increased flow – and that costs disproportionately much with a heat pump. In well-insulated buildings you are better off with continuous, low operation.
Where to read off the flow temperature at all
Before you adjust anything, you should know what is currently applied. There are three places for that:
At the heat generator. Heat pumps and modern gas appliances show flow and return temperature in the display or in the app, mostly under "system status" or "heating circuit". That is the most convenient way.
At the heating circuit manifold. There are often thermometers in the flow and return bars. The difference between them is the spread – if it is well above 8 kelvin, too little water is flowing through the circuits.
At the flow meters. The small sight glasses on the manifold show the volume flow per heating circuit in litres per minute. Strongly differing values with circuits of similar length are the clearest sign of a missing hydraulic balancing.
Important when reading off: measure in running operation, not directly after domestic hot water production. Heat pumps briefly run considerably higher temperatures for that – anyone who reads off at that moment takes 50 °C for the normal heating setting.
The role of the individual room controllers
Almost every underfloor heating has room thermostats that switch off individual heating circuits via actuators on the manifold. Their function is regularly misunderstood.
An individual room controller is a limiter, not a control. It can keep a room cooler than the system would deliver it – but it cannot make a room warmer than the flow temperature allows. Anyone who turns the controller higher when a room is too cold therefore changes nothing if the flow is too low.
Conversely: if the controllers switch off constantly, the flow is too high. That is the most useful signal when lowering – as long as the valves keep closing, there is still room to go down.
A second point concerns inertia. Because the screed stores heat, an individual room controller acts with a considerable delay: it switches off, but the floor goes on giving off heat for hours. Rooms with high internal loads – south side with solar gain, a lot of technology, many people – overheat easily as a result. There it is not a more sharply set controller that helps, but a lower flow for the whole system.
Rule of practice: First lower the heating curve far enough that the individual room controllers only rarely have to intervene. Then the rooms are evenly tempered, the heat pump runs in its most favourable range and the controllers take over their actual task again – the fine correction of individual rooms.
If lowering does not work
Sometimes the flow cannot be lowered without individual rooms cooling down. Then there is usually one of four things behind it:
Missing hydraulic balancing. Without balancing, the short heating circuits get too much water and the long ones too little. One room is too warm, another stays cold, and the flow has to be set so high that even the worst circuit is still supplied. Balancing is therefore almost always the most effective single step.
Pipe spacing too large. In older systems 25 to 30 cm was laid, today 10 to 15 cm are usual. A large spacing necessarily needs higher temperatures – that cannot be regulated away.
The floor covering. See above: carpet and thickly built-up parquet noticeably shift the necessary flow temperature upwards.
The building envelope. Where a lot of heat is lost, only more output helps. Uninsulated external walls and old windows are hard to heat with an underfloor heating – here the heating is not the problem but the symptom.
The special case: new screed
With a freshly laid heating screed, separate rules apply before the covering is laid. Functional heating to DIN EN 1264-4 is compulsory and is recorded: the screed is first run for several days at around 25 °C flow, then for several days at the maximum design temperature, then lowered in a controlled way.
The purpose is not drying but proof that the system works and the screed has relieved its stresses. For readiness for covering a moisture measurement is additionally necessary – functional heating and heating to readiness for covering are two different things.
Anyone who takes shortcuts here risks cracks in the screed and damage to the covering. On the screed build-ups themselves there are separate articles on Screed on a separating layer and on Bonded screed.
Frequently asked questions
What is the optimum flow temperature of an underfloor heating?
As low as possible without the rooms cooling down. In a well-insulated new building 30 to 35 °C are usual, in a renovated existing building 35 to 40 °C. The right value for your house can be found only by lowering step by step over a heating season.
How high may the flow temperature be at most?
What is limited is not the flow but the surface temperature of the floor: to DIN EN 1264 a maximum of 29 °C in the occupied zone, 33 °C in the bathroom, 35 °C in edge zones. Which flow suits that depends on pipe spacing, screed and floor covering.
What flow temperature with tiles?
Tiles are the most favourable covering – their thermal resistance is about 0.01 m²K/W. Tiled surfaces therefore manage with the lowest flow temperature; in a new building 28 to 32 °C are realistic.
What flow temperature with parquet?
Usually 2 to 5 kelvin higher than with tiles, because wood slows the heat. In addition the manufacturer's limit for the surface temperature applies, usually 27 to 29 °C. Glued parquet transfers considerably better than floating parquet.
Is a flow temperature of 25 degrees enough?
In very well-insulated buildings – KfW-40 standard and better – yes, there that is a usual value on mild days. In a normal new building it is usually enough only in the transitional season, in an existing building not.
How do I calculate the right flow temperature?
Arithmetically it results from the heat load of the room, the available area, the pipe spacing and the floor covering – that is in the design record of the system. Without these documents the empirical route is the practicable one: lower in 2-kelvin steps and observe for two to three days each time.
Why does my room not get warm although the flow is high?
The most frequent cause is a missing hydraulic balancing – the heating circuit gets too little water. Further reasons: air in the system, a floor covering that is too thick, a pipe spacing that is too large, or an individual room controller that throttles down.
Is night setback worthwhile with underfloor heating?
Usually not. The screed stores heat and hardly cools down at night, but in the morning has to be reheated with an increased flow. Especially with heat pumps this ramp-up costs more than the setback saves.
Where do I read off the flow temperature?
On the display or in the app of the heat generator, alternatively on the thermometers in the flow and return bars of the heating circuit manifold. Measure in normal heating operation – directly after domestic hot water production, heat pumps briefly show considerably higher values.
What does hydraulic balancing achieve?
It distributes the heating water evenly over all heating circuits. Without it, the short circuits supply themselves at the expense of the long ones, and the flow has to be set so high that even the worst room still gets warm. Balancing is therefore usually the most effective single step, before you turn the heating curve at all.
Why do my room thermostats switch off constantly?
Because the flow temperature is higher than necessary. That is a good signal: as long as the controllers keep closing, the heating curve can be lowered further. The aim is a state in which the individual room controllers only rarely have to intervene.
