Flow temperature: the number on which almost everything depends

If you are interested in a heat pump, you will be asked about the flow temperature sooner or later. The answer determines more than most expect: the power consumption for the next twenty years, which devices are even an option — and in several federal states whether a public subsidy is available or not.

What the flow temperature is

The flow temperature is the temperature of the water that the heating system sends towards the radiators or underfloor heating. The return flow is what comes back cooled down.

The crucial factor is not a fixed value, but the design temperature: the flow temperature your house needs on the coldest day of the year to get warm. On all other days it is lower — modern controls adjust the temperature up and down via a heating curve based on the outside temperature.

Exactly this design value counts for planning and public subsidies.

Why a heat pump reacts so sensitively to this

A heat pump pumps heat from outside to inside. The greater the difference between the temperature of the heat source and the temperature to which it has to heat the heating water, the more electricity it costs.

A gas boiler hardly notices whether it delivers 45 or 60 °C — it just burns a bit more gas. With a heat pump, every degree has a noticeable impact on the electricity bill. That is why the flow temperature for a heat pump is a completely different metric than for the boiler it replaces.

This is also the reason why the question “does a heat pump fit in my house?” almost never depends on the year of construction, but on this: It is not the age of the house that decides, but the temperature it needs.

How to find out your own

Three ways, from rough to reliable:

1. Read it on the controller. Almost every heating controller shows the current flow temperature and the set heating curve. The value read on a mild day says little — but the heating curve reveals what the system aims for on cold days.

2. The practical test in winter. On a really cold day, turn the flow temperature down gradually, in small steps over several days. If it still gets warm enough in all rooms, your house is better than expected. This costs nothing but patience and is more meaningful than any estimate.

3. The heating load calculation. The only reliable way, and the basis of any serious design. An installer calculates room by room what is needed, and derives the necessary flow temperature from this. Anyone who receives a quote without this calculation should ask for it.

As a rough guide: underfloor heating typically works in the range of 30 to 35 °C, sufficiently large radiators in the range of 45 to 55 °C, old, undersized radiators above that. These are empirical values for orientation and do not replace the calculation.

Where the public subsidy depends on this number

This is where it gets concrete, and where the federal states differ significantly. Three examples, each read in the exact wording of the applicable regulation:

Vienna is the strictest. Heat pumps receive a public subsidy provided the flow temperature provided by the heat emission system is “a maximum of 40 degrees Celsius”. Higher temperatures are only permitted with proof — this is considered provided if the device achieves a seasonal space heating efficiency of at least 140 per cent at 55 °C according to the GET-Produktdatenbank (GET product database).

Tyrol sets the limit directly on the heat source type: a maximum of 55 °C for water-to-water and brine-to-water heat pumps, 50 °C for air-to-water heat pumps. So anyone planning with an air source heat pump has five degrees less leeway there than with geothermal energy.

Upper Austria links the efficiency requirement to the design: for a design temperature up to 35 °C, the efficiency value for 35 °C must be met, above that the one for 55 °C. The design temperature is explicitly requested in the technical data sheet.

And one state breaks ranks: Vorarlberg does not link the public subsidy to the flow temperature, but to the sound power level of the device. Anyone planning there must keep a completely different number in mind — more on this in the article about sound values.

⚠️ The other federal states are deliberately not listed here. Not because they have no requirements, but because we have not yet checked their exact wording. What applies in your federal state can be found in the public subsidy overview — there you can see for each state when we last checked. The subsidy authority is always binding.

How the flow temperature can be lowered

If your house needs 60 °C today, a heat pump is not ruled out — but it is worth working on the number beforehand. In order, from cheapest to most expensive:

Hydraulic balancing. This adjusts how much water flows through each heating circuit. Without it, some rooms get too warm and others too cold — and the system has to raise the flow temperature until even the worst room gets warm. Balancing costs little and often brings several degrees of improvement.

Enlarging individual radiators. Usually it is not the whole house that requires a high temperature, but one or two rooms. Larger radiators exactly there are significantly cheaper than a full renovation — and have the same effect.

Windows, roof, top floor ceiling. Classic insulation measures reduce the demand and thus the necessary temperature. More expensive, but they also pay off for everything else.

Retrofitting underfloor heating. The biggest intervention, usually only sensible if a renovation is pending anyway.

The order is important: first lower the temperature, then design the heat pump. Anyone who buys the device first and renovates afterwards ends up with a machine that is too large — which cycles, wears out faster and consumes more than it needs to.

Three questions for the quote discussion

What design temperature is the system calculated for? A concrete number, not a range.
Is there a heating load calculation? If not: why not.
Does this design meet the public subsidy conditions of my federal state? The installer should be able to answer this without hesitation.

If one of these questions remains open, it is no reason to worry about the device — but a reason to ask before signing. The flow temperature can only be changed with effort after installation.