Lowering the Flow Temperature: The Underrated Efficiency Lever
There is one setting on your heating system that hardly anyone knows about, yet it saves real money: lowering the flow temperature. Even a small reduction cuts energy consumption noticeably, and with a heat pump it even has a disproportionately large effect on electricity costs. Nevertheless, many heating systems run unnecessarily hot.
In this guide, we explain in plain terms what the flow temperature is, why lowering it achieves so much and how to optimize it step by step via the heating curve without feeling cold. This way, you can tap into the savings potential of your system without any new equipment.
What is the flow temperature?
The flow temperature is the temperature of the heating water flowing from the heat generator to the radiators or the underfloor heating. The higher it is, the more energy the boiler or heat pump has to expend to bring the water to this temperature. The cooled water then comes back at the return temperature and is heated again.
Many systems are set to 60 to 70 degrees at the factory or out of habit, although much less would be sufficient. The basic principle is: as low as possible, as high as necessary. Every degree of flow temperature you save reduces energy use, as long as the rooms still get warm. Closely related to this is hydronic balancing, which is explained in our guide to hydronic balancing.
Why lowering it achieves so much
A well-adjusted flow temperature is one of the most effective and at the same time cheapest efficiency levers. In poorly adjusted systems, optimizing the heating controls can save up to 15 percent of energy. This is possible without investing in new equipment, simply through the right settings.
The effect is particularly strong with a heat pump. The lower the flow temperature, the more efficiently it works, because at high temperatures its coefficient of performance (COP) drops and electricity consumption rises disproportionately. Lowering it by just a few degrees can therefore improve the seasonal performance factor significantly. You can read how a heat pump works in an existing building in our article Heat Pumps in Older Buildings.
Understanding and adjusting the heating curve
The flow temperature is controlled via the so-called heating curve. It determines how much the flow is heated depending on the outdoor temperature. When it is cold outside, the flow temperature rises; on mild days, it falls. This weather-compensated control adjusts the heating automatically and is the basis of any optimization.
Two values define the heating curve: the slope and the level. A flatter heating curve means lower flow temperatures and is ideal above all for heat pumps. If the curve is set too steep, you heat unnecessarily hard on mild days. You will find the setting in the controller of the heat generator. Proceed carefully and in small steps.
Guide values: which flow temperature for which system
How far you can lower the flow temperature depends on the heat emitters and the insulation. The following table provides orientation:
| System | Typical flow temperature |
|---|---|
| Underfloor heating | approx. 30 to 40 °C |
| Modern, large radiators | approx. 45 to 55 °C |
| Old, small radiators | approx. 60 to 70 °C |
| Heat pump (ideal) | below 50 °C if possible |
Especially in well-insulated houses and with surface heating systems, low flow temperatures are perfectly sufficient. In older buildings with small radiators, the limit is higher, but even there it can usually still go a little lower. Larger heating surfaces can reduce the required flow temperature even further.
Lowering the flow temperature step by step
Proceed in stages, ideally on a cooler day, so that you can judge the effect well. First lower the heating curve or the maximum flow temperature by about two to three degrees. Wait one to two days and check whether all rooms still get comfortably warm.
If the rooms continue to get warm, lower it again by a few degrees. As soon as individual rooms stay cool, go back one small step. It is important that the radiator thermostats are fully open, otherwise the result will be distorted. Bled radiators and hydronic balancing are prerequisites for success. Our guide Bleeding Your Radiators shows you how to bleed them properly. You will find suitable accessories and spare parts in our Heating category.
What if the rooms no longer get warm?
Sometimes you hit a limit when lowering the temperature: a single room stays cool while all the others are sufficiently warm. This is almost never due to a low flow temperature alone, but to the way the heat is distributed. The most common cause is missing hydronic balancing, which leaves distant radiators with too little water. Air in the system or an undersized radiator can also be behind it.
So before you raise the flow temperature again, it is worth checking these points. Hydronic balancing ensures that each radiator receives exactly the right amount of heating water and often makes an even lower flow temperature possible. In stubborn cases, a larger radiator that delivers the same heat at a lower temperature helps. This way, you combine comfort and efficiency instead of simply turning up the temperature and losing the savings again.
Frequently asked questions
How much can I save with a lower flow temperature?
In poorly adjusted systems, optimizing the heating controls can save up to 15 percent of energy. With heat pumps, the savings are often even greater thanks to the better coefficient of performance.
Will I be cold if I lower the flow temperature?
No, not if you proceed carefully. As long as all rooms still get warm, the temperature is sufficient. If one room cools down, simply go back one small step.
What does the heating curve have to do with the flow temperature?
The heating curve controls how much the flow temperature rises or falls depending on the outdoor temperature. A flatter curve means lower flow temperatures and greater efficiency, especially with heat pumps.
Do I need to adjust anything else first?
Ideally, hydronic balancing has been carried out and the radiators have been bled. Only then is the heat distributed evenly, allowing you to lower the flow temperature reliably.
Does this also apply to an old gas or oil boiler?
Yes. A condensing boiler also works more efficiently when the flow temperature, and therefore the return temperature, is low, because it can then make better use of the heat in the flue gas. The savings are smaller than with a heat pump, but they are real and cost nothing.
Conclusion
Lowering the flow temperature is one of the easiest ways to save on heating costs, without any new equipment. Using the heating curve, you set the temperature so that the rooms get just warm enough, and no warmer. With heat pumps, every degree pays off particularly well. What matters is a careful approach in small steps and a well-adjusted system with hydronic balancing and bled radiators. This way, you save up to 15 percent of energy and still stay comfortably warm. You will find suitable heating technology, accessories and spare parts in our shop under Heating.
