Lowering the Flow Temperature (Heating Curve Optimisation)

Q: How much does one degree less flow temperature save?

One kelvin less flow temperature reduces a heat pump's electricity consumption by around 2.5 % and a condensing boiler's gas consumption by 1 to 1.5 %. With low-temperature gas boilers without condensing, the effect is only about 0.5 % per kelvin. A 5 K reduction on a heat pump with a heat demand of 15,000 kWh brings around 500 kWh of electricity per year. Sufficiently large heating surfaces are a prerequisite.

Q: How low can the flow temperature be with a heat pump?

Heat pumps operate most efficiently at flow temperatures between 35 and 45 °C, as is typical with underfloor heating. With radiators, 45 to 55 °C is realistic. The lower limit is set by the rooms' heat demand: the flow temperature may only be reduced to the point where all rooms still reach their set temperature on the coldest day. Hydraulic balancing and larger radiators allow lower values.

Q: Can you lower the flow temperature yourself?

The flow temperature can be adjusted yourself on the heating controller via the level and slope of the heating curve. A step-by-step approach is recommended: lower the level by 2 to 3 K, observe for a few days whether all rooms get warm, then lower further. If in doubt or with complex systems with several heating circuits, the heating engineer should assist with the setting; the cost for this is at most about 300 €.

Q: What is the difference between flow and return temperature?

The flow temperature is the temperature of the heating water as it leaves the heat generator; the return temperature is the temperature after it has passed through the heating surfaces. The difference, known as the temperature spread (delta T), is typically 10 to 20 K for radiators and 5 to 8 K for underfloor heating. For condensing boilers, a low return temperature below 55 °C is decisive so that the flue gas condenses; for heat pumps, a low flow temperature matters most.

Q: Does hydraulic balancing need to be done before lowering the flow temperature?

Hydraulic balancing should precede the reduction, but is not a formal requirement. Without balancing, the worst-supplied radiator determines how far the flow temperature can be lowered; all other rooms are then oversupplied. After balancing, each radiator receives the correct amount of water, so the flow temperature can typically be lowered by a further 5 to 10 K. The two measures therefore complement each other ideally.

Reducing the flow temperature as far as comfort allows — a direct lever for condensing operation on gas and for a higher seasonal performance factor on heat pumps.

How does lowering the flow temperature work?

The flow temperature is the temperature at which the heating water flows from the heat generator into the heating surfaces. It is set via the controller's heating curve as a function of outdoor temperature. In many existing systems, the heating curve is set higher than necessary – as a safety margin or because it has never been adjusted. Every excess kelvin costs efficiency: with gas condensing boilers, a lower flow temperature improves flue gas condensation and reduces gas consumption by 1 to 1.5 % per kelvin. With heat pumps, the effect is even stronger because a smaller temperature lift between heat source and flow relieves the compressor: around 2.5 % less electricity per kelvin. The reduction is made step by step via the level and slope of the heating curve, accompanied by room temperature checks.

Which buildings benefit from heating curve optimisation?

In principle, all water-based heating systems, especially those with a heat pump or condensing boiler. The heating surfaces must be large enough to heat the rooms even at a lower flow temperature – which is the case with underfloor heating, generously sized radiators, or after thermal refurbishment has reduced the heating load. Hydraulic balancing should be carried out first, otherwise the worst-supplied radiator limits the possible reduction. Severely undersized radiators in uninsulated rooms set limits to the measure. A good starting point is to check whether the rooms still get warm during frost with a flow temperature of 55 °C.

What does lowering the flow temperature actually deliver?

Final energy demand falls by 3 to 15 %, depending on heat generator and achievable reduction. A heat pump with a heat demand of 15,000 kWh saves around 500 kWh of electricity per year with a 5 K lower flow temperature, and the seasonal performance factor rises from 3.8 to about 4.3. Costs are 0 to 300 € for the controller adjustment, with support from the heating engineer if required. The measure therefore pays back in less than a year and is one of the cheapest efficiency measures of all.

Energy carrier

Erdgas, Strom → Erdgas, Strom

Savings potential

3-15 %

Worked example

A detached house is heated by a heat pump with a seasonal performance factor of 3.8; the annual heat demand is 15,000 kWh. The heating curve was set with a safety margin at commissioning and has not been adjusted since. As part of the optimisation, the level of the heating curve is lowered in several steps over the heating season, each accompanied by room temperature checks; in total, the flow temperature can be reduced by 5 K without the rooms becoming too cold. A saving factor of 0.025 per kelvin applies to heat pumps. The electricity saving is 15,000 × 5 × 0.025 / 3.8 = 1,875 / 3.8 = 493 kWh per year. At the same time, the seasonal performance factor rises from 3.8 to about 4.3. Since only the controller setting is changed, costs are at most 300 €, and the measure pays back within the first heating season.

Investment & payback

0-300 € (Reglereinstellung, ggf. Begleitung Heizungsbauer) · Service life: 10 a · Payback: <1 a

Applicability

Requirements

Heizflächen ausreichend dimensioniert (oder thermische Sanierung erfolgt)
Hydraulischer Abgleich

Exclusion criteria

Stark unterdimensionierte Heizkörper in ungedämmten Räumen

Typical buildings

Alle wassergeführten Systeme - besonders mit WP

Frequently asked questions

How much does one degree less flow temperature save?

One kelvin less flow temperature reduces a heat pump's electricity consumption by around 2.5 % and a condensing boiler's gas consumption by 1 to 1.5 %. With low-temperature gas boilers without condensing, the effect is only about 0.5 % per kelvin. A 5 K reduction on a heat pump with a heat demand of 15,000 kWh brings around 500 kWh of electricity per year. Sufficiently large heating surfaces are a prerequisite.

How low can the flow temperature be with a heat pump?

Heat pumps operate most efficiently at flow temperatures between 35 and 45 °C, as is typical with underfloor heating. With radiators, 45 to 55 °C is realistic. The lower limit is set by the rooms' heat demand: the flow temperature may only be reduced to the point where all rooms still reach their set temperature on the coldest day. Hydraulic balancing and larger radiators allow lower values.

Can you lower the flow temperature yourself?

The flow temperature can be adjusted yourself on the heating controller via the level and slope of the heating curve. A step-by-step approach is recommended: lower the level by 2 to 3 K, observe for a few days whether all rooms get warm, then lower further. If in doubt or with complex systems with several heating circuits, the heating engineer should assist with the setting; the cost for this is at most about 300 €.

What is the difference between flow and return temperature?

The flow temperature is the temperature of the heating water as it leaves the heat generator; the return temperature is the temperature after it has passed through the heating surfaces. The difference, known as the temperature spread (delta T), is typically 10 to 20 K for radiators and 5 to 8 K for underfloor heating. For condensing boilers, a low return temperature below 55 °C is decisive so that the flue gas condenses; for heat pumps, a low flow temperature matters most.

Does hydraulic balancing need to be done before lowering the flow temperature?

Hydraulic balancing should precede the reduction, but is not a formal requirement. Without balancing, the worst-supplied radiator determines how far the flow temperature can be lowered; all other rooms are then oversupplied. After balancing, each radiator receives the correct amount of water, so the flow temperature can typically be lowered by a further 5 to 10 K. The two measures therefore complement each other ideally.

Related measures

Standards & sources

DIN EN 12831

EEff-RL-VO-2016 · Anlage 1 Methode Heizungsoptimierung

Metering points in the EDM Toolbox

Ideal mit EDM Toolbox monitorbar: VL/RL-Temperatursensoren + Wärmemengen/Stromzähler. Stufenweise VL-Absenkung mit Raumtemperatur-Monitoring (Stichproben) iterativ ermitteln.

All measures