Heating circuit

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Scheme of a simple heating circuit. It can have a positive effect on the service life of the pump if it is arranged in the colder return instead of - as is the case here - in the flow.

In heating technology, the pipe system for distributing the water heated in a hot water heating system is called a heating circuit , heating circuit or heating circuit .

Gravity heating systems consist of a pressureless heating circuit ( open system ) with large pipe diameters in which the water (heat transfer medium)heatedin the heat exchanger ( boiler , buffer storage , district heating transfer point) rises due to the influence of gravity. On the heating surfaces ( radiators or radiators , surface heating systems , fan heaters ), the hot water releases heat energy to the environment and, when cooled, sinks back to the heat exchanger automatically. In the past, the increased volume of water due to heating was mostly taken up by an open container at the highest point of the system. In order to avoid the entry of oxygen into the heating circuit, a closed membrane expansion tank ispreferred for pressure equalizationtoday.

Pump heaters are much easier to manufacture. A circulating pump also transports the thermal energy over longer horizontal or branched sections and through pipes with a small diameter to the heating surfaces.

The path from the boiler to the radiators is referred to as the flow ; the way from the radiators to the boiler as a return . The temperature difference between the flow and return is called the spread .

... spread from forward to return
... flow temperature
... return temperature

If the connection lines are laid in such a way that the common path of the flow and return pipes for each individual radiator (or other consumer) is approximately the same length, this is called a Tichelmann system .

If the flow and return are connected ( short-circuited ) by a three-way mixer or a hydraulic switch , the part of the circuit that is closer to the boiler is also called the boiler circuit . Heating circuit then only refers to the remaining part of the circuit. A distinction is then made between boiler flow and heating flow . The boiler flow always has a higher temperature than the heating flow. The short-circuit line is also known as a stub line or bypass .

Filling, emptying and venting

Ventilation options are provided at the highest points in the heating circuit. Filling is simplified by using automatic venting valves through which air can still escape while the system is in operation. Since automatic ventilators can develop a slight leak over time, they usually have loosely screwed on plugs or caps to be able to close the valves. These valves are then only opened when the system is filled.

The new or changed parts of the system must be rinsed before filling. For this purpose, filling and discharge valves must be provided at a sufficient number of points with a nominal width that allows the required flow.

If the building is not heated in winter and the system is to be taken out of operation, freezing must be prevented by draining the fill water. In order to be able to drain the water from system parts for repair purposes, drainage options should be provided at the lowest points of the heating circuit. If the filling valves are positioned accordingly, they can also be used for emptying.

Risers or connection lines are usually provided with shut-off valves in the flow and return lines above the boiler or above the heating distributor so that the filling water of the entire system does not have to be drained off during repairs in the boiler circuit or in a single line.

Shut-off valves with small integrated venting valves can be used in order to be able to fill deeply drained pipelines after completion of the work without having to completely ventilate higher system parts. (If there are screw connections at a suitable point, venting can alternatively also take place by loosening the screw connections.)

If there are pipe loops at the highest point in the heating circuit without the possibility of ventilation, the air must be flushed out by a sufficiently strong flow of liquid. This is made much easier if all lines can be shut off individually so that one after the other can be opened and flushed. For the purpose of hydraulic balancing , a so-called line regulating valve is usually installed in each line , which can also be used to shut off. Only a second shut-off valve is then required per line.

As unwanted oxygen is introduced with the newly filled water and the heating filling water nowadays often contains expensive corrosion protection agents ( inhibitors ), it is caught when the air is flushed out and returned to the circuit via a pump.

Mixed circuits

Scheme of a heating circuit with return mixer, which lowers the temperature of the heating flow compared to the boiler flow. If the three-way mixer should instead be used to increase the boiler return temperature, it would have to be arranged to the right of the bypass (here running horizontally) and the pump would have to be installed in the boiler circuit.

In a mixing circuit, a certain part of the volume flow is branched off from the main flow and fed back again in order to limit the flow or return temperature in a part of the heating circuit or to keep it constant.

The branched volume flow is usually regulated by a thermostatically controlled or an electric servomotor controlled mixing valve , which is also referred to as a mixer for short . In order to be able to better control temperature and volume flow, an additional pump is often used in the mixing circuit. If both pumps can be precisely controlled, the mixing valve can also be dispensed with. Up to now, however, this solution has generally been more expensive than using a mixing valve.

Surface heating

Mixing circuits are often used to lower the temperature level of the flow for wall or floor heating, if radiators that require a higher temperature are also connected to the same heating circuit.

Thermal solar systems

Since the energy input can usually not be controlled in a solar system , very high temperatures can occur in the solar circuit when there is strong sunlight. By installing a mixing valve, cooler water from the return is added to the flow in order to limit the flow temperature to the desired value.

Wood heating and older gas and oil boilers

The temperature of wood heating systems cannot be controlled at will either. Log firing systems are usually heated up periodically and bring a buffer storage tank to a high temperature level at regular intervals. As with solar energy generation, a mixing circuit is often required in order to lower the high temperature of the boiler or the buffer tank to the desired level.

In wood boilers and older boilers for liquid or gaseous fuels, a return increase prevents the water flowing back into the boiler from cooling too much. If the return temperature is too low, thermal stresses arise, in the worst case cracks, the formation of glossy soot reduces the efficiency and the condensate that forms causes corrosion of the boiler. Often, thermostatic mixing valves are used to mix warmer water from the flow into the boiler return until the minimum temperature required by the boiler manufacturer is reached. The return flow increase is often mandatory for solid fuel boilers.

Since, on the other hand, low return temperatures are desired in modern condensing boilers or bivalent solar systems in order to increase efficiency, existing mixing valves to increase the return flow are often removed when the heating system is modernized.

If both an increased return temperature in the boiler circuit and a low flow temperature in the heating circuit are required, both the boiler and the heating circuit must usually be equipped with a pump. The temperature controls are either via

Temperature control of a mixing circuit

In the simplest case, a thermostat is used to keep the temperature level at a fixed, preset level. The expansion element also serves to measure the temperature and to control the valve. It is either integrated in the valve body or an external contact thermostat is connected to the valve via a fine hydraulic line.

In the case of outside or room temperature-controlled heating systems, the outside or room temperature as well as the current flow temperature of the heating circuit are recorded by temperature sensors . Based on the values ​​and the selected heating curve, the control determines the individual setpoint for the heating circuit flow and controls the mixer accordingly.


  • Mixer flow temperature control . In: IKZ-Praxis - magazine for trainees in building and energy technology . Strobel, March 2007, p. 12–13 ( ikz.de [PDF; 1 kB ; accessed on October 27, 2018]).
  • Ernst-Rudolf Schramek, Hermann Recknagel: Pocket book for heating + air conditioning . tape 73 . Oldenburg-Industrieverlag, Munich 2007, ISBN 3-8356-3104-7 ( control with heating mixers in the Google book search).


  1. Martin Schlohbach: How does a closed heating system work? , In: HaustechnikVerstehen.de
  2. Technology and types of heating mixers in a large expert check - What is a heating mixer? What is its task and how does it functionally meet it? What are the advantages and disadvantages of using a heating mixer? When do you need the component, when not ?; In: Energie-Experten.org