Hydronic balancing

The term hydraulic balancing is generally used in the field of hot water heating systems, but is also true for cooling systems and potable water distribution. Here the term will be explained in connection with the hot water heater.

Hydraulic balancing describes a method by which a heating system within each radiator or heating surface heating to a certain flow of warm water is adjusted. This is to ensure that at a certain flow temperature of each room is furnished exactly with the amount of heat as the operating point of the heating system, which is needed to achieve the desired room temperature and the return of each radiator has the same temperature.

Consequences in the absence of hydraulic balancing

If there is no hydraulic balance, so are radiators that are close to the heat source, better care, more distant radiators for example in upper floors will not warm. The control behavior in the remote rooms is poor.

In a wired network without hydraulic balance of a heat source (boiler, buffer tank, heat exchanger) is reached at the nearest radiator with minimum flow resistance, more and more easily traversed, and his return water leaves the radiator with a comparatively high temperature. The lower flow resistance will cause more water to flow through the radiators as needed. The causes:

  • A disadvantage of remote radiator or other rooms with high heat demand,
  • The distant radiators are not or only late warm and the control response in the remote rooms is poor. It was not until the closing of the valves on the radiators close means that more distant radiators are warm.
  • The entire heating return to the hot water generator consists to a large extent from the return of the first heater and its temperature is thus hotter than uniform distribution.

If the heater return to hot,

  • Incur unnecessary thermodynamic losses in the distribution network,
  • Turns a simple kind of heating control from the pump for the heat supply,
  • Or it is switched off the burner of the boiler.

Turns the heater control prematurely, more distant radiators are not heated. In most cases the performance of the heating water circulation pump and / or the flow temperature is then increased as a remedy. This solves the problem of poorly heated radiator, but hot water from the first radiator then flows more quickly or even hotter back and leads to even more burner stops, cooling of the burner with heat loss up to restart ( " cycles " ) of the burner.

The main drawbacks and heating losses result:

  • From the frequent burner stops,
  • The subsequent cooling of a gas or oil burner by flushing the burner space with cold air ( which heat energy is lost in the exhaust gas passes ) or pellet boilers a residual burn, without the use of heat,
  • Subsequent cooling of boilers by heat radiation or heat conduction into the boiler room,
  • And the incomplete combustion (fuel waste or formation of carbon monoxide with residual calorific value ) in the first minutes of burner start-up phase, thus with each burner stop unnecessary energy is wasted.

The idle time between Stop burner and burner start is called " clock ", the boiler or burner " overclocked " too short.


In the by the German Federal Environmental Foundation ( DBU) OPTIMUS program savings at 92 buildings in the area of ​​Northern Germany were determined at single and multi- family homes in the practice and the heat losses then minimized with a limited catalog of services. The investigated objects were in the mean among other things a power oversizing the pump of about 3 relative to the sufficient electrical capacity.

The countermeasures were (2003) with Aufwänden of € 2, - to € 7, - per square meter comparatively inexpensive .. The success of the improved the OPTIMUS project individual heaters, extrapolated to the whole of the Federal Republic of Germany showed a potential savings 20000-28000 GWh per year

Were carried out only:

  • Preset thermostatic valves for flow limitation ( = hydraulic adjustment),
  • Settings of the heating circulation pumps (lower power)
  • Setting the heating controls

Evidence of lack of hydraulic balancing

  • Individual radiators do not heat up, while other parts of the plant are oversupplied ( "hydraulic short circuit ");
  • Burner of the boiler overclock too often;
  • Heating costs (per square meter and year ) are relatively high;
  • Radiator valves making noises because the differential pressure in the valve is too large.
  • Radiator valves and pipes making noises, since the flow velocity is large.
  • The control behavior of thermostatic heads is bad by strong " overshoot ".
  • The heating system is operated at high flow temperatures to compensate for the under-supply in this way.
  • The radiators are too hot. In particular, the use of heat pumps and with the supportive Electricity heating, the efficiency deteriorates.
  • The return temperature is unnecessarily high. In particular, the use of modern condensing technology, the efficiency deteriorates.
  • The efficiency of the heat generator is deteriorating as the plant with high return temperatures and strongly fluctuating airflow is operated.

From the non-optimal operating performance resulted in a significant increase in consumption of electricity and heating energy. The Energy Saving Ordinance in Germany writes for this reason before the hydraulic balance for issued or to be rehabilitated facilities.

Adjustment methods

Basis of the hydraulic balancing is a corresponding mathematical model. It must

  • The heat output of each heating element
  • The size of the room
  • The exchange of air through doors and ventilation
  • Heat loss through the walls and windows

Be known. The less known, the worse the computational model is supplied. With the hydraulic balancing a better distribution of heating water is reached.

Hydraulic balancing is achieved by good planning, review and adjustment during commissioning of the plant at a new construction of the building or the heating system.

A later hydraulic balancing is possible if the necessary fittings for flow reduction at individual radiators are available through

  • Adjustable return throttle valves
  • Strand with differential pressure controllers.

Hydraulic balancing is a task for heating contractors, energy consultants and inspectors. They can advise and carry out the calculations or measurements.

For promoting the modernization of heating systems by KfW and the market incentive program for renewable energy (MAP ) of the BAFA hydraulic balancing is a requirement.

The hydraulic balance in the theory

Since 1 April 2004, in Germany, the DIN EN 12831 ( June 2003). After that, a professional planning with heat load, pipe network, and Heizflächenberechnung of a planner is required. From the planning stage heat demand and volume flows arise.

In Germany, craftsmen, within the meaning of the associations want to completely execute their work, committed after the award of contract procedures (VOB ) Part C, to match heating pipe network hydraulic. This is necessary in particular for heating systems without flow control.

A stationary (quasi -static) hydraulic balancing is achieved when all parallel systems (such as radiators on one strand or apartments in a building ) each have the same hydraulic resistance. Fundamentally, however, this is only for one operating point ( desired temperature ) and at constant system conditions, ie a certain flow rate possible. For example, the pump flow rate must not vary or individual heater must not be closed. Therefore, the stationary hydraulic adjustment is made for a particularly critical state: the maximum heating load, in which all heating surfaces are traversed.

Especially in modern heating systems with regulated pump pressure and variable gas flow rate and Office thermostatic valves on each radiator and single variable flow rate and variable heat loss is the stationary hydraulic balancing of lesser importance. Instead, where the maximum possible flow rate for the individual heating elements must be limited. Such a dynamic balance is reached, the pressure to be shut off at each thermostatic valve, in particular reduced, and thus prolongs the life of the spindle drive.

The hydronic balancing in practice

To preset the flow rate for each radiator, either thermostatic valves with flow coefficient (adjusted kV cone) used and the calculated value can be set or through the return fittings, the flow resistances are regulated. Here is possible to set corresponding to a model calculation.

This setting is useful only possible with special lockshields that support fine-tune and serve not only shutting off.

It can also radiator valves with integrated flow regulators are used. In these valves, the maximum required for the radiator flow rate is set once. Thereafter, the thermostat is mounted on the valve. The thermostat controls now only in the range of zero to the preset flow. A so- equipped system operates stably at any time when the influences of other parts of the system have no effect on the radiator. It must only be ensured that the heating body is present, a sufficient differential pressure.

Hydraulic balancing in hot water circulation systems

For secondary hot water circulation a network is often installed, which maintains a back line leading the temperatures up to the ultimate consumer. In large systems, it is also useful, similar to reconcile this pipe network where the heating. The circulation pump in the recycle line can run up to 50% more efficient and thus save power, and the Legionella problem is reduced because the better blood flow also comes in distant consumers to bear. The DVGW worksheet W 553 provides information on the proper sizing of the corresponding line dimension. Here, the term " large system " also specified for the range of hot water: Thus, small plants within the meaning of the worksheet, if the owner with only lives in buildings with 1 WE (WE = units ) or 2 WE in the house, to be regarded as such. In addition, content play of the water heater ( WWB ) and the content of the installed pipe installation an important role. If the nominal volume of WWB 400 liters or more is automatically spoken by a large system. Likewise, do not have more than 3 liters of hot water in the tube content area defined as a small-scale system installation.

The most important difference to the small and large systems consists in the operating temperatures. A hot water system in large-scale systems must always be operated with at least 60 ° C, cooling down to the re-entry of the circulation in the WWB must not be more than 5 Kelvin. In small systems, the water heater may be operated at 50 ° C, but are also here because of the Legionella problem 60 ° C is recommended.