What requirements does the respective HVAC application place on the circulation pump?

Circulation pumps for HVAC applications

In a modern heating, cooling or DHW supply system there are different circuits in which circulation pumps of different designs can be used. A distinction must be made between heat generation (or cold generation) and its distribution.

The system can contain only one circulation pump or be divided into two circuits - a primary and a secondary side.

On the generator side, there are applications in which the circulation pumps are installed in an upstream or internal circuit, e.g. for brine circuits in water-water heat pumps or in thermal solar systems, which is hydraulically separated from the rest of the system, e.g. by a heat exchanger.

If, however, only the heat generator in a primary circuit flows through, but is hydraulically connected to the distributor side, these should be decoupled from each other by a hydraulic switch or a buffer tank so that the volume flows and / or temperatures on both sides remain largely independent of each other.

With some heat generators (e.g. condensing boilers, heat pumps or district heating) the return temperature must be kept as low as possible. It is therefore necessary to keep the flow rates in the primary and secondary circuits in the correct balance. Without a hydraulic switch, the primary pump generates a differential pressure in the secondary circuit, which can lead to imbalances in the heating circuits. As a result, under certain circumstances a generator effect can also occur in the secondary pump, which should be protected electronically against it.

Primary-side circulation pumps are mostly integrated in the heat generator. The pump control

usually takes place via a control signal (e.g. digital PWM signal) from the heat generator in order to optimize the pump operation or the combustion process.

Gas or oil heaters and multi-purpose water heaters

Most of the systems installed for space heating and hot water production still use fossil fuels, such as B. natural gas or oil. However, the proportion of gas and liquid fuel obtained from biomass plants is increasing continuously. Heat generator with high water content, such as B. floor-standing boilers are heated regardless of the actual volume flow requirement. Wall-mounted boilers, on the other hand, are only heated up when a minimum volume flow is flowing through the heat generator. In multi-purpose water heaters, the circulation pump supplies both the heating system and the domestic hot water preparation.

Today, modulating condensing boilers are increasingly being used, which are temperature-controlled by a weather-compensated controller with various time programs. In systems with hot water supply, which are designed either as combination devices with integrated hot water supply, with external hot water storage tanks or with heat exchangers, the water temperature of the central heating is often lower than the domestic hot water temperature. Therefore, the medium temperature must be temporarily raised above the desired hot water temperature. In smaller residential buildings with good insulation, the heat requirement for domestic hot water production is usually higher than that for space heating.

Internal (or external) circulation pumps for this type of primary circuit must be controlled by the heat generator in order to optimize the combustion conditions and the condensation process. This can often only be achieved by using a speed-controlled circulation pump, which is controlled by an external control signal supplied by the boiler control.

Solid fuel boiler

Most of these heat generators use biomass, such as B. pellets or wood chips, which as

Renewable energies can be viewed and are carbon neutral. They usually respond slowly to changing heat demands. Therefore the medium temperature can rise above the demand. Therefore, the flow rate should be kept constant and the excess heat energy should be stored in a buffer tank.

Heat pumps

Different types of heat pumps are available on the market:

• Compressor heat pumps with electric motor or combustion engine

• Sorption heat pumps. Sorption is a physico-chemical process in which a liquid

or a gas absorbed by another liquid or from the surface of a solid

is adsorbed. Both processes are reversible and only occur under certain physical conditions (pressure, temperature). Often a circulation pump with customized specifications is installed in the process cycle.

The use of circulation pumps in the primary circuit depends on the heat transfer principle:

• Air-to-air heat pumps (often used in air conditioning systems)

- Circulation pumps are not installed.

• Air / water heat pumps for space heating and cooling or hot water preparation

- The main source of heat is the outside air up to an outside temperature of -20 ° C. The heat energy is extracted from the outside air directly by an air evaporator supplied with a fan, which is a component of the heat pump. Sometimes there is a brine primary circuit between the outdoor air unit and a brine / water heat pump. The circulation in this circuit takes place with the help of a circulation pump, which must be designed for media temperatures down to -20 ° C.

• Water / water heat pumps for space heating and cooling or hot water preparation

- As a heat source z. B. Groundwater between 7 ° C and 12 ° C. The groundwater will

Mainly pumped out of extraction wells with the help of an underwater pump and returned via a return well. If the water quality is not sufficient, a primary circuit is arranged between the heat exchanger and the heat pump. The circulation in this circuit takes place with the help of a circulation pump, which must be designed for media temperatures of up to 2 ° C.

• Brine / water heat pumps for space heating and cooling or hot water preparation

- Horizontally installed geothermal collectors or probes installed vertically in the ground transport the solar energy stored in the ground via a water-brine mixture to the evaporator of the heat pump. The circulation in this circuit takes place with the aid of a circulation pump, which must be designed for media temperatures of up to 2 ° C or lower.

The circulation pump in the secondary circuit hardly differs from a circulation pump used in boiler applications. As a rule, the media temperature does not exceed 60 ° C and the spread is small in most cases (e.g. ∆T = 5 K). Heat pumps with a compressor that runs at a fixed speed often require a constant volume flow and long cycle times. The cycle time can be extended using a memory. The storage tank also ensures a flow through the heat pump that is independent of the volume flow in the heating distribution circuit. Variable speed compressors are often used in conjunction with a speed-controlled circulating pump that is controlled externally by the heat pump controller.

Micro / mini combined heat and power units

In the case of small combined heat and power plants (BHWK), heat and electricity are generated either with the help of combustion engines, Stirling engines or fuel cells. In the combined heat and power plants with fuel cells, metering and circulation pumps are often installed, which have to meet certain customer-specific requirements. The circulation pump in the secondary circuit, however, hardly differs from a circulation pump used in boiler applications. However, the vibrations, media temperatures and ambient temperatures can be relatively high. A constant volume flow is often required, so a buffer storage tank is advantageous.

Thermal solar systems

Solar collectors convert sunlight into heat that can be used for heating or hot water supply to a building. The primary circuit is only in operation when the temperature difference between the collector and the heat exchanger or storage tank is positive. In the case of pressurized solar thermal systems, the circulation pumps must be designed for media containing glycol and a wide temperature range from 2 ° C to 110 ° C (briefly up to 130 ° C). When the system is started up, the temperature of the medium can sometimes cause condensation to form in the pump.

The flow and head requirements can vary depending on the design of the system components. With drain-back systems, the pump must be able to fill the system each time it is started up. Therefore, the delivery head of the pump must correspond to the geodetic height of the system. The solar circulation pumps can be installed in all types of solar thermal systems with variable or constant volume flow. However, high-efficiency pumps with an electronically commutated motor (ECM) must not be controlled via an external speed controller that changes the supply voltage. The speed can be controlled via a PWM low-voltage signal with a C profile supplied by the solar system control in order to optimize the solar energy yield and the system temperature. At the same time, the power consumption of the pump is significantly reduced. If no PWM signal is available, the pump should be set to a constant speed. It is then only switched on and off via the control.

District heating systems with heat exchangers

District heating systems supply all types of buildings either directly or using a house transfer station with heating and domestic hot water. In systems with heat exchangers, the circulation pumps on the secondary side hardly differ from those in boiler applications

used circulation pumps because the pressures and temperatures are similar.

Space heating (radiators or underfloor heating)

Internally regulated, externally arranged pumps are often used on the secondary side. B. are mounted in a connection group and to the changing flow requirements of the heat consumer, such. B. radiators or underfloor heating circuits react. Hydraulic balancing is required so that the best possible operating conditions exist for all components

extremely important for the system. In the case of two-pipe heating systems with thermostatic valves, in particular, hydraulic balancing helps to avoid noises, over- and undersupply, and too high a delivery rate. At the same time, energy is saved.

If an automatic overflow valve is installed to maintain a minimum volume flow, the differential pressure control of the pump must be set in such a way that the function of the overflow valve is guaranteed. To do this, a constant pressure control curve must be selected that is higher than the set differential pressure of the valve. The maximum permissible media temperature and differential temperature depend on the system design. Tmax is usually 30 to 90 ° C. ∆T is between 5 and 20 K.

Combined space heating and cooling systems

Underfloor heating or ceiling cooling systems in connection with reversible heat pumps can

Heat rooms in winter and noticeably lower the temperature of the room air by 4 ° C to 6 ° C in summer. When cooling, the medium temperature must be kept above the dew point of the air in order to avoid condensation on the floor, the walls or the ceiling. Occasionally, condensation can form in the pump. For this reason, circulation pumps protected from condensation should be installed in such systems.

Domestic hot water production

In directly heated drinking water heating systems, the circulation pumps can be used on both sides of the heat exchanger. Drinking water is stored in a hot water tank and is usually heated by the heating water that is either fed from the heat generator via an external plate heat exchanger or through an internal heating coil. Especially when using renewable energies, the heating water is increasingly being stored and the hot water is generated by a fresh water module using the continuous flow principle.

Domestic hot water circulation systems

A circulation pump pumps the heated drinking water back on the secondary side from the taps to the water heater in order to increase comfort and avoid the formation of legionella. All pumps in contact with drinking water must have a drinking water approval and are offered either with a rustproof metal housing (brass, bronze or stainless steel) or a plastic housing (eg PPS). Most pump housings for the European market have the drinking water approval KTW (DE), DVGW W270 (DE), ACS (FR) and WRAS (GB).