NL1027140C2 - Installation for heating tapwater comprises a storage tank, a low-power heating unit, a boiler and a common feed pipe for supplying water from the tank to the heating unit and boiler - Google Patents

Abstract

Installation for heating tapwater comprises a storage tank (10), a low-power heating unit (12), a boiler (14), a common feed pipe (11) for supplying water from the tank to the heating unit and boiler, a first return pipe (26) for recirculating water from the heating unit to the bottom of the tank, and a second return pipe (14a) for recirculating water from the boiler to the top of the tank. An independent claim is also included for heating tapwater by storing water in a tank, supplying water from the tank to a low-power heating unit and a boiler through a common feed pipe, heating water drawn from the tank at a first level (L1) in the heating unit, heating water drawn from the tank in the boiler, and returning the water heated in the boiler to the tank at a second level (L2), where L2 is higher than L1.

Description

Title: Installation for mixed with solar energy and heating of tap water

The invention relates to an installation with a storage tank for heated tap water, a high-power heating unit and a low-power heating unit.

Heated tap water can be supplied by heating when tapping, such as for example when using a geyser or a combi boiler, or by pre-heating the tap water such as when using a boiler. In the latter case, the water is stored warm before use in a storage vessel. The advantage of using a storage vessel is, among other things, that it is possible to suffice with gradual heating with low power (ie with a small burner and / or a limited electrical heating current).

On the other hand, when using a storage tank with tap water, special measures are needed to ensure that sufficient hot water is available. Almost simultaneously with the draining of hot water from the storage vessel, cold water is introduced into the vessel, which, when mixed with the remaining heated water in the storage vessel, could result in that only water of reduced temperature remains available. To prevent this, it is common to introduce the make-up water at the bottom of the storage tank in such a way that a thermal layering is maintained in the storage tank between the cold make-up water below and the hot water above. By heating the water in the storage tank, this front is gradually cleaned up, and if tap water is required before, this is tapped from the top of the storage tank to be sure of warm water for as long as possible.

Also in installations with low power heating units such as solar water heater installations and heat pump installations use is made of a storage tank for tap water. In addition, the temperature of the 1027140 2 tap water is generally only gradually increased with a limited charging capacity. In this case, water is heated at the bottom of the storage tank. Lightly heated water thereby rises, whereby the body of water below the temperature front slowly warms up until the water in the entire storage vessel is up to temperature again.

With solar water heater installations, the available charging capacity also depends on the amount of sunlight, which can vary greatly. However, this means that heating takes longer than is acceptable for many users during peak use. Therefore, an additional (gas) firing unit is usually used to be able to reheat tap water from a solar-heated pre-heated storage tank before use if the water is not yet hot enough. However, if a large hot water flow is required, for example at the same time for a shower and a kitchen, a heating unit with relatively large flow capacity is required.

Furthermore, when using a solar collector, special safety measures are required to prevent damage caused by frost and / or overheating (cooking). The usual solution for this is an emptying circuit, which contains an air bubble which, in the event of system failure (or deliberate switching off), moves to the solar collector 20 so that water cannot freeze or boil there. In order to realize such an air bubble and to be sure that it is moving to the solar collector, special requirements must be imposed on the course of the pipes that supply liquid to the solar collector. A separate circuit is generally used for this purpose, with a heat exchanger in the form of a heating coil at the bottom of the storage vessel. However, this imposes restrictive requirements on the placement of the storage vessel.

It is inter alia an object of the invention to reduce the required through-flow capacity of a heating unit that is used in combination with a storage vessel in which tap water is gradually heated.

It is inter alia a further object of the invention to simplify the incorporation of a high power heating unit in installations with a low power heating unit.

It is inter alia a further object of the invention to simplify the requirements for placing a storage vessel in a solar boiler installation with a minimal loss of efficiency.

It is inter alia a further object of the invention to provide installations with a low-power heating unit in which a comfort position and an economy position are available.

It is inter alia a further object of the invention to provide solar boiler installations in which a minimum number of pumps is required.

The heating installation according to the invention is claimed in claim 1. This invention is applicable to use with low-power heating installations such as solar boiler installations and heat pump installations. However, solar boiler installations in particular will be discussed by way of example. The invention provides a common storage vessel with separate heating by solar heat and fired heat, with which tap water, which is separately extracted from the same storage vessel via a common supply line, is heated and the heated water is returned to the storage vessel at various points. The common supply line simplifies the construction of the installation. The solar collector heats water that is returned to the bottom of the storage tank and with the heating unit the water is directly heated to the desired storage temperature and returned to the top of the tank. This achieves heating with maximum efficiency.

Use is preferably made of a common pump 30 for the heating circuit and the heating circuit, coupled to the common supply line, instead of separate pumps for both 1027140 4 circuits. Use can be made for this of switchable valves which, depending on the desired type of heating, connect one circuit or the other circuit to the pump.

Preferably the heating unit is only switched on if there is insufficient hot water and / or hot water of too low a temperature at the top of the vessel. For detecting this situation, use is preferably made of a sensor at a certain height in the storage vessel which causes the heating unit to switch on if the temperature at the sensor falls below a threshold value.

In one embodiment use is made of several temperature sensors at different heights in the storage vessel. This makes it possible for the user to be able to switch between a comfort position and an economy position, whereby the heating is switched on if the temperature at the lower or higher sensor drops too much.

With intensive use, the user can then switch the installation to the comfort position where a larger volume of hot water is kept in stock, to switch back to the economy mode economy mode with a smaller stock volume of hot water, for use at the time of a lower demand for hot tap water.

In another embodiment, an indicator for the size of the cold water supply (flow sensor) is also used to measure the instantaneous extraction of hot water, and to calculate the available amount of hot water, to determine whether the heating unit should be switched on. In this embodiment, the heating unit can for instance also be switched on when the measured temperatures do not yet give rise to this, but there is more than a threshold current of cold water supply and hot water extraction.

The tap water from the storage vessel that is circulated externally for heating with the solar collector is preferably drained lower from the storage vessel 30 than for heating with the heating unit.

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In a further embodiment, switching of the circuit is achieved by changing the pump speed. For this purpose use is made, for example, of a three-way valve which is adapted to allow water to flow to the solar heating unit or to the heating unit, depending on the pressure difference over the valve.

In one embodiment, use is also made of a common suction pipe for the pump, which taps out of the storage tank at a first height and has a small lateral suction point at a lower height. This has the effect that with an increasing temperature difference between the two suction openings, relatively more water is taken from the lower tap point. This temperature difference is created with the introduction of cold water at the bottom of the storage tank with every tap. This increases the efficiency of heating with the solar collector in particular.

Furthermore, use is preferably made of a closed liquid circuit, separated from the tap water circuit, which is arranged locally at the solar collector. This circuit includes an idle circuit and a heat exchanger to transfer the water to heat the data from the storage tank to the local circuit of the solar collector. Because the closed liquid circuit with the draining is arranged locally at the solar collector and the tap water is circulated with a separate circuit between the local circuit and the storage vessel, no requirements have to be imposed as to the arrangement of the storage vessel. This simplifies the installation.

In another embodiment, the water from the storage vessel 25 is led directly through the solar collector. This leads to a higher efficiency. In this embodiment, the installation is preferably provided with safety valves, to switch the installation from an operating position to a switch-off position. In the switch-off position, the tap water circuit is interrupted by the storage tank and the solar heating unit, so that 1027140 6 no more tap water flows through the solar heating unit and the solar heating unit is connected to a drain connection.

These and other objects and advantages of the invention will be described on the basis of exemplary embodiments with the aid of the following figures.

Figure 1 shows an embodiment of a tap water heating installation. Figure 2 shows a mouth for use in a storage vessel

Figure 3 shows an embodiment of an installation with a shared pump

Figure 4 shows a T-valve unit Figure 4a shows an alternative T-valve unit Figure 5 shows a transfer circuit for a solar collector

Figure 6 shows another transfer circuit for a solar collector i

Figure 1 shows a tap water heating installation, provided with a storage tank 10 with a cold water supply line 9, a solar heating unit 12, a heating unit 14, a control unit 16, a first circulation pump 17, a second pump pump 22 and a tap water line 19. Supply line 9 opens into a mouth 28 in a lower zone of storage vessel 10. First circulation pump 17 is coupled to a supply line 11, which draws water from a central zone of storage vessel 25. An output from pump 17 is coupled to combustion unit 14.

Second circulation pump pump 22 is coupled between drain line 20 and a line 24 to solar heating unit 12, and a return pipe 26 from solar heating unit 12 to storage tank 10. The return pipe opens out at the bottom of storage tank 10. Furthermore, the installation comprises a lower 1027140 7 temperature sensor 16c for measuring of a water temperature at the bottom of storage tank 10.

Heating unit 14 is coupled to a second return line 14a which opens onto an upper zone of storage vessel 10. Tap water line 19 draws water from the upper zone of storage tank 10. Storage tank 10 is provided with a first and second sensor 16a, b which are coupled to control unit 16. Heating unit is, for example, part of a central heating boiler, so that the heating unit can also be used for heating water in a separate heating circuit (not shown).

10 In operation, (for example when showering), hot tap water is withdrawn from the upper zone of storage tank 10 via tap water pipe 19. The extracted water is replaced by the supply of cold water to the lower zone of the storage tank via supply pipe 9. Through the supply of cold water at the bottom creates a vertical layering of the water in storage tank 10, with a temperature front between cold water at the bottom and warm water above it.

The water in storage tank 10 is heated in two ways: with solar heating unit 12 and with heating unit 14. Solar heating unit 12 gradually heats water at the bottom of storage tank 10 via spiral 8. Only after a long time will the water in the storage tank 10 return to full temperature. Below the front with the warmest part of the water of storage tank 10, a gradually warmer body of less warm water forms.

Control unit 16 activates firing unit 14 when the temperature 25 in the storage tank 10 on the upper sensor 16a falls below a threshold temperature. In that case, pump 17 is switched on, whereby water is pumped from the central zone of storage tank to firing unit 14, where this water is heated and then returned to the upper zone of storage tank 10.

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As shown, the water for firing unit 14 is drawn from a central zone of storage tank 10 which is substantially higher than the supply and return line of the solar heating circuit. Thus, the amount of water having the coldest temperature (at the bottom of storage tank 10) remains the same when draining before heating unit 14. As a result, a maximum efficiency of the solar heating unit is achieved. Control unit 16 is preferably switchable between an economy mode and a comfort mode. For operation in the comfort mode, a lower temperature sensor 16b is coupled to control unit 16 which measures the temperature of the water in the storage tank at a lower point than the first temperature sensor. In economy mode, control unit 16 activates heating unit 14 only if the upper temperature sensor 16a indicates a temperature below a threshold value. In the comfort mode, the control unit 16 also activates the heating unit 14 if the temperature on the lower temperature sensor 16b indicates a temperature below a threshold value. Thus, a greater short-term hot water requirement is taken into account, which must be met with hot water from above storage tank 10.

When such a larger short-term need is provided, the control unit is switched to the comfort mode, for example by a user (with the aid of a control switch (not shown)) or with a timer (not shown). This ensures that in comfort mode a larger quantity of sufficient hot water can be counted on.

In another or further embodiment, control unit 16 is coupled to a supply or discharge detector (flow sensor), and ie control unit 16 adapted to activate heating unit 14 if lower temperature sensor 16b indicates a temperature below a threshold and the supply or discharge detector detects that the volume of water taken from storage tank 30 is above a threshold value. Thus, it is ensured that with 1027140 9 a greater decrease can be counted on a larger quantity of sufficient hot water. This can be limited to the comfort mode, or used separately from the comfort mode.

Storage vessel 10 can further be provided with a third temperature sensor (not shown) in the lower zone and coupled to switch off a heat flow from the solar heating unit if there is insufficient solar heat to heat water from storage vessel 10. Heat supply can also be switched off if the temperature of the water in the storage tank including the lower zone exceeds a threshold temperature. Thus, if necessary, overheating is prevented.

Second circulation pump 22 is switched on, for example, when lower temperature sensor 16c indicates that the temperature of the water at the bottom of storage tank 10 falls below a threshold value. In that case, water is pumped from storage tank 10 through solar heating unit 12, where it is heated and from there the water is returned to the bottom of storage tank 10. After hot water has been drained from storage tank 10 via tap water pipe 19 and cold water has been supplied via supply pipe 9 the cold water body thus formed at the bottom of storage tank 10 is gradually heated. When the temperature at the top of storage tank 10 drops too much, water from storage tank 10 with heating unit 14 is heated and added back into storage tank 10, so if necessary it is maintained above a hot water volume.

Figure 2 shows an embodiment of mouth 28 in which one end of this mouth is closed and a part 280 of the mouth 28 is made of a porous material, such as sinterbrone. The sintered bronze ensures that the cold water can only flow into storage tank 10 at a low flow rate, so that the thermal layering of the water in storage tank 10 is minimally disrupted. It has been found that a highly efficient installation 30 can be realized in this way. If required, the whole of the 1027140 10 outlet pipe 28 in the storage vessel can be manufactured from sintered bronze, or only a part, such as the last part of a few centimeters up to the end of the outlet pipe 28. Of course, the sintered bronze can be replaced by another porous material . If instead of a porous outlet 28 an open outlet is used, such as an open pipe or a pipe with a closing mesh, there is a greater risk that a flow will arise that disrupts the layering of the tap water. However, by keeping the inflow speed sufficiently low or by sufficiently spreading the inflow, for example by splitting the mouth 18 into a frequently spread narrow mouth channels, open ends can also be used.

After tapping and topping up with cold water several times, an extra cold body of water can be created each time, so that a plurality of water bodies are created in storage vessel 10, with vertically successive temperature fronts between successive bodies of water and with an increasingly higher temperature upwards. In that case, the fronts with the solar heating unit 12 are cleaned from below one by one by heating the lower body each time until the front with the next body of water has disappeared.

As shown, the water for furnace unit 14 is preferably drained from a central zone of storage vessel 10, substantially above the point from which water for heating with heat is drained from the solar collector. This ensures that in the case of multiple fronts, where the coldest water body is below the draw-off point, the amount of water in the coldest water body is not reduced by draining for heating with heating unit 14. This increases the efficiency of heating with solar heating unit 12.

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However, the invention is not limited to this. Alternatively, the water for heating through firing unit 14 can also be drawn from the bottom of the storage tank.

Control unit 16 uses lower sensor 16c at the water tap 5 to switch off pump 22 if the temperature difference between the water in storage tank 10 is higher than the temperature in solar heating unit 12, so that water in storage tank 10 cannot be heated with it (for this heating unit becomes 12 preferably also provided with a temperature sensor (not shown) for determining the difference).

In principle, return line 26, like the mouth 28 of the cold water supply, can be provided with a porous nozzle or a nozzle with a plurality of scattered narrow channels. In practice, however, an open mouth tube is usually sufficient because such a small inflow can be used that no annoying flows arise.

Figure 3 shows an embodiment in which the drainage of water for heating is combined with solar heating unit 12 and for heating with heating unit 14. This embodiment comprises a 3-way valve unit 18 which is connected behind pump 17 and has outputs 20 which are respectively coupled to heating unit 14 and solar heating unit 12 (the latter via a circulation line 24).

Figure 4 shows an embodiment of a 3-way valve unit 18. This unit has an input 34 and two outputs and contains coupled non-return valves 30, 32 in both outputs. The unit is arranged such that a first non-return valve 30 in the output to firing unit 14 only opens at a higher pressure from input 34 than a second non-return valve 32 in the output to solar heating unit 12. Moreover, first non-return valve 30 is arranged such that opening of this first non-return valve 30 closes the second non-return valve 32.

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In operation, the installation operates broadly as described for figure 1. Under the circumstances in which heating with solar heating unit 12 is desired, but no heating with heating unit 14, control unit 17 switches pump 17 to a low pumping speed. As a result, water flows to solar heating unit 12 via second check valve 32, but the first check valve 30 remains closed. Under circumstances where heating with heating unit 14 is desired, the control unit switches pump 17 to a high pump speed, with which first non-return valve 30 is pressed open and water flows to heating unit 14. Thus, one pump is sufficient, and the 3-way valve switches based on the different pressures and flows created at different pump speeds. Alternatively, a 3-way valve operated electrically by control unit 16 may be used, but this is more expensive.

Supply line 11 of pump is preferably provided with a side branch (possibly no more than one hole) which opens into the lower zone of storage vessel 10, with which water can be tapped laterally from the lower zone. XXXX explanation needed In this way, it is ensured that with an increasing temperature difference across the two openings, relatively more (cold) water is tapped from the lower zone, in particular for heating by solar heating unit 12 after a tapping. Because heating with solar heating unit 12 thus drains colder water, this increases the efficiency of heating with solar heating unit 12. The ratio of the amounts of water drained from the low side branch and the higher main branch is influenced by the difference in density between the (warmer) ones. water column in supply line 11 above the side branch and the (colder) water column above the side branch in storage tank 10. There is a higher pressure behind the water coming from the side branch. This pressure determines the fraction of the water that comes out of the side branch to a higher extent as the pumping speed is lower.

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If desired, a non-return valve can also be provided in the side branch (which need not be more than a hole in supply line 11), the non-return valve being arranged to close at the high pumping speed. It is of course also possible to use valves directly controlled by control unit 16 which, when heated, activate supply from the lower zone with solar heating unit 12 and, when heated, with heating unit 14, supply from the middle zone.

The invention is not limited to the use of a side branch. As an alternative, supply line 11 can, for example, completely drain from underneath storage tank 10.

Figure 4a shows an alternative embodiment of T valve unit 18. This includes a ball valve with a ball 36 in the exit to the solar heating unit 12, which ball closes the valve when the ball 36 is pressed against a closing edge by sufficient pressure from input 34. The non-return valve 30 in the output to firing unit 14 comprises a spring which ensures that this non-return valve does not open until the pressure is higher from input 34.

Figure 5 shows a transfer circuit for a solar collector 60. The transfer circuit comprises a heat exchanger 54, an emptying 52 and a circulation pump 56. Circulation pump 56 pumps liquid From heat exchanger 54 to a lower connection on solar collector 50 emptying 52 collects liquid from an upper connection of solar collector and is connected to heat exchanger 54.

In operation, incident sunlight heats liquid in solar collector 50. The heated liquid flows to heat exchanger 54, where water is heated for storage vessel 10. Circulation pump 56 pumps the liquid back to solar collector for new heating. Drain 52 provides protection against freezing and overheating in a manner known per se. Emptying vessel 52 contains an air bubble. When pump 56 is switched off, the liquid flows back from the solar collector, air being drawn from draining 52 1027140. Thus, the liquid disappears from solar collector 50. The liquid level falls back to a level that is below the level of the lower connection to solar collector 50.

Heat exchanger 54 is preferably arranged close to solar collector 50, a possible distance between storage vessel 10 and solar collector 50 being bridged by pipes from heat exchanger 54 to storage vessel 10 (with a separate liquid circuit from and to a coil 8, or with a direct tap water circuit from and to the tap water in the storage tank 10). The circuit between heat exchanger 54 and solar collector 50 thus forms a compact unit with predetermined height differences (preferably determined height differences independent of the installation of storage vessel 10). Due to the separation of the circuits, no special requirements have to be imposed on the arrangement of storage vessel 10. Storage vessel 10 can for instance be arranged at an arbitrary height of deflating 52.

As previously stated, the invention can also be applied instead of to an installation with a solar heating unit to an installation with a heat pump instead of the solar heating unit or in addition to the solar heating unit.

Figure 6 shows an alternative transfer circuit for use with a solar collector 50. A supply line coming from 3-way valve 18 is coupled to the solar collector 50 via a first valve unit 60. Solar collector 50 is coupled via a second valve unit 62 to the return line 26 to storage tank 10. First valve unit 60 further has an outlet 64 to the sewer or another water outlet. Second valve unit 62 has a venting input. A control unit 66 controls first and second valve unit 60, 62.

In operation, incident sunlight heats tap water in solar collector 50. The heated tap water is pumped directly by pump 17 through solar collector 50. Valve units 60, 62 provide protection 1027140 15 against damage due to frost or overheating. In normal operation, valve units 60, 62 allow water to flow to and from storage vessel 10. In a shut-off position, valve units 60, 62 block the connections to storage vessel 10. In this case, first valve unit 60 connects water collector 5 from solar collector 50 to drain 64, while second valve unit 62 the water circuit of solar collector 50 at the aeration input switches. As a result, the water can drain from solar collector 50 without causing damage. The valve units 60 and 62 can also be integrated into one unit. Control unit 66 switches valve units 60, 62, for example, from normal operation to the shut-off position if a temperature sensor (not shown) indicates that the temperature of the water in solar collector 50 falls below a threshold (as protection against frost) or exceeds a threshold (as protection against cooking).

1027140

Claims (20)

1. Heating installation for tap water, provided with a storage vessel, a low-power heating unit and a heating unit and a common supply pipe for supplying water coming from the storage vessel, wherein the low-power heating unit and the heating unit via the common supply pipe are coupled to the storage vessel for heating water withdrawn from the storage vessel, the storage vessel being provided with a first and second supply point, respectively for supplying water heated with the low-power heating unit substantially at the bottom of the storage vessel and with the heating water heated substantially at the top of the storage tank. 2. Heating installation as claimed in claim 1, provided with a common pump, which serves both for circulating water from the storage vessel to the low-power heating unit and for circulating water from the storage vessel to the heating unit. 3. Heating installation as claimed in claim 2, provided with a control unit adapted to switch the pump to a first pump speed when the water in the heating unit is heated and to switch to a second pump speed that is lower than the first pump speed when heated with the layer heating unit power. 20. Heating installation as claimed in claim 3, which installation is furthermore provided with a pressure-dependent valve unit, with an input which is coupled to the pump and outputs which are respectively coupled to the firing unit and the low-power heating unit, which valve unit is adapted to conducting a pressure water generated by the pump to the output to the firing unit or the low-power heating unit, and simultaneously to close the other output. 1027140 5. Heating installation as claimed in claim 4, wherein the valve unit comprises a 3-way valve, with a ball valve between the input and the output to the low-power heating unit and non-return valve between the input and the output, but the heating unit, wherein the ball valve is 5 adjusted to open to a low-power heating unit under a predetermined pressure difference between the input and the output, and to close above that pressure difference, and wherein the non-return valve is arranged to open the heating unit above a further predetermined pressure difference between the input and the output open up. 6. Heating installation as claimed in any of the claims 2 to 5, with a common drain pipe for withdrawing water for heating with the low power heating unit and heating with the heating unit, which drain pipe ends at a drain height in the storage vessel and has a side branch that below the drain height in the storage vessel 15. 7. Heating installation as claimed in any of the foregoing claims, provided with a control unit adapted to activate the heating unit until a hot water body of at least a predetermined temperature is formed in at least an upper part of the storage vessel, which control unit is switchable between a comfort mode and an economy mode. mode, wherein a limit of the upper part in the comfort mode is lower in the storage vessel than in the economy mode. 8. Heating installation as claimed in claim 7, provided with a temperature sensor for sensing a water temperature at a sensing height in the storage vessel at the level of the limit in the comfort mode, which temperature sensor is coupled to the control unit, wherein the control unit is arranged around the heating unit. activate the comfort mode until the temperature sensor indicates a temperature above the predetermined temperature, and in economy mode deactivate the heating unit when the water is heated to the limit for the economy mode, without the temperature sensor having a temperature above the predetermined temperature need to indicate. 9. Heating installation as claimed in claim 8, provided with a further temperature sensor for sensing a temperature above the sensing height in the storage vessel, the control unit being adapted to activate and deactivate the firing unit in the eco-mode as a temperature on the further temperature sensor or below and is above a further threshold value. 10. Heating installation as claimed in any of the foregoing claims, provided with a sensor for determining an inflow speed at which cold water is added to the storage vessel, and a control unit adapted to switch on the firing unit in response to detection that the inflow speed is above a threshold value. 11. Heating installation as claimed in any of the foregoing claims, wherein a mouth for supplying cold water to the storage vessel is made at least in part of porous material and does not contain an open pipe mouth. 12. Heating installation as claimed in any of the foregoing claims, wherein water for heating by the low-power heating unit is withdrawn from the storage tank below an affcap height for withdrawing water for heating by the heating unit. 13. Heating installation as claimed in any of the foregoing claims, wherein the low-power heating unit is provided with a local circuit in which a solar collector, an emptying, a heat exchanger and a pump are included, and wherein the water from the storage vessel outside the local circuit via the heat exchanger is heated, whereby a difference in height between the local circuit and the storage vessel is overcome with a pump between the heat exchanger and the storage vessel. 14. Heating installation as claimed in any of the foregoing claims, wherein the low-power heating unit is included in a tap water circuit which runs through the storage tank and the low-power heating unit in an operating position, and safety valves in the ap water circuit are arranged between the storage tank and the low power heating unit, to switch the installation from an operating position to a shut-off position, wherein in the shut-off position the safety valves interrupt the tap water circuit through the storage tank and the low power heating unit and connect the low power heating unit to a drain connection. 15. Heating installation as claimed in claim 14, provided with a common pump both for circulating water from the storage vessel to the low-power heating unit and for circulating water from the storage vessel to the heating unit. A method for heating tap water, comprising the steps of - storing the tap water in a storage vessel; - heating the tap water from the storage tank to a heating height with a low power heating unit; heating the tap water in the storage vessel with a heating unit, wherein water extracted from the storage vessel for heating and heating is fed via a common supply line to the heating unit and the heating unit and the water extracted from the storage vessel after heating at a return level is returned to the storage vessel, the return height being above the heating height and a height of water being withdrawn for heating. 19. Method as claimed in claim 18, wherein water from the storage vessel is circulated to the low-power heating unit and to the heating unit with a common pump, the pump being switched to a first pumping speed when the water in the heating unit is heated and to a second pump speed, which is lower than the first pump speed, on heating with the low-power heating unit. 20. Method as claimed in claim 18 or 19, wherein the heating takes place until a hot water body of at least a predetermined temperature is formed in at least an upper part of the storage vessel, and wherein switching is carried out between a comfort mode and a saving mode, wherein a limit of the upper part in comfort mode is lower in the storage tank than in saving mode. The method of claim 18 or 19 wherein the firing unit is turned on in response to detection that a replacement rate at which cold water is supplied to the storage vessel is above a threshold value. A method according to any one of claims 17 to 20, wherein water is withdrawn from the storage vessel for heating by the low power heating unit and after heating is returned to the heating level. 1027140