Is the water flow always even

Hot water circulation system

Lexicon> Letter W> Hot water circulation system

Definition: a system of central hot water preparation, which supplies hot water soon after opening a hot water tap, even at distant consumption points

English: hot water circulation system

Categories: building services, heating and cooling

Author: Dr. Rüdiger Paschotta

How to quote; suggest additional literature

Original creation: 03/12/2011; last change: 10.01.2021

URL: https://www.energie-lexikon.info/warmwasser_kreisulationssystem.html

If a central hot water system (with water heating in the central heating system) has simple hot water pipes to the points of consumption, it can take some time for hot water to come when a hot water tap is turned on. This is simply because the pipe from the heat generator to the point of consumption is still filled with cold water. Only when this has drained is warm water available - regardless of the set warm water temperature.

This problem is used in many buildings Circulation lines used (see Figure 1). Here a hot water pipe leads from the heat generator to the point of consumption, and parallel to this another pipe leads back again. A circulation pump (usually installed in the return line in the boiler room) pumps the water in this circuit regularly (for example, once an hour for 5 minutes), whereby it is reheated in the heat generator. With this system you can get warm water quickly after turning on a hot water tap; at most, a short, simply run piece of line leads to a slight delay. Such a circulation system increases comfort and saves water, but increases energy consumption (see below).

In many cases, the use of a circulation system (or alternatively a pipe heating system, see below) is required for hygienic reasons - in Germany this is when the pipe content between the water heater and the most distant tapping point is greater than 3 liters.

In larger buildings there can also be several circulation lines. They can either be supplied with separate pumps or with a single one, if the system is hydraulically balanced accordingly.

Gravity circulation systems are rarely used today - for good reasons.

In the past there were also gravity circulation systems that did not require a pump. Here, however, due to the principle involved, there was a relatively high temperature drop from the water heater to the draw-off points, and circulation could only occur if the water in the return line cooled down significantly. (If the temperature in all pipes is uniform, natural circulation is not possible.) Therefore, this technique is no longer recommended. The disadvantages far outweigh the advantages of the saved circulation pump.

In the case of branched circulation systems, the problem can arise that parts of the system with low flow resistance have a high flow rate, while other parts have less. This can be countered with hydraulic balancing, for example, using regular valves (but this is not easy), or with thermostatic valves or by using separate pumps in the lines.

Energy consumption of hot water circulation systems

Unfortunately, such circulation systems can have considerable energetic disadvantages:

Modern circulation pumps are quite economical, but there are still many old energy wasters.
  • The circulation pump needs valuable electrical energy as long as it is running. In the case of new pumps for single-family houses, the power consumed is usually 10 to 30 watts, but in older pumps it can be considerably higher and can then increase the electricity bills considerably. (An early replacement of the pump can pay for itself quite quickly.) There are also pumps that switch themselves off via an integrated thermostat as soon as a certain temperature is reached.
The heat losses in the system are usually the bigger problem - especially with pipes in uninsulated external walls.
  • As long as the hot water pipes (supply and return pipes) are kept warm, they are constantly losing heat. In winter, however, this heat release is not necessarily a loss; it often contributes to the heating of the building, as long as the lines run largely in heated rooms and not in uninsulated outer walls. In summer, on the other hand, this heat is lost and can even heat the house undesirably. The size of this heat loss depends on the length of the pipes, the quality of their thermal insulation and the hot water temperature and the ambient temperature. Most of the time, it is about amounts of energy that are massively higher than the power consumption of the circulation pump.
  • If a circulation pump is operated continuously in order to keep the entire system at a sufficiently high temperature to kill Legionella, the heat supply required for this is associated with a high return temperature. This reduces the energy efficiency z. B. from heat pumps and partly from condensing boilers.

When operating a solar thermal system, the heat loss from the pipes in summer is of course less of a consequence, since excess heat often occurs here anyway.

The energy losses from circulation systems can be significantly reduced in various ways:

A favorable layout of the room layout of a house allows short hot water pipes. This saves a lot of energy over time.
  • When building the system, care should be taken to ensure that hot water pipes are as short as possible, and that they should not run in external walls (especially not in uninsulated). This is made easier in houses in which several bathrooms and kitchens are arranged directly one above the other and are not distributed over the entire area.
  • Hot water pipes should have good thermal insulation. Unfortunately, this is not very easy; In particular, considerable thermal bridges can occur at the seams of the insulation, so that the heat losses are far greater than one would expect from a naive calculation using the U-values ​​of the insulation material.
  • The hot water temperature should not be set higher than necessary. Since temperatures that are too low promote the proliferation of Legionella bacteria in the water pipes, the required temperature can often not be measured solely on the basis of the temperature required for use on the hot water taps. However, for legionella prophylaxis, it is sufficient that the temperature in the system briefly reaches 60 ° C a few times a day; At other times, the temperature can also be kept a little lower. (Depending on the country, legal regulations may also have to be observed, which we will not discuss here.)
Every circulation pump should have a timer that limits its operating time to the necessary extent, or a thermostatic control. This can also be easily implemented at a later date.
  • The circulation pump should not work all the time, which unfortunately is still the case in many buildings. This leads to a high demand for electricity and (mostly more important) to high heat losses. From an energetic point of view, it is ideal to activate the circulation pump specifically for a few minutes a few minutes before water is required - by pressing a button or simply by briefly turning on the hot water tap. Unfortunately, after activation, it takes a certain amount of time (a maximum of a few minutes) for the pump to convey the hot water to the points of consumption, and this solution is not very practical for apartment buildings. Another possibility is to operate the circulation pump via a timer (or the heating control) specifically only at certain times, for example in the early morning hours, at lunchtime and again in the evening. The savings in electrical energy and heat are so great that the costs for this measure can be amortized in a very short time. There are now also “self-learning” pump controls that automatically determine the appropriate operating times over time based on the registered user behavior. It should be noted, however, that the temporary cooling of the lines increases the risk of legionella multiplying in the system. In this regard, it is more beneficial to switch off the circulation pump via a thermostat as soon as the temperature in the return line is sufficiently high.
  • Of course, a circulation pump should also be correctly dimensioned. The use of an unnecessarily powerful pump increases power consumption and under certain circumstances (especially with copper pipes) can cause problems due to excessive flow rates.
  • The heat losses of the circulation system are particularly unfavorable in summer, since they then do not even contribute to covering the heating requirements of the building. However, a system for solar hot water preparation can also cover such losses, so that they become less important.

As already mentioned, the heat losses of a circulation system can partly be used to heat the house and are therefore not to be regarded as real losses at all. The proportion of the losses actually used for heating, i.e. a reduction in the heating requirement, depends heavily on the respective circumstances. Obviously, this effect is only relevant within the heating season. It also depends on where the escaped heat goes. In the case of pipes in external walls with good external thermal insulation, most of the heat gets inside and is therefore useful for heating. Without externally insulated walls, or even with internally insulated walls, more heat can escape uselessly. Many circulation lines are also located in basements, where an additional heat source is less useful or completely useless.

Circulation system and water heater

In many cases, a circulation system does not go very well with a water heater. As long as the circulation pump is running, the instantaneous water heater must be activated frequently to compensate for heat losses. It now depends on what type of water heater it is:

Not all instantaneous water heaters are suitable for operation with a circulation system.
  • If the heating is carried out via the heating system (e.g. a natural gas boiler), the heating operation may be continuously interrupted (with priority being given to hot water preparation) and the boiler often has to be brought up to the required (often higher) temperature. However, almost only the compact natural gas boilers are operated in this way anyway, and with these this is naturally less of a problem due to the low thermal mass.
  • If the heating is done via an electric instantaneous water heater, it must be thermostatically controlled, otherwise it would become much too hot when the circulation system is working and it is fed with already warm water.

In both cases, it is particularly important to use a timer to limit the operation of the circulation pump to periods of time that are not too long per day in order not to disturb the heating operation too much.

Disturbances in circulation systems

The functioning of circulation systems can have disorders for various reasons that are not always easy to diagnose:

Circulation systems can malfunction for a variety of reasons.
  • A circulation pump or the associated control can fail so that the pump does not run or does not run at the desired times. More often, however, the cause is likely to be that a control (possibly only its clock) is set incorrectly.
Check valves can become inoperable due to limescale or rust particles, which can result in malfunctions.
  • The circulation pump must be equipped with a suitable non-return valve. This is intended to prevent cold water from being added to the hot water via the circulation line when hot water is withdrawn (especially when the pump is idle). (A centrifugal pump can even be damaged if a water flow is forced through it when it is switched off.) However, such a check valve can be defective or rendered inoperable by foreign bodies (e.g. small rust particles from the pipes) (especially with old pipe systems, e . with galvanized pipes that show corrosion). A possible consequence of this is that the hot water is then mixed with lukewarm water, which flows in the wrong direction through the return line. In other cases, the non-return valve causes a large loss of pressure, i.e. slows down the circulation too much.
Non-functional check valves on mixer taps can cause a variety of problems.
  • A defective mixer tap (or one that has been affected by rust particles) can create a connection between cold and hot water pipes. This can result in the circulation pump conveying water in undesired ways: up the hot water pipe, then through the defective fitting into the cold water pipe and from there back to the boiler room. One of the consequences is that you can then briefly get warm water yourself from the cold water tap (which otherwise should never happen). In addition, cold water can inadvertently be added to the hot water (even if it is drawn off at other points), so that the desired water temperature is no longer reached at the tap. You should react to this by locating and correcting the defect and not simply setting the hot water temperature even higher.
  • If the hot water is produced using a flow heater, the circulation mode only works if the circulation pump generates a sufficiently strong flow. Otherwise the instantaneous water heater won't even start and only cold water will be pumped around. This problem can also arise if the water can circulate in undesired ways due to a defective fitting (see above).

Some of these disturbances can occur more frequently if there are many rust or lime particles circulating in the pipe system. You can then come back soon after the problem has been resolved at the respective location (e.g. in a non-return valve). Please note that components such as check valves can react very differently to such contamination, depending on their design.

Colonization with Legionella

Inadequately thermally insulated return pipes increase the risk of colonization with legionella.

Legionella bacteria can not only colonize hot water storage tanks, they can also establish themselves in hot water systems. This not only applies to, but also to circulation systems. Areas that do not reach a sufficiently high temperature over a long period of time to kill Legionella are particularly problematic. This risk also arises when old circulation lines are inadequately insulated, so that the return line in particular never gets particularly hot.

Thermal disinfection is not necessarily effective as there are often cooler spots where bacteria can survive.

One can use a thermal disinfection try, for example, in which the hot water temperature is increased to 60 ° C or more for one hour and the circulation pump runs continuously. However, such measures are not always effective. For example, there may be line branches that are not heated or not heated sufficiently. The bacteria then survive in these cooler areas and can re-spread from there.

Because of the legionella problem, z. B. in Germany a whole series of regulations concerning the obligation to use a circulation system and various details of its operation. For example, the water returning from the return line must not be cooled by more than 5 Kelvin, and the circulation pump must not be switched off for too long times per day. Unfortunately, compliance with these regulations leads to increased energy consumption, but without at least eliminating the legionella problem with certainty.

Electric heating tapes as an alternative solution

Instead of circulation pipes, electrical heating tapes (as Pipe heating) that keep the (simply routed) hot water pipes warm (e.g. via built-in PTC thermistors, with decreasing heating power when the target temperature is reached) At first glance, this solution appears attractive, as only simple water pipes are required, which saves costs and significantly reduces heat losses. In addition, a number of possible disturbances that are typical for circulation systems (see above) are avoided from the outset.

Caution: Electric heating tapes for hot water pipes can be enormous energy wasters!

Unfortunately, however, such heating tapes can lead to a high consumption of valuable electrical energy if the thermal insulation of the lines is not very good. As mentioned above, there are often massive losses through thermal bridges, even with sufficient insulation thicknesses. It is true that the total heat losses are lower if a return pipe can be dispensed with thanks to a circulation system. The consumption of final energy can therefore be significantly lower than with a circulation system.However, the electrical energy required for this is much more valuable and expensive than the heat provided by a heating system (except, of course, in systems with an electrical instantaneous water heater or electric boiler); the primary energy requirement is therefore usually higher than with a circulation system. The resulting electricity consumption can easily become a substantial part of the total consumption in the house.

It should also be noted that a solar thermal system can cover the heat losses of a circulation system (provided there is enough solar radiation), but cannot prevent the power consumption of the heating cables mentioned.

Dispensing with a circulation system

If the pipe lengths between the water heater and the consumption points can be kept short enough, it is also possible to dispense with a circulation system. This can also be achieved with centralized hot water preparation if the rooms supplied with hot water are arranged close to one another in a small house and allow a correspondingly small water content in the system.

In larger houses, on the other hand, semi-decentralized water heating is an option. For example, the hot water for each apartment in an apartment building can be prepared separately with a gas boiler in the bathroom. However, this does not make it possible, for example, to produce solar hot water. This allows the alternative approach of using decentralized fresh water stations that are centrally supplied with heating water.

Otherwise, you could in principle choose a completely decentralized solution - with an electric boiler at every point where warm water is required. Then you can only use electrical energy, which is not very efficient - except when the amount of hot water is very small and the device has very good thermal insulation to keep standby losses low.

Problems with hot water billing

In apartment buildings, the cost accounts for hot water should be based at least in part on the measured water volumes. However, this measurement becomes difficult or at least expensive if every household draws hot water through several branches on a common circulation system. Then it becomes necessary to install a water meter in each of these branches. This causes considerable installation costs and also increases the effort required to read the meters.

Unfortunately, it is not practical to place water meters within the circulation line in order to determine the sum of all branches between any two such meters by means of a differential measurement. Since the amount of water circulated is usually much larger than the amount of water used, even small measurement errors of the meters have a massive effect on the accuracy of the differential values. In addition, the continuous operation of the circulation would put a lot of strain on the meters.

If there is only one meter for hot water in the cold water supply line of the water heater (or in the hot water line after it), it is of course not possible to determine the consumption of different parties separately.

Questions and comments from readers

15.05.2020

We have just recently renovated and partially rebuilt a house. Now we have noticed that all newly drawn or laid pipes do not provide hot water directly. It takes a long time to get warmer. This is also the case in the master bathroom, where we have changed almost everything. On the same floor, but in the guest shower, the water warms up immediately, here we have left everything in its original place.

Can something like this be due to the circulation pump, or can it be ruled out because the old pipes all have hot water immediately?

Answer from the author:

Presumably, the new pipes were not included in the circulation, so that they always have to be filled before warm water comes in. Apparently they are also relatively long, so that becomes a nuisance. That should have been considered and, if necessary, discussed before the relocation; Sometimes a good solution in this regard is much more complex to set up, so that you have to decide between higher costs and the problem you mentioned. In any case, something like this should be considered and discussed in advance.

But there are also cases where, for another reason, you don't get hot water for far too long - for example when cold water is mixed with hot water somewhere due to a defective valve (e.g. on a shower fitting). Since the problem often occurs elsewhere (far from the defective valve), it is sometimes not easy to find it.

15.11.2020

There are debris and traces in my dishwasher and faucet filters and even in my Brita water filter; it looks like lime and rust. My landlord claims that this would come into his house from the community, but he also says he has a circulation pump against lime. Can it come from his old management or from the community?

Answer from the author:

Apparently you have bothersome lime and rust particles in the water. This happens often and can occur in the house - especially in older lines, especially galvanized ones. Normally the water supplied by the community should be fairly clean, so the problem should have originated in the house itself.

A fairly safe solution would be to replace the entire pipe system, but that would be very time-consuming - with weeks of construction work throughout the house. Instead, I would first have it checked whether the pipes are suitable for the water used and vice versa; For example, you shouldn't lower the degree of hardness too much with a water softener if you still have old galvanized pipes. In some cases old pipes are decalcified with a rinsing action, but usually only when the free cable cross-section has already become too small.

Otherwise you can use various types of filters, but sooner or later they will be clogged if they are not cleaned regularly. A “circulation pump against lime” probably means a pump that is equipped with such a filter.

I would not expect any health problems, as rust and lime are problem-free in this regard.

30.11.2020

I am trying to understand some strange behavior of our hot water system at the moment. We have a hot water circulation system with a pump in the hot water return, a flow sensor in the cold water supply and a heat accumulator as a “flow” heater, the fresh hot water of which is mixed with the return by a three-way valve. For some time now a problem has arisen: When showering, the water gets colder and colder, so you have to keep the tap on hot - until finally only lukewarm water comes in here. Switching off and waiting a few minutes helps - especially when the pump starts up again.

What I find really strange, however, is: If the water in the shower is only lukewarm and I turn on the hot water at the sink in the same bathroom at the same time, hot water immediately comes back in the shower! (The fact that at some point only lukewarm water comes out also occurs with the bathtub, so it does not depend on the shower faucet). If I turn off the shower and turn on the sink, it is only lukewarm here, too. It cannot be due to the performance of the heat storage tank, its temperature remains more or less constant. Do you have a suggestion? Rather a fitting, pump, mixer or fitting? I'm grateful for every tip!

Answer from the author:

I suspect the problem is caused by an unwanted addition of cold water. I suspect the through fitting, which presumably contains a mixer tap. The fact that the problem also occurs in a bathtub does not in any way prove that it is not the problem with the shower mixer: Cold water could also flow into the hot water network at the shower mixer and thus lower the temperature in the bathtub. Apparently the mixing of cold water is influenced by the pressure conditions, even if I cannot interpret the matter in detail.

The hot water circulation should not play a role here; it just has to prevent the line network from cooling down if it is not used for a long period of time.

22.12.2020

For some time now we have had violent hammer blows when we turn up the hot water (strongly) on the upper floor. I suspect the circulation line or possibly a defective check valve. At the same time, there is now the phenomenon that after a short turning off, the water is cold again for a long time before warm water comes back. We installed the circulation line about 9 years ago, but never let the pump run because the way is not really that long. I've tried turning the pump on, but I get the impression that it's not working.

All of this apparently occurred after the waterworks turned off the water several times and there were a lot of air pockets in the pipe anyway.

Answer from the author:

I also suspect that it is due to a valve. Such malfunctions occur frequently when work has been carried out on any fittings - but not necessarily as a result of a temporary shutdown of the water from the waterworks.

You should feel whether the pump is running by the vibrations. Whether it promotes water is of course another question; this can usually be determined by changing the temperatures in the pipes.

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