Compulsory circulation water heating system: schemes, implementation options, technical details

Alexey Dedyulin
Checked by a specialist: Alexey Dedyulin
Author: Mikhail Yashin
Last update: August 2019

Many modern solutions for water heating of houses require the use of a pumping group. Design and installation of a heating system with forced circulation must be carried out taking into account technical aspects arising from the rapid movement of the coolant.

High pressure in the heating circuit allows you to implement many wiring diagrams. Agree, this is an important advantage of a forced circulation heating system. However, the arrangement of such a scheme requires competent design.

We will tell you what characteristics the main working units of the system are selected for, and we will also describe in detail the possible wiring options for the main and how to organize the heating circuit.

Technical features of the main components of the system

The forced circuit differs from the natural one by the addition of one or more circulation pumps. Due to the increase in pressure and velocity of the coolant, the rules for the formation of nodes and the location of the circuit elements change.

This fact must be taken into account in order to ensure high-quality heating during forced circulation.

General requirements for the pump group

Circulation pumps are selected based on the requirements for the volume of distilled water (cubic meter per hour) and pressure (meter). The calculation of both parameters depends on the cubic capacity of the heated housing and the heating method, as well as the length of the water circuit and the diameter of its pipes.

The pump should be chosen so that its parameters are not “close to” the requirements of the system. This will make it possible to add elements to the circuit if necessary without replacing the pump.

Basically, the pumps are designed for a voltage of 220 volts, but there are also with support for 12 volts. During power surges, it is necessary to install a stabilizer to prevent the device from failing.

In case of frequent power outages, care must be taken uninterruptible power supply. There is no need to take a powerful UPS - for heating private houses rarely use devices with a consumption of more than 150 watts per hour.

Conditionally circulating pumps can be divided into two types according to the position of the motor. Devices with a dry rotor have a higher efficiency, but have a higher noise level and lower resource than with a wet rotor.

If the wiring of the system provides an opportunity for the natural movement of the coolant along the circuit, then the pump must be mounted through the bypass. In this case, when it breaks or a power outage is possible, the heating can switch to the gravitational circulation mode.

Water can also move through an idle pump, but it will create a strong resistance to its movement.

Calculation of the operating point of the pump
The choice in favor of the pump model in relation to a specific heating system is carried out by determining the operating point and matching it with the required values ​​of the coolant flow (+)

Particularly relevant is the problem of stopping the pump when using stove or fireplace heating. In this case, the furnace will continue heating the heat exchanger and it is possible to boil water in it and permanently break down the entire system.

Better to do pump installation on the return pipe, because a lower water temperature will extend its service life. If it is not possible to install the pump in a place other than on the pipe leaving the boiler, then use a pump with ceramic seals.

Although they can withstand temperatures up to 110 ° C, but when the system boils, they may have problems with functioning.

Bypass circulation pump
Having mounted the pump in the circuit through the bypass, it is possible to achieve not only normal working conditions on the principle of natural circulation, but also the possibility of removing the pump without draining the water

Useful information on the selection of circulation pumps for the heating system is given in the articles:

  1. Selection of a circulation pump: device, types and rules for choosing a pump for heating
  2. Circulation pump for heating: top ten models and tips for customers

The subtleties of choosing boilers and furnaces

The use of electric and gas boilers and long-burning furnaces as a heat generator is attractive from the standpoint of simplicity of controlling the flow of heat through a heat exchanger.

Application solid fuel furnaces, especially home-made designs, is fraught with insufficient or excessive heat. However, their use is often justified from the point of view of cheapness and availability of fuel.

Many models of electrical and gas boilers with integrated pump. On the one hand, the built-in circulation system is selected according to the capacity of the boiler and eliminates the need to purchase and install a separate pump. On the other hand, if the built-in pump breaks down, it will not be so easy to repair or replace it as a separate one.

Electric boiler with integrated pump
The electric boiler with integrated pump is a ready-made and compact solution for incorporating it into a forced circulation circuit

The requirements for the boiler when using forced circulation are the same as for natural:

  • Calculation of boiler power - the indicator should satisfy the needs of heating the house in the most severe conditions for a particular area. It is advisable to have a small margin in power (10-20%) due to possible force majeure circumstances that may arise in the heating system.
  • Prevention of boiling coolant in the heat exchanger. It is easier to fulfill this requirement when using the furnace – pump combination than with the gravitational model of fluid motion.

To prevent boiling water in the heat exchanger of the boiler, it is enough to set the power control depending on the temperature of the outgoing liquid. This method works with any kind of circulation.

Solid fuel boiler control unit
The control unit for the operation mode of the solid fuel boiler includes the function of starting the pump when critical temperatures are reached at the outlet of the heat exchanger

For furnaces with natural circulation, there is no way to prevent the coolant from boiling in the event of an excessive amount of loaded fuel. The only option in the presence of a pump is an increase in the volume of the liquid being driven through the heat exchanger.

Moreover, such an emergency system can be made automatic using a thermostat and a pump speed control unit.

Installation and inspection of the water circuit

With a heating circuit using forced circulation, there will be higher water speeds than in the gravity model. Therefore, you can use a smaller pipe diameter with the same heating parameters of the building. This reduces the cost of water heating in terms of costs for pipes, fittings and fittings.

In addition, smaller contour elements are easier to hide in technological niches or fit into the interior of the premises.

Compared to natural circulation, increased hydrodynamic pressure of the flow will be added to the hydrostatic pressure of the liquid column. Therefore, in order to avoid the formation of leaks or, moreover, a breakthrough of the system, it is necessary to adhere to some rules.

In the case of a transition from gravitational to forced circulation, it is necessary to eliminate all, even minor leaks in the circuit. With increasing pressure, the flow rate will increase, which, in addition to problems in the room, will cause a decrease in the amount of coolant and its excessive aeration (air saturation).

Before the onset of the heating period, it is necessary to carry out hydraulic tests of the circuit strength with the maximum used or even slightly higher pressure. This will allow you to identify problems and eliminate them before the onset of cold weather, when a long stop of heating for repair is undesirable.

Leaking radiator under the ceiling
Heating radiators can leak in the most unexpected places and it will take a long time to fix the problem, so it’s better to check the integrity of the system in advance

Since the velocity of the coolant will be more than 0.25 m / s, according to SNiP 41-01-2003 there is no need to maintain a constant pipe slope to remove air from the circuit. Therefore, with forced circulation, the installation of pipes and radiators is slightly simpler than with a gravitational circuit.

Forced circulation heating options

The use of forced circulation allows you to move away from the principle of designing wiring with the obligatory allowance for the difference in hydrostatic pressure, which is necessary for functioning in the gravitational scheme.

This adds variability in modeling the geometry of the water circuit and makes it possible to use solutions such as collector heating or large floor heating.

Top and bottom wiring

Any heating scheme can be conditionally attributed to the upper or lower wiring. At the top wiring, hot water rises above the heating appliances, and then, flowing down, heats the radiators. At the bottom - hot water is supplied from below. Each option has its positive aspects.

Top wiring is also used for natural circulation. Therefore, heating circuits of this type allow you to use both types of circulation. This, firstly, provides a choice, and secondly, increases the reliability of the system.

In the event of a power outage or breakdown of the pump, the movement of water along the circuit will continue, albeit at a lower speed.

Upper and lower wiring
A good pressure allows you to make a choice between the upper and lower wiring, taking into account the convenience of conducting pipes that supply coolant to the radiators (+)

When using the lower wiring, the total length of the pipes is less, which reduces the cost of creating a system. In addition, there is no need to lay risers on the top floor, which is good from the standpoint of room design. The lower hot water feed pipe is laid either in the basement or on the floor level of the first floor.

Varieties of single-pipe connection schemes

The one-pipe scheme uses the same pipe to supply hot water to the radiators and to discharge cold water to the heating boiler. With this wiring, the length of the pipes used is almost halved, the number of fittings and valves is reduced.

However, heating of radiators occurs sequentially, therefore, when calculating the number of sections, it is necessary to take into account a gradual decrease in the temperature of the supplied coolant.

Connection of a heating radiator with a single pipe scheme
Serial connection of radiators using a single pipe for supplying coolant is often used in modern homes to minimize material costs and simplify installation work

Single pipe schemes can be implemented in horizontal and vertical versions.With forced circulation, in the case of using vertical risers, it is possible to supply hot water not only from above, but also from below.

The feasibility of using one or another option depends not only on the convenience of the pipes, but also on the maximum allowable number of radiators on one riser of a single pipe circuit.

You can connect heating radiators in two ways:

  • Serial connection - the coolant flows through all radiators. In this case, the minimum number of pipes is necessary, however, if it is necessary to turn off one of the radiators, you will have to stop the entire branch of the system.
  • Connection bypass - the coolant can flow around the radiator according to the installed discharge. Using a system of taps, you can redirect the flow past the radiator, which will allow for its repair or dismantling without stopping the heating.

A single-pipe scheme is often used for heating, but in the case of a large number of radiators, another option is used to uniformly heat them.

Single pipe implementation options
Single pipe schemes have many options for forced circulation, so choosing the right solution for a particular room geometry is quite easy (+)

Ways to use the two-pipe version

The heating circuit diagram using a second pipe to drain the cooled water to the boiler is called double pipe system. Pipe length increases, as does the number of connections and devices.

However, the system has an important plus - a heat carrier of the same temperature is supplied to each radiator. This makes the two-pipe version very attractive.

With water heating with forced circulation, both horizontal and vertical wiring are used. Moreover, with the vertical version, it is possible to use the upper and lower hot water supply.

Diagonal radiator connection with two-pipe design
A two-pipe water supply and drainage system in conjunction with a diagonal radiator connection gives maximum heat transfer to the room

Since the temperature of the supplied water to all radiators is the same, the geometry of the circuits depends only on the following factors:

  • material saving - minimize pipe footage and number of connections;
  • ease of contour heating through walls and floors;
  • aesthetic appeal - the ability to fit heating elements into the interior of the premises.

Depending on the movement of hot and chilled water, two-pipe schemes are divided into two types:

  1. Associated. The movement in both pipes occurs in one direction. The cycle of the coolant cycle has the same length for all radiators in this part of the system, so their heating rate is the same.
  2. Dead ends. In the associated circuit, radiators located closer to the boiler heat up faster. However, for systems with forced circulation, this is not very important due to the significant speed of the water in the circuit.

When choosing between the associated and dead-end option, they are guided by the condition of the convenience of carrying out the return pipe. In vertical circuits, with a lower wiring, a dead-end system is obtained, and with an upper one, a passing system.

Using a heating distribution manifold

Another popular way of organizing heating is now collector circuit design. To some extent, this scheme can be called a two-pipe subspecies, although it is also used in the organization of single-pipe heating circuits.

Only the distribution of hot coolant and the collection of chilled does not take place from the main riser, but from special distribution node devices - collectors. Such a system works stably only with the use of forced circulation.

Beam and two-pipe scheme
Radial wiring compared to a two-pipe requires a collector, a greater total pipe length, the number of fittings and valves

The distribution unit for the two-pipe system is a complex combination of the supply and return manifolds, with the help of which the coolant is balanced in temperature and pressure.

Each branch of the device feeds one heating element or a small group of them. Branches are usually located under the floor; each floor of a multi-story building is served by one collector installed in the center.

Despite the obvious advantages of this option for organizing heating, the collector system has two significant disadvantages:

  • longest pipelines, therefore, this option of organizing a water circuit requires considerable financial investments;
  • difficulty changing contour - pipes with this option are usually located under the floor or in the walls, so in the case of adding heating appliances it will be very difficult to make any corrections.

All collectors are mounted, as a rule, in a special cabinet, since the shutoff valves are located there and access is required. Placing cranes in one place is very convenient.

In the case of the need to turn on or off the radiators or an emergency, it is enough to have access to the cabinet and there is no need to visit all the rooms.

Distribution manifolds can have a simple structure consisting of two combs and a minimum of shutoff valves. Complex assemblies can also include automatic thermostats, electronic valves, mixers, automatic air exhaust devices, sensors and control units, a valve for draining water, and a separate circulation pump.

These systems can most accurately adjust the temperature in the house, but require a good understanding of the basics and nuances of the work of water heating.

Underfloor heating

One of the most comfortable ways of heating is considered warm floor organization. It should be noted that the installation of this option for heating living rooms, showers, kitchens and other rooms is quite complicated.

Water floor heating of a large area is possible only with the organization of forced circulation, since it is necessary to create pressure in a long system of narrow tubes.

Pressure is needed to overcome the resistance of narrow pipes with many bends. In addition, it is necessary to achieve a pressure that allows you to remove air from the pipes of the warm floor, which are located horizontally.

There are many tube-laying combinations:

  • for small rooms apply schemes with one inlet for hot water and an outlet for chilled;
  • for large rooms organize more complex underfloor heating systems using a distribution manifold.

Often for fragments of a circuit with a warm floor, separate circulation pumps are installed.

Distribution manifold for underfloor heating
The use of the collector is justified for large areas of underfloor heating, when calculations show that one pipe may not cope with heating

Conclusions and useful video on the topic

A detailed presentation of a two-pipe and rather complex heating scheme for a two-story house:

Closed system for a three-story house based on a gas boiler:

The use of pumps for water heating of premises greatly facilitates the design of the circuit, making possible options that are not available for the gravity model. The correct selection of equipment will solve the issue of heating homes, making this process convenient and simple.

Have something to supplement, or have questions about organizing a heating system with forced circulation? Please leave comments on the publication and participate in discussions. The contact form is located in the lower block.

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Visitors Comments
  1. Denis

    We have central heating, but since we live in a private house, we can turn on the heating at any time. But the problem is that it is going very weakly, there is no pressure, and they give it only when the heating season begins. Therefore, when it is cold, you have to turn on the pump. They installed it at the end of the branch, for 5 minutes the pipes are already hot, I think that for us the pump is the most optimal option, it does not have much electricity, you can drive around the clock.