Calculation Of Heating Radiators: How To Calculate The Capacity Of Batteries And Their Number

A competently arranged heating system will provide the dwelling with the necessary temperature and all rooms will be comfortable in all weathers. But in order to transfer the heat to the air space of the living spaces, you need to know the necessary number of batteries, right?

Find out this by calculating the heating radiators based on the thermal power required from the heating devices being installed.

You have never done such calculations and are afraid of making a mistake? We will help you with the formulas - the detailed algorithm of the calculation is explained in the article, the values of the individual coefficients used in the calculations are explained.

To make it easier for you to understand the details of the calculation we have selected thematic photo materials and useful videos explaining the calculation of the heating capacity.

The contents of the article:

  • Simplified calculation of heat loss compensation
  • Detailed formula for calculating heat output
    • Orientation of rooms according to sides of the world
    • Calculating the influence of outside walls
    • Dependence of radiators on insulation
    • Climate is an important factor in arithmetic
    • High room calculation
    • The role of ceiling and floor calculation
    • The quality of the frame is the key to heat
    • Window size matters
    • The influence of the closed radiator closing of the radiator
    • Efficiency of the radiator connection
  • Heating capacity calculation example
  • The thermal capacity of the battery sections
  • Calculating the number of radiator sections
  • Heating efficiency
  • Conclusions and useful video on the topic

Simplified calculation of heat loss

All calculations are based on certain principles. The calculation of the required heat output of radiators is based on the understanding that well-functioning radiators must fully compensate for the heat loss resulting from their operation due to the characteristics of the rooms to be heated.

For living rooms in a well insulated house situated in a temperate climate zone, a simplified calculation of heat loss compensation is in some cases suitable.

In these rooms, the calculation is based on the standard power of 41 W required to heat 1 cubic metre. It is recommended when in doubt about calculations to give preference to values with larger values.

It is easier then to reduce the temperature of radiators with the help of thermostatic controls than to freeze when their thermal capacity is insufficient.

Afterwards each factor involved in the formula for calculating the thermal capacity of batteries is detailed.

In the end of the article, information is given about the characteristics of collapsible radiators of different materials, and the calculation procedure of the required number of sections and batteries themselves based on the basic

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A simplified method of calculating the capacity of radiators needed for normal room heating suggests that for every 10 m3 one kW of heat should be delivered

In order that the room owners have a reserve for unexpected heat losses, the calculated capacity value is multiplied by 1.15, i.e. increased by 15%

Compact radiators used in low temperature heating circuits are as efficient as traditional appliances. Compact radiators, used in low-temperature heating circuits, are as effective as traditional devices. Their output should be calculated in the same way

If a room is restricted by two outer walls and has one window, the calculated thermal output should be increased by 20%

The output of a heating system device installed in a room with an exit to a terrace or a winter garden The power of heating device must be multiplied by the correction factor 1.15

If heating battery is masked by a box or a screen, its power increases by 15 - 20% depending on the heat conducting characteristics of the material,

When calculating the capacity of radiators for a mansard with large panoramic windows the result is increased by 25 - 35%

The average heat output of the radiators in the room with two outside walls

The heat output of the radiators

Low temperature compact radiators in the interior

Radiators in the room with two outside walls

Radiators in the terrace

Heat in the corner

Calculation for the radiator in the box

Heat in the attic

Are the rooms oriented to the cardinal points

And on the coldest days, the thermal balance inside the apartment is still affected by the sun.

The orientation of the rooms in one direction or the other determines the \"R\" coefficient of the heat output formula.

  1. A room with a window to the south is R = 1.0. During the daylight hours it will receive the maximum amount of added external heat compared to the other rooms. This is the basic orientation, and the value added is minimal here.
  2. West-facing window - R = 1.0 or R = 1.05(for regions with a short winter day). This room will also get its portion of sunlight. Although the sun will look there closer to evening, but still the location of this room is more favorable than the eastern and northern ones.
  3. The room is oriented to the east - R = 1,1. Rising winter light is unlikely to have time to properly heat such a room from the outside. Extra watts will be needed for battery power. Accordingly we add to the calculation a tangible correction of 10%.
  4. With only north of the window - R = 1,1 or R = 1,15 (no mistake resident of the northern latitudes, who will take additional 15%). In winter, such a room does not see direct sunlight at all. Therefore it is recommended to adjust the calculation of the required heat efficiency of radiators also by 10% upwards.

If the winds of a certain direction prevail in the living area, it is advisable for rooms with windward sides to increase the R value by another 20% depending on the blowing force (x1,1÷1,2) and for spaces with walls parallel to the cold streams to increase the R value by 10% (x1,1).

Calculation of heating radiators: how to calculate the capacity of batteries and their number

Houses with north and east facing windows and windward facing rooms need more heating power

Consider the influence of outside walls

In addition to the wall with its window or windows, other walls in the room may also have contact with the outside cold.

The outside walls of the room determine the \"K factor\" of the calculated radiator heat output formula:

  • The presence of one outside wall in the room is a typical case. Here the coefficient is simple - K = 1.0.
  • Two outside walls will request 20% more heat to heat the room - K = 1.2.
  • Each additional outside wall adds 10% to the calculations. For three street walls, K = 1.3.
  • If the room has four external walls, this also adds 10 % - K = 1.4.

Depending on the nature of the room for which the calculation is carried out, a corresponding coefficient must be taken.

Reflector insulation dependence

Cut the heating budget of the internal space by properly and correctly insulating the house from the winter cold and significantly.

The degree of insulation of the outside walls is determined by the \"U\" coefficient decreasing or increasing the design heat output of the heating devices:

  • U = 1.0 - for standard outside walls.
  • U = 0.85 - if the external walls have been insulated in a special calculation.
  • U = 1.27 - if the external walls are not cold enough.

Standard walls are those with materials and thickness appropriate to the climate. As well as reduced thickness, but with a plastered exterior surface or with surface exterior thermal insulation.

If the area of the room permits, it is possible to make insulate the walls from the inside. And there is always a way to protect the walls from the cold outside.

Calculation of heating radiators: how to calculate the capacity of batteries and their number

Well insulated by special calculation corner room will give a significant percentage of savings on heating costs of the whole living area of the apartment

Climate is an important factor of arithmetic

Different climatic zones have different indicators of minimum low outdoor temperatures.

In calculating the heat output of radiators, a \"T\" factor is provided to account for temperature differences.

See the values of this factor for different climatic conditions:

  • T = 1.0 to -20 °C.
  • T = 0.9 for winters with frost to -15 °C
  • T = 0.7 to -10 °C.
  • T = 1.1 for frosts up to -25 °C,
  • T = 1.3 - to -35 °C,
  • T = 1.5 - below -35 °C.

As seen from the list above, winter weather down to -20 °C is normal. For the areas with the least cold, a value of 1 is taken.

For warmer regions, this calculation factor will lower the overall result of the calculations. But for regions with harsher climates the amount of heat required from the heaters will increase.

Peculiarities of calculating high rooms

It is clear that of two rooms with the same area, more heat is needed for the room with the higher ceiling. The correction for the size of the heated space can be taken into account in the calculation of the heating capacity by the coefficient \"H\".

You can see at the beginning of the article that a normative room is mentioned. A room with a ceiling of 2,7 meters and less is considered as such. Here the coefficient is 1.329>


  • H = 1.0for 2.7 m ceilings.
  • H = 1.05 - for a room up to 3 meters high.
  • H = 1.1 - for a room with a ceiling up to 3.5 meters high.
  • H = 1,15 - up to 4 metres.
  • H = 1,2 - heat requirement for a higher room.
  • As you can see, for rooms with high ceilings 5% has to be added for every half metre of height from 3,5 metres.

    Law of nature says that warm heated air rushes upwards. In order to move it all upwards, the radiators have to work hard.

    Calculation of heating radiators: how to calculate the capacity of batteries and their number

    A room of equal size may require more radiators to be connected to the heating system

    Regulation of the ceiling and floor

    It is not only well insulated outer walls that reduce the thermal capacity of radiators. A ceiling in contact with the warm room also helps to minimise the room heating loss.

    The \"W\" factor in the calculation formula is just that:

    • W = 1.0 - if there is an unheated and uninsulated loft for example upstairs.
    • W = 0.9 for an unheated but insulated attic or other insulated room above.
    • W = 0.8 for a heated room above.

    W can be corrected upwards for first floor rooms if placed on the ground above an unheated basement or basement space. Then the figures are: floor insulated +20% (x1,2); floor not insulated +40% (x1,4).

    Frame quality is the key to heat

    Windows were once the weak point in insulating living space. Modern insulated frames have made it possible to significantly improve the protection of rooms from the outside cold.

    The degree of window quality in the formula for calculating heat output describes the \"G\" coefficient.

    The calculation is based on a standard frame with a single chamber insulated glass unit that has a coefficient of 1.

    Let us look at the other variations of the coefficient:

    • G = 1.0 - frame with single glass unit.
    • G = 0.85 - if the frame is equipped with double or triple glass unit.
    • G = 1.27 - if the window has an old wooden frame.

    If the house has old frames, the heat loss will be considerable. Therefore, more powerful batteries will be required. Ideally it is desirable to replace these frames as this represents an additional cost for heating.

    Window size does matter

    According to logic, the more windows a room has and the wider the view, the more sensitive the heat loss through them is. The \"X\" coefficient in the formula for the heat output required from the radiators reflects this.

    Calculation of heating radiators: how to calculate the capacity of batteries and their number

    In a room with big windows, the radiators should be of the same size and quality as the room frame

    The normal value is the total of the window area divided by the room size which is between 0.2 and 0.3.

    Let us give the basic values of the coefficient X for different situations:

    • X = 1.0 - for a ratio of areas from 0.2 to 0.3.
    • X = 0.9 - for a ratio of areas from 0.1 to 0.2.
    • X = 0.8 - when the ratio of areas is up to 0.1.
    • X = 1.1 - when the ratio of areas is from 0.3 to 0.4.
    • X = 1.2 - when it is from 0.4 to 0.5.

    If the size of the window openings (for example in rooms with panoramic windows) exceeds the suggested ratios, it is reasonable to add another 10% to the X value when the area ratio increases by 0.1.

    A door in a room that is regularly used in winter to access an open balcony or loggia makes its own adjustments to the heat balance. For such a room, it is right to increase X by another 30% (x1,3).

    The loss of heat energy can easily be compensated by a compact installation of a water channel or an electric convector under the balcony door.

    The influence of the closed battery

    All the radiator which is less obstructed by various artificial and natural obstacles will certainly give the heat better. In this case, the formula for calculating its heat output is extended by the coefficient \"Y\", taking into account the conditions of the battery.

    The most common location of radiators is under the window sill. In this position the coefficient is 1.

    See the typical situation where the radiators are placed:

    • Y = 1.0 - just under the window sill.
    • Y = 0.9 - if the radiator is suddenly completely open from all sides.
    • Y = 1.07 - when the radiator is obscured by a horizontal wall ledge
    • Y = 1.12 - when the radiator under the window sill is covered by a front cover.
    • Y = 1.2 - when the radiator is enclosed on all sides.

    Long, thick curtains also cause the room to get cold.

    Calculation of heating radiators: how to calculate the capacity of batteries and their number

    The modern design of radiators allows them to be operated without any decorative covers - thus ensuring maximum heat output

    Efficiency of radiator connection

    The way the radiator is connected to the interior heating system directly affects its efficiency. Often homeowners sacrifice this indicator in favor of the beauty of the room. The formula for the calculation of the required heating power takes all this into account by means of the \"Z\" coefficient.

    The following are the values for different situations:

    • Z = 1.0 - connection of the radiator to the heating circuit in a \"diagonal\" connection, the most defensible.
    • Z = 1.03 - the other, most common because of the short distance of the supply pipes, is the connection \"from the side\".
    • Z = 1.13 - the third method \"from below on both sides\". Thanks to the plastic pipes, it has quickly caught on in new construction, despite its much lower efficiency.
    • Z = 1.28 is another, very low efficiency method \"from below on one side\". It is only worth considering because some radiator designs are supplied with prefabricated units with both flow and return pipes connected at one point.

    Installing air vents in the radiators would increase the efficiency and in due time save the system from \"choking\".

    Calculation of heating radiators: how to calculate the capacity of batteries and their number

    Before you hide the heating pipes in the floor using ineffective radiator connections, you should think of the walls and ceiling

    The working principle of any water heater is based on the physical property of the hot liquid to rise up and after cooling to move down.

    Hence, it is strongly recommended not to use radiator connections with the supply pipe at the bottom and the return pipe at the top.

    Practical example for heat output calculation

    Basic data:

    1. A corner room without balcony on the second floor of a two storey plastered cinder block house in a windless area in Western Siberia.
    2. Length of room 5.30 m X width 4.30 m = area 22.79 sq.m.
    3. Window width 1.30 m X height 1.70 m = area 2.21 sq.m.
    4. Height of room = 2.95 m.

    Calculation sequence:

    Area area in sq. S = 22.79
    South window orientation:R = 1.0
    Number of outside walls - two: K = 1.2
    The insulation of the outer walls is standard:U = 1.0
    Minimum temperature is to -35°C: T = 1.3
    Height of room - up to 3 m:H = 1.05
    Upper room - uninsulated attic:W = 1.0
    Frames - single-glazed G = 1.0
    Window-to-room ratio - up to 0.1:X = 0.8
    Radiator position - under sill:Y = 1.0
    Diagonal radiator connection: Z = 1.0
    Total (remember to multiply by 100):Q = 2,986 Watts

    The following describes the calculation of the number of radiator sections and the number of batteries required. It is based on the thermal capacity results obtained taking into account the dimensions of the intended installation locations of the radiators.

    Independent of the results, it is recommended that in corner rooms not only the sill niches are equipped with radiators. Batteries must be installed next to \"blind\" external walls or near corners which are subject to frost penetration due to the cold outside.

    Specific heat output of battery sections

    Even before a general calculation of the required heat output of radiators, it must be decided on the collapsible radiator material to be installed in the rooms.

    The choice must be based on the heating system characteristics (internal pressure, temperature of the heating medium).

    How to correctly calculate the right number of different batteries for heating, and we will talk further.

    For a 70°C heating medium the standard 500 mm sections of radiators made of different materials have different specific heat power \"q\".

    1. Cast iron - q = 160 Watt (specific power of one cast iron section). Radiators made of this metal are suitable for any heating system.
    2. Steel - q = 85 watts. Steel tubular radiators can handle the harshest conditions. Their sections are beautiful in their metallic luster, but have the lowest heat output.
    3. Aluminum - q = 200 watts. Lightweight, aesthetic aluminum radiators should be installed only in self-contained heating systems where the pressure is less than 7 atmospheres. But in terms of heat emission their sections have no equal.
    4. Bimetal - q = 180 Watts. The insides of bimetallic radiators are made of steel and the heat dissipating surface is aluminum. These radiators will withstand all kinds of pressures and temperatures. The specific heat output of bimetal sections is also at a high level.

    The values of q are rather relative and are used for a preliminary calculation. You can find more exact figures in the data sheets of the radiators to be purchased.

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    The advantages of the sectional assembly principle

    The basic rules for assembly of heating appliances

    Section of the old cast iron radiator

    Powder-coated color sections

    Calculating the number of radiator sections

    Assembled radiators of any material are good that individual sections can be added or subtracted in order to reach their calculated heat output.

    For determining the required number of \"N\" radiator sections in the selected material the following formula is followed:

    N = Q / q,


    • Q = the previously calculated thermal capacity of the devices for heating the room,
    • q = thermal capacity of the individual sections assumed for installation of batteries.

    Calculating the total number of radiator sections required in the room, it is necessary to understand how many radiators in total are to be installed. This calculation is based on a comparison of the dimensions of the intended installation locations of the radiators and the size of the batteries including the supply pipes.

    Calculation of heating radiators: how to calculate the capacity of batteries and their number

    Battery elements are connected with nipples with different male threads using a radiator spanner and gaskets are placed in the joints simultaneously

    For the preliminary calculations you can obtain data about the width of the different radiator sections:

    • cast iron = 93 mm,
    • aluminum = 80 mm,
    • bi-metal = 82 mm.

    When making collapsible steel pipe radiators, manufacturers do not hold themselves to certain standards. If you want to put such radiators you have to approach the issue individually.

    You can also use our free online calculator to calculate the number of sections:

    Heat efficiency

    When the radiator heats the inner air of the room it also heats the outer wall in the area behind the radiator. This leads to additional unjustified heat loss.

    It has been suggested that a heat reflective screen should separate the radiator from the outside wall to increase the heat transfer efficiency.

    The market offers many modern insulating materials with a heat reflective foil surface. The foil protects the warm air warmed by the radiator from contact with the cold wall and directs it inside the room.

    For proper operation, the boundaries of the installed reflector should exceed the dimensions of the radiator and protrude 2-3 cm on each side. A space of 3-5 cm should be left between the heater and the heat shield surface.

    Isospan, penofol, alufoam can be recommended for making a heat reflective screen. Cut a rectangle from the purchased roll with the necessary dimensions and fix it to the wall where the radiator is mounted.

    Calculation of heating radiators: how to calculate the capacity of batteries and their number

    The heat reflective screen of the radiator is best fixed to the wall with silicone glue or liquid nails

    It is recommended to separate the insulation sheet from the outer wall with a small air space, e.g. with a thin plastic grating.

    If a reflector is joined with several pieces of insulation material the joints on the foil side have to be covered with metallized adhesive tape.

    Conclusions and useful videos on the subject

    Small films present the practical implementation of some engineering tips in the home. In the following video you can see a practical example of calculating heating radiators:

    Changing the number of radiator sections is considered in this video:

    The next video will tell you how to mount a reflector under the radiator:

    Gained skills in calculating the thermal capacity of different types of heating radiators will help the home master in a competent arrangement of the heating system. And homeowners will be able to control the correctness of the process of installing batteries by third-party specialists.

    Have you been calculating your own heating capacity for your home? Or have you encountered problems as a result of installing low wattage radiators? Tell our readers about your experience - please comment below.

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