How to measure a radiator

Proper selection of radiators requires knowledge and analysis of data concerning the size, type and conditions in the room. The parameters of the installation and the device itself are also important, such as, for example, the heat output of the radiator. How important is the calculation of the radiator power and what else is worth paying attention to when choosing a radiator?

Heater heating power

The first important parameter when selecting a radiator for a specific room is the radiator’s heat output. This is a value that determines the ability of the device to dissipate heat to the room. The radiator’s heat output is expressed in watts [W] and should be equal to or greater than the space’s heat requirement.

It is worth remembering that when calculating the power of the radiator, the size, type and conditions in a specific room (room dimensions), and not in the entire building, are taken into account. The demand for heat is different in the bathroom, where the highest level of humidity is recorded, and in the kitchen, where the additional heat source is an oven or a hob. Yet another demand for heat occurs in the hallway, bedroom or living room (even when the rooms are of similar dimensions).

For example, the optimal temperature should be:

  • 22-24 ° C in the bathroom,
  • 20-22 ° C in the living room and dining room,
  • 18-20 ° C in the kitchen,
  • 16-18 ° C in the bedroom and hall.

The above temperature ranges are legally defined. However, the very value of the optimal temperature will not always mean that, for example, for a bathroom you should choose a radiator with the highest heating power, and for a bedroom with the lowest heating power. It is worth considering additional conditions, for example, whether the bathroom has underfloor heating or whether the windows in the bedroom are tight.

How to calculate the power of the radiator also depends on other parameters:

  • the prevailing external conditions (temperature and climatic zone),
  • the location of the building in relation to the directions of the world,
  • heat transfer coefficient of walls, ceiling and floor,
  • tightness of windows and doors,
  • the type and purpose of the room.

Which radiator power should be chosen for individual rooms?

Each room in your home has its own heat demand. Although all rooms may have similar dimensions, the needs of the kitchen, bathroom or hall will be different. This should be borne in mind and the power of heating system should be adjusted to a given room.

The temperature in the bathroom should, for example, be slightly higher than in the living room or bedroom. In the kitchen, on the other hand, we can afford a smaller heater power, because household appliances are additional sources of heat. The heat demand of the rooms in the house is as follows:

– For rooms with no more than one window or external wall and their optimal air temperature is no more than 20 degrees Celsius (living room, hall, kitchen), choose a heater with a power of 70-80 W / m2.

– For rooms with more than one window or external wall and their optimal air temperature is no more than 20 degrees Celsius (living room, hall, kitchen), choose a heater with a power of 80-100 W / m2.

– For rooms that do not have windows and external walls and their optimal air temperature is approx. 24 degrees Celsius, e.g. a bathroom, choose a heater with a power of 100-120 W / m2.

-For rooms with an external window or an external wall and their optimal air temperature is approx. 24 degrees Celsius, e.g. a bathroom, you should choose a heater with a power of 130-150 W / m2.

It is also important to install the radiator in the right place. The best location will be under a window or exterior wall of the building. The place where the radiator is to be placed also affects the power that should be chosen. If you plan to install a radiator, its power should be even half that.

The radiator power depends on the supply temperature, i.e. the temperature of the water that flows through the heating device. The greater the power, the greater the temperature that can be achieved in the room. Examples of parameters of various installations:

  • District heating network: 75/65/20 ° C
  • Solid fuel boilers: 75/65/20 ° C
  • Standard gas boilers: 65/55/20 ° C or 75/65/20 ° C
  • Gas condensing boilers: 55/25/20 ° C

How to calculate the power of a radiator – formula and calculator

The radiator’s heating capacity is often calculated using an estimate method. Usually it is the product of the area of ​​a given room and the average heat demand. On average, the heat demand in a room ranges from 60 to even 200 kWh / m 2 . The worse the building insulation, the higher the coefficient. In well-insulated houses, heat losses are lower, so a radiator with a lower heating power is enough to heat them.

Building’s heat demand estimations

  • 15 kWh / m 2  (passive house)
  • 20-60 kWh / m 2 (energy-efficient building)
  • 70-100 kWh / m 2 (modern building)
  • above 120 kWh / m 2 (building without insulation)

For example, you can pre-calculate the power of the radiator for a room with an area of ​​15 m 2 in an energy-efficient building. In this situation, you can use the heating power calculator or the previously quoted estimated formula:

  • X [kWh / m 2 ] * Y [m 2 ] = Z [W]
  • X – average demand for m 2 
  • Y – room area
  • Z – the required heating power of the radiator

40 kWh / m 2 x 15 m 2 = 600 W.

The radiator power per m 2 calculator in many cases also helps in calculating the heat loss of a given room based on the characteristics of the space or the already installed device (ultimately to be replaced).

British thermal units

British thermal units (BTU) is the standard unit of energy used in the United States and sometimes in the United Kingdom. BTU is often used as a quantification specification for energy production or the energy transfer capacity of heating and cooling systems such as ovens, ovens, refrigerators, and air conditioners. The thermal power of IT equipment is often determined so that it can be taken into account when planning the size of climate control systems in buildings.

Selection of heaters – power table


Calculating the power of a radiator based on a formula or radiator power calculator has an alternative: it is a table of the radiator power from a specific manufacturer. If the radiator is to be the main source of heat in the room, it is worth adding about 10-15% of the value to the calculated radiator power.

When you are unsure of the results, it is always a good idea to seek professional help. Designers and specialists in central heating installations can make a professional heat balance.

Pipe centre measurements – calculation of diameters of heating pipes and available pressure

In order for individual radiators to achieve the required power with the assumed temperature drop (e.g. 20K), it is necessary to provide an appropriate mass flow of water for each radiator. This condition is fulfilled by appropriate adjustment of the installation in the design conditions (initial adjustment).

In general, the following are distinguished:

– initial adjustment,

– operational regulation.

Is only pipe centre measurement needed?

The calculation method consists in establishing by the designer the appropriate settings on the control valves. Then the contractor sets the selected settings on the individual radiator valves.

 With accurate measurement method, the designer determines the required flows and the contractor then operates the control valves to obtain the required flows. In this case, it is necessary to use appropriate fittings that enable flow measurements to be carried out.

Generally, pre-regulation aims to distribute the factor “fairly”. Here, “fair” means: “to everyone (the radiator) as needed”. Ie. a larger radiator, which is supposed to provide more heat, should receive a greater flux than a smaller radiator.

Initial adjustment (sometimes also called assembly or permanent adjustment) is aimed at ensuring appropriate mass streams of water in particular sections of conduits in the design conditions. Operational regulation (also known as current regulation) is the continuous adaptation of the heating power to the temporary heat needs.

If the pre-adjustment is not performed, the water jets flowing to the individual radiators will be random, and consequently their power and temperature drop will also differ from the design values. The lack of pre-regulation may to some extent be compensated by the operational regulation (e.g. radiator thermostats), but it significantly lowers the quality of the operational regulation. Too large stream, which may be throttled by the radiator valve, is less dangerous. On the other hand, if the stream is too small, the radiator’s power drops, and the radiator valve, even when fully open, is not able to “help” anything. Generally, the installation is not pre-tuned, even if operating in a way that is acceptable to the user,

Radiator replacement

replacement radiator

It is no coincidence that the radiators, as a rule, are located on the outer wall under the window sill. They work best in places where the greatest heat losses occur. However, if we consider radiator replacement and we would like to place the it on an internal wall away from the window, we should choose a model with a capacity of 10%. bigger. The distance between its edges and the floor and the window sill is also very important for the radiator’s efficiency. In the case of plate devices, they should be at least 10 centimeters. Its performance decreases at smaller distances. The aluminum ones should be even 12 cm above the floor. Modern cast iron radiators require 5 centimeters of space on both sides.

Radiator height

replacement radiator

New radiator will be manufactured in several heights (most often 30, 60 and 90 cm) and several lengths – from 40 to 300 cm (width measure). When installing, first the height of new radiator is determined, then its length and the central heating radiator is selected from the table. With the power closest to your needs. Radiators with a height of 60 cm are best suited for hanging under a typical window (window sill at a height of 80-85 cm).

After installation, the distance between their upper edge and the sill, as well as the lower edge and the floor is approximately 10 cm, which provides them with the best working conditions.

Summary: what to consider when choosing a radiator? Size radiator as not the only aspect when buying new radiator

Calculation of the radiator power is not the only parameter that determines the choice of the radiator. Also important are: heating technology in the building, expected temperature, individual expectations and even visual issues. After all, the radiator performs not only a practical but also an aesthetic function.

Another indicator of the selection of a radiator is the determination of its dimensions. The height most often depends on the distance between the floor and the window sill, because this is where the radiators are most often installed. There should be 10 cm of free space from the floor to new radiator and the same space from the radiator to the window sill.

Heating specialists should inform about these and other parameters and values. A properly selected and installed radiator will guarantee optimal operation and heating of the room. It is worth reporting to professionals who will help you choose the best heaters.