Calculation of heat output of radiators

Calculation of heat output of radiators


As has been mentioned, the warmth transmitted by radiators to the air of the room must compensate heat losses of the room and in a simplified form, this corresponds to the fact that for every 10 square metres of floor space, the radiators with heat output of at least one kilowatt must be installed. In practice this figure is increased by 15%, i.e. the radiator power must be multiplied by 1,15. There are more accurate calculations of the required radiator power which are performed by professionals, but the above guide is enough for a rough estimate. With this method of calculation, the radiators will be a little bit more powerful than necessary, but the quality of the heating system will be improved in that more precise tuning and lower temperature of the heating regime are possible.

When buying radiators in stores, in the section in the manual on technical details the warmth power can be specified in kilowatts or by the supply (litres per minute) of the warmth carrier. If the supply of warmth carrier is specified, we already know that a supply of one litre per minute corresponds approximately to the power of one kilowatt.

Usually in the section on technical details the size of the radiator is shown in millimetres. Nowadays the radiators you can buy in the stores come in heights of 20, 30, 40, 50, and 60 cm. Devices with a height of 20 cm and less are called «baseboard» or «moulding» radiators. The height of 60 cm is the traditional height of old cast iron radiators, and new radiators of this height are good as a simple replacement. Nowadays, radiators of 50 cm in height are often used because high windows with low sills are common and installation of radiators under the window requires a regulation gap between the bottom of the sill and the radiator of at least 5 cm and the distance between the floor and the radiator should be at least 6 cm. Low radiators look more compact but with the same power output will be longer and the size of the room may not allow for the installation of long radiators.

In the section on technical specifications, next to the power (for example 1905 watts) the calculated temperature difference is indicated (for example 70/55). This means that during cooling from 70 to 55 degrees, the radiator gives out 1905 watts of thermal power from its surface. However, many sellers show the power of their radiators only for the differential 90/70. When using such radiators for a medium temperature heating system with a difference 70/55, the power of the warmth output of this radiator will be less than stated in the technical details. Therefore when choosing radiators for medium and low temperature (55/45) heating systems, the actual power needs to be recalculated.

The power of the heating device is determined by the formula:

Q = k×A×ΔT,

where k is the coefficient of heat transfer of the heating device (W/m²×°C); A is heat transfer surface area of the heating device (m²);
ΔT is the temperature difference (°C) (see Figure 82).

For example, we need to choose radiators for a room of 16 square metres. For heating this area, we need radiators of power 1,6 kW. Multiply this number by 1,15 and we get 1,84 kW. We go to the store and choose a radiator suitable for us with respect to size and power. Assume we find such a heating device and in the technical details the power 1905 watts (1,9 kW) is indicated. Reading further in the technical details, we find that this power is for a temperature difference of 60 degrees C (90/70). Therefore, for a low temperature heating system (ΔT = 30 degrees C) with quality regulation of temperature of warmth carrier, for example using three-way mixers in a regime (55/45), the power of the radiator needs to be recalculated. Using the formula or technical specifications, we find that the value of the k×A = 31,75 W/degrees C and we can insert the updated data into the formula for power.

Q = k×A×ΔT = 31,75×30 = 956 watts which is only about half of the power we need.  Then we can proceed in several ways: buy two radiators instead of one; calculate the power of one section of the radiator and on the basis of this calculation, choose a radiator with the necessary number of sections; search for other radiators that meet our requirements. It should be added that while purchasing radiators for a low-temperature heating system (ΔT = 30 degrees C) when the temperature difference is indicated at 60 degrees in the technical details, the result will be the same — the number of sections of radiators must be doubled. In other cases, when in the technical details different temperature differences are used or you have your own requirements for calculated temperature difference, the power of the radiator needs to be recalculated.

The place where the radiator is situated and the way in which it is connected to the pipelines influence the supply of warmth from the radiators into the room.

Radiators are usually placed under the light apertures. Even when ultra-modern window glass arrangements are installed in the window frames, this is still the place of maximum heat loss. A radiator which is placed under a window heats the air around it. As the hot air rises up, it creates a warm «curtain» in front of the window which prevents the spreading of cold from the window. Besides, the cold air from the window immediately mixes with the warm air rising from the radiator and increases the convection in the whole room, contributing to a more rapid warming of all the air in the room. It is desirable that the radiator «accordion» should have length of the whole width of the window. If this is not possible, then it should be at least 50% of the width of the aperture. The central vertical axis of the window aperture and of the radiator must be aligned. The permissible deviation should be not more than 50 mm. In rooms at the corner of a building, extra radiators along a wall without a window must be placed as close as possible to the external corner. In using vertical heating systems, the vertical pipelines must be put in the corners of the rooms, and it is especially important to place the vertical pipelines in the external corners of the corner rooms. The point here is that the external corners of houses are subject to cold air from two sides, in contrast to non-corner walls. By placing the vertical heating lines in the corners, you provide heat to them and dramatically reduce the probability of dampness and blackening of the material of the walls — development in the corners of fungal growth.

The heating devices should be placed so inspection, cleaning and repairs can be done. If a screen or decoration of the devices is used, then in the calculation of the radiators’ power, corrections must be made. The power of the purchased radiators must be calculated with a correction factor (see Figure 83).

Effect of placement on the warmth power output of radiators
Figure 83. Effect of placement on the warmth power output of radiators

Pipes which are joined to the radiators can be on the same side (unilateral) or on opposite sides (bilateral). When pipes are connected on opposite sides, the heat transfer of the devices increases, but from the construction point of view, it is more rational to make unilateral pipe connections. If there is more than one radiator coupled together, or if  there are more than 20 sections in any radiator, join the pipes from different sides.

The heat flow in radiators depends on the places where the supply and return of warmth carrier are attached. Heat transfer increases if the supply of warmth carrier is attached at the high part of the device and the return at the low part (direction of flow from top to bottom) and decreases if the direction of motion is from bottom to top (see Figure 84). When installing heating devices in several tiers of height (different floors), it is recommended that consistent motion of warmth carrier be provided, from the top down.

Different radiator warmth power output according to pipe connection
Figure 84. Different radiator warmth power output according to pipe connection

Individual regulation of the warmth output of heating devices can be manual or automatic. Thermostatic valves regulate the flow of warmth carrier in a way that achieves the best exchange in all parts of the thermal unit.