The actual heat dissipation of various types of heating radiators is often discussed at construction forums. Participants argue which batteries are better in terms of thermal performance - cast iron, aluminum or steel panels. To clarify this issue, it is proposed to calculate the power of different heating devices and to compare radiators for heat transfer.
How to correctly calculate the real heat dissipation of batteries
First of all, study the technical data sheet of the battery. In it, you will definitely find the parameters of interest - the thermal power of one section or a whole panel radiator of a certain size. Do not rush to admire the excellent performance of aluminum or bimetallic heaters, the figure indicated in the passport is not final and needs to be adjusted, for which you need to calculate heat transfer.
Mistaken judgment: the power of aluminum radiators is the highest, because the heat transfer of copper and aluminum is the best among metals. The thermal conductivity of aluminum is really high, but the heat transfer process depends on many factors. The second nuance: heating appliances are made of silumin - an aluminum alloy with silicon, whose performance is much lower.
The heat transfer specified in the passport of the heater corresponds to the truth when the difference between the average temperature of the coolant (tfiling + treturn) / 2 and room air is 70 ° С. The value is called the temperature head, denoted by Δt. Settlement formula:
Substitute the known value of the temperature head and get the following equation:
(tfiling + treturn) / 2 - tair = 70 ° C
Reference. In the documentation of products from various companies, the Δt parameter can be designated differently: dt, DT, and sometimes it is simply written "at a temperature difference of 70 ° C".
What heat transfer will we get if the documentation for a bimetallic radiator says: the thermal power of one section is 200 W at DT = 70 ° C? The same formula will help to understand, we substitute the value of room temperature +22 ° C into it and carry out the calculation in the reverse order:
(tfiling + treturn flow) = (70 + 22) x 2 = 184 ° С
Knowing that the temperature difference in the supply and return pipelines should not exceed 20 ° C, we determine their values as follows:
- tfeed = 184/2 + 10 = 102 ° C;
- treturn = 184/2 - 10 = 82 ° C.
Now it is clear that 1 section of the bimetallic radiator from the example will give 200 watts of heat, provided that the water in the supply pipe heats up to 102 ° C and the air temperature in the room reaches +22 ° C.
The first condition is not feasible, because modern domestic boilers are heated to 80 ° C (maximum). This means that the radiator section will never give up the declared 200 watts of heat. And the temperature of the coolant in the system of a private house rarely rises above 70 ° C, then DT = 38 ° C, and not 70 degrees. That is, the actual heat transfer of the device is twice lower than the passport.
The procedure for calculating heat transfer
So, the real power of the heating battery is much less than declared, but for its selection it is necessary to understand how much. There is a simple way for this: applying a reduction coefficient to the passport value of the heater’s thermal power. Below is a table of coefficients by which the declared heat transfer of the radiator is multiplied, depending on the present value of DT:
The algorithm for calculating the real heat transfer of heating devices for your individual conditions is as follows:
- Determine what should be the temperature in the house and the water in the system.
- Substitute these values in the formula and calculate your temperature head Δt.
- Find in the table the coefficient corresponding to the found DT.
- Multiply the passport value of the heat transfer of the battery by it.
- Count the number of sections or whole heating appliances for heating a room.
In the above example, the thermal power of 1 section of a bimetallic radiator will be 200 W x 0.48 = 96 W. Approximately 1000 W of heat or 1000/96 = 10.4 ≈ 11 sections will be used to heat a room of 10 m² (rounding up).
The presented table and the calculation of the heat transfer of the batteries should be used when Δt equal to 70 ° С is indicated in the documentation. But it happens that manufacturing companies give the radiator power for other conditions, for example, at Δt = 50 ° С. Then you can’t use the coefficients, it’s easier to dial the required number of sections according to the passport characteristic, only take their number with one and a half margin.
Reference. Many manufacturers indicate heat transfer values under these operating conditions: tfeed = 90 ° C, treturn = 70 ° C, tair = 20 ° C, which just corresponds to Δt = 50 ° C.
Comparison of thermal power
If you carefully studied the previous section, you should understand that the heat transfer is very much influenced by air and heat carrier temperatures, and these parameters are little dependent on the radiator itself. But there is a third factor - the heat exchange surface area, here the design and shape of the product plays a big role. A clear comparison of a steel panel heater with a cast-iron battery will not work, their surfaces are too different.
The fourth factor affecting heat transfer is the material from which the heater is made. Compare yourself: 5 sections of an aluminum radiator GLOBAL VOX with a height of 600 mm will give 635 W at DT = 50 ° C. A DIANA cast-iron retro battery (GURATEC) for 5 sections of the same height will transfer only 530 W to the room under similar conditions (Δt = 50 ° C). These data are published on the official websites of manufacturers.
Note. The power characteristics of aluminum and bimetal heaters differ little, it makes no sense to compare them.
You can try to compare aluminum with a steel panel radiator by taking the closest frame size suitable for the dimensions. The battery length of 5 aluminum sections of GLOBAL with a height of 600 mm will be approximately 400 mm, which corresponds to a KERMI 600 x 400 steel panel.
Even if we take a three-row steel panel (type 30), we get 572 W at Δt = 50 ° C versus 635 W for 5-section aluminum. Also note that the GLOBAL VOX radiator is much thinner, the depth of the device is 95 mm, and the KERMI panels are almost 160 mm. That is, the high heat transfer of aluminum sections allows to reduce the dimensions of the heater.
In an individual heating system of a private house, batteries of the same power, made of different metals, will work differently. Therefore, the comparison is quite predictable:
- Bimetal and aluminum products quickly warm up and cool down. Giving more heat over a period of time, they cool the water returned to the system more strongly.
- Steel panel radiators occupy an average position, since they transfer heat not so intensely. But they are cheaper and easier to install.
- The most inert and expensive ones are cast iron heaters, they are characterized by long heating and cooling, which causes a slight delay in the automatic control of the coolant flow by thermostatic heads.
The conclusion is simple: no matter what material the radiator is made of. The main thing is to choose the right battery for power and design that suits the user. In general, for comparison, it does not hurt to get acquainted with all the nuances of the operation of a particular device, as well as where which is better to install.
Comparison by other characteristics
One feature of battery performance — inertia — has already been mentioned above. But to make the comparison of heating radiators look objective, in addition to heat transfer, other important parameters should be taken into account:
- working and maximum pressure of the heat carrier;
- amount of enclosed water;
- weight.
The working pressure limit determines whether a heater can be installed in multi-story buildings, where the height of the water rising by network pumps can reach hundreds of meters. The parameter does not play a role for private houses, where the pressure in the system is low, a maximum of 3 Bar.
A comparison of the capacity of the radiators can give an idea of the total amount of water in the network that will have to be heated. Well, the mass of the product is important when choosing the installation location and method of mounting the battery.
As an example, a comparative table of the characteristics of different heating radiators of the same size is shown below:
Note. In the table, a heater from 5 sections is accepted for 1 unit, except for steel, which is a single panel.
Conclusion
If we compare products from a wide range of manufacturers, it will still turn out that aluminum radiators hold the first place in terms of heat transfer and other characteristics. Bimetallic ones win by working pressure, but they cost more, it is not always advisable to buy them. Steel batteries are rather a budget option, but cast-iron ones, on the contrary, are for connoisseurs. If you do not take into account the price of Soviet cast-iron "accordions" MC140, then retro radiators are the most expensive of all existing.