How to choose the radiator power?
An optimally selected radiator allows for the delivery of enough heat during operation to maintain the room temperature at the level expected by the user.
To determine the optimal thermal power of a radiator for a given room, you should know:
- room size
- heat demand
- supply and return water temperatures in the central heating system
- required room temperature – e.g., bathroom 24°C, room 21°C
Additional factors that may affect the choice of radiator power are:
- building location in Poland (temperatures of climate zones)
- building location and wind conditions
- type of floor on which the room is located
- radiator placement
If the house is under construction, it's best to check these data in the installation project. However, when modernizing the heating system, we usually don't have access to such calculations. In that case, the power of replaced radiators is estimated. In the cards below, we focus only on the most important factors.
Determine the floor area of the room in square meters (m²).
Heat demand should be known from the project. If we don't know this parameter from the project, we must estimate it. However, let's try to avoid \"roughly\" situations, e.g., choosing the size of cast iron radiators based on square meters, where usually one section (rib) corresponded to one square meter of the room. Homeowners with radiators selected this way have repeatedly found that they were incorrectly chosen. A better solution is to determine the heat demand. To choose the size of radiators, you need to classify your house into one of the mentioned \"categories\".
Energy classification of buildings:
- passive (A+) - up to 15 kWh/m2/year
- low-energy (A) - 15 to 45 kWh/m2/year
- energy-efficient (B) - 45 to 80 kWh/m2/year in contemporary built houses, well-insulated, for example with a 15 cm layer of polystyrene
- moderately energy-efficient (C) - 80 to 100 kWh/m2/year in contemporary built houses, well-insulated, for example with a 10-12 cm layer of polystyrene
- moderately energy-intensive (D) - 100 to 150 kWh/m2/year since 1999 (approx. 5 cm of polystyrene)
- energy-intensive (E) - 150 to 250 kWh/m2/year until 1998 in houses without thermal insulation
- very energy-intensive (F) - over 250 kWh/m2/year until 1982 in houses without thermal insulation
However, we must remember that even in a well-insulated house where the average heat demand is 60 W/m2, there may be rooms where 45 W/m2 is enough and those where 80 W/m2 will be too small. Therefore, when choosing radiator sizes by estimation, you should:
- increase the power of radiators located on the ground floor by about 10%,
- for rooms located at the end of the building (having at least two external partitions), choose higher power radiators than those heating better-located rooms (with one external partition).
When choosing radiators without a project, it's best to oversize them slightly. We will then avoid situations where certain rooms are underheated.
Before buying radiators, you need to determine the operating parameters of the home central heating system. You should specify the maximum temperature of the supply and return water from the radiators. Lower system operating parameters force an increase in radiator surface area. When choosing a radiator, points 3 and 4 must be considered simultaneously. Manufacturers provide the power of individual radiator models mostly for 75/65/20°C or 90/70/20°C parameters, where the first number is the supply temperature, the second is the return temperature, and the third is the internal room temperature. Until mid-1999, radiator manufacturers provided power for 90/70°C. Currently, due to the establishment of the new PN-EN 442 standard, manufacturers provide the power of their radiators differently. Some - still according to the old standard - 90/70°C, others - according to the new one - 75/65°C. An additional difficulty is the change in the method of testing the thermal power of radiators. Their power tested according to the PN-EN 442 standard is a few, and sometimes even a dozen or so percent lower than the power of the same radiators tested according to old standards. Just like temperature, some manufacturers provide power according to the old testing method, others according to the new one. It is worth noting how their power was specified. If according to the old method, we usually won't find any mention of it; if according to the new one, the manufacturer usually includes information - \"power given in accordance with the PN-EN 442 standard\". Correctly, the power of radiators should be given for 75/65/20°C parameters, according to the testing method compliant with the PN-EN 442 standard.
The final stage of radiator power selection is the correction of radiator power, taking into account the system's operating parameters. If the manufacturer provides power for 75/65/20°C parameters, and these are to be our system's operating parameters, we can choose the radiator size directly from the table in the manufacturer's catalog without calculation. If it's different (for parameters deviating from standard, e.g., 90/70/24°C), we must multiply the power demand by the correction factor and only then select the radiators. Correction factor tables are provided by manufacturers.
calculation example:
Q = 6m2 x 90 W/m2 x 0,8 = 432 W
Therefore, when choosing a radiator, correction factors should be used to account for the change in power at parameters different from the catalog ones.
The greatest thermal power of bathroom radiators is obtained for a cross top-bottom connection. In the case of a bottom-bottom connection, the radiator power is 5–10% lower than the previously mentioned one. The least favorable in terms of thermal efficiency is a bottom-top connection. In such a configuration, the radiator power drops by 30–40%. Therefore, it is not recommended.
Table of climate zone temperatures.
Climate zone I: -16 °C
Climate zone II: -18 °C
Climate zone III: -20 °C
Climate zone IV: -22 °C
Climate zone V: -24 °C
