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UNI EN 1264-3:2009 is the part of the standard that offers help in dimensioning the radiant surface system. The updated version contains advice on how to design a cooling-only system, as well as a combined heating/cooling system.
This is the part of the standard that enables dimensioning of the radiant surface. To limit power loss, in Paragraph § 126.96.36.199 the standard states that the insulation panel of the radiant surface system must have a thermal resistance Rλ,ins value at least equal to that stated in UNI EN 1264-4.
The standard states that to verify what the thermal power should be of a radiant surface, the heat requirements of the room (calculated according to EN 12831) must be divided by the heating surface; if the figure derived is greater than the maximum power suppliable by the radiant surface, the standard calls for another installation surface to be considered.
UNI EN 1264-3 establishes that the maximum specific thermal output of a radiant heating surface derives from assuming limits on surface temperature values:
Different specific outputs correspond to a maximum floor temperature of 29°C according to the degree of uniformity of the temperature reached on the surface of the floor (dependent on the type of surface finish and the distance between circuits); the so-called limit curves define the specific output limits.
For a uniform floor temperature equal to 29°C, the maximum specific output qG that can be supplied to a room for which the desired temperature 20°C is calculated by:
qG = 10.8 (29-20) = 97 W/m2
At a mean ceiling temperature equal to the limit value of 29°C, the maximum specific output that can be supplied to a room at 20°C can be calculated with:
qG = 6.5 (29-20) =59 W/m2b (see § 188.8.131.52 of UNI EN 1264-3:2009)
At a mean wall temperature equal to the limit value of 40°C, the maximum specific output that can be supplied to a room at 20°C can be calculated with:
qG = 8 (40-20) =160 W/m2 (see § 4.3.1 of UNI EN 1264-3:2009)
Once the specific heat demand for each room to be heated has been calculated according to EN 15243 and checked if it is greater or less than the maximum specific output qG, you can start sizing the radiant surface. With regard to the radiant floor, the standard recommends not using, if possible, coverings that have a thermal resistance, Rλ,B higher than 0.15 m2K/W. However, if you don‘t know the type of covering that will be used or whether the covering will have a thermal resistance below than 0.1 m2K/W, standard UNI EN 1264-3:2009 suggests dimensioning the floor system for heavy operating conditions to ensure the room always achieves conditions of thermal comfort, even if the floor covering is changed during the lifetime of the radiant system.
Therefore, the standard advises using a floor with a thermal resistance equal to 0.1 m2K/W for all rooms. If the thermal resistance of the floor covering is greater than 0.1 m2K/W, the actual resistance value will have to be considered when designing the floor system.
Sizing a floor system consists of determining: spacing between circuits, supply temperature and circulation flow rates in the individual loops of the system. Supply temperature and circuit spacing is determined by comparing the specific heat requirements that need to be met with the working capacity of the floor system chosen. System performance depends on its geometry, the characteristics of the tubing used and the spacing between tubing. A "high-output" floor system means it is made to operate at the lowest flow temperature possible.
UNI EN 1264-3 recommends designing circuit spacing according to the room where heat demand is highest, the "problem room". This cannot be the bathroom. The spacing used and output supplied to the problem room determine a system‘s supply temperature. Any of the spacing options available for the chosen system can be used, provided that floor temperature limits are not exceeded. A narrow spacing design results in a lower flow temperature, which benefits system efficiency. The flow temperature must be the temperature at which the temperature drop of the heating loops in the room with the highest heat demand is equal to 5 K.
For example, for a room with a specific heat demand, q 60 W/m2, the flow temperature is 38 °C for 10 cm spacing and 42°C for 20 cm spacing.
In reality, if in the problem room we install a ceramic floor covering with a thermal resistance of 0.01 m2K/W and use 10 cm spacing, the actual flow temperature will be 33°C.
If however, we were to install a floor covering with a thermal resistance greater than 0.1 m2K/W, for example 0.125 m2K/W, a design with 10 cm spacing would give a temperature of 40°C.
Once the system flow temperature has been established based on the power and spacing requirements of the room with the highest heat demand, the spacings for the other floor-heated rooms can be worked out by comparing their specific thermal requirements with the performance of the radiant floor. To facilitate thermal and hydraulic calibration of the radiant floor, it is advisable to use wider spacing for rooms that have lower heat demands. To demonstrate this concept, we plot the output of the radiant floor, which has a covering with a thermal resistance of 0.1 m2K/W, on a graph taking W/m2 as the y co-ordinate and the logarithmic mean temperature difference as the x co-ordinate, for which, as a good approximation, we can use data from the mean temperature difference between the water in the circuits and the room. If you have two rooms with specific heat demands q equal to 70 and 50 W/m2 the average water temperatures necessary to meet the demands are 40°C for the room with the greater thermal need, with 10 cm spacing, and 37°C for the room with the lower thermal need, with 20 cm spacing. This means that with a flow temperature of 43°C, the temperature difference will be 6 K in the first example, and 12 K in the second.
If 10 cm spacing is used in a room that has lower heat requirements, the system will have a higher output. It is therefore important to make sure the mean water temperature is lower if you do not want to have a surplus of supplied power. The graph shows that the mean temperature must be no higher than 33.5°C and the temperature difference 19 K.
Dimensioning the radiant floor concludes with the determination of the water flow rate. For example, if in our ‘problem room‘, which has a thermal heat demand of 60 W/m2, tubing is installed at 10 cm spacing, the water needs to be supplied at 33°C to have a temperature difference of 5 K with a ceramic floor. If the room is on the first floor above a garage with a temperature of 10°C, the specific power lost is 3.7 W/m2 if an insulating panel with a thermal resistance of Rλ,ins = 1.48 m2K/W is fitted. Finally, from knowing the heating surface AF, we can determine the water flow rate needed to meet the total thermal capacity demand.
If the radiant floor also has a cooling function, UNI EN 1264-3 recommends determining the output of the system in cooling mode, in accordance with UNI EN 1264-5:2009. Dimensioning is done using the same criteria as for heating systems. The summer heat load calculated according to EN 15243 is compared with the output of the system in cooling mode.
The standard recommends using a flow temperature no lower than 1 K with respect to the value of the dewpoint temperature, calculated on ambient air conditions. It must be equipped with humidity sensors if operated in cooling mode to prevent reaching dew point temperature. The procedure remains the same for ceiling or wall systems in heating and/or cooling mode.
UNI EN 1264-3 recommends determining the output of the ceiling, wall or floor system in accordance with UNI EN 1264-5:2009. Working and flow temperatures are calculated using the same criteria already given for floor radiant systems in heating and/or cooling mode. Insulation panels with a thermal resistance Rλ,ins at least equal to that stated in UNI EN 1264-4 are also recommended for radiant wall and ceiling systems.