Pre-calculation of the heat contour in floor heating

Pre-calculation of the heat contour in floor heating

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Determination of density of heat flow per square metre of «warm floor»: q = Q/F, where q is the density of heat flow (W/m²); Q is the total heat losses from the room (W); F is the active floor area (m²).

The total heat loss from the room is determined by technical thermal calculations of the building, but as it has been mentioned several times before, for a rough calculation, it can be taken as 1000 W for one square metre.  If the floor heating will be used as an addition to radiators, then the total heat loss can be determined as a percentage. For example, 40% of heat loss will be reimbursed by radiator heating and 60% by floor heating. For calculating the area of the floor, one should consider only that part of the floor which will participate in the heating of the room. For example, along the internal walls where the furniture is located, one should leave borde regions with a width of 400–500 mm. So, for total floor area, say 20 m², the active area of the floor can be 14–16 m².

Next we find the average temperature of the warmth carrier in the heating contour (C): Δt = (ti + to)/2 where ti is the inlet temperature of the heating contour and to is the outlet temperature of the heating contour. The recommended temperatures for the warmth carrier on the inlet and outlet temperatures of the system (ti/ to) are 55/45, 50/40, 45/35, 40/30 degrees C. You can use your parameters to calculate the average temperature, but the temperature of the supply should not be more than 55 C and the temperature of the return must be 10 degrees lower (optimally 5 degrees). Based on the density of the heat flow (q) and average temperature of the warmth carrier in the heating contour (Δt), according to the graph shown in Figure 101, select the diameter and layout density of metal-plastic pipes.

Graph of dependence of specific heat flow from average water temperature in metal-plastic pipes with screed thickness of 70 mm, room air temperature of 20 degrees and with floor covered with ceramic tiles
Figure 101. Graph of dependence of specific heat flow from average water temperature in metal-plastic pipes with screed thickness of 70 mm, room air temperature of 20 degrees and with floor covered with ceramic tiles

How do we use this graph? On the axis of average temperature, we put the value of the average temperature which we obtained and make a horizontal line. Then, on the axis of density of heat flow, we put the density which we calculated according to the formula and make a vertical line. The point where the lines cross shows the diameter of the pipes which is suitable for us (the solid line shows the diameter of 16 mm and the dashed line 20 mm) and by the colour of the line we determine the recommended distance between pipes. If the drawn lines do not intersect in the region of coloured lines, then we should take the closest dependence point, on the side of increasing the distance between pipes, or change the average temperature of the warmth carrier.

It should be noted that this graph is valid for floors with cement-sand screeds with total thickness of 7 cm and covered with ceramic tiles. For other floor coverings and other thicknesses of screeds, it is necessary to correct the calculation. For example, a carpeted floor, instead of tiled, requires an increase in the temperature of warmth carrier by 4–5 degrees C and every extra 10 mm of screed thickness, decrease the density of heat flow by 5–8 percent. Nevertheless, the rough calculation of «warm floors» can be done according to the formulas shown here along with the graph, and the final regulation of temperature of warmth carrier should be done with three- and four-way mixers or thermostats after the installation of the heating contour. However, if you want the exact calculation of «warm floors», then you should ask a hydronic heating engineer. Since in the calculation a lot of different data are taken into account from technical specifications of the pipes to the heat calculation of the layers of the walls and floors

Next the approximate length of the pipes is calculated: the active area of the floor (m²) is divided by the distance between pipes (m). To this result is added the length of bending pipes and the length of connections to the collectors.

Now that we know the length of the pipes and their diameter, we can calculate the volume of the warmth carrier in them. The maximum speed of motion of warmth carrier in the pipes of the «warm floor» should be in the range 0,15–1 m/sec. Knowing the supply of warmth carrier (Q) (we recall that 1 KW = 1 litre/sec) and the volume of water in the pipes, we check the speed of motion of the warmth carrier. If it is within the required range, then we take these pipe diameters; if the speed is to great, we must increase the diameter.

We choose the pump of the heating contour by the supply of warmth carrier with 20% increase for hydraulic resistance in the pipes. If several «warm floors» with circulation from one pump are «sitting» on the collector system, then the pump is selected for the general supply of the heating contour.

Let me repeat one more time that the method above of the calculation of «warm floor» is very approximate; the actual calculation can be quite different. For example, if you make «warm floors» according to this method, you can increase or decrease the temperature of warmth carrier and thereby change the warmth output of the floor but you cannot increase the temperature of the floor without limit. The floor will be converted into a hot frying pan but will not heat the room. Or, for example, if you make a mistake in selecting the pump (by not considering a sufficient hydraulic resistance of the pipes) then the weak pump will have to be replaced with a more powerful one. Therefore it is better to use this method for making «warm floors» as auxiliary heating to the main radiator heating and for the full calculation of the «warm floors» as the main heating, refer to specialists.

 

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