Calculation of the heat of the underfloor heating to optimize the heating system
Calculation of the heat of the underfloor heating to optimize the operation of the heating system
The calculation of the heat of the underfloor heating is carried out taking into account the heat loss through the enclosing structures and the useful area of the rooms. Errors in calculations affect the operation of the system, increase energy consumption and home maintenance costs. The errors are caused by the use of aggregated indicators. The efficiency of insulation and tightness of structures (foundation, loadbearing walls, ceilings, roof, doubleglazed windows, entrance doors) guarantees economical consumption of energy resources in the water underfloor heating system.
Purpose and calculation of underfloor heating
The lowpressure heating circuit is able to optimize radiator heating or provide equivalent heating of the house and reduce energy consumption.
The heating element and the heat carrier are the design features by which water and electric underfloor heating are distinguished. You can calculate the power of electric underfloor heating using online calculators, which are placed on specialized services on the Internet. In this article we will take a closer look at the purpose and calculation of the power of water underfloor heating.
Table 1. Recommended specific capacity of water underfloor heating per unit area:
Design features of a residential building  Underfloor heating power, W/m2 (min/max)  
Additional (comfortable) heating  
The year of construction of the building was before 1996, the climatic region is the European part of Russia  80/120  
Year of construction of the building — after 1996 (improved external insulation, thermal insulation of the basement and roof, double—glazed windows), climatic region – European part of Russia  50/80  
In rooms with wooden floors (rough floor and finishing flooring)  80/80  
Loggias (balconies), in which double glazing and insulation are provided  140/180  
The main heating of the house  
Kitchens, living rooms on the first and second floors (at least 3/4 of the heated area)  150/∞ 
Heat Q (W), which generates 1 square meter of lowpressure water circuit, is the total flow of radiant (≈ 4.9 W/m2) and convective (≈ 6.1 W/m2) energy:
Q =
[ al×(tpola − toc) + ak×(t floor − t air) ]× S, (W), where
al and ak — radiant and convective energy flows, W/m2;
t floor — the temperature of the floor covering, °C;
toc — wall and ceiling temperature, °C;
air temperature — room temperature, °C;
S is the useful area of the contour, m2.
Explanation of schemes 1 and 2 of the calculation of underfloor heating:


The calculation of underfloor heating determines the heat consumption of a residential building according to regulatory documents on the thermal protection of buildings and construction heat engineering:
Q = (al + ak) × S ×(t floor − t air), (W);
tpola = Q/[(al + ak) × S] + t air, (°C);
at S = 1m2, t floor = Q/(al + ak) + t air, (°C).
When the room temperature is heated by 1 degree, the heat from the floor surface is transferred to the air:
∆t = t floor − t air =1°C;
Q =(al + ak) × S×∆t = (4,9 + 6,1) × 1× 1 = 11 ( Tue).
The ideal conditions under which the heat transfer of the water circuit on one square meter of underfloor heating for heating the air in the room at 1 ° C is 11 W/ m2. The higher the room temperature, the faster the room will warm up and the lower the energy consumption of the coolant. The underfloor heating system is preferred in order to heat residential insulated houses with permanent residence. The average allowable value of heat loss is 65 W/m2.
To calculate the heat transfer of underfloor heating, there are special programs that can be found on the resources on the network. To clarify the issue, we suggest reading the video “Calculation of the heat transfer of underfloor heating”.
Coolant temperature
The temperature of the coolant in the circuit depends on the heat load, the laying step, the diameter of the pipes, the thickness of the screed and the material of the floor covering. The minimum temperature values in the contour are taken for parquet boards and smallpiece wood products. Tile, metlakh, ceramic tiles, granite, marble withstand the maximum permitted temperature of the coolant (55 ° C). Low—pressure heating schemes, which are used in practice, have an operating range of 45/35 °C.
Sanitary standards define a comfortable (26°C) and permissible temperature limit for a person’s foot:
 28°C in living rooms for permanent residence;
 35°C along the perimeter of the loadbearing walls of a residential building;
 33°C for kitchens, baths and sanitary rooms.
Underfloor heating bases
The type of overlap affects the materials and the choice of the thickness of the layers above and below the pipe. The basis for underfloor heating is cement screeds and flooring systems made of polystyrene or wooden inter—pipe boards. The aluminum profile in rack modules serves as insulation of wood from direct contact with the heating element and for fixing pipes.
Related article:
Water underfloor heating with your own hands, video and description of the process. Description of the installation process of water underfloor heating. Its advantages and disadvantages are in contrast to other types of floor heating systems. Selection of materials. Video lessons.
The wiring of the contour pipes on concrete slabs is arranged in the body of the concrete screed. The volume of the material and installation calculations of underfloor heating are determined after preliminary marking of the surface (hydraulic or laser level). The layout plan is carried out on paper (scale 1:50). The accuracy with which the calculation is carried out depends on the material consumption and the speed of work.
The surface cleaned and treated with a polymer primer is leveled in advance, waterproofing is done on the soils and the first floors. Glue the walls around the perimeter with a damping tape to a height that will go under the screed (with a small margin). Thermal insulation material with a foil base shields the specific heat flow upward in a given direction. Heat loss through the foil does not exceed 5%.
The reinforcement is laid on top of the insulation, the frame stiffens the screed and allows you to achieve the correct fixation of the step. The pipe contour is laid out, fixed, the contour is tested under pressure and filled with a screed solution.
Lightweight modular systems are used for wooden structures (rough floor or logs) that do not have the ability to withstand high static loads.
Calculations of pipes for water underfloor heating (length, diameter, pitch and methods of laying and pipes)
The limited length of the lowpressure heating circuit is associated with the “closed loop” effect, in which the pressure loss exceeds 20 kPa (0.2 bar). Increasing the pump power is not an option in this case — the resistance will increase in proportion to the increase in pressure.
The estimated length of pipes for underfloor heating is determined by the formula:
L = (S/a×1,1) + 2c, (m), where
L — contour length, m;
S — area, contour, m2;
a — laying step, m;
1.1 — increase in the size of the bending step (margin);
2c — the length of the supply pipes from the collector to the contour, m.
Important! The useful area of the room takes into account the area of the contour with the addition of half the pipe pitch.
The heating circuit is laid, retreating 0.3 m from the walls. Take into account the open floor area, which transmits a uniform radiation flux. Experts do not recommend installing a heating circuit in places where furniture is placed. Prolonged static load can cause deformation of pipes.
With a large area of the room, the heating circuit is divided into sectors. The basic rules of zoning are the ratio of the lengths of the sides 1/2, the heating of the area of one sector is not more than 30 m2 and the observance of the same length and diameter for the circuits of one collector.
Table 2. Ratio of lengths and diameters of contour pipes:
Diameter, mm  Pipe Material  Recommended contour length, m 
16  metalplastic  80 ÷ 100 
18  crosslinked polyethylene  80 ÷ 120 
20  metalplastic  120 ÷ 150 
The diameter and pitch of the pipe layout depends on the heat load, purpose, size and geometry of the room. The heat propagation zone is proportional to the radius of the pipe. The pipe heats a section of the floor in each direction from the center of the pipe. Balanced pipe pitch: Dy 16 mm — 0.16 m; 20 mm — 0.2 m; 26 mm — 0.26 m; 32 mm — 0.32 m.
In the passport data of the products, the maximum throughput of the pipes is indicated, on the basis of which the linear pressure change is calculated. The optimal value of the coolant velocity in water heating pipes is 0.15 – 1 m/s.
Table 3. Dependence of the step on the area and load of the sector:
Diameter, mm  Distance along the axes (pipe pitch), m  Optimal load, W/m2  Total (or divided into sections) usable area of the room, m2 
16  0,15  80 ÷ 180  12 
20  0,20  50 ÷ 80  16 
26  0,25  20  
32  0,30  less than 50  24 
Pipe laying options: simple, angular or double loops (snakes), spirals (snails). For narrow corridors and rooms of irregular shape, snake laying is used. Large areas are divided into sectors. Combined laying is allowed: in the marginal zone, the pipe is laid out with a snake, in the main part — with a snail.
Along the perimeter, closer to the outer wall and near the window openings, there is a contour feed. The laying step in the edge zones may be less than the distances between the pipes in the central part of the room. Connection of edge zone reinforcements is necessary to increase the power of the heat flow.
Important! Bending the pipes by 90 ° in the spiral connection scheme of the water underfloor heating, reduces the hydraulic resistance less, in comparison with laying loops (snake).
In the calculations of pipes for water underfloor heating, diameters of 16, 20, 26, 32 mm are used.
For systems of warm water floors, corrugated, stainless steel, copper, metalplastic, crosslinked polyethylene pipeline is used. Corrugated pipe for underfloor heating began relatively recently in order to facilitate the installation of the structure and reduce the cost of rotary length increases.
Polypropylene pipeline has a large bending radius, so it is rarely used in underfloor heating systems.
Floor coverings
Types of finishing flooring for underfloor heating: selfleveling surface, linoleum, laminate or parquet, tile, ceramic and metlakh tiles, marble, granite, basalt and granite.
Wooden flooring is contraindicated by constant humidity in the room, so it is not used in bathrooms with warm floors.
Table 4. Thermal conductivity of floor coverings:
Type of material  Layer thickness δ, m  Density γ, kg/m3  Thermal conductivity coefficient λ, W/(m °∁) 
Insulated linoleum  0,007  1600  0,29 
Tile, broom, ceramic  0,015  1800 ÷ 2400  1,05 
Laminate flooring  0,008  850  0,1 
Parquet board  0,015 ÷ 0,025  680  0,15 
Insulation (ursa)  0,18  200  0,041 
Cementsand screed  0,02  1800  0,76 
Reinforced concrete slab  0,2  2500  1,92 
Pumping equipment in the calculations of underfloor heating
Reducing the temperature of the coolant allows you to achieve efficient operation of circulation pumps.
The heating circuit of underfloor heating is located horizontally and covers a large area. The force that the circulation pump gives to the flow is spent on overcoming linear and local resistances. The calculation of the pump for underfloor heating depends on the diameter, roughness of the pipe, fittings and the length of the contour.
The main calculation parameter is the pump performance in a lowpressure circuit:
H = (N×L + ΣK)/1000, (m), where
H — head of the circulation pump, m;
P — hydraulic loss per linear meter of length (passport data from the manufacturer), pascal/meter;
L is the maximum length of pipes in the circuit, m;
K is the coefficient of power reserve for local resistances.
K = K1 + K2 +K3, where
K1 — resistance on adapters and tees, connections (1,2);
K2 — resistance on the shutoff valve (1,2);
K3 is the resistance at the mixing unit in the heating system (1,3).
The degree of productivity possessed by the circulation pump is determined by the formula:
G= Q/(1,16 ×∆t), (m3/h), where
Q — thermal load of the heating circuit (W);
1.16 — specific heat capacity of water (Wh/kgF);
∆t — heat removal in the system (for lowpressure circuits 5 ÷ 10 ° C).
Table 5. Dependence of the power of the unit on the area of heated rooms (for hydraulic calculation of underfloor heating):
Floor area, m2  The capacity of the circulation pump for underfloor heating, m3/h  
80 ÷ 120  1,5  
120 ÷ 160  2,0  
160 ÷ 200  2,5  
200 ÷ 240  3,0  
240 ÷ 280  4,0 
Useful advice! The power of the unit consists of the sum of the expenses of all circuits. In case of abnormal cold weather, it is necessary to provide a margin of pump performance of 1520%.
Calculation of the cost of underfloor heating
The gas boiler and the floor hydraulic circuit connect the collector. The uniform flow of the coolant is provided by automatic adjustment, using balancing and thermostatic valves. The check valve protects the pumpmixing unit.
Table 6. Elements of a complete set of underfloor heating:
Position name  Size and unit of measurement  Price per unit of goods (rubles) 
Waterproofing  roll (1.5×50 m)  from 2000 
Damping tape  25 m  from 500 
Shielding thermal insulation (expanded polystyrene)  1100×800×38 mm  769 
Tube  16 ÷ 20 mm  50 ÷ 80 
Concrete screed: cement dry mixes 
50 kg 25 kg 
125 200 
Collector group assembly  2 outputs  4600 
Pumping and mixing unit: thermostatic head balancing and thermostatic valves, circulation pump  set  from 20000 
The total cost of underfloor heating is determined by the area of the room, the equipment configuration, the quality of the material and the method of work. The batch formation of underfloor heating ensures the compatibility of elements and effective heating in temperature ranges. Factory equipment reduces the cost of materials by 1.52 times.
The owner of the house can make a calculation of water underfloor heating, install the system with his own hands, if he has sufficient knowledge in heat engineering, hydraulics, materials science and experience in performing plumbing work. A lot of positive examples from life inspire. However, everyone should carry “their own briefcase”, their own house is not a springboard for experiments.