Designing a cable heater
The main difference between electric underfloor heating. heating elements consisting of cables or cable sections. Consider the varieties and methods of calculation.
Resistive heating cable. a heating element of one or two-core cable in a protective shield, with a constant resistance, which is laid on the floor area.
The cable has standard lengths and therefore resistance and heat generated. The cable length must not be changed, this would result in a change of current and malfunction.
Specific wattage and length
This is the capacity of one square.m of underfloor heating. Under this parameter, the length of the heating electric cable is selected.
For instance, the wattage of a cable system for proper heating should be approx. 100-150W/m2; if the floor heating is planned as the main heating, you’ll need 150-200W/m2. If we want to heat 10 m2, we need a cable with power 10100=1000W.
This will already depend on the cross-section. The thicker the wire, the greater its power, and the greater will be the pitch of the installation. A thinner wire will have to be laid in shorter increments to meet the selected specific power, respectively the cable consumption will be higher.
For the convenience of calculation and laying electric heating mats are sold in the form of mats, rolled up in a roll. The cable in them is laid in a snake with a certain pitch and fixed. The width of such a “mat” can’t be changed, as a rule, it’s equal to 50 cm. So the power density is defined by the manufacturer, for example 130 W/m2. The buyer only needs to choose a suitable area of the heating element from those available for sale.
Consider how the calculation of hydraulic. It is necessary to find out the power of the pump to be purchased. The loss for a straight pipe of 10 m length, 16 mm diameter and 2 mm wall thickness is 1600 Pa. 180 degree turns: 40 Pa each. Then for a room with an area of 18 m 2 with wall length and width of 5.6 m and 3 m respectively when installing a water tube flooring system in a snake, the hydraulic loss is 18 680 Pa. The figure is obtained by the following calculations: Room width 3 divided by a step of 0.15. 20 straight pipe sections. Losses of all straight sections: 20×5,6×160 = 17,920 Pa. At the turns the GP will be 19×40 = 760 Pa. Thus, adding 760 and 17,920 Pa, we obtain a value of 18,680 Pa.
This means that in order for the system to work properly, at least 2.4 liters/hour of coolant must flow through 1 m of length. Exactly calculate the capacity can be as follows: coolant flow RTN = 0,86xMK/RT, where MK. circuit capacity in kW, RT. the temperature difference in the supply and return sections of the pipe. Based on the above calculations, for that room will be useful pump that can pump 0.172 m 3 / hr (0, 86×2/10).
The length of the contour of the water underfloor heating. the most important parameter that must be determined before starting the installation work. On it depends on the future power of the system, the level of heating, the choice of components and structural units.
To ensure that the warm floor created the right climate and did not become a cause of inconvenience or utility accidents, the room in which this heating circuit will be installed, must meet the following requirements:
- the height of the ceilings from the subfloor should be such that its reduction by 20 cm does not cause discomfort;
- The door opening must be at least 2.1 m high;
- The subfloor must be strong enough to withstand the cement screed that will cover the heating circuit
- if the subfloor is laid on the ground or under the heated room is unheated, it is necessary to lay an additional layer of insulation with a shielding coating;
- The surface on which the planned installation of the heating circuit and all components of the “pie” of the floor heating must be flat and clean.
The principle of operation of a typical closed-loop heating system is quite simple.
Boiler equipment heats the coolant, which passes through the heaters, releasing thermal energy into the surrounding space. If at the construction will be used the natural circulation of the coolant, it will be necessary to lay the pipeline at a certain angle to the horizon. This will allow the working fluid to move independently.
But with this method it is impossible to ensure a sufficiently high speed of coolant movement, so it returns to the boiler strongly cooled, and it forces it to work continuously at maximum load. In this regard, the floor heating without a pump, the connection diagram of which can be found on the websites of companies, can cause certain difficulties in operation.
In order to increase the flow rate, circulation pumps are used. Their use makes it possible to achieve a temperature difference of several degrees at the inlet and outlet of the pipeline line. Accordingly, the boiler ceases to operate at full load, so energy costs are reduced.
Structurally, the pump consists of: the body, for the manufacture of which copper and stainless steel alloys are used; electric motor; impeller (impeller). At its rotation there appears centrifugal force. As a result, the desired set is formed at the outlet of the casing, and the working fluid is fed into the pipeline.
There are two types of pumps. dry and wet. They differ from each other by the structure of the rotor. In the design of the wet wheel is located directly in the working environment, but the electrical part of the unit is securely sealed in a metal sleeve separating the stator and rotor.
But this type of units should not be installed for pumping hot water, over time, the salts dissolved in water, will hammer a micron gaps between the rotor and the stator, as a result of which the engine will stop working.
In the motor of the dry type, the impeller is also immersed in the working environment, but the element is completely isolated from it. It should be noted that the devices of the latter type are characterized by high performance.
The homeowner should understand that the calculation of the circulating pump for the underfloor heating, it is quite a complicated matter and it will be better if it is performed by specialists of heat engineering. By the way, after the calculations will be clear and the connection diagram of the floor heating pump.
As a rule, in suburban homes heating systems of two types are used with forced flow of coolant and natural. The first type provides a circulating pump. Its task is to ensure the supply of coolant at a given speed. To perform the calculations of the circulating pump you will need the following data:
- The volume of the coolant that must be pumped through the piping system for a certain amount of time, that is in m.cubic meter./ч.
- The amount of heat required to heat the room, this parameter is called the heat output, it is measured in watts.
When performing the calculation must take into account the temperature difference in the pipeline, ie the tube leaving the heater and the one through which it is fed back. The difference can be up to 20 degrees for long pipelines, if short circuits are used in the heating system, the value is 10 degrees. If the floor heating is performed with a small area, the temperature difference is assumed to be 5 degrees.
Do not forget about the type of coolant. If the piping is filled with water, then the calculation takes a coefficient of heat capacity, it is 1.163. If antifreeze is used in the system, this factor has a different value and is determined by special literature.
In addition to the data mentioned above, the following data will be required for the calculations:
- Type of building materials used in the construction of the building.
- The area of the room to be heated.
- Whether additional heating equipment will be used.
Number of loops
In the laying of underfloor heating systems one-piece pipe is used. The presence of connections increases the likelihood of damage to the pipe at the joint, and this leads to additional costs for repair and restoration of the heating system.
That is, the homeowner must know the total length of the heating circuit. In fact, this is the simplest calculation, but in order to make it you will need to prepare a detailed diagram of the room showing all the lines and the distance between them.
To carry out such a calculation, several techniques are used:
- According to the average. For one square meter of the floor is mounted 5 p. м. pipe. That is, you need to multiply the area of the room by 5.
- According to the size of the average pitch. To do this, multiply the area of the room by the average pitch size in meters and add 10% to the obtained value for corners and turns. If the wall distance between the lines is 100 mm, then in the center it is 300 mm. That is, the average pitch will be 200 mm.
- You can use the size of the width of the room. It is necessary to multiply by the number of steps and add the length of the room for turns. This method of calculation is used when installing the floor in a serpentine manner.
Note that the optimum length of the piping system is 80 120 p.м. That is, with these parameters the coolant will heat the room, and at the same time will not cool to the temperature at which the drop in pressure in the system. If the calculated length will be greater than this value, it makes sense to mount a second heat loop.
The hydraulic resistance of the pipe
The resistance to the movement of the coolant flow, which the piping system has, is called hydraulic. It is estimated as the amount of lost thermal energy consumed by the friction forces.
Any pipe construction is not only made up of straight sections, but also of turns, branches etc., For their formation various connecting devices are used. All this leads to hydraulic resistance. It also depends on the material used to manufacture the pipeline.
Conducting the appropriate calculations will reduce heat loss and thus avoid unnecessary energy consumption. Hydraulic calculation is carried out for the following purposes:
- Calculation of pressure losses in the heating system sections.
- Calculation of the optimal size of the pipeline, while it is necessary to consider the recommended flow rate.
- Calculation of heat losses and minimum pressure resistance in the pipe system.
- The correct assembly of parallel placed lines and installed fittings.
In the course of movement on a closed circuit, the flow must overcome a certain resistance. With its increase the pump capacity must be increased.
Actually there is no point in buying high capacity equipment, as energy costs will increase. If it is insufficient, the pump will not be able to provide the required pressure, and this will lead to an increase in heat loss.
For the correct selection of pumping equipment that is designed to provide forced movement of the heat carrier, it is required to understand its technical characteristics. It is also necessary to understand what information is encoded in its marking.
In fact need to pay attention to two key properties. the head and the capacity (flow).
The head is the resistance created by the system, overcome by the unit. To measure this characteristic we use the meters of water column. For the most part, the pressure limit is given by the upper point of the pipework through which the heat transfer medium flows.
The capacity tells how much heat transfer medium can be carried through the pipe in a certain amount of time. The capacity is measured in cc.meters per hour.
The nameplate, which is attached to the pump casing, shows the following data:
When calculating the length of the pipework the construction length of the pump, i.e. the distance between the pump ends, must be taken into account. If there is an error in the calculation or the size is too short, you will have to strain the pipes too much. This can cause damage to the hose.
Example of pump calculation
Assuming that per square meter of pipework the pressure limit is given by the top of the pipe system to which the heat transfer medium is transported. m it would be necessary to lay five linear meters of hose in a room of 50 sq. m it will be necessary to lay 250 p. 50 m of hose, plus 37 m reserve for curves. As the standard delivery is 120 meters, three pieces, two of 120 meters and one of 37 meters, must be installed.
At 50 m.kv.(1 circuit)
If used, it is necessary to install one circulation pump. Its capacity should be determined by the expression
Pn. heating circuit capacity, kW,
tobr.t is the temperature of the heating medium in the return line,
tpr.t. temperature in the direct flow line.
At 50 m.sq. (2 loops)
In a system with two circuits, we have to calculate for each of the pumps using the same formula as in the previous section
IMPORTANT! THE CONNECTION CAN ONLY BE MADE AFTER THE UNDERFLOOR HEATING MANIFOLD GROUP WITH PUMP HAS BEEN INSTALLED.
Calculations and design are based on several room characteristics, as well as the choice of heating. main or supplementary. Important factors are the type, configuration and area of the room in which the planned installation of this type of heating system. The best option is to use the floor plan with all the parameters and dimensions required for the calculations. It is allowed to independently perform the most accurate measurements.
To determine the value of heat loss, you will need to have the following data:
- type of materials used during construction;
- type of glazing, including the type of profile and insulating glass unit;
- Temperature indicators in the region of residence;
- Use of additional heating sources;
- the exact dimensions of the area of the room;
- the expected temperature conditions in the room;
- storey height.
In addition, the thickness and insulation of the floor is taken into account, as well as the type of floor covering to be used, which has a direct impact on the efficiency of the entire heating system.
When performing the calculations, the desired temperature for the room to be built up should be taken into account.
Flow rate of underfloor heating depending on the loop pitch
Calculation of underfloor heating
The temperature should be no higher than 55 o C, and in some cases no higher than 45 o C.
precisely: the temperature should be in accordance with the temperature calculated in the project, which takes into account the need for heat in the room and the material used for the floor covering.
Monitor the temperature with this thermometer, or preferably two.
One thermometer shows the supply temperature of the underfloor heating medium (mixed water temperature), and the other. return temperature.
If there is a 5 mm difference between the two thermometers. 10 o C, so your underfloor heating system is working correctly.
What the surface temperature of the underfloor heating should be?
29 o C. in rooms with a long stay of people;
Different shapes are used for laying underfloor heating pipes: serpentine, corner serpentine, snail-shaped, double serpentine (meander).
You can also combine these shapes when laying a single circuit.
For example, the edge area can be arranged in a snake shape, and then the main part of the coil.
Which laying is best for underfloor heating?
For large rooms with a square, rectangular or circular shape without geometric exclusivity it is better to use a snail.
For small rooms, complex-shaped rooms or long rooms, use a serpentine pitch.
What should be the pitch?
The paving pitch should be in accordance with the design calculations.
For the edge zones, a pitch of 10 cm is used. For the other zones with a difference of 5 cm. 15 cm, 20 cm, 25 cm. But no more than 30 cm.
This limitation is due to the sensitivity of the human foot. With a larger pipe pitch your foot starts to feel the temperature differences in the floor areas.
How to calculate the length of the pipe?
For this you can use a very simple formula: L = S / N 1,1, where
S. the area of the room or the circuit, for which the pipe length is calculated (m 2 ); N. laying spacing; 1.1. 10% reserve of pipe for turns.
Do not forget to add the length of the pipe from the collector to the underfloor heating, including supply and return.
As an example, let us consider a task in which we have to calculate the pipe length for a room in which the floor has a usable area of 12 m2. Distance from manifold to underfloor heating. 7 м. Pipe pitch 15 cm (do not forget to convert to m).
What is the maximum length of one circuit?
Everything depends on the hydraulic resistance or pressure loss of the specific circuit, which in turn depends directly on both the diameter of the pipes used and the volume of the coolant that flows through the cross-section of these pipes per unit time.
In the case of underfloor heating (if the above factors are not taken into account) you can get a so-called “trapped loop” effect. The situation in which no matter how powerful the pump is, circulation through the loop will not be possible.
In practice, it has been found that a pressure loss of 20 kPa or 0.2 bar leads to this effect.
To avoid going into calculation, here are some recommendations that we use in practice. For plastic pipes with a diameter of 16 mm we do the circuit of not more than 100 m. Generally stick to 80 m. The same applies to PE pipes. For 18 cross-linked polyethylene pipe the maximum loop length is 120 m. In practice we stick to 80. 100 м. For 20 mm metal plastic pipe the maximum length of the loop is 120. 125 м.
Can floor heating circuits be of different lengths??
The ideal situation is when all loops are the same length. You do not need anything to balance, adjust.
In practice, this can be achieved, but more often than not is not practical.
For example, at the facility is a group of rooms, where you want to make a heated floor. Among them, there is also a bathroom with a usable area of the warm floor in which 4 m 2. Accordingly, the length of the pipes of this circuit together with the length of the pipes to the collector is only 40 m. Is it really necessary to adjust all the rooms to this length, crushing the usable area of the remaining rooms by 4 m 2 ?
Of course not. This is not practical. And then there’s the balancing fixture, which is precisely designed to help balance out pressure loss across the circuits?
Again, you can use the calculations, through which you can see up to what the maximum variation of the lengths of individual circuits at a particular facility with this equipment can be allowed.
But again, without immersing you in complex boring calculations, we say that we at our facilities allow a variation of pipe lengths of individual circuits in the 30. 40%. You can also “play” with pipe diameters, pitch and “cut” large areas into medium sized chunks rather than small or large ones, if necessary.
How many loops can be connected to a mixing unit with one pump?
The physical meaning of the question is like asking “How much weight can be carried by the machine??”
What else would you want to know if someone asked you this question?
Absolutely right. You would ask: “Which machine are we talking about??”
So the question “How many loops can be connected to a floor heating manifold?”, it is necessary to consider diameter of a collector and what volume of the heat carrier is capable to pass through itself the knot of mixing for a unit time (it is accepted to consider m 3 / hour). Or, what is also the equivalent heat load of the mixing station you have selected??
Suppose you have a Valtec Combimix as a mixing unit. For which heat load it is calculated? Let’s take its certificate. See excerpt from the datasheet.
Its maximum capacity factor is 2.38 m3 /hour. If we put the pump Grundfos UPS 25 60, then at the third speed at a given ratio this unit is able to “pull” the load of 17000 W or 17 kW.
What this means in practice? 17 kW is how many circuits?
Let’s imagine that we have a house in which there are some (unknown) premises of 12 m2 of usable area of floor heating in each room. The pipes we have laid with a pitch of 20 cm, which leads to the length of each circuit, given the length of the pipes from the underfloor heating to the collector, 86 m. According to the design calculations, we also got that the heat take-off from each m2 of this floor heating gives 80 W, which brings us accordingly to the heat load of each circuit
How many rooms or similar circuits can our mixing unit provide heat??
17000 / 960 = 17.7.
In practice, however, in most cases it is not necessary to calculate the maximum values. So let’s stick with number 15.
Valtec itself has a manifold with the maximum number of outlets to this node. 12.
Do I need to do multiple underfloor heating circuits in large rooms?
In large rooms, the design of the floor heating must be divided into smaller areas and make several circuits.
This need arises for at least two reasons:
Limiting the length of the circuit pipe is necessary so as not to get a “locked loop” effect, in which the coolant will not circulate through it;
the correct operation of the cement pouring slab itself, which must not exceed 30 m 2. The ratio of the lengths of its sides should be 1/2 and the length of one of the edges should not exceed 8 m.
How do I know how many underfloor heating circuits I’ll need for my house?
To understand how many loops of the floor heating are needed and on this basis to choose a suitable collector with the same number of outputs, you need to start from the area of the rooms where you plan to use the system.
You then calculate the usable area of the underfloor heating. How to do this is described in question 12 “How to calculate the usable area of the underfloor heating?”.
Then, use the following method: based on the pitch of the underfloor heating, divide the usable area of the underfloor heating in each room into the following dimensions:
If the floor area in the room is smaller than the specified dimensions, it does not need to be broken down. We recommend reducing these values by 2 m 2. if the length of the pipe connection from the underfloor heating to the manifold exceeds 15 m. Breaking up the usable floor area in the rooms, try also to achieve that the length of the pipes in these circuits is either the same, or the difference between the individual circuits did not exceed 30. 40 %. To find out the length of the pipes in each circuit, see question 6 “How to calculate pipe lengths?”.
How to calculate the usable area of the underfloor heating?
To calculate the usable area of the future floor heating, you need to draw a plan of the room where it will be located. It is better to make a plan to scale.
Step back from each of the walls and draw a line 30 cm. Draw the hatched area. Mark on the plan areas where there will always be furniture: refrigerator, furniture wall, sofa, large closet, etc.д. Then use the hatched areas as well. The white shaded part of the floor plan is the usable area of the floor heating that you are looking for.
How to design Underfloor Heating by LoopCAD (Manual J method).
For clarity, let’s calculate the usable area of the dining room, where there will be a warm floor. The total size of the dining room is 20 m2. The length of the walls is 4 m and 5 m respectively. In the kitchen there will be a kitchen unit, a refrigerator and a sofa. Do not forget to space out the walls by 30 cm. Check the shading of the areas to be shaded. See drawing.
And now let’s calculate the usable area of the underfloor heating.
What is the total thickness of the underfloor heating pie?
It all depends on the thickness of the insulation, since the other values are known.
With the following thickness of insulation you will have these values (the thickness of the finishing coat is not taken into account)
What do you use to calculate your underfloor heating system?
For calculating radiator heating systems as well as underfloor heating systems we use the company’s Audytor CO.
Below we lay out a screenshot of the module of this program for the preliminary calculation of the floor heating and a screenshot of the module for calculating the layers of the floor heating pie.
When you look closely at these screenshots, you can see how serious the proper calculation of underfloor heating is.
You can also see the operation of the program itself, which makes it possible to visually control important parameters such as pipe length, pressure loss, floor surface temperature, heat escaping uselessly downwards, useful heat flow, etc.д.
How to assess the dimensions of a manifold box in order to place all the necessary components in it?
Determining the dimensions of a manifold cabinet is not difficult. Once again, we suggest that you take advantage of the Valtec products and their ready-made recommendations in the table below, provided you already use ready-made floor heating assemblies from this manufacturer.
Manifold cabinet selection
At what height should the manifold cabinet be installed?
On the one hand, it is clear that when installing the manifold cabinet, you need to consider the height of the future screed and trim, so you do not get a situation where you can not even open the cabinet door.
On the other hand, you need to consider the ease of maintenance and the need for possible replacement of individual elements of the system with the likelihood of disconnection of the pipeline.
The shorter the pipe section, the greater the stiffness and vice versa.
Given this factor, it is possible to make the manifold cabinet rise by 20. 25 cm from finished floor level.
However, a very important design element must not be forgotten. If raising the cabinet leads to unacceptable disturbance to the design and it is not possible to solve this problem in another way, lower the cabinet to the floor level, but with the calculation so that it can open.
Underfloor heating power calculation
To determine the required capacity of the underfloor heating in the room is affected by the heat loss indicator, for the exact determination of which it will be necessary to make a complex thermotechnical calculation according to a special methodology.
The method to perform such accurate calculations requires extensive theoretical knowledge and experience, and therefore thermal calculations are best left to professionals.
After all, only they know how to calculate the capacity of water underfloor heating with the smallest error and optimal parameters. This is particularly important when designing heated built-in heating in rooms with a large area and a large height.
Underfloor heating can be installed and operated efficiently only in rooms where the heat loss is less than 100 W/m². If the heat loss is higher, the room must be insulated to reduce the heat loss.
However, if the design engineering calculation costs a lot of money, in the case of small rooms approximate calculations can be carried out independently, taking 100 W/m² as an average value and the starting point in further calculations.
For a detached house, it is usual to correct the average heat loss figure on the basis of the total floor area of the building:
The power of underfloor heating per square metre is influenced by the system load, the hydraulic resistance and the level of heat transfer, for example:
Pipes can be copper (which have the best thermal and performance characteristics, but are not cheap and require special skills and tools).
They can also be flexible polyethylene, polypropylene reinforced, polybutene (the most affordable, but the least durable option, in addition, difficult to install), metal-plastic (the best option in cost and durability).
The area, heated by one circuit should not exceed 20m ². If the heated area is larger, it is advisable to split the pipeline into 2 or more circuits, connecting them to the distribution collector with the possibility of regulating the heating sections of the floor.
The total length of one circuit should not exceed 90 m. In this case, the larger the chosen diameter, the greater the distance between the strands of pipes. As a rule, pipes with a diameter greater than 16 mm are not used.
Each parameter has its own coefficients for further calculations, which can be viewed in the.
Calculation of heat output: calculator
To determine the capacity of the water floor, you need to find the product of the total area of the room (m²), the temperature difference between the supply and return fluid, and coefficients depending on the pipe material, the floor covering (wood, linoleum, tiles, etc.).д.), other elements of the system.
The power of water underfloor heating per 1 m², or heat output, should not exceed the level of heat loss, but not more than 25%. If the value is too low or too high, you need to recalculate, choosing a different pipe diameter and distance between the threads of the circuit.
The power indicator is the higher the larger the diameter of the selected pipes, and the lower the greater the step between the strings. To save time, you can use electronic floor calculators or download a special program.
General recommendations before installing the system
Before you buy a water heating system, you need to make a heat map of the house with the help of a specialist. Such a map will help to identify the heat loss of the room. Thus, if they are more than 100 watts per square meter, then before calculating the length of the pipe, you need to insulate the building.
You can do the calculation of the warm water floor on your own, using the calculator. But here an important point is that the heating system can not be placed under the oversized furniture and stationary equipment. Otherwise, the heating system will quickly fail. But at the same time, the water structure should still occupy at least 70% of the floor area, otherwise the room will be poorly heated.
Also the efficiency of heating will depend on the requirements of the premises.