Principle of the calculation method
You will find the complete range in the underfloor heating category – system insulation boards, pipes, manifolds, controls and electric heating mats.
The calculation method for basic cases is based on the following assumptions:
• the temperature distribution (air temperature and design temperature) is uniform,
• heat losses are calculated for steady-state conditions, assuming constant properties such as temperatures, characteristics of the building elements, etc.
• its result also determines the price of underfloor heating
The calculation procedure for basic cases can be used for most buildings:
• in which the room height does not exceed 5 m,
• which are heated, or are assumed to be heated, to a required constant temperature,
• in which the air temperature and the operative temperature are assumed to be equal,
In poorly insulated buildings, or during the heating-up period, with heating systems:
• with a large convective share of heat transfer, e.g. warm-air heating,
• large-surface heating systems with a significant radiant component, e.g. underfloor or ceiling heating, in which there may be a marked difference between the air temperature and the operative temperature, as well as a deviation from uniform temperature distribution in the room, this may lead to a substantial deviation from the basic case. The heat losses are calculated first. These results are then used to determine the design heat load, which is important in determining the price of underfloor heating.
To calculate the design heat losses of a heated space, the following must be taken into account:
• the design transmission heat loss, which is the heat loss to the outside by conduction through the enclosing elements, as well as heat transfer between heated spaces resulting from the fact that adjoining spaces are to be heated, or are usually assumed to be heated, to different temperatures; for example, adjoining rooms in a neighbouring flat are assumed to be heated to a set temperature corresponding to an unoccupied flat,
• the design ventilation heat loss, which is the heat loss to the outside as a result of ventilation or infiltration through the building envelope, or heat transfer by ventilation from one heated space to another heated space inside the building. (STN EN 12831)
The design heat losses may not affect the price of underfloor heating, but they have a marked effect on achieving thermal comfort.
The steps of the calculation procedure for a heated space are as follows (see figure):
• determining the values of the external design temperature and the annual mean external temperature,
• determining the status of the spaces (heated or unheated) and the values of the internal design temperature of each heated space,
• determining the dimensional and thermal characteristics of all building elements for all heated and unheated spaces,
• calculating the transmission heat loss coefficient and multiplying it by the design temperature difference; the result is the design transmission heat loss,
• calculating the ventilation heat loss coefficient and multiplying it by the design temperature difference; the result is the design ventilation heat loss,
• calculating the total design heat loss of the heated space as the sum of the design transmission heat losses and the design ventilation heat losses,
• calculating the heating-up capacity of the heated space, i.e. the additional power required to compensate for the effects of intermittent heating,
• calculating the total design heat load of the heated space as the sum of the total design heat loss and the heating-up capacity. (STN EN 12831)
Once these values have been determined and calculated, they are the decisive factor in calculating the price of underfloor heating.
Calculation procedure for part of a building or a whole building
To size the heat source, e.g. a heat exchanger or boiler, the total design heat load of the part of the building or of the whole building must be calculated. The calculation procedure is based on the results of the room-by-room calculation. The steps of the calculation procedure for part of a building or a whole building are as follows:
• adding up the design transmission heat losses of all heated spaces, disregarding heat transfer inside the defined system boundaries, in order to calculate the total design transmission heat loss for the part of the building or the whole building,
• adding up the design ventilation heat losses of all heated spaces, disregarding heat transfer inside the defined system boundaries, in order to calculate the total design ventilation heat loss for the part of the building or the whole building,
• adding the total design transmission heat loss and the total design ventilation heat loss in order to calculate the total design heat loss for the part of the building or the whole building,
• adding up the heating-up capacities of all heated spaces in order to calculate the total heating-up capacity for the part of the building or the whole building, which is needed to compensate for the effects of intermittent heating,
• calculating the total design heat load for the part of the building or the whole building as the sum of the total design heat loss and the total heating-up capacity. (STN EN 12831)
• finally, the price of underfloor heating is calculated on the basis of these findings
Calculation and design of a radiant heating surface
The characteristic feature of radiant heating surfaces is that the radiant component of the heat output predominates over the convective component. To achieve this, radiant heating surfaces usually take the form of smooth flat panels, without any fins or projections that would increase the share of heat transfer by convection.
The surface temperature of these surfaces (flat panels in terms of heat transfer), which are in fact heated locally by pipes, is not uniform. The primary task in the theoretical calculation of radiant heating surfaces is therefore to determine:
• first the mean surface temperature tP (for surfaces heated by a heating medium flowing through pipes) and then,
• the heat output Q (°C), or the specific heat output q (W. m-2) (calculated in a similar way as for conventional heat emitters).
The calculation of an underfloor heating surface is based on the assumption that the mean surface temperature of the floor will not exceed the values permissible on health grounds, while the heat output of the underfloor heating surface will cover the heat losses of the heated space. The level of heat output and the price of underfloor heating are related to each other.
The calculation of a radiant heating surface is based on the assumption that the mean surface temperature will not exceed the values permissible on health grounds, while the heat output of the heating surface will cover the heat losses of the heated space. The radiant heating surfaces of low-temperature heating can be constructed:
• with embedded pipes,
• with lamella heating surfaces.
The calculation of water-based underfloor heating is based on the following criteria:
- the thermal balance of the heated interior,
- the heat losses of the rooms,
- the thermal calculation of the heated floor,
- the hydraulic calculation of the heated floor,
- a comprehensive algorithm for heat and mass transfer.
The last, but no less important, factor when buying is the price of underfloor heating.
THERMAL BALANCE OF AN INTERIOR WITH UNDERFLOOR HEATING
To ensure the thermal balance of an interior with underfloor heating, we define the fundamentals of heat transfer, its mathematical formulation, the conditions for thermal comfort and the heat load.
HEAT TRANSFER IN UNDERFLOOR HEATING
With radiant heating of an interior (fig. below), the heating surface P delivers the heat flow by radiation, without the room air acting as an intermediary, to the surrounding cooled surfaces of the interior, and it always holds that the effective temperature here is higher than the air temperature.
The radiant heating surface P is located in or on one of the building elements (in the first case P is built into, or inseparable from, the building element; in the second case it is separate), which is the ceiling, a wall or the floor. We then speak of large-surface radiant ceiling, wall or underfloor heating.
Prices of underfloor heating, i.e. of the specific products for preparation and installation, can be found on our website.
References:
Dušan Petráš: Water-Based Underfloor Heating
Dušan Petráš: Low-Temperature Heating and Renewable Energy Sources
PhD. Štefan Papučík, PhD. Radovan Nosek, PhD. Richard Lenhart: Heating
