Radiant heating systems for passive buildings


1.Effect of radiant heating systems on the energy performance of buildings (EHB)

Buildings account for 40% of total energy consumption in the EU. This sector is growing, which is associated with an increase in its consumption. Therefore, reducing primary energy consumption and the use of renewable resources in the buildings sector are an important measure to reduce the Union's energy dependence and greenhouse gas emissions.

The goal in the field of construction and major reconstruction of buildings is to build almost zero-energy buildings - passive buildings. Mandatory construction of passive buildings has been determined since 2020.

1.1.Passive buildings from the perspective of architecture

Photographs of passive buildings with distinctive shading elements - Vienna

In terms of architecture, the concept of building construction is the same as in the previous period. Changes occur in the reduction of the heat transfer coefficient U of the materials used, the size of the glazing and the orientation of these areas in order to obtain as much outdoor heat profits as possible during the winter period. For summer, buildings are equipped with external shading elements to reduce solar heat profits.

1.2.Passive buildings from the energy point of view

In passive buildings, the heat sources and the heating system only cover up to 20% of the heat consumption for heating. The decisive share of heat consumption of the building is covered by heat profits of up to 80%. Heat profits can be external - mainly solar profits and internal heat profits supplied from electrical appliances, lighting, human activities such as food cooking and human biological heat.

Graph 1 Coverage of heat consumption for heating in passive buildings
In order to ensure the energy-efficient use of heat profits, it is necessary to ensure the priority of heat profits use over heat supplied from the heating system.
The priority of heat profits over the heat supplied by the heating system is a new technical task for designers of heating systems. Energy efficient use of heat profits are declared in the energy certificate of the building. It is therefore binding on developers and investors in the construction of new buildings, but also in the major renovation of buildings. Energy-efficient use of heat profits means that they will be used to heat the building to a calculated interior temperature and not be degraded to overheat the building.

2. Example for comparison of radiant heating systems in relation to the accumulation load and the possibility of using heat profits

To compare the properties of thermal systems and to determine the assumption of utilization of thermal profits, a method of monitoring the heat accumulation in the heating system is suitable. The heating systems are not insulated towards the interior of the building and we cannot regulate the discharge of accumulation. Discharge of the accumulation takes place depending on the current heat loss of the building.

In this case, the actual heat output and the surface temperature of the system are dependent on the system's accumulation status.

The energy-efficient use of heat profit is decisively influenced by the thermal delay = accumulation load of the heating system. The accumulation load of the heating system is the charging time and the discharge time of the system accumulation. The heating system is made up of a heat-carrying medium - heating water and a construction material that forms a thermal system. These materials accumulate heat and transfer it to the interior with a time shift = accumulation load.

2.1.Ideal thermal system

In order to ensure use the priority of the heat profits, it is necessary for the heating system to have zero accumulation load, or to minimize this load. An example is the ideal heating system on the following chart.

Graph .2. The accumulation of an ideal heating system

Description of graph No.2

  1. At 0hr, the ideal heating system is able to react immediately to a regulatory stimulus and achieve nominal heat output. The accumulation load when charging the accumulation is 0 hours.
  2. The red horizontal bar  represents the running time of the system. The system transfers heat and reaches the required interior temperature.
  3. Upon reaching the desired indoor temperature, the system immediately loses energy and ceases to supply heat to the space. Accumulation load for discharging accumulation is 0 hours. Immediately stopping the heat supply does not raise the interior temperature above the setpoint = we do not overheat the space!

2.2.Example - calculation of accumulation and accumulation load of heating systems for a building with a calculation area of ​​100m²

Entry conditions:

  1. Heat loss of building at design temperature Qs= 3200W. Actual heat loss for calculation of accumulation load (50%Qs) Qa=1600W .
  2. The heat source is a heat pump with a capacity 8000W
  1. The heating systems are designed to cover the heat loss of the building

-WH water infrared heaters                                      3 pcs WH20, 4pcs WH 15

-Plasterboard ceiling heating system                         15mm (9,9x1,1 ,r= 100mm)

-Concealed ceiling heating system                             24mm (9,9x1,1 ,r= 100mm)

-Floor system in screed with thickness                       45 mm ( 17x2, r= 150mm , R= 0,1 m².K/W)

-Thermally activated building structure TABS            150mm ( 17x2, r= 150mm )

For surface radiant systems, the ratio of active area to total area 0,7.

Formula for calculating heat storage Q=c.m.∆ϴ   (W)

c- specific heat capacity of the material W/kg/K

m- weight of material kg

∆ϴ -temperature difference before and after heating (ϴ1 - ϴ0) , K

Table 1 Physical properties of thermal system materials used

System construction materialSpecific weight ρ [kg/m3]Heat capacity of material C [w/kg/K]Heat capacity of water C [W/kg/K]
AL alloy270020,2561,16

Table 2 Source table for calculating the accumulation of heat systems

Heating systemSystem volume [m3]System weight [kg]Water volume [kg]Heat storage [W]Charging accumulation [hr]Discharging accumulation [hr]
WH-water infrared heaters 1075818930,21,1
Plasterboard heating system1,512604752640,73,3
Concealed ceiling heating system2,420164762900,84,0
Floor heating system in concrete4,6772862338894,221,2
Thermally activated building structure TABS1525200628644610,854,0


Graph 3 Graphical comparison of the accumulation of heating systems during heating
Description of operation of comparable thermal systems

Methodology of system comparison
All systems achieve the same heat output and transmit the heat for the same time - heat system operation .This interval connects all systems and it is thus possible to compare different charging and discharging times - accumulation loads of the systems. In practice, these accumulation cycles begin at the 0 hour point and the charging time of the accumulation is a time delay for the regulatory stimulus. The accumulation status is directly related to the actual heat output and system surface temperature.
If we chose to compare the operation of heating systems from point 0 hours in graphical representation, the transparency would be lost due to extremely long time intervals for the Thermally activated building structure TABS and Floor system in concrete.

Description of Graph No.3
1.Charging accumulation
The accumulation load when charging the accumulation ranges from 0.2 hours for WH water infrared heaters to 10.8 hours for Thermally activated building structure TABS. Charging time of accumulation is influenced by the heat output of the power supply, the charging time will always be considerably shorter than the time of discharging the system. The longer the charging time, the less thermal comfort in the interior.
2.Discharging accumulation
The accumulation load for discharging accumulation ranges from 1.1 hours for WH water infrared heaters to 54 hours for Thermally activated building structure TABS. The actual heat loss of the building influences the discharge time.
Extreme accumulation loads have mainly Thermally activated building structure TABS and Floor system in concrete. These extremes are a prerequisite for overheating the interior by releasing heat from the accumulation of the heating system even after reaching the upper limit of the desired interior temperature.
The accumulation load temperature curves from graph 3 will be used in graph 4 for two systems with the lowest accumulation load WH water infrared heaters and Plasterboard ceiling heating system .

2.3. Influence of accumulation load on interior temperature for selected thermal systems

Graph no.4. Graphical representation of the influence of the accumulation load on the indoor temperature ϴr for WH - water infrared heaters and plasterboard ceiling heating system.

Description of graph no.4
Desired interior temperature ϴr=21°C with hysteresis ±1K is indicated in the graph by yellow lines. The heat profits are shown in red

1.The plasterboard ceiling heating system
The blue line indicates the course of the interior temperature when heated by a plasterboard ceiling heating system. The system operation is non-harmonious and has a 3-fold accumulation load compared to WH water infrared heaters. This adversely affects the supply of thermal energy even after it reaches the upper limit of the indoor temperature interval ϴrmax. All the energy supplied above this level is undesirable and only contributes to the overheating. Heat profits are acting simultaneously supplies thermal energy with discharging accumulation heating system and thus contributes to extreme overheating interior.

2.WH water heaters
The green line indicates the course of the interior temperature when heated by WH-water heaters. The operation of this system is harmonious with minimum supply of thermal energy to ensure the required temperature. The system is able to respond to the regulatory stimulus in a very short time and does not deliver any thermal energy in the event of heat profit. These features ensure the use of thermal energy from the system and heat profit in an energy efficient way only to heat up the space without spending energy for overheating interior.

In this graphical representation, two thermal systems, which have the lowest accumulation loads of the evaluated systems, are compared and can be clearly displayed. Other evaluated thermal systems reach even greater extremes than plasterboard ceiling heating system and graphical comparison would be confusing.

2.4.Summary of knowledge from comparison of radiant heating systems during heating

The longer the discharging time of the thermal system, the more thermal energy delivered from the accumulation of the thermal system is merely spent to overheating the space. At the same time, the longer the discharge accumulation time, the less likely it is to use energy profit for energy efficiency. When combined with extreme accumulation load and heat profits, extreme indoor temperature and poor comfort for people will occur.
The heating system is becoming a major element in the energy system of passive objects. Thermal system with low accumulation load WH system ensures mainly energy-efficient use of renewable energy as well as heat recovery from indoor heat profit. This significantly reduces energy costs and the consumption of primary energy for heating passive objects. In extreme cases, in comparison with a system with a large accumulation load, the heat savings can reach up to the entire volume of heat profit at 80% of the heat consumption of the building. Heating systems with low accumulation load are a means of meeting the heat consumption values for heating stated in the building energy certificate.


3.1. Description of cooling effects in radiant heating systems

To describe the phenomena occurring in radiant heating systems during cooling, we will use an example of measuring and calculating the course of dew point temperature in exteriors and interiors of a family house measured during the peak day of the summer season.

Graph no.5. Graphical representation of the dew point temperature of outdoor air and indoor air in summer time in a family house and surroundings 30.7.2018.
Note: The temperature and humidity of the indoor air were measured in the most heavily loaded area in the living-room with the kitchen. The temperature and humidity sensors were placed 9 m from the hob and a cooker hood was drawn over the hob during cooking to extract the air into the exterior. A weather station was used to assess the temperature and humidity of the outdoor air.

Description of graph No.5
The graph shows that both dew point temperatures have a non-harmonic course with several extremes during the day and night. The dew point of the interior is most influenced by human activity. The largest indoor peaks are achieved during lunch preparation after 10:00 to 12:00 and in the evening after 17:00. The temperature of the outdoor dew point is influenced by nature and the highest peaks are reached at night. The temperature dew point of the outdoor air during the day was higher than the temperature of the indoor dew point. By human activity, which increases the temperature and relative humidity of the air in the summer mainly by cooking and ventilation by means of recuperation (depending on the operating system selected), we will raise the level of the dew point of the interior in a dynamic way. Radiant heating systems mainly constructed in the wet way and the plasterboard ceiling heating system must operate from a safety point of view above the dew point of indoor air with a sufficiently large temperature reserve. It is the only prevention against the development of mildew , damage on heating system and possible injury to people! The requirement to maximize the cooling capacity results in a short reaction time for charging and discharging the heating system during cooling = minimum accumulation load.
Photo of mildew on ceiling and walls
3.2.Accumulation of cooling systems

Graph 6 Graphical comparison of the accumulation of heat systems during cooling
Description of graph no.6
The accumulation in graph 6 is indicated by a negative value (- ).From the point of view of energy flow during cooling, thermal energy is taken from the interior therefore it has a negative value. When heating energy we supply to the interior has a positive value (+).If the system has -100% accumulation status it means that it is able to take heat from its surroundings with nominal = projected output.
Methodologies for comparing systems in cooling are the same as those described in heating. Phenomena in thermal systems during cooling take place in minus accumulation values in a mirror-image graph.
All systems achieve the same cooling capacity and take the heat from the interior for the same time - the heating system runs.This interval connects all systems and it is possible to compare different charging and discharging times - accumulation loads of systems. In practice, these accumulation cycles begin at the 0 hour point and the charging time of the accumulation is a time delay for the regulatory stimulus. The accumulation status is directly related to the actual cooling capacity and system surface temperature.
In contrast to heating, we will assess the effect of the accumulation load when charging and discharging the accumulation in relation to the dew point of the indoor air.

3.3. Accumulation load of heating systems during cooling in relation to the internal dew point temperature and recommended regulation of water supply temperature

To compare two thermal systems with the lowest accumulation load WH water infra-red heaters and plasterboard ceiling system SDK we use the dew point temperature curve from graph No. 5 and the curve of the accumulation load curve from graph No. 6. In graphs 7 and 8 we will also describe two methods of regulation of water supply temperature used in radiant heating systems. Control by dew point sensor for drywall ceiling systems SDK and control by dew point evaluation and copying for WH water heaters.

Figure 7 Cooling water supply temperature control for WH water systems and SDK gypsum ceiling system using dew point sensor at 16 ° C
Description of graphs 7 and 8
Gypsum ceiling system SDK
The gypsum plasterboard system uses a dew point sensor and a controller with the option of setting a constant or equithermically controlled supply water temperature to control the water supply temperature.The weather-compensated control is based on the outdoor air temperature. The external air temperature has no relation to the indoor dew point of the air, this design of the controller program is only auxiliary and is based on the concept for the equithermal control of the flow temperature for heating. For a simplified graphical representation, a constant supply temperature of 16 ° C is used in the graph. The blue line shows the control of the water supply temperature to a constant temperature of 16 ° C to achieve the highest cooling capacity. The red line shows the area of dew point exceeding due to the accumulation load of the system.
The dew-point sensor reacts when the relative humidity of the system surface reaches 85% by opening the power supply circuit of the circulating pump of the cooling circuit. When the sensor is open, the cooling capacity is reduced by the power supply. The cooling capacity from the accumulation of the system continues and the condensation of the system - red line. The longer the system has a discharge time - the accumulation load will be less inclined and the longer and more often it will cross the dew point temperature curve. After heating the system surface above the dew point, the controller must suspend cooling from the power supply for a preselected time - outage. This time is necessary to prevent the system from repeatedly falling below the dew point line. After the outage time has elapsed, the controller starts the cooling circuit circulation pump. During the day the dew point was exceeded 3 times. After 6pm, the dew point of the outdoor air begins to increase, which may cause condensation to form when the system is used for recovery. This type of regulation is strongly influenced by the input conditions entered by man, which can have a very negative impact on the operation of the system.
Setting the water inlet temperature as low as possible will always exceed the dew point temperature and condense the heating system. Such operation may lead to the formation of molds, destruction of the system and endangerment of people occurring under such a thermal system.The most common safe solution of regulation of water supply temperature for heating systems with high accumulation load is a constant temperature of + 2K above the maximum dew point temperature during the day shown in Figure 8.

WH water heaters
The green line shows the flow temperature for the WH system, which follows the temperature curve of the indoor dew point with little heat reserve. A temperature and relative humidity sensor is used to evaluate the interior dew point. Based on these values, the controller calculates the dew point temperature in real time and regulates the water supply temperature by the selected thermal safety.It is a continuous and efficient regulation of the water supply temperature with maximum cooling capacity throughout the day.

Figure 8 Progress of cooling water supply temperature control for WH water systems and SDK drywall ceiling system using dew point sensor at safe supply temperature.
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