Automatic and dynamic hydraulic balancing

The first law of thermodynamics always applies

In circulating water heating systems, the following simplified rule of three applies to the thermal power Qꞌ transported or transferred by heating surfaces, the flow rate Vꞌ and the temperature difference ΔT, where c is a constant representing the heat capacity of the heating water: Qꞌ = c · Vꞌ · ΔT

Vꞌ · ΔT Vꞌ · ΔT

The heat output Qꞌ delivered to a consumer is therefore proportional to the product of the flow rate Vꞌ and the flow/return temperature difference or spread ΔT:

Qꞌ ~ Vꞌ · ΔT (power rule of three)

The same amount of heat can therefore be transported, transferred or stored by cooling (or heating) a large amount of water slightly, or by cooling (or heating) a small amount of water significantly, see Fig. 1. A good approximation for the heat capacity of water is

c ≈ 4.2 J/(g·K) = 1 cal.

This means that 1 gram [g] of water can absorb (or release) 4.2 joules of heat by cooling it by 1 kelvin [K]. The heat capacity of water is therefore approximately 4.2 J/(g·K).

This means that 4.2 joules of heat can be extracted (added) from 1 gram [g] of water by cooling (heating) it by 1 Kelvin [K]. In the same way, ½ g of water could be cooled by 2 K or ¼ g of water by 4 K. This amount of heat is also called “one calorie”. Since a watt is the power at which one joule of heat is transferred or transported in one second (1 W = 1 J/s), the above-mentioned power triangle can be expressed as follows: ΔT = P × m (ΔT = power × mass) (ΔT = power × mass) Since a watt is the power at which the amount of heat of one joule is transferred or transported in one second (1 W = 1 J/s), the above power triangle can be written in common units as follows:

Qꞌ [kW] = 7/6 · Vꞌ [m³/h] · ΔT [K]2

For example, a consumer with a nominal power of QꞌN = 28 kW, which is designed for a nominal spread of ∆TN = 20 K (e.g. 80/60°C or 50/30°C), has a nominal volume flow of:

VꞌN = 7/6 · 28 kW / 20 K = 1.2 m³/h.

So far, nothing new.

Non-automatic and static hydraulic balancing …

now consists of regulating the flow through this consumer, after calculating it beforehand, to this nominal volume flow. By “non-automatic”, we therefore mean that hydraulic balancing cannot be carried out without knowledge of all individual nominal volume flows, which in itself represents a hurdle that should not be underestimated in the case of refurbishment. But what if the consumer, which has been “correctly” adjusted in this way, “correctly” regulated consumer consumes less than the nominal power at partial load? For example,

If the water quantity Vꞌ is not adjusted to the reduced power output Qꞌ, then, because Qꞌ ~ Vꞌ · ∆T , the temperature difference ∆T must be reduced! By “static” we therefore mean that at partial load, the flow rates Vꞌ are not adjusted to the actual thermal output Qꞌ.

Speaking of partial load

At this point, we would like to make a clear distinction between two very different types of “partial load”:

Weather-compensated partial load

In the case of weather-compensated partial load, we assume that, due to the heat conduction of the building envelope, the heating load generally increases in proportion to the difference between the outside temperature and the room temperature . The heating curve is then used to increase the flow temperature as the outside temperature falls. The slope of the heating curve indicates by how many Kelvin the flow temperature is increased when the outside temperature falls by one Kelvin. The partial load is therefore controlled via the flow temperature with almost constant water circulation.

Since the transfer capacity of the heating surfaces is approximately proportional to the difference between their average temperature and the room temperature, the return temperature follows a second, flatter heating curve. Since the transfer capacity of the heating surfaces is approximately proportional to the difference between their average temperature and the room temperature, the return temperature follows a second, flatter heating curve, which intersects the first heating curve at a heating load of zero. At this point, the spread is also zero. The heating load Qꞌ is therefore proportional to the spread ∆T, while the water circulation Vꞌ remains almost constant over the entire weather-compensated load range:

Qꞌ ~ ∆T | Vꞌ = const. (Delta-T control via the flow temperature), see Fig. 2.

Fig. 2: With weather-compensated partial load, the flow temperature is lowered with the load, while the flow rate remains almost constant. Since the average heating surface temperature also drops, the return temperature also follows a heating curve, albeit a flatter one.

Media-controlled partial load

For media-controlled partial load, we assume that in any weather-controlled load case – i.e. at any given but fixed assumed outside temperature and a sufficiently dimensioned but also constant flow temperature derived from the correct heating curve – there is a – there is a target/actual deviation in the temperature of the target medium, e.g. the room temperature. Ideally, the target temperature controllers then throttle the flow through the heating surfaces: Qꞌ ~ Vꞌ |∆T = const. (flow control) For example, the thermostatic valves of the radiators in two-pipe systems work in this way if the flow rate is controlled.

Qꞌ ~ Vꞌ |∆T = const. (flow control)

This is how, for example, the thermostatic valves of the radiators in two-pipe systems work if the bypasses in the tap blocks are closed. In the media-led partial load case with flow control, the flow through the heating surfaces decreases and the spread remains at least constant. In the case of thermostatic valves, the return temperature even decreases because the average heating surface temperature decreases with the heating load. The network is hydraulically relieved and its thermal efficiency increases, see Fig. 3. Unfortunately, there is also the other case : Heat consumption is reduced at constant flow:

Qꞌ ~ ∆T | Vꞌ = const. (Delta-T control via the return temperature)

Fig. 3: If, for example, the thermostatic valves throttle the flow through the radiators of a two-pipe system when the room temperature is too high, the spread increases even more because the average heating surface temperature drops with the transferred power, which also causes the return temperature to fall. The system is therefore relieved hydraulically (water circulation) and thermally (return temperature).

This happens, for example, when the fan of an air heating register is switched off by the electric room thermostat or in single-pipe radiator circuits: Now the spread ∆T must decrease with the thermal output Qꞌ , which means that the return temperature rises at a constant flow temperature. The network is not hydraulically relieved; its thermal efficiency decreases, see Fig. 4. Fig. 4: If, on the other hand, the power transfer is reduced without lowering the flow rate, as is the case, for example, with air heating coils or in single-pipe heating circuits, the temperature spread ∆T increases.

Fig. 4: If, on the other hand, the power transfer is reduced without lowering the flow rate, as is the case, for example, with air heating coils or in single-pipe heating circuits, the return temperature rises because only the spread decreases with the transferred power. The network is therefore not hydraulically relieved (water circulation) and its thermal efficiency decreases (return temperature rises). The network is therefore not hydraulically relieved (water circulation) and its thermal efficiency decreases (return temperature increases).

How does this affect the individual heating surfaces?

Almost all heating surfaces have separate individual controllers to regulate the temperature of their target media:

The following three cases can be distinguished:

^I The flow rate remains constant across the entire partial load range (no flow control at all).

^II The flow rate remains constant above the partial load of zero (two-point flow control on/off).

^III The flow is controlled across the entire partial load range (continuous flow control 0-100%).

Obviously, case I is the worst, case II is the second worst, and only case III is the best solution for the desired goal of maximising supply reliability and hydraulic and thermal network efficiency.

Two-point control and thermal efficiency

Since the essence of two-point control is to switch heat transfer completely on and off, heat transfer can only be reduced by limiting the transfer time. This means that

Two-point control is therefore inferior to proportional or continuous control not only for reasons of thermal comfort (occasionally cold floor surfaces are often criticised in new buildings), but also because of the more efficient use of scarce heating surfaces.

Automatic and dynamic hydraulic balancing

If a thermostatic return temperature limiter (RTB) , which acts as an automatic control valve to throttle the flow depending on the return temperature, the return temperature increase associated with the two worst cases I and II leads to a reduction in the flow through the heating surface, thereby compensating for the return temperature increase. This occurs both at full load in the “automatic” nominal state – i.e. without knowledge of the nominal volume flow – and at media-controlled partial load. The return temperature increase is compensated for by reducing the flow through the heating surface. This occurs both at full load in the “automatic” nominal state – i.e. without knowledge of the nominal volume flow – and at media-controlled partial load. This occurs both at full load in the nominal state “automatic” – i.e. without knowledge of the nominal volume flow – and at media-controlled partial load “dynamic” – i.e. depending on the temperature of the target medium. Thus, the two undesirable cases I and II mentioned above are converted into the desired case III without having to calculate all individual flow rates. This means that the two undesirable cases I and II mentioned above are converted into the desired case III without having to undergo the procedure of calculating all individual flows, which is often not possible in renovation projects and even with a complete pipe network calculation in new buildings is quite a challenging task when you occasionally look at the computer-generated figures that are handed over to the fitters for this purpose. pipe network calculation in new buildings, it is quite a challenging task if you occasionally look at the computer-calculated tables of figures that are handed over to the fitters for this purpose.

The same applies in particular to peak load boilers.

For example, if a peak load boiler is to maintain the top zone of a buffer storage tank at a minimum temperature without immediately charging the entire buffer, which is usually reserved for weaker regenerative heat generators, it must, of course, charge the buffer with at least this temperature (plus a surcharge for transport losses and hysteresis). But how can this be ensured if, in particular,

of the peak load boiler are not known? The only solution is then to measure the boiler flow temperature and use this to control the boiler volume flow. We have called these valves, which, unlike those of the heat consumers, must open when the temperature rises, flow temperature limiters (VTB).

What about the control quality?

We have found that it is advantageous to equip these return temperature limiters (RTB) as well as the flow temperature limiters (VTB) with a minimum circulation (MUL) in the order of one per cent of their nominal flow rate, so that the flow rate can never become zero. Otherwise, in the event of an overshoot following a sharp drop in load – for example, when the fan motor of an air heating register is switched off, as mentioned above – there would be a risk that the return temperature sensor would be disconnected from the the actual heat consumption of the heating surface, meaning that the return temperature limiter (RTB) would not open or would only open too late if there was a renewed load demand in the meantime. Especially for fan-operated heating surfaces, the associated warm start is not only a comfort requirement, but also an operational safety requirement in the event of frost. Of course, as always, the quality of the control depends significantly on the quality of the temperature measurement. For this reason, the RTB should be equipped with a temperature sensor with a response time of less than 10 seconds. outside air supply in the event of frost.

Of course, as always, the quality of control depends largely on the quality of the temperature measurement. Therefore, the sensors must be installed close to the outlets of the heating surfaces (minimum dead time!) and – especially in the case of controllers without auxiliary power – be completely surrounded by heating water as immersion sensors ; appropriate work preparation is therefore essential. Furthermore, each individual parallel strand of a network must be calibrated in the same , i.e. every air handling unit, every air heating coil, every single-pipe radiator circuit, every surface heating loop, every drinking water storage tank and every swimming pool water heat exchanger.

How does this work in conjunction with the heating curve?

The use of return temperature limiters (RTB) results in a flattening of the heating curve of the return temperature and thus also of the average heating surface temperature over the control range of the weather-compensated partial load, compared to the design case. This must be compensated for by a corresponding increase in the flow temperature or the steepness of the heating curve. Below are a few examples of this, see Fig. 5. heating curve. Here are a few examples, see Fig. 5.

Fig. 5: If heating surfaces with return temperature limiters (RTB) are operated on weather-compensated heating curves, the return temperature is constant. However, the heating surface output can only be maintained at the same average heating surface temperature, which is why the flow temperature must be increased. Due to the higher spreads, the output is then transferred at significantly lower flow rates and significantly lower return temperatures, thus reducing the hydraulic load on the network and making it more thermally efficient. The condition that the flow rates remain almost constant across the entire weather-compensated load range is maintained , as can be seen from the fact that the spread increases and decreases in proportion to the weather-compensated load.

What significance does this have for the self-regulating effect?

For the underfloor heating systems marked with an asterisk in Table 1, the return temperature regulated by the RTB is practically the room temperature. As a result, the so-called self-regulating effect of the underfloor heating is enhanced. The term “self-regulating effect” refers to the fact that, with underfloor heating, the average heating surface temperature is only a few Kelvin above the room temperature. In a new building, for example (design: 35/28 °C), this is 26 °C at 50% weather-compensated partial load (28/24 °C) and thus 6 K higher than the room temperature of 20 °C. If the room temperature now rises by 1 K to 21 °C, this temperature difference increases to 7 K. In contrast, the temperature difference between the heating surface and the room temperature is only 1 K at 20 °C. 26 °C, which is 6 K higher than the room temperature of 20 °C. If the room temperature now rises by 1 K to 21 °C, this temperature difference decreases by 1 K to 5 K, i.e. by 1/6 or 17 %. However, this temperature difference is approximately proportional to the heat output emitted by the heating surface, so that the increase in room temperature is compensated by a reduced heat supply.

As already explained, however, a heating surface reacts to a constant flow and reduced heat emission with an increase in the return temperature and thus with an increase in the average heating surface temperature. The self-regulating effect is therefore partially cannibalised in static hydraulic balancing without RTB. In dynamic balancing with RTB, on the other hand, these cancel out the increase in return temperature caused by the reduction in flow rate, so that the self-regulating effect can take full effect, see Fig. 6. Fig. 6: In surface and underfloor heating systems, the return temperature can be kept so close to the room temperature that the self-regulating effect is supported and the heat output can be reduced to a minimum.

Fig. 6: With panel and underfloor heating, the return temperature can be kept so close to the room temperature that the self-regulating effect is supported and, by means of the return temperature limiter (RTB), an “automatic device for room-by-room control of the room temperature” within the meaning of the EnEV , but which does not have a remote control or a switch-off function. As continuous controllers, the return temperature limiters (RTB) are superior to two-point controllers in terms of comfort and efficiency.

What exactly does the EnEV require?

According to § 14 (2) EnEV, “heating systems using water as a heat transfer medium … must be equipped with automatic devices for regulating the room temperature in each room when installed in buildings”. It is therefore does not state that the setpoint must be entered in the room. Since most individual room controllers for underfloor heating systems are equipped with two-point controllers, they are superior to continuous RTBs in terms of thermal efficiency, as described above. Only when the room is used as a living room and bedroom would the switch-off function of the usual individual room controllers be an additional advantage. And what do BAFA and KfW say about this? Under point 5.25 “Opening clause for innovative technologies” of the “Appendix to the information sheets” of KfW, the following is stated:

And what do BAFA + KfW say about this?

Under point 5.25 “Opening clause for innovative technologies” of the “Appendix to the information sheets” of KfW for “Energy-efficient renovation – loan (151/152)”, “Energy-efficient renovation investment grant (430)” and “Energy-efficient construction (153)”, it states: “If technical components are used in residential buildings for which there are no recognised technical rules or reliable empirical values published in accordance with EnEV § 9 (2) sentence 2, clause 3 for their energy assessment, components may be used for this purpose which have equivalent or poorer energy characteristics.” The phrase “components may be used for this purpose” refers to conventional methods and components depicted in the EnEV calculation standards. have equivalent or poorer energy properties.” The phrase “components may be used for this purpose” refers to the conventional methods and components of non-automatic and static hydraulic balancing as described in the EnEV calculation standards. The innovative technology must therefore be equivalent or better in terms of thermostatic return temperature limitation, automatic and dynamic hydraulic balancing of the heating circuits, and may be used for itself. For this reason, the installation and correct adjustment of the thermostatic return temperature limitation must be documented. return temperature limitation, as automatic and dynamic hydraulic balancing of the heating circuits, for itself. For this reason, the installation and correct adjustment of the RTB (VTB) is subsidised by both the BAFA and the KfW.

Fig. 8: The installation of return temperature limiters (RTB) in air heating coils results in an automatic adjustment of the flow rate at full load (full fan speed), partial load (reduced fan speed) and when the fan is switched off. the fan is switched off. The thermostatic control function with fixed minimum circulation guarantees a warm start and frost protection.

A final remark on control energy

Control energy is the mechanical work required to open or close the control valve close the control valve. In large systems, it is often applied by means of auxiliary electrical energy, i.e. via electric drives. In small systems in particular, a large number of control valves without auxiliary energy are used for hydraulic balancing. However, this only means that they do not require an additional energy supply, not that they do not need energy to function. But where do they get the energy they need to function?

Summary

The installation of thermostatic return temperature limiters