Thermostatic balancing with RTB valves

The purpose of hydraulic balancing is to provide each consumer in a distribution network with the correct amount of water. This must not be too low, otherwise the consumer will not be supplied with sufficient heat. However, it must not be too high either, otherwise the return temperature will rise too much, affecting the supply to other consumers and reducing network efficiency. But what is the “right” amount of water? and reducing the network efficiency. But what is the “right” amount of water?

As a general rule,

in circulating water heating systems, the following applies to the thermal output Q’ delivered by a heat exchanger, the flow rate V’ and the temperature difference ΔT, where c is a constant representing the heat capacity of the water:

Q’ = c · V’ · ΔT

The heat output delivered to a consumer is therefore proportional to the product of the flow rate and the temperature difference.

Q’ ~ V’ · ΔT

The same power can therefore be delivered by cooling a large amount of water slightly or cooling a small amount of water correspondingly more.

A good approximation for the heat capacity of water is

c = 4.2 J/(g·K) = 1 cal

This means that 4.2 joules of heat can be extracted by cooling 1 g of water by 1 K. Similarly, ½ g of water could be cooled by 2 K.

This means that 4.2 joules of heat can be extracted by cooling 1 g of water by 1 K. Similarly, ½ g of water could be cooled by 2 K or ¼ g of water by 4 K. This amount of heat used to be called a calorie.

1 W = 1 J/s

Since a watt is the power at which the amount of heat of one joule per second is transported, the above power triangle can be written in standard units as follows: Q’ [kW] = 7/6 · V’ [m³/h] · ΔT [K]

This means, for example:

a consumer with a nominal power of 28 kW, designed for a nominal temperature difference of 20 K, has a nominal volume flow of 1.2 m³/h:

V’ [m³/h] = 6/7 · 28 kW / 20 K = 1.2 m³/h

These nominal values can always be found on the rating plate or in the data sheet!

Hydraulic balancing

now consists of limiting the flow through this consumer to this nominal value, for which volume flow and differential pressure control valves are used.

But what if the consumer draws less than the nominal power? For example:

because it is an air heater whose fan is switched off by an electric room thermostat?
because it is a drinking water storage tank that only has to cover the standby losses of the circulation?

If the water quantity V’ is not adjusted to the reduced power output Q’, then, because Q’ ~ V’ · ΔT, the temperature difference ΔT must be reduced! However, a reduction in the temperature difference can only mean an increase in the return temperature, because the flow temperature in the network remains constant. However, a reduction in the temperature difference can only mean an increase in the return temperature, because the flow temperature in the network remains constant.

Because without the installation of RTB valves (return temperature limiters), the volume flow is constant (cV’) even with properly performed hydraulic balancing, the return temperature can only assume the lowest value if the fans of all air heaters are running at the same time or the drinking water tank is heating up fresh cold water. However, as this is not always the case, a condensing boiler, for example, can never run continuously in condensing mode, nor can a solar system ever fully exploit its yield potential.

Thermostatic balancing is

a very simple method of solving this problem and consists of installing a return temperature limiter (RTB) in each individual parallel consumer branch. This thermostatic valve limits the return temperature to the set maximum value. If the temperature falls below this value, the valve is fully open; however, if it exceeds this value, the valve is closed to a minimum flow rate of approx. 1% of the nominal value. The sensor must be installed as close as possible to the outlet of the consumer in order to keep the reaction time as short as possible, which is particularly important for air heaters. The minimum flow rate not only shortens the reaction time, but also ensures an immediate warm start and guarantees frost protection.

xml-ph-0028@dee but also ensures an immediate warm start and guarantees frost protection.

A pleasant side effect

of this simple and efficiency-enhancing measure is that you can save yourself the trouble of installing volume flow

and differential pressure control valves in future: simply set all consumers, such as air heaters or

drinking water storage tanks in the pipe network, to a specific temperature difference, set the corresponding flow temperature and set all RTB valves to the corresponding return temperature.

For example: Air heaters

Assuming that the air heaters are all designed for 70/50°C at rated output. Then select the slope of the heating curve accordingly and set the RTB valves to 50°C. If it is too cold in severe cold weather, then increase the slope of the heating curve; if the return temperature rises too much in mild cold weather, then lower the maximum values of the return temperatures at the RTB valves. Lower the maximum values of the return temperatures as much as possible. As a practical value, 40/30°C has proven to be suitable. lower the maximum return temperatures on the RTB valves. Lower the maximum return temperatures as much as possible. A practical value has been found to be 40 to 45°C with a flow temperature of 75 to 80°C in the design case (nominal load). The fan speed should be set to the lowest possible value, which increases the outlet air temperature and reduces draught and noise. The return temperature must never fall below the temperature of the air drawn in by the air heaters.

For example: Drinking water storage tank

The most important thing when charging the storage tank is that it is charged in one go without interrupting the burner. To do this, the boiler flow temperature must be set as high as possible in charging mode. The return temperature can now be limited to a value just below the circulation return temperature. Practical values for small systems (according to TrinkwV) have been found to be 45 to 55°C. For large systems (according to TrinkwV), a minimum of 55°C should be selected, and 70°C for thermal disinfection. In all of the above cases, the flow temperature in charging mode should be at least 85°C. These specifications correspond to the conclusion of In all of the above cases, the flow temperature in charging mode should be at least 85°C. This information corresponds to the conclusion of the

statement by the Federal Environment Agency “Energy saving in hot water preparation – compatibility of energy saving and hygiene requirements for drinking water” from September 2011 . We would also like to take this opportunity to refer to the connection between drinking water hygiene and container surface quality.

The circulation pump

is dimensioned as before for the total water volume of all consumers at nominal power and set to the lowest possible setpoint in the “constant differential pressure (cΔp)” operating mode. Only in special exceptional cases are values above Δp = 250 mbar = 2.5 mWS

Δp = 250 mbar = 2.5 mWS

are recommended. Such exceptions include, for example, decentralised fresh water modules in apartment stations without their own pump with proportional valves without auxiliary energy.