Calculating the size of the expansion tank for heating is crucial for efficient heating. If it is too big, sufficient pressure cannot build up. If it is too small, overpressure occurs.
Factors
The selection of the appropriate pressure expansion tank is decisive for the function of the heating system. Efficient heating and the avoidance of problems and damage is only possible if the expansion vessel is correctly matched to the system.
Various factors must be taken into account:
- Expansion volume
- required water reserve
- Filling pressure of the heating system
- Nominal volume of the expansion tank
- Pre- and final printing
- Water content of the heater
Different conditions and calculations must be considered for each factor.
Expansion volume
The expansion volume plays an important role in the selection of the correct expansion tank and must also be calculated. Two factors are important for this. On the one hand, the so-called system volume, i.e. the amount of water in the heating system. On the other hand, the flow temperature.
Because these factors determine how much the water volume can change or increase when heated. The system volume will
depending on this, multiplied by a further factor. This is a factor that depends on the flow temperature. The following factors apply to heating systems without frost protection:- 0.0093 at 40 ° C
- 0.0129 at 50 ° C
- 0.0171 at 60 ° C
- 0.0222 at 70 ° C
It must be noted, however, that the values can differ depending on the type of heating system. Here you should pay attention to the information provided by the manufacturer or, if necessary, ask the provider.
as Sample calculation A heater with a system volume of 200 liters can be used, which runs at a flow temperature of 70 ° C.
- 200 liters x 0.0222 = 4.44 liters of expansion volume
The compensation tank should therefore have at least such a capacity. If a vessel of the appropriate size is not available for the heating system, a pressure equalization tank of the next larger capacity should be used.
If you want to calculate the expansion volume (Ve) for other temperatures, you can use the following formula and table as a guide:
That example with a system of 200 liters and a temperature of 120 ° C can illustrate how the calculation is carried out:
- Ve = (e x VSystem): 100
- Ve = (5.93 percent x 200 liters): 100
- Ve = (1,186): 100
- Ve = 11.86
Required water reserve
The water reserve is to be understood as a reserve that can bridge the gap between the maintenance intervals. Regardless of the system volume, at least three liters should be planned as a water reserve.
For larger systems, 0.5 percent of the volume should be kept ready and included in the calculation. With a heating system with 200 liters, 0.5 percent would be just 1000 milliliters. Nevertheless, three liters should be added and planned in order to compensate for the normal losses.
Filling pressure of the heating system
In order to find the right expansion tank for the heating system, the filling pressure must also be known. In order to calculate this it is first necessary to use this formula:
As a result, the heater is filled to such an extent that the minimum filling pressure is reached.
Nominal volume of the expansion tank
The expansion tank needs a corresponding volume in order to be able to absorb the necessary amount of water. The volume can be calculated using the following formula:
Pre- and final printing
When the expansion volume has been calculated, the pre-pressure and final pressure can also be determined. The pre-pressure should be at least 0.7 bar. To calculate it, the altitude pressure is added to the vapor pressure.
The altitude pressure results from the system altitude,
which is divided by ten. With a distance of five meters between the expansion tank and the system, the following calculation results:5 m: 10 = 0.5 bar
The flow temperature must be taken into account for the steam pressure:
- 0.2 bar at 60 ° C
- 0.3 bar at 70 ° C
- 0.5 bar at 80 ° C
This value is now also added to get the form. In our example calculation for a system with a flow temperature of 80 ° C this means:
- 5 m: 10 = 0.5 bar
- 0.5 bar + 0.5 bar = 1.0 bar
The final pressure can easily be determined from the response pressure of the safety valve and should be 0.5 bar below this limit. With a response pressure of 3 bar, the final pressure should therefore be 2.5 bar.
Water content of the heater
How much water there is in the heater determines how big the pressure equalization tank has to be. However, the capacity depends not only on the amount of water, but also on the temperatures and the type of heating.
- 36.2 liters per kilowatt for tubular radiators at 70/50 ° C
- 26.1 liters per kilowatt for tubular radiators at 60/40 ° C
- 20 liters per kilowatt for underfloor heating
- 14.6 liters per kilowatt for panel radiators at 60/40 ° C
- 11.4 liters per kilowatt for panel radiators 70/50 ° C
The so-called system volume is achieved by multiplying the characteristic values and the output of the heating system. In heating systems with a particularly large buffer storage tank, this buffer must also be included in the calculation. It is added to the result to account for the total volume of water.