Excess pressure

The maximum permissible working pressure of a boiler is the overpressure determined by the construction and strength of the materials used in the boiler.

Safety valve

This information can be found on the boiler’s manufacturer data plate, in the documentation supplied and in the acceptance documents. The overpressure must be limited by a safety valve on the boiler. The safety valve opens when the maximum permissible pressure is reached in order to prevent a further increase in pressure in the boiler body. After the safety valve has been activated, it closes again when the pressure in the boiler is reduced.

Only safety valves with a closed spring cover and closed lift mechanism must be used for safety devices on hot water boilers and flue gas heat exchangers.

This design prevents media leakage during normal operation and increases operational safety.

When protecting against liquids – i.e. when the safety valve is connected to a water-bearing space – a pressure relief and water separation pot must be installed on the discharge side. This enables phase separation between the liquid and vapour phases, and protects the connected piping system against damage caused by entrained liquid. The relevant legal, regional regulations and official requirements must be taken into account.

In accordance with DIN ISO 4126-1, the following permissible tolerances apply to the set excess pressure:

• ± 3 % of the excess pressure or
• ± 0.1 bar,
• whereby the higher value prevails in each case

When the safety valve is activated, a pressure increase above the excess pressure is permissible in order to be able to discharge the required mass flow rate through the connected pipe system. In accordance with the Pressure Equipment Directive, this pressure increase must not exceed 10 % of the excess pressure of the safety valve.

Expansion vessel for safety valves

If the boiler falls within the scope of the Pressure Equipment Directive (safety temperature limiter > 110 °C), an expansion vessel on the boiler and flue gas heat exchanger is mandatory. Only the expansion vessel ensures safe drainage of the hot water, including phase separation.

For boilers according to the Gas Appliances Regulation (safety temperature limiter ≤ 110 °C), the expansion tank can be omitted in accordance with EN 12828 – Part 4 under the following general conditions:

• Boiler output ≤ 300 kW
• For boiler output > 300 kW: A second safety temperature limiter and a second pressure limiter are installed for each heat generator