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Central heating system
Open and Closed Loop Central Heating Systems
Closed or open loop?
In Poland, the installation of solid fuel boilers is most often done in open (old-type) systems. At the highest point of such an installation, we mount an open expansion vessel as a safety device. The expansion vessel should be selected based on the amount of water in the heating system (at a level of 5-10% of the total water in the heating system). An example of an open vessel installation (element no. 8 in the drawing) is shown on the side.
If the country's legislation and the boiler's operating instructions allow installation in closed systems, then this solution is worth choosing. However, proper protection of the boiler and installation against excessive pressure and water boiling is necessary:
↺ Simplified diagram of an open central heating system
⥀ Simplified diagram of a closed central heating system
When boiler installation in closed systems is permitted, this solution is worth choosing. However, proper boiler protection and installation protection against excessive pressure and water boiling is necessary. The protection should be triple:
1️⃣ In a closed loop, an open vessel is not used, but rather a so-called membrane expansion vessel. The membrane expansion vessel should be selected based on the amount of water in the heating system (at a level of approx. 5-10% of the total water in the heating system).
2️⃣ A safety valve must also be installed:
- 1.5 bar for boilers with output up to 65 kW,
- 2 bar for boilers with output from 65 kW to 78 kW.
It is worth securing the installation by using two safety valves. On the supply side, we recommend installing a safety group (safety valve + air vent + pressure gauge), and on the return side, a safety valve alone.
3️⃣ The installation must also have protection against boiler overheating. One of the two following solutions is used:
a) Cooling coil with a single-function valve (cools the entire installation when the temperature exceeds 97 degrees Celsius, does not allow water into the installation). In some boilers, the cooling coil is already built into the boiler, e.g. Atmos, Attack, Orlan Super. In other steel boilers and in all cast iron boilers, the coil must be integrated into the installation as a separate element. The most commonly used single-function valves are Regulus BVTS, Watts STS-20.
b) Dual-function valve (allows cold water into the boiler return and releases hot water from the boiler supply). This solution is typically used for larger boilers, e.g. with output above 50kW, for which an external cooling coil would not be sufficient. The most commonly used dual-function valves are Regulus DBV-1, SYR 5067 and Caleffi 544.
Every installation is worth securing by using 1.5 bar safety valves and 2 bar. On the supply side, we recommend installing a safety group (safety valve + air vent + pressure gauge), and on the return side, a safety valve alone.
