A vessel in the attic, thick steel pipes, and a manually fed boiler in the basement. This system heated Polish homes for decades and still operates flawlessly in many of them. The question, then, is not whether it is bad, but whether it keeps up with changing standards for home heating. In this guide, we explain how an open system differs from a closed one, what the weak points of an old gravity-fed installation are, and when modernization really makes sense.
How an open system works and where it came from
In an open system, the water has constant contact with air. This is ensured by the open expansion vessel, located at the highest point of the installation, usually in the attic, above all the radiators. The pressure in such an installation is close to atmospheric pressure and simply results from the height of the water column, which is why it increases with the height of the building.
An open expansion vessel performs three functions at once. It accommodates the increase in the volume of water as it expands when heated, makes up for ongoing losses, and, in the event of boiling, releases excess steam. This is a simple and effective solution, designed at a time when a coal- or wood-fired manually fed boiler was the standard. Such a boiler has considerable thermal inertia, and a gravity-fed installation could operate even without electricity, driven solely by the difference in density between hot and cold water.
The closed system, or how the newer solution differs
A closed system is hermetically sealed. The water has no contact with the atmosphere, and the role of the open vessel is taken over by an expansion vessel, in which a membrane separates the water section from the gas cushion. When the water heats up and expands, it flows into the vessel and tensions the membrane; after cooling, it returns to the circuit. Any excess pressure, should it arise, is released by the safety valve.
However, the most important difference between an open and a closed system does not lie in the vessel itself, but in the water's contact with air. In an open system, this contact is constant; in a closed system, it does not occur. This seemingly minor distinction determines the durability of the entire installation. Instead of an attic vessel, a closed system has a compact unit by the boiler, and the pressure is maintained at a constant operating level rather than depending on the height of the building.
Advantages of an open system
An open system has real advantages and can still be a sensible choice in some situations.
- It can operate without electricity. With a solid-fuel boiler, a gravity-fed system distributes heat through the difference in temperature alone, which can be invaluable where the power supply is unreliable.
- It is simple and maintenance-free. It contains no pressure safety components that could fail.
- It is not at risk of overpressure. Steam always has an escape route through the open expansion tank, so a system rupture caused by a pressure increase is practically impossible.
One safety point to remember with this type of system: no shut-off fittings may be installed on the safety pipe connecting the boiler to the open expansion tank. Accidentally closing it while the boiler is hot could cause the system to rupture.
Drawbacks of an open system
The greatest weakness of an open system is something that seemed harmless for years: constant contact between the water and the air. This is what causes accelerated corrosion throughout the installation.
The mechanism is simple. Water in an open expansion tank continuously absorbs oxygen from the air, and this oxygen oxidizes the steel components of the circuit: the boiler, pipes, and radiators. Evaporation adds to the problem. Water gradually evaporates from the open tank, so it has to be topped up from time to time, and each top-up brings another portion of fresh water containing a new load of oxygen and dissolved minerals. Oxygen drives corrosion, while minerals build up as limescale on the boiler's heat exchanger. This vicious cycle continues throughout the season.
In a closed system, this process practically does not occur. Isolated from the air, the water releases dissolved oxygen and deaerates after a few days of operation, and because it does not evaporate, it does not need to be topped up. Without a constant supply of fresh, oxygenated water, corrosion almost stops.
Corrosion is not the only problem. An open expansion tank in an unheated attic is at risk of freezing, releases heat into a cold part of the building, and takes up space. Having to top up the water can be a nuisance, while an open system itself is incompatible with modern heat sources and underfloor heating, where deposits and corrosion quickly damage the thin pipes in the floor.
What you gain by switching to a closed system
Modernization is not only about getting rid of the old system's drawbacks, but also about a set of tangible everyday benefits.
- Longer installation life. Water without a constant supply of oxygen causes much less damage to the boiler, pipes, and radiators, resulting in fewer failures.
- No more topping up the water. In a sealed circuit, water does not evaporate, so there is no need to regularly replenish the system.
- Compatibility with modern heating systems. A closed system is required for a condensing boiler, heat pump, and underfloor heating to operate.
- Recovered space and no risk of freezing. The tank in the attic is removed, along with the risk of freezing and heat loss in an unheated part of the building.
- More stable and quieter operation. Maintained operating pressure means that the installation operates more smoothly and predictably.
When modernization is necessary and when it is only optional
Not every gravity-fed installation requires immediate modification. It all depends on what you use, or plan to use, to heat your home.
Modernization is necessary if you are planning a modern heat source. Gas boilers, especially condensing boilers, and heat pumps are designed exclusively to operate in a closed system. Connecting them to an open installation requires rebuilding it or separating the circuits with a plate heat exchanger. The manufacturer's instructions always specify the detailed requirements for the equipment.
Modernization is optional if you are keeping a conventional solid-fuel boiler. Such a boiler can continue to operate in an open system, and this is a proven solution.
It is worth clearing up a common misconception here. You may come across the claim that a solid-fuel boiler must never be connected to a closed system. This information is outdated. Since the regulations changed in 2009, a solid-fuel boiler may operate in a closed system, provided that its output does not exceed 300 kW and it is equipped with a device for releasing excess heat, such as a cooling coil working with a thermal valve. In the event of overheating, such a device removes excess heat and protects the installation. If the solid-fuel boiler does not have such protection, a mixed system is also used, in which the boiler operates in an open circuit while the rest of the installation, separated by a plate heat exchanger, operates in a closed circuit.
What needs to be replaced when switching to a closed system
The most important message for a homeowner is this: modernisation usually involves rebuilding the boiler connection point, not replacing the entire installation. Old pipes and cast-iron radiators can usually remain, as replacing them rarely makes economic sense. The actual changes concern several components grouped around the heat source, and some of them may already be factory-fitted in the new boiler, so the actual scope of purchases depends on its equipment.
- Diaphragm vessel in place of the open vessel in the attic. It is selected according to the installation’s capacity, approximately at ten percent of the system’s water volume, while the exact value is calculated based on the system’s output and parameters. Wall-hung gas boilers often have a factory-fitted vessel, and an additional one is only needed when the system volume exceeds the limit specified by the boiler manufacturer. Our guide on how to select a diaphragm vessel will help, while the differences between the series are explained in the article about IBO expansion tanks for C.O. and C.W.U. Once the vessel has been selected, the guide on how to inflate a diaphragm vessel will help with commissioning it.
- Safety valve, pressure gauge and automatic air vent, separately or combined in a single, compact safety group mounted on a common rail. Wall-hung gas boilers usually have a factory-fitted safety valve, so only an air vent at the highest point of the installation is then missing.
- Circulation pump, if the installation has so far operated solely by gravity and the new heat source does not have its own built-in pump.
- Thermal protection additionally, if a solid fuel boiler is to operate in the closed system.
Finally, the open expansion vessel is dismantled from the attic and the pipe leading to it is capped. This entire modification is carried out by a qualified installer, who will select the components for the specific installation and confirm that the connection is correct, which can be important for the new boiler’s warranty. You can find diaphragm vessels, safety groups and other fittings in our offer in the vessels and tanks category.
Frequently asked questions
What is the difference between an open and a closed system?
In an open system, water is in constant contact with the air through an open expansion vessel located in the attic, and the pressure is close to atmospheric pressure. In a closed system, the circuit is hermetically sealed, water expansion is absorbed by a diaphragm expansion vessel, and a safety valve protects against excess pressure. This difference determines, among other things, the rate of corrosion in the system.
Can an open system be converted to a closed one?
Yes. In most cases, this involves rebuilding the boiler connection point rather than replacing the entire system. The open vessel is removed from the attic, and a diaphragm expansion vessel, safety group, and, if necessary, a circulation pump are installed by the boiler. Old pipes and radiators can usually remain in place. The conversion must be carried out by a qualified installer.
Can an old solid-fuel boiler operate in a closed system?
Yes, provided that its output does not exceed 300 kW and it is equipped with a device for discharging excess heat, such as a cooling coil with a thermal valve. Without such protection, a solid-fuel boiler must remain in an open system or be separated from the rest of the system by a plate heat exchanger.
What is the purpose of the expansion vessel in the attic?
In an open system, the vessel located at the highest point of the system absorbs the increase in water volume as it heats up, replenishes water losses, and vents steam if the water boils. Its position above the radiators and contact with the atmosphere are characteristic of an open system. In a closed system, this role is performed by a diaphragm expansion vessel installed by the boiler.
Do the entire system and pipes need to be replaced?
Usually not. Converting an open system to a closed one mainly involves rebuilding the boiler connection point. Old pipes and cast-iron radiators can usually be left in place, as replacing them is rarely cost-effective. The expansion vessel is replaced, safety devices are added, and a circulation pump is installed if necessary.
Can a condensing boiler be connected to an open system?
In practice, no. Condensing boilers are designed to operate in a closed system, with forced circulation and stable pressure. Connecting one to an open system risks improper operation and usually means losing the manufacturer's warranty.
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