✅ Open and Sealed Heating Systems - Key Differences
When undergoing a major heating renovation or building a new home, we often encounter the terms open and sealed central heating systems. It's important to understand what they mean and how they differ. Generally, there are two basic categories used in Europe and around the world for central heating systems - open and sealed systems. Knowing the basic differences between them is valuable.
☑️ Open and sealed systems in central heating are the two fundamental types of heating systems. There are essentially no other types in practical use.
Both types of central heating systems differ significantly in their characteristics and functionality - so choosing between them is not arbitrary. Each works with specific types of heating appliances and has certain advantages over the other, or in some aspects, disadvantages. Sealed systems are generally considered more modern, but this doesn't mean they are always applicable. In this article, we'll explore and discuss in detail the main differences between open and sealed systems in central heating.
An open vented central heating system operates in such a way that it is not completely sealed - it must have a point where it comes into contact with the air. The hot water that heats our radiators and warms our homes circulates through a system of pipes, typically culminating at its highest point (often in the attic or loft) with an open-type expansion tank. This vessel is a simple device, resembling a bucket in construction (though it can be of any shape), with a lid. When the water in the open central heating system is heated (usually by a boiler, fireplace, etc.), the pressure in the system increases, causing the lid of the expansion vessel to rise. This allows the system to oxidize - maintaining contact with air, which is crucial.
It's often heard that open systems are outdated and that modern heating systems should be sealed. It's hard to disagree with this thesis; however, in some cases, open central heating systems have their advantages. They may not necessarily outweigh the disadvantages, but it's worth mentioning the benefits of an open system.
✅ Firstly, an open heating system allows for gravitational operation, which basically does not work in modern closed CH systems that require circulation pumps. The gravitational system operates based on pressure variations in different parts of the heating system - thus, water begins to circulate naturally as the heating is activated (in simplified terms, when we start the heating boiler or light a fireplace). The absence of a need for a circulation pump represents some savings (a good CH pump costs at least 120-150 €), but the real savings come from the fact that the circulation pump does not consume electricity over the years.
Heating devices, considered more primitive in their construction, more inert - are often designed for an open system. These are often fireplaces with a water jacket, but also old-fashioned boilers - e.g. coal-fired, solid fuel, wood-burning, so-called "junk". Most of these devices do not have automation that turns off the device when the house is heated. Usually, they continue to heat our house until the fuel runs out - e.g. when the wood in the fireplace or combustion chamber runs out.
Since these devices do not turn off - they can lead to overheating of the CH system and dangerous pressure increase in the pipes. Pressure, underestimated by laymen - could even burst these pipes. Therefore, installing old boilers in closed systems - is downright dangerous.
✅ An open CH system has the advantage that in the event of overheating of the system and pressure increase - it will open the expansion vessel causing an automatic pressure drop in the installation and draining some water into the sewage system. Some of the water also simply evaporates through the open expansion vessel. This will save our system from damage.
☑️ Unfortunately, the drawbacks of open systems are quite significant and basically often disqualify them from modern heating systems.
Although some devices can only operate in an open system - unfortunately, the drawbacks of an open system in a central heating system are so significant that they outweigh the advantages. Over the years, it is increasingly difficult to find its advocates among installers of heating devices, and further use of open systems often results from necessity (e.g. we have a heating boiler that requires it).
Below we will discuss the specific drawbacks of open CH systems.
The fact that in an open system water has access to air has a beneficial effect on protecting the CH system from excessive pressure buildup - but it is also the main reason why open systems are considered more harmful. Water evaporation from the system requires water to be supplied. As we know, water containing certain minerals (mainly magnesium and calcium) causes scale deposition in the boiler. Scale is a huge problem affecting the efficiency of heating systems. Old, scaled-up CH systems in extreme cases can increase the amount of fuel needed to heat the house - even by 60% (of course, we are talking about scale deposits after many years).
➡️ The potential for boiler scale is not the only problem, air ingress into water also causes a corrosive potential for devices in the system, radiators, heat exchangers, boilers - leading to a faster need for their replacement and generally speaking - contamination of the installation. Comparing the cleanliness and efficiency of closed and open systems - e.g. after 5 or 10 years - always turns decisively in favor of closed systems, which maintain relative cleanliness.
✅ For the above reasons - the potential harm of open systems - manufacturers of modern heating devices, equipped with automation, such as condensing boilers (gas) or heat pumps - generally do not honor the warranty if you use the device in an open system. The manufacturer usually informs in which system their supplied system should be installed.
Underfloor heating consists of thin pipes, so the hygiene of such an installation is very important. Maintaining underfloor heating in a system that carries regular scale deposits over time would result in some heating circuits becoming non-functional. This would essentially prevent the underfloor heating from working effectively - and this cannot be effectively repaired other than by tearing up the floor, scraping off the screed, and laying new pipes.
☑️ What are the basic differences between a sealed and an open vented system?
A sealed CH system is the opposite of an open system. It also must have the ability to neutralize excessive pressure in case of system overheating - however, these are sealed expansion vessels. They may not be as effective as open overflow vessels, but - to put it simply - they don't need to be as effective. A sealed CH system is designed to work with modern heating devices equipped with temperature sensors that shut down the system to protect it from overheating. Therefore, inert heating boilers or fireplaces should not operate in a sealed system, but rather in an open one.
✅ Hygiene of a sealed heating system
The primary advantage of a sealed system is the absence of harmful corrosion potential and boiler scale deposits. Water in regions with high water hardness has the potential to generate scale due to its physico-chemical structure. However, in a sealed system, water does not come into contact with air and thus does not evaporate. In such a scenario, new water is rarely introduced into the system; it remains the same water. After the initial calcium deposition, it cannot generate more scale deposits, making the sealed system healthier for the CH system overall.
As mentioned, an open system is not recommended for underfloor heating. A sealed system, due to the lack of real potential to clog the thin pipes of underfloor heating, can be used with both underfloor heating and radiators. Therefore, it is undoubtedly much more functional.
✅ Heat pumps and condensing boilers operate in a sealed system, not an open one
In fact, there are hardly any manufacturers of gas boilers or heat pumps that allow their devices to be used in open systems. In a sealed system, the environment is very safe - thus, it is absolutely the only solution if you want to install a heat pump or modern gas boiler. There may be very few exceptions to this rule, but as a rule of thumb, these devices are not installed in open systems, only sealed ones.
✅ The disadvantages of a sealed system are definitely fewer than the advantages.
Unfortunately, a sealed central heating system also has some drawbacks:
✅ Therefore, if you want to have a "healthier" and "cleaner" sealed system in your home, you won't connect a coal boiler or so-called "junk" directly to it (there are ways to connect a sealed and open system in one system - through a plate heat exchanger - but more on that later).
We mentioned that gravity-fed operation is possible in open CH systems. However, it's neither a requirement nor a recommendation. Systems without pumps (definitely not recommended for sealed models) can manage in specific conditions and were once the norm. However, it's not always the case that they will function efficiently if we opt for an open system. This depends, for example, on the size of our CH system (i.e., the extent of the building) and whether it's a multi-story building.
In our opinion, if you do choose an open vessel system, we recommend one with a pump. This setup will be much more efficient and there's a chance of avoiding underheated rooms. Gravity systems are an unattainable dream but also somewhat utopian when it comes to larger homes. An open system with a pump, if you opt not to seal the system, is a realistic and generally better solution.
Yes, technically it's possible to have both an open and sealed system within one building. Moreover, this isn't a strange or extravagant solution but rather commonly used during the renovation of old houses. Typically, in new home installations, we tend to opt for heat pumps, gas boilers, energy-efficient electric boilers, modern pellet boilers - hence, we immediately opt for a sealed system, especially conducive to underfloor heating projects.
Conversely, older buildings generally originally had open CH systems - that's just how things were done back then. Hence, nuances arise, especially when wanting both a coal boiler and, for example, a gas boiler in one house. Or when wanting both a fireplace and a heat pump.
Certainly, there are many potential examples, but essentially, the scheme for combining sealed and open systems is usually similar or almost the same. Thanks to heat exchangers, devices that are quite inexpensive (unless it's a heat pump exchanger), we can have both systems, each with its own heat source.
Definitely, a sealed system is better and shows significantly less harm to the entire CH system. It remains healthy for many years and doesn't tend to degrade with each season by collecting larger amounts of dirt and scale. It doesn't cause a drastic increase in fuel consumption or electricity as it ages.
However, this isn't a matter of free choice - whether you choose an open or sealed system - it actually depends on the heat source that will heat your home. It's not excluded that there will be two heat sources that will require both systems within one house.
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