Open Heating Systems: How to Spot Them and Upgrade Your Portfolio

An open heating system is a system with an open expansion vessel, where the heating water stays in constant contact with ambient air, usually through a tank in the loft. The oxygen this lets in drives corrosion and sludge formation throughout the pipe network, making these systems noticeably more prone to faults in daily operation.

For existing building portfolios, this is more than a minor technical detail. Every system with an open expansion vessel needs recurring attention, tends toward pump and valve damage, and burns more energy over the years than a healthy, closed system.

For technical managers in the housing industry, what matters most is how quickly an open system can be identified across the portfolio and replaced in a way that makes economic sense.

What Is an Open Heating System, and How Do You Spot One in Your Portfolio?

Technically, this is a hot water heating system whose expansion vessel is open to the atmosphere, usually a simple tank in the loft or attic space, connected to the heating circuit through a riser pipe. Because the water there is in direct contact with air, part of it evaporates over time, and the system needs regular top-ups.

In existing buildings, an open system reveals itself through several signs at once.

Open vs. Closed Heating Systems: The Key Differences

The key difference lies in pressure maintenance. An open system holds pressure through the atmosphere and constant air contact, while a closed system handles this through a diaphragm expansion vessel that never touches outside air.

Diaphragm expansion vessel (DEV): a sealed pressure vessel with an elastic membrane that absorbs volume fluctuations in the heating water without the water ever touching outside air. Also known as a pressure expansion vessel.

Open systems are practically never installed new today. Because of the ongoing oxygen ingress, specialist contractors usually recommend converting to a closed system whenever work is already being done on the plant.

What Damage Does Oxygen Ingress Cause in Open Systems?

Constant oxygen contact drives one reaction above all: iron ions from pipes, radiators and the boiler oxidize into magnetite, a fine black sludge. Magnetite sludge settles in pumps and valves and narrows the flow cross-sections there. In heat exchangers, it also blocks heat transfer, which encourages operational faults.

For context: Oxygen-saturated fill water contains around 10 grams of oxygen per cubic meter, which can theoretically produce up to 36 grams of magnetite sludge per cubic meter of system water. That figure is a calculated benchmark for the theoretical upper limit. In practice, larger sludge volumes usually result from defective expansion vessels and leaking valves. Poorly balanced hydronic systems add to the effect.

Wherever magnetite builds up, wear on pump shafts and bearings rises measurably, in extreme cases all the way to complete pump failure. Even 1 to 2 millimeters of limescale or rust deposits on a heating surface can cut heat transfer by up to 20 percent, driving energy demand up in turn. Consistent degassing, conversely, can boost efficiency by up to 6.5 percent, a figure from studies on single-family homes that doesn't automatically carry over to large portfolio systems.

In daily operation, that translates into more faults and higher maintenance costs. Pumps and valves wear out noticeably faster. Heat exchangers also lose performance earlier, which shortens the lifespan of the entire system. Our article on system diagnostics for existing heating installations breaks down which fault patterns put the biggest financial strain on a portfolio, with concrete cost examples.

How Do You Retrofit, Separate or Upgrade an Open System?

Two technical routes lead out of an open system, supplemented by one supporting measure for the fill water itself.

System separation decouples the generator and consumer circuits completely via a heat exchanger. This pays off above all where the consumer circuit runs on plastic piping that isn't oxygen-tight, such as older underfloor heating, whose oxygen ingress would otherwise spread into the entire generator circuit.

The classic conversion replaces the open compensation tank with a diaphragm expansion vessel, along with additional components such as a safety valve and pressure gauge. Effort and cost depend heavily on system size and the pipe network. For multi-family or portfolio-scale systems there's no reliable market range for this; only a specialist contractor can give binding figures after an on-site inspection.

Water treatment complements both routes. VDI 2035 sets limits for the pH value and hardness of the fill water, as well as for electrical conductivity. It isn't a legal requirement. As a recognized standard of good practice, though, many manufacturers still require it for warranty claims. At a conductivity above 100 microsiemens per centimeter, the guideline recommends an oxygen content below 0.02 milligrams per liter, and for closed systems in general a target value below 0.1 milligrams per liter.

A full conversion makes the most economic sense wherever a renovation, boiler replacement or heat pump installation is happening anyway. If the rest of the system is otherwise sound, targeted water treatment combined with degassing can be a cheaper interim step. Our article on heating optimization in existing buildings describes how individual operating parameters can improve even before a larger conversion.

How Do You Track Down Open Systems Across Your Entire Portfolio?

In a single building, an open system shows up through symptoms that seem unremarkable on their own. These include frequent top-ups and a slow pressure drop. Recurring pump faults often come on top of that. Spread across an entire portfolio, patterns like these become almost impossible to track by hand.

Good to know: according to the BDH's 2026 half-year report, over 40 percent of Germany's roughly 21.5 million heat generators no longer meet the state of the art, with around 4 million older than 30 years. The BDEW puts the average age of German heating systems at 13.7 to 13.9 years, with one in three systems older than 20 years. There's no separate official statistic on the share of open systems specifically, but the age distribution offers a useful clue as to where to keep looking across your portfolio.

This is exactly where digital system diagnostics comes in. KUGU's VIS Anlagendiagnose (VIS stands for Visuelles-Informationssystem, the German term for Visual Information System) assesses the hardware condition and operating behavior of a heating system. Hydraulics and efficiency are factored in too, based on more than 20 criteria in total, translated into concrete recommendations for action. Symptoms such as unusually frequent top-up needs or recurring fault messages become visible this way across many properties before they lead to a breakdown. The approach was tested in practice with Gewobag ED, among others, and presented at KUGU Days 2026.

Our article on heating monitoring in multi-family buildings shows which metrics and alerts make the real difference day to day.

From Single Cases to Systematic Portfolio Checks

In a single building, an open expansion vessel can often be replaced with reasonable effort. The real challenge starts once a portfolio spans several hundred properties of different construction years, where nobody knows exactly anymore which buildings still run open systems.

Waiting until a system announces itself through a fault or breakdown ends up costing twice: once for the repair, and once for the energy consumption that went unnoticed and too high for years. Digital system diagnostics makes that connection visible before it turns into an emergency.

The practical next step is a systematic stocktake, starting with the oldest systems in the portfolio, complemented by a system diagnostic for the properties with the most conspicuous operating data.

Does an Open Heating System Have to Be Replaced Immediately?

No, there's no legal replacement requirement. DIN EN 12828 still permits open systems as a special case, for example with slow-response solid fuel boilers that lack thermal drain-off safety devices. Specialist contractors recommend converting anyway, because the constant oxygen ingress noticeably speeds up corrosion and wear over the years.

How Often Does an Open System Need Water Topped Up?

There's no blanket figure, since the interval depends on system size and tank design, but several weeks to a few months is typical. If top-up needs rise noticeably or become irregular, that usually points to an additional leak or a technical fault.

Why Can't an Old Underfloor Heating System Simply Be Connected to an Open System?

Because many older underfloor heating systems use plastic pipes that aren't oxygen-tight, letting additional oxygen into the system. Combined with an already-open expansion vessel, oxygen ingress into the generator circuit rises so sharply that system separation via a heat exchanger becomes the recommended fix.

How Can You Spot Magnetite Sludge Before a Pump Fails?

Typical warning signs are a pump running louder than before and falling flow temperatures despite unchanged settings. Rising pressure loss in the system is another one. Digital system diagnostics often catches these shifts in the operating data well before they become audible or noticeable at the unit itself.

Is an Open Heating System Still Permitted Under VDI 2035?

Yes, VDI 2035 doesn't explicitly ban open systems, it only defines limits for the pH value, hardness and oxygen content of the fill water. In an open system, though, constant air contact makes those limits considerably harder to meet than in a closed, oxygen-tight system.