New study: Air in radiators can affect both heating performance and costs
A study conducted in collaboration between Brunata and DTU shows that air in radiators can affect heat output, return temperature and heat cost allocation measurements. And because an excessively high return temperature can lead to additional costs, what may seem like a minor operational issue can have wider implications.
A radiator that does not heat up properly is often regarded as a local comfort issue. However, a new study shows that air in radiators can also affect the radiator’s heat output, return temperature and, in some cases, the readings recorded by heat cost allocators. This can have implications for both the operation and the costs of a building’s heating system.
The study was carried out as part of a collaboration between Brunata and DTU. The practical experiments were conducted at INRiM, Italy’s National Metrology Institute in Turin, where a test facility with 40 radiators made it possible to measure parameters including flow, temperature, pressure and energy consumption at radiator level. The facility was also equipped with Brunata Futura Heat heat cost allocators.
The results provide new insights into an issue that many property managers and technical staff encounter in day-to-day operations: What actually happens in a heating system when air accumulates in the radiators?
What happens when air gets into a radiator?
Air in a radiator prevents hot water from circulating through the entire heat-emitting surface. This is clearly visible in the thermal images from the study. While a radiator without air has a relatively even temperature distribution, a radiator containing air can have large areas that are significantly colder.
As a result, the radiator’s effective heat-emitting surface area is reduced.
This affects not only how much heat the radiator can provide. The study also shows that air in the radiator can lead to a higher return temperature. For two radiators with significant air accumulation, differences in return temperature of 6.6°C and 10°C respectively were observed under the operating conditions examined.
From an operational perspective, this is relevant because a high return temperature can both indicate less efficient heating system operation and have financial consequences if the district heating supplier has requirements regarding the building’s cooling of the district heating water.
Air and hydraulic imbalance can affect the return temperature
One of the key findings of the study is that air cannot be considered in isolation.
The consequences also depend on how the rest of the heating system is operating.
In one of the test scenarios, the heating system was hydraulically imbalanced. Here, a radiator containing air received a relatively high flow rate, and the impact could also be seen in the return temperature of the associated riser.
In a more balanced riser, the effect on the riser’s overall return temperature was less pronounced.
This highlights an important relationship: both the conditions at the individual radiator and the hydraulic balance of the overall heating system matter. While residents can respond to radiators that are not heating properly, technical staff have an important role in ensuring that the system is correctly balanced and operates effectively as a whole. Together, these measures create better conditions for a low return temperature and good thermal comfort.
Air in radiators can ultimately affect heating costs
The study shows that air in radiators can affect the return temperature. And return temperature is important not only for the efficiency of the heating system – it can also have financial implications.
Many district heating suppliers set requirements for how effectively a building must cool the district heating water. If the return temperature is too high, this can therefore result in additional costs. Conversely, some district heating suppliers reward effective cooling.
Any additional costs resulting from insufficient cooling become part of the building’s overall heating costs. This means that the connection between a radiator that is not operating optimally and overall heating costs may not necessarily be visible to the individual consumer.
The study does not document the amount of money that can be saved by bleeding radiators. However, the results demonstrate an important relationship: air in radiators can affect the return temperature, and an excessively high return temperature can cost money.
Air can also affect heat cost allocation measurements
The study also shows that air in a radiator can affect the conditions under which a heat cost allocator operates. Air accumulation changes the temperature distribution across the radiator’s surface, meaning that the temperature at the point where the allocator is installed may differ from the radiator’s normal temperature profile. This can affect the readings recorded by the heat cost allocator, and the extent of the effect depends, among other factors, on where the air accumulates in the radiator.
The experiments support this relationship. When the radiators were free of air, the calculated and measured temperatures corresponded closely. When air was present, part of the radiator’s surface was no longer effectively emitting heat, changing the way the radiator operated.
Under the conditions examined, this could significantly affect the readings recorded by the heat cost allocator. This does not mean that air in a radiator automatically results in an incorrect heating bill, but it does show that a properly functioning radiator is an important prerequisite for reliable heat cost allocation.
What do the results mean for day-to-day operation?
For property managers and technical staff, the main practical takeaway is that air in radiators should be taken seriously, even if the immediate problem appears to be limited to insufficient heating for an individual resident.
Particularly as the heating season approaches, it is worth paying attention to:
- Radiator performance: Cold areas, gurgling noises or radiators that only become warm across part of their surface may indicate trapped air.
- Bleeding and correct system pressure: If air problems occur regularly, it is important not only to bleed the individual radiator but also to investigate why air is entering or accumulating in the system.
- Hydraulic balance: Significant differences in flow between radiators and risers can amplify the consequences of other issues in the system.
- Return temperature: An unusually high return temperature may indicate that the heat distribution or flow within the system should be investigated further.
- The overall conditions for heat cost allocation: If a radiator is operating abnormally, this can also affect the conditions under which the heat cost allocator operates.
Why air in radiators should be considered in the context of the entire heating system
Above all, the study shows why it makes sense to consider the heating system as a whole.
Air in a single radiator may seem like a minor operational issue, but the study demonstrates why the problem should be considered in relation to the rest of the heating system.
For property managers, the message is therefore simple: efficient heating operation is not only about supplying heat. It is also about ensuring the right conditions throughout the entire system – from radiators and risers to hydraulic balancing and measurement.
By regularly focusing on radiator performance, bleeding and system balance, technical staff can help support a more stable heat supply while creating better conditions for reliable heat cost allocation.