27/09/2026Heat recovery

Why run-around coil heat recovery falls short of its design efficiency

The building automation trend shows 45 % for heat recovery, although the coils were once sized for considerably more. In run-around coil heat recovery this is a common situation. In March 2026 Belok, the network of the Swedish Energy Agency and Sweden's largest property owners, published a study that gathered property owners' experience through interviews and a survey and tested a new control strategy at three sites. In most of the systems in the study, efficiency was 40–60 %, often below 50 %.

For comparison: the EU Ecodesign Regulation 1253/2014 requires new non-residential ventilation units with run-around coil heat recovery to reach a thermal efficiency of at least 68 %.

The design efficiency is a ceiling

The highest efficiency a heat-recovery coil can reach is set at the design stage. Coil size, flow velocities and design data determine it, and no control strategy raises efficiency above this ceiling. In operation, however, the operating point easily drifts below the ceiling, because conditions do not stay at the design point.

Airflow varies with demand-controlled ventilation, filter fouling and fan speed steps. The temperature difference is large in frost and small in mild weather. When moisture in the extract air condenses on the coil surface, it releases heat, and its share of the total output varies with the weather. The coil's heat transfer also changes over time through fouling and corrosion. Each of these changes the fluid flow at which the coil works best.

What most often lowers efficiency?

According to research at Chalmers University of Technology, the most common cause of low efficiency is an incorrect fluid flow. It matters more the higher the efficiency the coils were sized for and the more the airflow varies. In a system sized for about 50 %, the fluid flow matters little as long as it is reasonable, at least when the airflow is constant and high. In a system sized higher, especially with demand-controlled ventilation, the suitable fluid flow range can be very narrow at low airflows.

The study's interviews and survey kept returning to the same findings. Circulation pumps mostly ran at constant speed even though the airflow varied. The glycol concentration and age of the fluid were often unknown, and where the fluid had been tested it often held too much glycol. Owners did not trust the placement of the temperature sensors or the measurements, and few knew what efficiency their coils had originally been sized for.

Why external heat in the loop can weaken recovery

Many systems also feed external heating or cooling into the run-around loop instead of using separate coils. In winter this can set off a spiral. When heat is brought into the loop, the fluid going to the supply air coil gets warmer, and so does the returning fluid. In the extract-air coil the temperature difference between the fluid and the extract air shrinks, so less heat is recovered from the extract air. To hold the supply air setpoint, the loop then needs ever more external heat. At one of the study's test sites the spiral was limited by allowing external heat only once the supply air temperature had fallen a set amount below the setpoint, and with a delay.

What the study's tests showed

The control strategy developed at Chalmers sets the circulation pump speed from a temperature ratio: the rise in supply air temperature across the supply air coil divided by the drop in fluid temperature across the same coil. The control needs no data on the fluid's properties and no measured fluid flow. In laboratory tests it reached the same efficiency as flow-based control with a simpler implementation, and the best efficiency was reached at ratios of 1.1–1.3.

At the three sites in operation, the results depended above all on the condition of the equipment:

At the Vasakronan site far less external heat was needed than before, and heat recovery alone was enough down to an outdoor temperature of about −3 °C. At the site with the worn coil the improvement remained small, because the control cannot remove the limits of the coil or the pump. At the rebuilt site the control ran stably also when the airflow changed.

When fluid flow control helps and when it does not

According to the study, correct fluid flow control clearly improves efficiency when the coils are sized for a high enough efficiency, are in good condition and the pump can deliver the flow needed. At one test site the pump was judged to need about 0.5 l/s of fluid per cubic metre per second of airflow. If the coils are sized for an efficiency of only about 50–55 %, optimising the fluid flow does not usually raise efficiency. In that case it is worth checking first whether the fluid flow is too low, and then the condition of the coils, the connections and the sensors.

An example from the study shows the scale: Vasakronan would save about half a gigawatt-hour a year for every percentage point by which it raises the average efficiency of its systems. Savings always depend on the site's airflows, temperatures and operating hours, but run-around coil heat recovery often has significant savings potential, especially when conditions vary.

Where to start in a building

The study recommends four steps for existing systems:

1. Find out what efficiency the coils were sized for, and document it. Measured efficiency cannot be judged without this reference.
2. Monitor efficiency in the building automation and make sure the temperature sensors are installed and measuring correctly.
3. Check the temperature ratio at the current airflow, for example by hand measurement. If the ratio is 1.0–1.3, the fluid flow is right. With variable airflow, check at several airflow levels.
4. Once the actual and the design efficiency are known, calculate the savings potential and decide on the measures needed.

In new-build and renovation projects it is also worth requiring a high design efficiency and a control strategy that maintains it in operation. In the study's survey, seven of eight respondents said they replace run-around coil heat recovery with other solutions wherever possible, but about half would keep it if the design efficiency could be ensured in a simple way. Run-around coil heat recovery is often chosen precisely because extract and supply air stay separate, so repairing an existing system is in many buildings a well-founded alternative to removing it.

Windis Zero

Windis Zero is a patented control system for run-around coil heat recovery. It holds the run-around loop at an operating point where heat recovery delivers what the coils were sized for, even as outdoor temperature, airflow and load change. It does not raise efficiency above the design value, but keeps it from sliding below it in operation. Integrated energy metering shows the recovered energy in the building automation as a measured figure.

Windis Zero IQ is a feature of Windis Zero that continuously seeks the operating point at which heat recovery transfers the highest power at each moment. In varying operating conditions it typically recovers roughly 5 to 10 per cent more energy than constant flow control.

Windis Zero is installed in a new or an existing air handling unit regardless of its make, and in a renovation the building automation needs no software changes. Site-specific sizing is done in Windis Select (select.windis.fi), which produces a sizing report, a functional description and a schematic for the design documents. The choice of product stays with the designer. Details of Windis Zero and Zero IQ are on the heat recovery page.

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Sources: Jangsten, M. and Olsson, D. (2026). Vätskekopplad värmeåtervinning: Erfarenheter bland användare samt tester av en ny reglerstrategi för högre temperaturverkningsgrad. Version 1.0, 5 March 2026. CIT Renergy / Belok, belok.se. Chalmers research: doi.org/10.1080/23744731.2025.2538372. Commission Regulation (EU) No 1253/2014 on ecodesign requirements for ventilation units.

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