Heat recovery in a Nordic winter: what 86% means
Efficiency figures on a data sheet are easy to quote and easy to misread. “86% heat recovery” is not a promise of an 86% smaller heating bill, and it is not a number that holds regardless of how the system is built. Here is what it actually describes when it is minus fifteen outside the wall.
Where the heat comes from
The extract air leaving the building is already warm. You paid for that heat once, through electricity, wood or a heat pump. In a system without recovery it goes straight out through the roof, and an equal volume of cold outdoor air has to be heated from scratch to replace it.
A heat exchanger puts the two air streams in thermal contact without mixing them. The warm extract air gives up its heat to the cold supply air on its way in. Nothing is burned, nothing is “produced” — the heat is simply not thrown away.
The number in practice
Temperature efficiency tells you how much of the difference between outside and inside is recovered. At 22 °C indoors it looks like this:
| Outside | Extract | Supply before post-heater | Recovered |
|---|---|---|---|
| −15 °C | +22 °C | ≈ +17.8 °C | 86% |
| −5 °C | +22 °C | ≈ +18.2 °C | 86% |
| +5 °C | +22 °C | ≈ +19.6 °C | 86% |
The cold row is the interesting one. Without recovery the system would have to lift the supply air by 37 degrees; with recovery, a little over four remain. The post-heater covers that remainder. That is the difference between a heating coil that works all winter and one that barely switches on.
Why a rotary exchanger suits a cold climate
A rotary heat exchanger transfers not only heat but also part of the moisture from the extract back into the supply air. In a Nordic winter that is an advantage in itself: outdoor air at −15 °C is extremely dry, and a system that returns no moisture at all can dry the indoor climate to below 20% relative humidity.
The second advantage is frost resistance. A plate exchanger condenses and freezes on the extract side at low temperatures and has to defrost often. The rotor moves that moisture across into dry supply air instead, and needs far less defrosting.
Defrosting: what happens below zero
Even a rotor can ice up in severe cold combined with high indoor humidity. The unit detects it through pressure drop or the temperature after the rotor, and responds by reducing rotor speed or lowering the supply airflow for short periods until the ice is gone.
This is worth knowing because it explains something many people notice: on the coldest days of the year the system may briefly run with less supply air than usual. That is not a fault — it is the protection doing its job.
What lowers the figure in a real installation
- Imbalance between supply and extract. Recovery assumes the two streams are roughly equal. If extract is 30% larger than supply, recovered heat falls accordingly.
- Clogged filters. Lower airflow moves the working point and changes the ratio between the streams.
- Uninsulated ducts in a cold loft. Heat recovered and then lost before it reaches the room is heat wasted twice.
- Leaky ductwork. A leak on the supply side inside a cold shaft delivers cold air to the room however good the exchanger is.
- Wrong rotor speed. A rotor stopped by a broken belt recovers nothing, and the system can still look like it is running normally.
Reading a data sheet honestly
- Look for measured efficiency, not nominal. Measured curves show what the unit does at several operating points, not only at one.
- Check the airflow the figure applies at. Efficiency falls as airflow rises.
- Look at SFP at the same point. High recovery bought with high fan power is a bad trade.
- Confirm the figure is dry temperature efficiency at balanced flow, which is the comparable definition.
86% is not a marketing number. It is a measurement at a defined point — and a system that is balanced, sealed and running clean filters is the one that actually delivers it.
Every Ensy unit is documented with measured curves rather than nominal values, so the figure you design with is the figure the system delivers. Your distributor holds the full documentation for the model.