Knowledge · Guide

Condensate and the risk of freezing in heat recovery

Why condensate occurs in heat recovery, when it freezes and what helps: dew point, choice of material, bypass and defrost strategies.

When warm, humid exhaust air cools inside the heat exchanger, it eventually reaches its dew point: water vapour condenses on the plates. In itself that is not a fault but additional recovery - the latent heat released benefits the supply air. It only becomes a problem when the condensate is not drained away, or when it freezes.

When does condensate occur?

Whenever the plate temperature on the exhaust air side falls below the dew point of the exhaust air. Typical in winter operation: air at 22 °C with 40 % relative humidity has a dew point around 8 °C - meeting plates cooled by outdoor air at -10 °C, it condenses reliably. The more humid the exhaust air (swimming pools, kitchens, dryers), the earlier and the more.

Our configurator calculates the dew points of both sides live; our sizing lists condensate as a separate note - including the structural consequence: the unit needs a condensate tray with a drain and an installation position that actually leads the water there.

When does that become a freezing risk?

The dangerous combination is plenty of condensate and outdoor air well below 0 °C: at the cold end of the plate stack the condensate can freeze. Ice first narrows the gaps (the pressure drop rises), then blocks levels and can damage the stack mechanically. The sizing therefore reports the freezing risk separately from the condensate - together the two determine the operating strategy.

What helps?

• Bypass control: part of the outdoor air is led past the exchanger; the plate temperature at the cold end stays above the critical limit. It costs recovery temporarily but protects the unit.
• Defrost intervals: temporarily reducing the outdoor air flow lets accumulated ice thaw before it blocks.
• Preheating: under persistent hard frost, a small preheating coil lifts the outdoor air above the critical temperature.
• Enthalpy design: because the membrane transfers moisture into the supply air, less water is left to condense - the freezing risk drops by design. Details in the design comparison.

What does this mean for the material?

Condensate is rarely pure water: it picks up whatever the exhaust air carries. In kitchen, process or laboratory exhaust air this turns into a corrosive film. For such cases coated aluminium, stainless steel or plastic plates are available; plain aluminium as the standard choice remains reserved for dry or uncritical extract air. When in doubt, describe the composition of the extract air in the enquiry field - the final material recommendation belongs in the quote, not in guesswork.