Frost Protection for Plate Heat Exchangers

In cold-climate ventilation, an exhaust-to-supply air-to-air plate exchanger can frost on the exhaust side when the supply outlet drops below the dew point of the humid exhaust. Frost narrows the channels, raises pressure drop and eventually blocks the core. Knowing when it forms — and which protection strategy to use — keeps the unit efficient through winter.

When frost actually forms

Frost needs two things: sub-freezing supply temperature and moisture in the exhaust. A typical case: exhaust at 20 °C and 40% RH, outdoor air at −10 °C. As the exhaust cools toward the supply outlet it can reach −5 °C, well below freezing, and the condensed moisture ices onto the plates.

• Effectiveness above ~70% in deep cold raises frost risk.
• High indoor humidity (pools, labs, kitchens) makes it worse.
• Frost shows first at the cold corner of the plate pack.

Four protection strategies

Method	How it works	Trade-off
Preheat supply inlet	Warm outdoor air before the core	Most reliable; uses energy
Exhaust bypass / recirculation	Waste part of recovery to keep plates warm	Simple; lowers seasonal effectiveness
Periodic defrost (wheels)	Reverse rotation or purge to melt ice	Not for plates; brief loss of recovery
Heat-pipe assist	Heat pipe pre-warms supply near the cold corner	Passive; adds cost and depth

For a plate exchanger, preheat at the supply inlet is the standard fix because plates have no moving parts to reverse. A rotary wheel can instead use a purge and periodic defrost cycle.

Sizing margin

Size the core for the design winter temperature, then add 10–15% face area so the bypass or preheat strategy has room to work without starving airflow. Our sizing guide walks through the calculation, and the kitchen recovery example shows the same logic on a warm-side load.

Related reading

Air-to-Air Heat Exchangers ·
Heat Recovery Units ·
Plate Heat Exchanger Guide

Frequently Asked Questions

Why does frost form on a plate heat exchanger?
Frost forms when the supply outlet temperature drops below the freezing point of the moisture in the humid exhaust, condensing and then freezing on the cold plates until the channels narrow or block.

What frost-protection strategies are used?
Common methods are preheating the incoming air, recirculating a portion of warm supply air, modulating or bypassing the exchanger at low outdoor temperatures, and using a defrost cycle on the exhaust side.

How should a unit be sized for cold climates?
Size the core and the frost-control strategy for the design winter condition, checking the supply outlet temperature against the exhaust dew point so the chosen method keeps channels clear.

Does frost protection hurt efficiency?
It trades a little recovered heat for reliability; a well-tuned bypass or preheat holds effectiveness high through most of the season while preventing blockage during the coldest hours.

Frequently Asked Questions

Why does frost form on a plate heat exchanger?

Frost forms when the supply outlet temperature drops below the freezing point of the moisture in the humid exhaust, condensing and then freezing on the cold plates until the channels narrow or block.

What frost-protection strategies are used?

Common methods are preheating the incoming air, recirculating a portion of warm supply air, modulating or bypassing the exchanger at low outdoor temperatures, and using a defrost cycle on the exhaust side.

How should a unit be sized for cold climates?

Size the core and the frost-control strategy for the design winter condition, checking the supply outlet temperature against the exhaust dew point so the chosen method keeps channels clear.

Does frost protection hurt efficiency?

It trades a little recovered heat for reliability; a well-tuned bypass or preheat holds effectiveness high through most of the season while preventing blockage during the coldest hours.