Plate HRS Frost Protection: Preheat vs Bypass vs Modulation

Full bypass is the cheapest frost protection to buy and the most expensive to run: at -12 °C outdoor with a 75 %-efficient counter-flow plate, bypassing throws away roughly 43 kW of recovery on a 5 000 m³/h unit, while a preheat coil holding the exchanger frost-free costs about 12 kW. That factor-of-three gap is decided at selection time, in a dialog most quotes click through in two seconds.

Why plate exchangers frost, and at what temperature

Frost forms on the extract side, not the intake side. Room air at 22 °C / 35 % RH has a dewpoint near 6 °C. As it gives up heat to the incoming outdoor air it drops below that dewpoint, condenses on the plates, and once the local plate surface falls below 0 °C the condensate freezes. Ice narrows the exhaust channels, pressure drop climbs, efficiency collapses, and eventually the exhaust fan stalls against its own curve.

The counter-intuitive part: the more efficient the exchanger, the earlier it frosts. A high-efficiency counter-flow core drives the exhaust air closest to the intake temperature, so its coldest corner reaches 0 °C sooner.

Core type	Typical dry efficiency	Typical frost onset (22 °C / 35 % RH extract)
Cross-flow plate	50-65 %	around -8 to -12 °C outdoor
Counter-flow plate	70-85 %	around -3 to -6 °C outdoor
Rotary wheel	70-85 %	several K lower; speed modulation usually suffices

Onset figures are indicative — manufacturers publish a frost-free limit per core, and it shifts with extract humidity. A humidified hospital or pool extract at 50 % RH frosts several kelvin earlier than a dry office. Always read the limit against the project's extract condition, not the catalogue's.

The three strategies, priced

Take a 5 000 m³/h unit (1.67 kg/s, ~1.68 kW/K), 75 % counter-flow core, 22 °C extract, 20 °C supply target, at an outdoor design point of -12 °C.

Strategy	What it does	Heat input at -12 °C	Notes
Full bypass	Damper routes 100 % of outdoor air around the core below a setpoint	~54 kW in the heating coil	Zero recovery exactly when the load peaks
Preheat coil	Lifts intake from -12 °C to -5 °C before the core	~12 kW preheat + ~8 kW reheat ≈ 20 kW	Adds 30-60 Pa air-side, all year
Modulating bypass	Bypasses only the fraction needed to hold the exhaust above 0 °C	~24 kW at 30 % bypass	No extra coil, no extra pressure drop

The ranking at design conditions is clear, but design conditions occur for a handful of hours. Over a season the picture flattens: in a maritime climate that drops below -5 °C for 100-200 hours a year, the preheat coil's permanent SFP penalty can eat most of the recovery it saves. In a continental climate with 800+ hours below -5 °C, it pays back several times over.

Two corrections engineers routinely skip:

• Compare energy carriers, not kilowatts. An electric preheat coil at €0.22/kWh against gas-fired reheat at €0.08/kWh reverses the arithmetic above. Price the strategy, don't just count kW.
• Charge the preheat coil its fan energy. 40 Pa across both a supply-side coil and its 8 760 hours of operation is a real number in the SFP_int budget, and it counts in every season — including the summer case where the coil does nothing at all.

Control: setpoint or pressure?

Frost control is triggered one of two ways. Temperature-based control acts on an exhaust-outlet or intake sensor crossing a fixed setpoint — simple, robust, and systematically conservative, because the setpoint has to cover the worst extract humidity the building will ever see. Differential-pressure-based control watches Δp across the exhaust side of the core and reacts to actual ice, not to a proxy for it. It runs the exchanger closer to the real limit and typically recovers meaningfully more over a heating season, at the cost of one sensor and a properly commissioned threshold.

A fourth strategy exists — unbalancing the fans, reducing supply flow so the warmer extract stream defrosts the core. It is common in residential heat-recovery units and unacceptable in most commercial work: it depressurises the building, breaks the balanced-flow assumption behind the EN 308 efficiency figure, and invalidates the airflow the room loads were sized on.

Note also that ErP Regulation 1253/2014 already requires a thermal bypass facility on non-residential ventilation units fitted with heat recovery, for summer free cooling. The hardware is therefore usually present anyway — which is exactly why "bypass" gets selected as the frost strategy by default rather than by analysis.

What to check before you sign the selection

1. What frost-free limit does the core carry at the project's extract temperature and humidity, not the catalogue's dry test point?
2. How many hours per year does the site actually spend below that limit? That single number decides preheat vs bypass.
3. Is the preheat coil's pressure drop already inside the quoted SFP, or was SFP taken from a bypass-only configuration?
4. Does the frost strategy change the winter Eurovent class? It usually does — see why units pass in January and fail in July.
5. If cross-contamination is not a constraint, has a rotary wheel been priced against the plate + preheat combination? Speed modulation is often the cheapest frost strategy of all.

Where the tooling matters

AirSelect3D carries the frost strategy as part of the HRS selection, so a preheat coil's air-side pressure drop flows straight into SFP_int and the ECP-05-2026 class calculation instead of being remembered later — and switching from bypass to preheat recalculates the whole psychrometric chain and both seasonal classes in the same pass.

Model your frost strategy in the 3D designer →