Winter vs Summer Eurovent Class — Why AHUs Pass in January, Fail in July

An AHU does not get one Eurovent energy class — it gets two fs-Pref ratios, one for winter duty and one for summer duty, and ECP-05-2026 grades the unit on whichever season is worse. A rotor sized to shine in heating mode can lose a full letter grade the moment you switch to the cooling coil test point, and most of that swing comes from three things: coil rows, heat-recovery operating mode, and which pressure drops the reference configuration actually assumes. Here is where the gap comes from and how to catch it before a customer does.

The same casing, two different pressure budgets

fs-Pref compares your unit's actual specific fan power against a reference SFP for the same airflow class, per EN 16798-3. The airflow doesn't change between seasons — the external pressure the fan has to overcome does, because the components doing the work are different:

• Heating coil (winter): typically 1–2 rows, low water-side resistance, air-side dP often 30–50 Pa at design face velocity.
• Cooling coil (summer): typically 4–8 rows to hit the latent load, wet-coil condensation adds friction, air-side dP commonly 100–160 Pa — three to four times the heating coil's contribution.
• Heat recovery: a rotary wheel run at full speed for winter frost/efficiency duty carries a different pressure drop than the same wheel bypassed or slowed for summer free-cooling — see plate vs rotary heat recovery for how frost strategy shifts this per climate.

Filter loading is usually held constant between the two seasonal test points (EN 13053 assumes an averaged clean/dirty state), so it isn't the variable that trips people up — the coil and HRS duty are.

A worked example

Same 10,000 m³/h unit, same casing, same fan — only the seasonal operating point changes:

Component	Winter dP	Summer dP
Filters (ePM1, averaged)	90 Pa	90 Pa
Coil (2-row heating / 6-row cooling, wet)	40 Pa	140 Pa
Heat recovery (full speed / bypassed)	120 Pa	30 Pa
External ductwork allowance	30 Pa	30 Pa
Total external pressure	280 Pa	290 Pa
SFP_int (approx.)	~1,480 W/(m³/s)	~1,620 W/(m³/s)
fs-Pref class	A+	C

The total pressure barely moves — but the composition flips. In winter the HRS wheel is the dominant term because it's running at full recovery efficiency; in summer it's bypassed, and the wet cooling coil takes over as the dominant resistance and pushes the fan further up its curve, where efficiency drops faster than pressure rises. That non-linearity, not the raw Pascal count, is why a 10 Pa difference in total pressure can cost a full class letter.

Why this gets missed at quote stage

Most selection workflows size the heating coil first, check winter fs-Pref, see A+ or A, and stop. The cooling coil gets sized later for capacity alone, with the pressure-drop consequence for the Eurovent class checked — if at all — at printout time. By then the customer has already seen a winter-only number in the first proposal. Two fixes:

1. Size both coils before you commit to a fan/wheel combination. If summer dP is going to dominate, you may need a shallower rotor pitch or a different EC motor operating point, not just a bigger motor.
2. Recompute both seasonal fs-Pref every time a coil or HRS parameter changes, not just once at the end. A tool that only reports the class at PDF export time hides the exact trade-off this example shows.

How AirSelect3D handles this

AirSelect3D computes winter and summer fs-Pref live, from the same position_metrics() geometry and the same DLL-sourced coil and fan curves used everywhere else in the model — not a separate spreadsheet pass. Change coil rows or HRS bypass strategy and both seasonal classes update immediately next to the 3D view, so a summer regression shows up while you're still configuring the unit, not after the customer asks why the July number doesn't match the January one.

Try it on your next project at app.airselect3d.com.