Heat Recovery Ventilation Winter Problems: Frost Protection, Preheaters and Filters (Rotary vs Plate)

❓ It’s freezing outside at night — do I need to do anything for a rotary HRV unit?

Usually nothing critical. Rotary units are typically stable in cold weather.

✅ Common action: enable or increase supply air heating (if available) so the airflow doesn’t feel cool.

❓ I see some condensation on the casing during cold weather — is that normal?

Often yes. In freezing conditions, parts of the casing or nearby duct areas can cool down and surface condensation can appear.

✅ It’s usually fine if it’s just light moisture and there’s no dripping water or ice.

⚠️ If you see ice, dripping, or wet insulation, check:

• Duct and casing insulation quality
• Whether outdoor intake/exhaust sections are properly insulated
• Condensate drainage (if applicable)

❓ Plate-type HRV without a preheater and it’s freezing outside — what can I do to keep it running?

Key point: without preheating, a plate-type unit may not ventilate comfortably during harsh cold spells.

✅ What you can do right now:

• Switch to a low/minimum mode
• Avoid continuous “boost”
• Watch for frost/protection warnings
• If possible, reduce supply airflow slightly (less cold air through the core)

❓ Can I turn off my HRV unit on a very cold night?

For a short time — yes. For long periods in winter — not recommended.

Main issue: if motorised dampers don’t fully close, cold air can still move through the ducts (wind/pressure differences) and overcool the system.

✅ Best option is usually minimum mode, not OFF.

If you do switch it off, keep it short and only if you have tight closing dampers on outdoor intake/exhaust.

❓ How can I reduce energy use during very cold weather?

3 quick wins:

1. ✅ Reduce airflow at night (e.g., 250 → 180 m³/h)
2. ✅ Don’t leave “boost” running continuously
3. ✅ Keep filters clean (dirty filters increase fan load and costs)

Extra tips:

• Use supply air heating only as needed for comfort (not max)
• For plate units: a smaller stable airflow is often better than a high airflow that keeps “cycling”

❓ My plate-type HRV gets noisy near freezing — is that normal?

Often yes. It can be a sign that frost protection logic is active:

• Fan speed changes
• Supply/extract imbalance cycles
• Higher resistance through the heat exchanger during icing risk

✅ First check: filters. ✅ Second: make sure outdoor intake grilles aren’t blocked by ice or snow.

❓ I got a “filter” warning after only 4 weeks in winter — is that possible?

Yes — especially for the outdoor (supply air) filter.

Common reasons:

• More soot and fine particles during the heating season
• Temperature inversions can keep pollution closer to ground level

✅ What to do:

• Check the filter — it’s often genuinely clogged
• In polluted areas, a prefilter can help protect the main filter
• A clogged filter means it captured particles that would otherwise enter your home

❓ Why can a plate-type unit show frost issues even around +2 to +4 °C?

Because it depends on indoor humidity. If the home is humid (new build, drying finishes, 60–70% RH), more condensation forms in the core and icing can start earlier.

✅ In that case, solutions often include:

• Lower airflow
• A stable mode (avoid frequent changes)
• Preheating (if available)

❓ Is it true that some plate-type units stop defrosting in deeper frost?

Yes — on some models, below a certain limit, the defrost cycle becomes ineffective, so the unit chooses other protection: limiting airflow, stronger imbalance, or a temporary stop.

✅ Simple explanation for customers: “It can handle moderate frost with cycles, but in deeper cold it can’t defrost fast enough — so it reduces ventilation to protect the core.”

❓ Windows started fogging up in winter — what should I check first?

It usually means real ventilation has dropped (filters / frost mode / airflow set too low).

✅ Steps:

1. Check filters
2. See if the unit is in frost/defrost/anti-ice mode
3. Temporarily increase ventilation during the day (when it’s warmer) and keep a stable low mode at night

❓ Can I reduce airflow to the minimum to save money — is it safe?

Yes, this is common in winter, especially with plate-type units.

✅ Just don’t overdo it:

• Keep at least a minimum level to avoid high CO₂ and humidity
• A small stable mode is usually better than OFF

❓ Why does a rotary unit feel “cooler” even though it works fine?

A rotary exchanger recovers heat, but in deep winter the supply air after the exchanger will still be cooler than room temperature.

✅ Solution: use supply air heating for comfort — this is normal winter operation.

❓ Is an enthalpy core really better in winter?

For indoor comfort, often yes (air may feel less dry). But in harsh freezing weather, without preheating it can still reach its limits and spend more time in protection mode.

❓ When do you really need a preheater?

If you have a plate-type unit (standard or enthalpy) and you want:

• Stable ventilation during freezing winter conditions
• To avoid constant protection/defrost cycling
• Less “mode switching” and more consistent airflow

✅ In colder climates, a preheater is often the practical standard for plate-type units — especially if you want normal airflow through winter.