Mechanical ventilation with heat recovery: how to secure good indoor air

Ventilation in a new residential building is in practice always built as mechanical supply-and-extract ventilation with heat recovery (HRV). Decree 1009/2017 sets the outdoor airflow at a minimum of 6 dm³/s per person and at least 0.35 (dm³/s)/m² of floor area; the minimum per dwelling is 18 dm³/s. In addition, heat must be recovered from the extract air in an amount equal to at least 55 per cent of the heat needed to warm the ventilation air.

For scale: in a 120 m² home, the floor-area requirement of 0.35 (dm³/s)/m² means 42 dm³/s, where a family of four would give 24 dm³/s on the per-person basis. The larger figure governs.

Why does an airtight house need mechanical ventilation?

A modern envelope is tight – q50 is typically 1–4 m³/(h·m²) – so air no longer changes through gaps in the structure. Good for energy saving, but everyday moisture, carbon dioxide and pollutants must be removed in a controlled way. Mechanical supply-and-extract ventilation is in practice the only way to guarantee indoor-air quality and moisture control. It also filters the supply air and carries moisture out of the wet rooms and kitchen, protecting the structures.

Airtightness and ventilation also link into the energy calculation: without an airtightness test, the calculation uses a leakage rate of 2.0, and a better measured value improves the E-value. The tighter the envelope, the larger the share of heat losses that pass through the ventilation – and the more the quality of the heat recovery matters.

Does a breathing log house need ventilation?

It does. A log wall's "breathability" means solid wood's ability to absorb and release moisture – it evens out indoor humidity swings, but it does not change the air. The decree's airflow requirements apply to a log house just as to any other dwelling. A modern log house is airtight too: quality log houses have measured leakage rates as low as 0.3 m³/(h·m²). Solid wood's moisture buffering and mechanical ventilation complement each other, and neither replaces the other.

The notion of a log house managing without ventilation stems from old, draughty buildings where air genuinely did change through gaps in the structure. That was ventilation of a sort – but uncontrolled and energy-wasting, not a quality to aim for in a new house.

Can you build a house with passive stack ventilation?

Passive stack ventilation is not forbidden, but meeting the airflow and energy-efficiency requirements with it is difficult: it relies on temperature difference and wind, so airflows vary with the weather and no heat is recovered. Mechanical HRV is therefore the standard in new construction, with passive solutions mostly in unheated and semi-heated spaces.

What does heat recovery save?

The recovery requirement is an annual efficiency of at least 55 per cent. If a project departs from it, the loss must be compensated in the heat-loss compensation calculation, for instance with a better envelope – in practice, units below the minimum are hardly ever chosen for new houses. Today's units clear the minimum comfortably: a rotary exchanger's temperature ratio reaches 60–85% and a plate exchanger's 60–80%, and a good installation achieves about 70% annual efficiency. Most of the heat that would escape with the exhaust air is returned to the supply air: at 70% annual efficiency, heating the ventilation air needs less than a third of the purchased energy it would take without recovery.

Be precise with the terminology when choosing a unit: the manufacturer's quoted temperature ratio is not the same thing as the year-round annual efficiency by which compliance is demonstrated. Compare units on annual efficiency, not on the biggest percentage in the brochure.

How is the ventilation used and maintained?

The ventilation unit is one of the few appliances in a house that runs every day of the year. Happily, its maintenance is simple:

• Keep the ventilation on at all times – at reduced speed when you are away or on holiday, never switched off entirely.
• Check and change the filters to the manufacturer's schedule, typically once or twice a year – a dirty filter degrades indoor air and reduces airflows.
• Never block the valves or alter their settings yourself – measuring and balancing the airflows is a professional's job.

Checklist for the builder

• Have an HVAC designer produce the ventilation plan as part of the permit documents – sizing follows occupancy and floor area.
• Choose the HRV unit on annual efficiency; a good level is about 70%.
• Position the unit so filter changes are easy – a plant room or utility room beats a loft space.
• Make the airflow measurement and balancing report part of the handover.
• Put filter changes in the calendar once or twice a year.

• Outdoor airflow at least 6 dm³/s per person and 0.35 (dm³/s)/m²; the dwelling minimum is 18 dm³/s (Decree 1009/2017).
• Heat recovery's annual efficiency must be at least 55% – good units reach about 70% in practice.
• A rotary heat-exchanger's temperature ratio is as high as 60–85%, a plate exchanger's 60–80%.
• A log house needs mechanical ventilation too: breathability buffers moisture but does not change the air.
• Passive stack ventilation is permitted, but meeting the requirements with it is difficult in practice.
• Filters are changed once or twice a year, and the ventilation stays on at all times.