Exhaust-Only Ventilation

What is exhaust-only ventilation, and how does it work?

Exhaust-only ventilation, called odsávacie (podtlakové) vetranie in Slovak, is a regime in which mechanical fans remove air from the wet rooms (kitchen, bathroom and WC) while fresh air enters through passive openings. Those openings are usually trickle vents in window frames (vetracie štrbiny) or small inlet grilles in external walls. Because the fans remove air faster than the openings can replace it, the building runs at slightly negative pressure (podtlak) compared with the outdoors, and that difference pulls fresh air inwards.

The method is cheap and has few parts to fail. It worked adequately in buildings that were never airtight, because leakage through windows, doors and floors supplied most of the air. Its weakness is that the supply side is left to chance. Once a building is sealed, the air the fans need no longer arrives easily, and that is where the trouble begins.

How does exhaust-only ventilation differ from balanced ventilation?

A balanced ventilation system has a supply fan and an extract fan whose flows are matched, so the building stays at neutral pressure. Exhaust-only ventilation has extract fans alone. It has no supply fan and no heat recovery (rekuperácia), so the warmth in the outgoing air is lost to the outdoors.

Fresh air also arrives differently. A balanced system delivers air through designed supply valves, usually in living rooms and bedrooms, and can filter it before it reaches the rooms. Exhaust-only ventilation admits air wherever the openings and leaks happen to be. The air often arrives cold, can cause drafts near windows, and may bypass the rooms that need it most.

Aspect	Exhaust-only ventilation	Balanced ventilation
Building pressure	Negative relative to outdoors	Neutral
Fresh air entry	Trickle vents, wall inlets and leaks	Designed supply valves, usually filtered
Heat recovery	None, so exhaust heat is lost	Possible through a heat exchanger
Open-flued appliances	Poor without a dedicated combustion air supply	Better, since pressure stays neutral, but still needs a check

Why does negative pressure create a combustion safety risk?

Every extract fan lowers the pressure inside the building. In a leaky house the gap is filled quickly from outside and the effect stays small. In an airtight house the air cannot come in easily, so the pressure falls further, and the fans can pull enough air out to reverse the flow in a flue whose combustion air comes from the same room.

An open-flued appliance, such as an open fireplace or an atmospheric gas water heater, burns fuel and depends on the chimney draught (ťah komína) to carry combustion gases up and out. The draught works because warm gases rise, and it works best when room pressure is close to outdoor pressure. If the room is depressurised, the flow can reverse, and combustion products, including carbon monoxide, spill back into the living space. Carbon monoxide has no smell, so occupants may not notice until they feel unwell. The risk is greatest when a fire is burning and several extractors run in a sealed house.

Can a wood stove or fireplace run alongside exhaust-only ventilation?

A room-sealed appliance takes its combustion air from outside through a dedicated pipe and has a sealed flue, so it is far less sensitive to room pressure. A non-room-sealed wood stove (krbová kachľa) draws its combustion air from the room where it stands. In an airtight house running exhaust-only ventilation, that stove does not mix safely unless its combustion air is supplied separately.

The safe arrangement needs three things. First, a dedicated combustion air supply, ducted from outside directly to the appliance, so the stove does not depend on leakage. Second, a pressure monitor in the room where the appliance stands, which shows when the room drops below outdoor pressure, ideally linked to an alarm or to an interlock that stops the extract fans. Third, an assessment of the chimney and the installation conditions by a chimney sweep (kominár) or a specialist designer. Any pressure limit should come from the appliance documentation and the designer, not from a general rule.

Appliance	Source of combustion air	Effect of negative room pressure	Typical response
Room-sealed boiler or stove	Outside, through a dedicated pipe	Little effect	Check that the air pipe and flue seals are intact
Open-flued fireplace	The room	Flue gases can spill into the room	Keep extract fans off while the fire burns, or add combustion air and monitoring
Non-room-sealed wood stove	The room	Flue gases can spill into the room	Dedicated combustion air duct, pressure monitor, CO alarm
Atmospheric gas water heater	The room	Spillage and backdraft risk	Replace with a room-sealed unit, or supply combustion air
Electric heating	Not applicable	No combustion risk	No combustion-specific action

Why do many Slovak flats already run an accidental exhaust system?

Many Slovak homes were built around shared ventilačné šachty (ventilation shafts) or individual exhaust flues in kitchens and bathrooms. A kitchen hood vented outside or into a shaft, a bathroom fan and a vented tumble dryer together form an exhaust system that nobody designed as one. Fresh air comes in through gaps around the windows and doors, which means the exhaust system has always depended on leakage.

The problem appears during renovation. When old windows are replaced with airtight units, the leakage paths that supplied the exhaust system disappear. The extract fans then depressurise the flat more than before, doors become harder to open, and any open flue or stove in the flat is exposed to backdraft.

Exhaust source	Effect on room pressure while running
Kitchen hood vented outside or into a shaft	Significant
Recirculating hood with a filter	Little or none, though it does not remove moisture
Bathroom or WC fan	Moderate, usually intermittent
Vented tumble dryer	Significant, usually short-duration

When is exhaust-only ventilation a reasonable choice?

It can be a reasonable choice for a modest renovation of a leaky older building where moisture in the wet rooms is the main problem, the heating appliances are room-sealed, and the envelope is not being tightened significantly. It is a weak choice for a new airtight house, for any home with an open-flued fire or a non-room-sealed stove, and for any project that replaces the windows. In those cases a balanced ventilation system is usually the better outcome. For a flat or a single room, a decentralized ventilation unit can supply and extract air without a full ductwork network.

What are the common misconceptions about exhaust-only ventilation?

• Extract fans bring in fresh air. They only remove air. Fresh air must enter through openings or leaks, and the fans cannot decide where it comes from.
• A trickle vent solves the problem. A vent admits air only when pressure drives it, so its performance depends on pressure, wind and weather.
• It is free to run. The lower purchase price hides the heat lost with the exhaust air and the moisture damage that follows when humidity is not removed evenly.
• If a fan runs, the whole home is ventilated. Air is pulled from wherever it can enter, so a bedroom with a closed door may receive little fresh air.