Ventilation used to be a hole in the wall; now it is a system. As buildings have become airtight in the name of energy efficiency, natural leakage no longer delivers enough fresh air, and mechanical ventilation has split into several distinct types. This guide walks through the main ones — and the role fans play in each.
Why buildings need mechanical ventilation
Every occupant adds moisture, CO₂ and odours; every kitchen and shower adds more. In a leaky older house, draughts carried this away uninvited. In a modern airtight building, nothing leaves unless a fan moves it — and what leaves must be replaced by an equal amount of air brought in, filtered, and ideally pre-warmed. That is the brief every mechanical ventilation system has to answer.
Exhaust-only and supply-only
The two single-sided types are the simplest:
- Exhaust-only systems put the fan on the extract side — bathroom and kitchen extract fans, or a central extract unit — and rely on leakage and trickle vents to let replacement air in. Cheap and retrofit-friendly, but the incoming air is unfiltered, unwarmed and uncontrolled.
- Supply-only systems do the opposite: a fan pushes filtered outdoor air in, and stale air escapes through leakage. Pressurisation keeps damp out of the envelope, but the extract side remains uncontrolled.
Both types shift the whole airflow through one fan, so that fan runs continuously — which puts noise and energy consumption under a spotlight.
Balanced ventilation with heat recovery: MVHR
A balanced system does both jobs at once: one fan supplies, another extracts, and the airflow through the building is deliberately matched. Add a heat exchanger between the two streams and it becomes MVHR (mechanical ventilation with heat recovery). The outgoing stale air pre-warms the incoming fresh air in winter — and pre-cools it in summer — without the streams mixing. MVHR is the standard choice for well-insulated new builds in cold and temperate climates, where recovering heat from air you have already paid to warm is too valuable to waste.
ERV: when moisture moves too
The ERV variant of a balanced system goes one step further: its exchanger transfers heat and water vapour. In winter the indoor humidity is partly kept rather than dumped outside; in humid summer weather the incoming moist air sheds part of its load to the drier exhaust. Buildings in warm, humid climates, and homes that suffer dry winter air, usually get the better indoor climate from ERV than from plain MVHR.
PIV: positive input ventilation
PIV (Positive Input Ventilation) is a supply-only system built for existing housing stock, and it is popular in the UK for one reason: it attacks condensation and mould without ripping out ceilings to run ductwork.
The principle is gentle pressurisation. A quiet fan unit — typically mounted in the loft, drawing air from above the insulation through a filter — pushes a continuous, low-speed stream of filtered fresh air into the hallway below. The house is held at a slight positive pressure, and that pressure finds its own way out: stale, moist air is pushed through existing leakage paths and trickle vents. No ducts through the living spaces, no building work beyond fixing one unit and a power supply.
PIV suits homes with the classic symptoms — condensation on windows, black mould in corners, stuffy bedrooms — where a full MVHR retrofit would be disproportionate. The fan is the heart of the system: an EC impeller running continuously at low speed, quiet enough to live with and frugal enough to run all year, with a temperature-sensitive control that backs off in the depth of winter.
Choosing between the types
Match the system to the building, not the other way round:
- New, airtight, cold or temperate climate: MVHR, sized for continuous balanced running.
- New or retrofitted, warm and humid climate: ERV, so outdoor humidity stays outside.
- Budget or retrofit, damp-prone existing housing: PIV for condensation control, upgrading later if the building is re-insulated.
- Simple wet-room extract in a leaky building: exhaust-only with trickle vents still earns its place.
The component view: what we supply
For ventilation-equipment manufacturers, these system types differ above the fan level: controls, exchangers, ducts. The fan and impeller are the shared core. SUNXFAN® supplies the core fan and impeller components behind ventilation equipment makers — EC dual-inlet centrifugal blowers for MVHR and ERV units, quiet continuous-duty impellers for supply and PIV units, and the speed-control interfaces that let each system respond to its sensors.
Bottom Line
Exhaust-only, supply-only, balanced, MVHR, ERV, PIV — six names, one decision tree: how airtight is the building, what does the climate demand, and how much of the structure can the project touch. Whichever branch the project takes, the fan inside decides the noise, the energy bill and the service life. For a component discussion around your unit, see our EC fan technology page or the double-inlet centrifugal range.