Choosing a Home Ventilation System: Why Pressure Direction Matters More Than the Spec Sheet

By the editorial team STRUCT · 25 October 2026

A brochure promising 90% heat recovery looks like the obvious winner. It isn't, not by itself. One of the most important factors in whether a ventilation system will work well in your house — and whether it contributes to damp problems over time — is which way it pushes air: does it pull air out of the building, push air into it, or move equal amounts both ways?

That question matters most once a house gets airtight. New windows, blown insulation, sealed roof junctions — all of it closes the gaps that used to let a house breathe on its own. According to the HVI (Home Ventilating Institute) overview of mechanical ventilation types, natural ventilation becomes unreliable as buildings are built tighter, which is exactly why mechanical systems become necessary rather than optional.

This article sets out the mechanical logic that explains why one system suits a given house and another doesn't — logic that holds regardless of what a specific installer quotes you.

Key Takeaways

• There are three main airflow categories of whole-house mechanical ventilation — exhaust, supply and balanced — plus energy recovery, which is a balanced-system variant with a heat exchanger added; what separates the airflow categories is direction, not just a stated efficiency percentage.
• Exhaust, supply and balanced systems don't recover heat or coolness from the air leaving the house, so tempering incoming make-up air can cost more over a season than with an energy-recovery system — though a supply system can still filter or pre-treat that incoming air with added equipment.
• Balanced systems run two fans and two separate duct networks, which usually makes them more expensive to install than a simple supply or exhaust setup — though with energy recovery, the heat reclaimed from outgoing air can offset that higher fan energy use over a season.
• Most energy-recovery units are central, whole-house systems tied into ductwork — but smaller wall- or window-mounted units exist for single rooms, without a full duct run.

How Each System Actually Moves Air

Exhaust ventilation depressurises the house. One or more fans pull stale air out — typically from bathrooms and kitchens — and the replacement air gets drawn in wherever it can: through gaps in the building envelope, trickle vents, or passive inlets. The HVI overview describes this as the defining mechanism of exhaust systems: negative pressure does the work of pulling in fresh air, rather than a fan pushing it.

Supply ventilation does the opposite. A fan pushes outdoor air into the house, which raises the indoor pressure slightly above outside. Air then finds its way back out through the shell, through ducting, and through vents, following the same leakage paths in reverse.

Balanced ventilation avoids picking a direction. It introduces and extracts roughly equal volumes of air at the same time, so the structure is neither pressurised nor depressurised. That neutrality is the main reason it avoids the moisture trade-offs the other two carry — covered in the next section.

Most balanced systems installed today add a fourth element: heat exchange. An energy-recovery unit transfers heat from the warm air being extracted into the cold air coming in during winter, and does the reverse in summer, cooling incoming air using indoor air as the reference.

This is also the point where the terminology installers use locally and the mechanism described in technical literature meet. According to Titon — a ventilation equipment manufacturer — in its guide on mechanical ventilation systems, benefits, types and installation, an MVHR unit extracts stale air from wet rooms while supplying fresh air to living spaces, passing both airstreams through a heat exchanger. MVHR is the term most often heard in Western Europe, close to what's sold in Belgium as a double-flow system.

Strip away the brand name and the acronym, and it's the same balanced, heat-recovering mechanism that North American literature calls an energy or heat recovery ventilator.

Why Pressure Direction Matters More in a Cold, Damp Climate

Climate suitability for each airflow type comes from North American guidance (HVI), built around a different set of regional categories than exist in Belgium. It's worth reading as an illustration of the underlying physics, not as a rule that applies directly to a Belgian house — but the physics itself travels.

The HVI material describes exhaust ventilation as best suited to colder climates; in warmer, more humid regions, the negative pressure it creates can draw moist outdoor air into wall cavities, where it condenses and causes damage. The same source also lists what exhaust systems can pull in alongside that make-up air: radon, mould spores, attic dust, garage fumes and flue gases — a risk that gets worse when a bathroom fan, a range hood and a dryer are all running at once and competing for the same replacement air.

Supply ventilation carries the opposite profile. Positive pressure keeps outdoor pollutants from being drawn in, stops combustion gases from back-draughting into the house, and lets the incoming air be filtered or dehumidified before it enters. According to HVI, this makes supply systems best suited to hot or mixed climates — in cold conditions, that same positive pressure risks pushing moisture into the attic or into exterior wall cavities, where it can condense.

Balanced ventilation sidesteps that asymmetry. Because it neither pressurises nor depressurises the structure, HVI describes it as suitable across all climate types — a relevant point in a climate where winters are cold and consistently damp, and where either kind of pressure imbalance carries a real moisture penalty.

Ductwork and Renovation Reality: What Actually Fits an Existing House?

Pressure behaviour decides moisture risk. Ductwork decides how much of your house gets opened up to install the system. Balanced ventilation, including energy-recovery versions, needs two separate duct networks and two fans — one to extract, one to supply — which HVI notes is usually more expensive to install and operate than a single-direction supply or exhaust setup.

That's the main reason balanced systems are easier to justify in new construction, where ducting can be planned into floor and ceiling voids before finishes go in, than in a renovation where every duct run means opening a ceiling or a wall.

There is a middle path. Most energy-recovery ventilators are central, whole-house units sharing ductwork, but HVI also notes that smaller wall- or window-mounted energy-recovery units exist — these serve single rooms without needing a full duct network, which matters for a renovation where ripping out ceilings room by room isn't realistic.

Titon's installation guidance adds three practical questions worth raising with any installer, regardless of system type: how the system is sized for the space, what ductwork route it will actually take through the existing structure, and where the unit and its air inlets will sit. None of those are decided by the brand on the box — they're decided by what your house looks like behind the plaster.

The row worth re-reading before a renovation is the last one: a central MVHR system asks the most of your ceilings and walls, but a wall- or window-mounted energy-recovery unit gets the same heat-exchange mechanism into a single room without that disruption.

What Actually Drives the Running Cost?

Installed and running costs vary with duct complexity, unit size and local labour rates, so the figure quoted by an installer for your specific house is the number to compare — not a general estimate. What follows is a qualitative hierarchy of what drives that cost up or down.

The first driver is whether the system tempers incoming air at all. HVI notes that exhaust, supply and balanced systems without heat recovery don't temper or remove moisture from the make-up air they bring in, which means that air needs to be fully heated or cooled once it's inside — a running-cost burden that energy-recovery systems reduce by using the heat already in the outgoing air.

The second driver is complexity. HVI is specific that simplicity, and reusing existing ductwork where it exists, keeps installation cost-effective, while complex systems with multiple branches cost more to install, need more maintenance, and draw more electricity to run.

A smaller, more specific point applies to supply systems: an in-line duct heater can be added to warm incoming air before it enters the house, which solves a comfort problem but increases operating cost — one more reason to ask what, exactly, is included before comparing two quotes against each other.

Heat Recovery Percentages: Why the Number on the Box Isn't the Number You'll Get

HVI and Titon don't agree on a single heat-recovery figure, and both numbers are defensible on their own terms. HVI states that most energy recovery systems can recover about 70-80% of the energy in the exhaust airstream. Titon — a ventilation equipment manufacturer — states in its guide to MVHR systems that advanced units with heat recovery can recover up to 90% of thermal energy.

Both figures can be true at once, because they're not describing the same thing. The HVI figure reads as a general description of how the category of energy-recovery systems performs. The Titon figure is a manufacturer claim for advanced units, phrased as an upper limit — "up to" — rather than a typical result, and it isn't tied to a stated test method or operating condition.

Neither number tells you what a specific unit will do in a specific house. A heat exchanger's real recovery rate depends on airflow balance, duct losses, and how the system is commissioned — none of which a headline percentage captures.

The practical move is to ask an installer for an independently tested efficiency figure for the exact unit being quoted, under a stated test condition, rather than accepting either a brochure percentage or a generic range as the number that will show up on a winter heating bill.

A Three-Question Checklist Before You Call an Installer

Before comparing brochures, three questions — asked in this order — do more work than any spec sheet.

1. Which way does it move air? Exhaust depressurises, supply pressurises, balanced does neither. This single fact predicts most of what follows.
2. What does that pressure direction do to moisture in my walls? A depressurising system draws in whatever air it can find, humid or not. A pressurising system pushes indoor air outward through the shell, where it can condense in cold cavities. A balanced system avoids forcing air through the structure in either direction.
3. What will installing it actually require of my house? A central balanced or energy-recovery system means two duct networks, usually run through ceilings and walls. A single-direction exhaust or supply system needs less ducting. A wall- or window-mounted unit sidesteps that disruption entirely.

Frequently Asked Questions

Is natural ventilation enough once I've air-sealed or renovated an older house? Once windows, insulation and roof junctions are sealed up, the gaps that used to let air move on their own are mostly gone. HVI's material is direct on this point: as buildings get tighter, natural ventilation stops being reliable, which is the point at which a mechanical system becomes the practical option rather than a precaution.

What should I actually ask an installer to check before accepting a quote? Beyond the system type, Titon's installation guidance flags three specifics worth confirming on paper: how the unit is sized for your floor area and room use, the exact route the ductwork will take through your existing structure, and where the unit and its air inlets will be positioned — inlet placement affects both how well a room gets served and how much noise reaches it.

Is a higher heat-recovery percentage always the better choice? Not on its own. A higher number without a stated test condition tells you less than a lower number backed by an independent test. Ask which standard a quoted figure was measured against before treating it as comparable to another unit's number.

What makes one ventilation quote more expensive than another? Two things mainly: whether the system tempers incoming air before it enters the house, and how complex the ductwork and fan setup is. Systems that don't pre-treat make-up air pass that heating or cooling cost on to your energy bill, while multi-branch, two-fan balanced systems cost more to install and maintain than a single-fan exhaust or supply setup. Installed and running costs otherwise vary with duct complexity, unit size and local labour rates, so compare quotes for the exact unit and ductwork route being proposed.

Conclusion

The clearest shift in this comparison isn't between the four system types — it's inside the balanced category itself. Small wall- or window-mounted energy-recovery units already deliver the same pressure-neutral, heat-exchanging mechanism as a full central system, without demanding a duct network through every ceiling. That narrows a gap that used to be absolute: balanced systems for new builds, exhaust or supply for renovations. As that boundary moves, the question worth asking a few years from now may not be which system type suits an old house, but how much of a full ducted system a handful of single-room units can replace.