How the two differ in kind
Every occupied building needs outdoor air, and the two common ways to provide it differ in one respect that drives everything else: whether the incoming air has a route of its own.
EXHAUST-ONLY runs a fan that removes air — continuously, or on humidity and occupancy — and lets the same volume arrive by itself. The house goes slightly negative, and outdoor air is pushed in through trickle vents, gaps, and every unintended opening in the envelope. It is one fan, one duct and one penetration, and it is by a wide margin the cheapest strategy to install.
BALANCED ventilation supplies and extracts matched flows, so there is no net pressure across the envelope, and passes the two streams through a heat exchanger on their way past each other. An HRV transfers sensible heat only; an ERV transfers heat and a share of the moisture, which is what makes it useful both in a humid summer, where it limits how much moisture comes in, and in a very cold dry winter, where it keeps the house from being dried out.
The consequential difference is that exhaust-only does not choose where its air comes from. If the shortest path to outside runs through a crawl space, an attached garage, or the flue of an atmospherically vented water heater, that is the path the air takes — and in the last of those cases the exhaust fan can pull combustion products back into the house. In a hot, humid climate the negative pressure draws moist outdoor air into wall cavities, where it meets a surface cooled by the air conditioning. Neither of these is a theoretical concern; both are why balanced systems are increasingly the default rather than the upgrade.
The factors that actually differ
Show
| Balanced ventilation with heat recovery | Exhaust-only ventilation | |
|---|---|---|
| Where the make-up air comes from | A duct you designed, through a filter you chose. | Wherever the envelope leaks. Nobody chooses, and the shortest path wins. |
| Pressure across the envelope | Near zero by design, which is what commissioning sets and verifies. | Negative while the fan runs, in proportion to flow and envelope tightness. |
| Heat recovery | Most of the heat in the outgoing air is transferred to the incoming air, so ventilating costs far less energy. | None. Every unit of heat in the extracted air leaves the building. |
| Filtration of incoming air | Possible and normal — the supply passes a filter before it reaches the rooms. | Impossible. Air entering through cracks brings whatever is on the other side of them. |
| Behaviour in a tight house | Unchanged. The supply fan provides its own air, so tightness helps rather than hinders. | Degrades. With nowhere for air to enter, flow falls and depressurisation rises instead. |
| Combustion safety | No net depressurisation, so no added backdraft risk. | A real risk where any atmospherically vented appliance shares the space, and one that must be checked rather than assumed. |
| Install cost and complexity | Two ducted paths, a unit, a condensate drain, filters, and a commissioning visit to balance the flows. | One fan, one duct, one grille. The cheapest ventilation there is. |
| Maintenance | Filters at intervals, the core cleaned periodically, the condensate path kept clear. Neglect shows up as reduced flow. | Almost none, which is genuinely an advantage where nobody will do any. |
| Cold-climate behaviour | The core can frost, so a defrost strategy is needed — and defrost costs energy and interrupts flow. | No core to frost, but cold outdoor air arriving through wall gaps creates draughts and condensation risk where it enters. |
| Distribution | Supply to bedrooms and living spaces, extract from wet rooms — air is put where it is wanted. | Extract from wet rooms only. Where fresh air reaches is not controlled. |
Which one, and when
Choose balanced ventilation with heat recovery when…
- The envelope is tight — new construction to any modern standard, or a deep retrofit.
- There is an atmospherically vented combustion appliance, or a chimney, sharing the conditioned space.
- The climate makes ventilation expensive: very cold winters, or hot humid summers where incoming moisture is the problem.
- Air quality is the objective rather than compliance, so filtering the incoming air matters.
Choose exhaust-only ventilation when…
- The envelope is leaky enough to supply make-up air without significant depressurisation.
- The budget or the building will not take ducted supply — a flat retrofit, a listed interior, a very small dwelling.
- There are no atmospherically vented appliances and no crawl space, garage or contaminated zone adjoining.
- Nobody will service a unit, and a strategy that keeps working unmaintained is worth more than efficiency.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- What is the actual risk of depressurising a house?
- It depends entirely on what is connected to the house, which is why it is a check rather than a blanket warning. The serious case is an atmospherically vented combustion appliance — an open-flue boiler, a gas water heater, a wood stove — whose flue relies on a weak buoyancy-driven draught. A house held negative by an exhaust fan can overcome that draught and pull combustion products back down the flue into the room, and that is a carbon monoxide risk rather than a comfort one. The lesser cases still matter: negative pressure draws air from crawl spaces, attached garages and the ground, bringing soil gas, radon and vehicle exhaust with it. Sealed-combustion and direct-vent appliances take their air from outside and are not affected, which is one reason they have become standard.
- ERV or HRV — which one?
- The difference is moisture. Both recover heat; an ERV also transfers a share of the water vapour between the two air streams, and the question is whether you want that. In a hot humid climate you do: it reduces how much outdoor moisture the ventilation system brings into the house, which is work the air conditioning would otherwise have to do. In a very cold dry winter you also do, for the opposite reason: it keeps some indoor moisture in the house instead of exhausting it, so the air does not become uncomfortably dry. Where an HRV is preferable is a cold climate with an indoor moisture EXCESS — a tight house with high occupancy where the aim is to get moisture out — because there an ERV is retaining exactly what you are trying to remove.
- Why does balancing matter so much?
- Because an unbalanced balanced system is an expensive exhaust-only system, and nothing about its appearance says so. The supply and extract fans are rarely identical in the resistance they see: duct runs differ in length, bends and terminal type, and filters load at different rates. If the extract moves more than the supply, the house is depressurised exactly as it would be by an exhaust fan, and every issue above applies while the owner believes they have solved it. Commissioning means measuring both flows and adjusting until they match, and it is the step that gets skipped when a system is installed by whoever is on site rather than handed over properly. It is also worth repeating after any change to the ducting.
- How much ventilation does a house actually need?
- It is set by a rate — typically a combination of floor area and occupancy, or an air change rate for the volume — and the figure comes from your local standard rather than from a manufacturer. Two things are easy to get wrong. The first is that the design rate assumes the system runs: intermittent boost-only operation on a tight house does not deliver a continuous rate, and a system switched off because it is noisy delivers nothing at all. The second is that extract points and supply points have to be in the right rooms — extract from kitchens and bathrooms where moisture and odour are generated, supply to bedrooms and living spaces where people are — because a nominally correct total that all passes through one corridor ventilates the corridor.
- Can I add balanced ventilation to an existing house?
- Yes, and the obstacle is ducting rather than the unit. A full system needs supply and extract runs to individual rooms, which in an existing house means a route through floor voids, a loft, or a boxed bulkhead — and the honest answer is that this is easy in some houses and very disruptive in others. Two intermediate options exist. Single-room heat recovery units serve one room each through a single wall penetration, and a set of them approximates a balanced system without any ducting. A partial system serving only the floor where routing is feasible is also better than nothing, provided the imbalance it creates is understood. What is not a good outcome is a ducted system with runs so long, so restricted or so leaky that it cannot achieve its flows, which is a common result of fitting the ducts into whatever space was left over.
- Does a tighter house need more ventilation?
- It needs more DELIBERATE ventilation, which is not quite the same claim. A leaky house is ventilated accidentally — through gaps, at a rate that varies with wind and temperature, unfiltered, and mostly when you least want it. Tightening the envelope removes that accidental exchange, which is the point: it is what makes the heating load fall. But the air still has to be replaced, so what was happening by accident now has to happen on purpose. The design rate does not increase because the house got tighter; what changes is that the mechanical system is now responsible for all of it rather than topping up a leaky baseline. This is why airtightness and ventilation are specified together, and why tightening a house without addressing ventilation reliably produces condensation and poor air quality.
- What happens to the core in freezing weather?
- Moisture in the outgoing air condenses as it gives up heat, and below freezing that condensate turns to ice inside the core, progressively blocking it. Units handle this with a defrost strategy, and each one costs something. Recirculation defrost closes the outdoor intake and circulates indoor air through the core to melt it — during which no fresh air is being supplied. A preheater warms the incoming air above freezing before it reaches the core, which works continuously but spends electricity to do it. Some units reduce supply fan speed to unbalance the flows deliberately, which depressurises the house while it runs. None of these is a defect; they are the reason a unit's rated recovery efficiency and its seasonal performance in a cold climate are different numbers, and why the defrost method is worth asking about before buying.
- Is a bathroom fan on a timer enough?
- It is an exhaust-only strategy with a small duty cycle, and whether it is enough depends on the envelope and the standard you are working to. In a leaky older house with modest occupancy it may well provide adequate exchange, and it is the arrangement most existing housing has. The cases where it is not enough are specific: a tight envelope, where intermittent extraction cannot achieve a continuous rate and simply depressurises the house harder while it runs; high occupancy relative to volume, where carbon dioxide and moisture accumulate between runs; and any building where the standard requires continuous mechanical ventilation. The intermediate upgrade is a continuously running low-rate extract fan with humidity-triggered boost, which is still exhaust-only but delivers a genuine background rate rather than bursts.