Balanced vs Exhaust-Only vs Supply-Only Ventilation

What it is

There are three ways to mechanically ventilate a whole house, and the only real difference between them is which direction they push the house pressure relative to outside:

• Exhaust-only removes air → house goes slightly negative → outdoor air leaks in wherever it can.
• Supply-only brings air in → house goes slightly positive → indoor air leaks out wherever it can.
• Balanced removes and supplies equal air → house stays roughly neutral.

That pressure direction drives everything that matters: combustion safety, what kind of air sneaks in, humidity behavior, and energy cost. Pick the strategy that fits the house's combustion situation, climate, and budget — not just whatever's cheapest to install.

How it works

Exhaust-only is usually one quiet, continuous-rated fan (sometimes a smart bath fan) pulling stale air out. Because air leaving has to be replaced, the house drops to a slight negative and pulls makeup air in through leaks, trickle vents, or passive inlets. Simple and cheap. The catch: the incoming air is unfiltered and uncontrolled (it comes from wherever the leaks are — including the attic, crawlspace, or an attached garage), and the negative pressure can backdraft atmospheric combustion appliances or pull in radon/soil gas. In a very tight house there may not be enough leakage to make up the air, so the fan just struggles against the envelope.

Supply-only is a fan pushing filtered outdoor air in, often dumped into the return side of the air handler so it mixes and distributes. The house goes slightly positive, so stale air leaks out through the envelope. Advantages: you get to filter and somewhat control the incoming air, and positive pressure keeps soil gas and garage fumes from being sucked in. Downside: you're pushing unconditioned (and in summer, humid) outdoor air into the house with no energy recovery, and in a cold climate positive pressure can drive moist indoor air into wall cavities where it can condense.

Balanced runs a supply fan and an exhaust fan moving equal CFM, so the house pressure stays neutral. The premium version is an ERV or HRV, which routes both streams through a recovery core so you reclaim most of the heating/cooling energy instead of throwing it out. Balanced is the most controlled and the most combustion-safe (no net pressure shift to backdraft appliances), and with an ERV/HRV it's the most energy-efficient. It's also the most expensive and the most install-sensitive — it only works if you actually balance the two streams.

In the field

• Start with the combustion audit. Are there atmospheric (naturally drafted) gas/oil appliances — a standing-pilot water heater, an 80% furnace on a B-vent? If yes, avoid exhaust-only; the negative pressure risks backdrafting CO into the house. Balanced or supply is far safer there. All-sealed-combustion or all-electric houses give you more freedom.
• Factor the climate. Hot/humid → supply-only dumps latent load inside; balanced with an ERV is much better. Cold → supply-only can push moisture into walls; exhaust-only or balanced with an HRV fits better. Mixed → balanced ERV is the safe generalist.
• Factor tightness. The tighter the house, the worse exhaust-only and supply-only behave (not enough leakage to make up or relieve the pressure) and the more balanced makes sense. A leaky house can sometimes get away with exhaust-only.
• Factor budget and ductwork. Exhaust-only is cheapest (one fan). Balanced with a fully ducted ERV/HRV is the most hardware and labor. There are middle options (a partially ducted ERV, or supply integrated into the return).
• Whatever you pick, commission the airflow and, on balanced systems, verify supply ≈ exhaust. An "unbalanced balanced" system is just a pressurizing or depressurizing fan with extra steps.

Normal values & targets

• Exhaust-only: house runs slightly negative. Cheap, simple. Worst fit for atmospheric combustion and tight envelopes. No filtration, no recovery.
• Supply-only: house runs slightly positive. Filters and controls incoming air; keeps soil gas/garage fumes out. No recovery; adds latent load in humid climates; can drive moisture into walls in cold climates.
• Balanced (plain): neutral pressure, two fans, no recovery core. Combustion-safe; energy-neutral on recovery.
• Balanced (ERV/HRV): neutral pressure plus ~60–85% energy recovery. Most combustion-safe and most efficient; highest cost and most install-critical.
• All strategies target the same whole-house rate: roughly (0.03 CFM/ft²) + (7.5 CFM × (bedrooms + 1)) continuous. The strategy changes the pressure and energy, not the required fresh-air quantity.

Common faults & what they mean

• CO alarm or backdrafting at a water heater after ventilation was added: almost always exhaust-only (or a big exhaust appliance) depressurizing a house with atmospheric combustion. Switch toward balanced/neutral. This is a life-safety issue, not a comfort tweak.
• Musty or garage-smelling makeup air with exhaust-only: the negative house is pulling its makeup air from the crawlspace, attic, or attached garage instead of clean outdoors. Supply or balanced fixes the source of the incoming air.
• Clammy house in summer with supply-only: pushing humid outdoor air in with no latent recovery. An ERV would knock down the moisture.
• Wet wall sheathing / condensation in cold climate with supply-only: positive pressure forcing humid indoor air into cold wall cavities. Wrong strategy for the climate.
• Balanced system not doing its job: the two streams aren't balanced (so it's secretly net-positive or net-negative), or a filter/core is plugged. Measure both streams.

Tech tips & gotchas

• Pressure direction is the whole story. Negative = exhaust = pulls unknown air in (combustion risk). Positive = supply = pushes air out (wall-moisture risk in cold climates). Neutral = balanced = the safe middle, and where recovery lives.
• Atmospheric combustion + exhaust-only = CO risk. This is the single most important gotcha. If there's a naturally drafted appliance, don't depressurize the house. When in doubt, go balanced.
• Exhaust-only doesn't filter or control its makeup air. People love it for the low cost, but you have no say over where the replacement air comes from. In a house with a damp crawlspace or attached garage, that's a real IAQ downside.
• Supply-only is a decent cheap upgrade for cold-climate, all-electric/sealed-combustion houses — you filter the incoming air and keep the house positive against soil gas — but watch wall moisture and don't use it in a hot-humid climate without considering the latent load.
• "Balanced" is only balanced if you balance it. The install quality matters more here than with the single-fan strategies. Measure, adjust, and document supply vs exhaust CFM.

Safety / code notes

• Do not create negative house pressure around atmospheric combustion appliances — ties to combustion-air and venting provisions in the IFGC/IMC. Worst-case depressurization testing confirms you're safe.
• Whole-house ventilation rates for all three strategies follow ASHRAE 62.2 (referenced by IRC/IMC).
• Outdoor intakes (supply and balanced) need proper location and clearance from contaminant sources per IMC intake provisions. Exhaust terminations go outdoors with required clearances — never into attics, soffits, or crawlspaces.
• Large exhaust appliances (range hoods) in tight houses may require makeup air per IMC/IRC provisions.