Even in very airtight homes with minimal intentional air leakage, some uncontrolled air movement still occurs due to wind pressure differentials across building surfaces and HVAC system pressure imbalances, requiring either perfect design or active pressurization strategies.

Hospital operating suites use continuous positive pressurization so that air always flows outward and no contaminated air from surrounding rooms can enter, establishing a model that could theoretically be applied to residential homes seeking perfect air quality control.

Modern ERV systems with EC (electronically commutated) motors allow independent variable control of incoming and outgoing fan speeds, enabling unbalanced ventilation strategies where supply and exhaust CFM can be set to different values within a single unit.

A specific ERV model with 150 CFM total capacity operating with EC motors can be used as a reference point for calculating whether unbalanced ventilation is feasible given typical home exhaust requirements and depressurization analysis.

ASHRAE 62.2 standard provides the calculation methodology for determining required ventilation rates based on building characteristics and occupancy, with newer versions (2013-2016) providing more current calculations than older standards.

Bathroom exhaust requirements in modern airtight homes should differentiate between shower exhaust (requiring more ventilation due to moisture generation) and toilet exhaust (requiring less), rather than applying the same CFM requirement to both as some industry standards suggest.

For a specific airtight home with a blower door test of 290 CFM50, 3000 square feet, and 47000 cubic feet of air volume, achieving a 3 Pascal positive pressurization requires 44 CFM of unbalanced ventilation (more supply than exhaust).

A typical residential home with a blower door test of 1500 CFM50 would require 230 CFM of continuous incoming air to achieve a 3 Pascal positive pressurization, which exceeds the capacity of most residential ERVs on the market and is essentially unfeasible in residential applications.

Depressurizing a home (creating negative pressure) is generally easier than pressurizing it in American residential construction because kitchen exhaust dampers open more readily to outflow than to inflow, and building paper/house wrap often have untaped bottom seams that prevent inflow but allow outflow.

Inexpensive supply-only ventilation systems (like products from Aprilaire and AC Infinity) that push air into the home through a duct between the return plenum and outdoors offer a more economical alternative to unbalanced ERVs for attempting modest home pressurization.

Adding 350 CFM of supply ventilation to a 2-ton air conditioner system (which normally moves 800 CFM) would require upsizing all ductwork by approximately 50 percent, representing a massive infrastructure investment that makes the ventilation strategy economically prohibitive even before considering operational costs.

A 10-inch round duct (approximately 78 square inches) connected to a 6-inch round duct (approximately 25 square inches) creates a 3:1 mixing ratio, allowing a ventilating dehumidifier to proportion outdoor supply air versus indoor recirculation based on duct diameter.

A ventilating dehumidifier unit rated at 350 CFM at zero water column pressure but only 280 CFM at 2.2 inches water column (realistic ductwork pressure) with 70 CFM from outside and 210 CFM from inside would only be able to pressurize a home with blower door test of 600 CFM50 to 2 Pascal, making it insufficient for most homes seeking pressurization.

ERVs are chosen for balanced ventilation primarily because they equalize airflow between incoming and outgoing air, not primarily for energy efficiency, and the balanced characteristic is the key functional requirement in airtight homes.

In a balanced ERV system with separate supply and exhaust fans, the location of supply and exhaust ductwork placement in different rooms creates exploitable pressure differentials that can be used to influence home air flow physics even though the system is nominally balanced overall.

Successful home system design requires understanding that 'everything is a system' and recognizing that incompatible design goals (like high pressurization without infrastructure upsizing) cannot be solved through equipment alone when fundamental physics constraints apply.

Using free online calculators and tools like the depressurization analysis tool on RedCalc and ASHRAE 62.2 calculations can help homeowners and builders understand the real-world requirements and constraints of ventilation system design before making equipment or design decisions.

When a home requires 120 CFM of bathroom exhaust and needs 44 CFM of unbalanced pressurization, the total required supply is 164 CFM, which exceeds the 150 CFM total capacity of the reference ERV, making it impossible to achieve the desired pressurization without purchasing a larger unit like a 180 or 200 CFM ERV.

For a very leaky home with a blower door test of 3000 CFM50, even achieving a modest 2 Pascal positive pressurization would require 350 CFM of continuous unbalanced ventilation, making active pressurization strategy completely infeasible without massive equipment and infrastructure investment.

To maintain continuous positive home pressurization (1-2 Pascal above ambient) using supply-only ventilation, the air handler must run continuously and the supply duct must remain open at all times, which creates ongoing energy consumption and dehumidification requirements.