A tight house can have very little random air leakage and still bring in a meaningful amount of outdoor air.
That is the point of mechanical ventilation.
Manual J needs to distinguish between infiltration and ventilation because they are not the same thing.
Infiltration vs. ventilation
Infiltration
Uncontrolled outdoor air leaking through cracks, joints, penetrations, and gaps.
Ventilation
Outdoor air intentionally introduced for indoor-air-quality purposes.
Examples include:
- ERV
- HRV
- Dedicated fresh-air duct
- Balanced ventilation system
- Certain exhaust-based strategies
The HVAC system may need to heat, cool, and dehumidify that air.
Why ventilation adds load
Outdoor air arrives at outdoor conditions.
If it is 10°F outside and you are maintaining 70°F indoors, fresh air creates a heating requirement.
If it is 92°F and humid outside, fresh air creates both:
- Sensible cooling load
- Latent moisture load
The amount depends on airflow and whether a heat/energy recovery device tempers the air.
HRV vs. ERV
HRV
A heat-recovery ventilator transfers sensible heat between outgoing and incoming air streams.
It reduces the heating or cooling penalty compared with bringing untreated outdoor air directly inside.
ERV
An energy-recovery ventilator transfers sensible heat and some moisture.
In humid cooling climates, that moisture transfer can reduce latent load compared with untreated ventilation air.
Neither device has 100% recovery.
The remaining outdoor-air load still belongs in the design.
Why tight homes make ventilation more important
In a leaky home, random infiltration may dominate outdoor-air load.
In a very tight home, infiltration may be low by design.
Mechanical ventilation then becomes the primary controlled outdoor-air source.
If you calculate a tight house with low infiltration and forget the ventilation system, you can understate the load.
Ventilation can matter a lot in small efficient homes
High-performance homes often have small envelope loads.
That means a fixed quantity of outdoor air can represent a larger percentage of the total.
For example, a modest ventilation load that barely matters in a leaky 1970s house may be significant in a compact, highly insulated new home.
As envelopes improve, "small" loads become less small.
Sensible ventilation load
The sensible component depends on the temperature difference between outdoors and indoors and the amount of air.
In winter, cold ventilation air needs heating.
In summer, hot ventilation air needs cooling.
Recovery equipment reduces that penalty according to its actual effectiveness and operating condition.
Latent ventilation load
Latent load becomes important when outdoor air contains much more moisture than indoor air.
The AC or dehumidification system has to remove that water.
An ERV can reduce some of the moisture load, but its latent effectiveness is not perfect.
Do not assume "ERV" means no latent ventilation load.
Exhaust-only ventilation can increase infiltration
An exhaust fan intentionally removes indoor air.
Replacement air has to come from somewhere.
Depending on the building and system design, that can pull outdoor air through the envelope.
The ventilation strategy should be modeled consistently rather than adding arbitrary infiltration on top of a full ventilation airflow without understanding the interaction.
Dedicated fresh air through the HVAC return
Some systems introduce outdoor air into the return duct.
That outdoor air becomes part of the equipment entering-air condition.
The designer should account for:
- Outdoor airflow rate
- Controls
- Runtime
- Mixing
- Filtration
- Moisture
- Whether the air is preconditioned
A motorized damper does not make the load disappear.
What about bath fans and range hoods?
Intermittent exhaust equipment is not always treated the same way as continuous designed ventilation.
Manual J uses standardized procedures rather than assuming every kitchen hood runs at full airflow on the design day.
Follow the applicable calculation method and project requirements.
Do not add every exhaust fan nameplate together and treat that as permanent outdoor-air load.
Common ventilation modeling mistakes
Forgetting it entirely
Common in tight new construction.
Treating ventilation as infiltration
This can hide the actual designed airflow and recovery performance.
Assuming 100% ERV/HRV recovery
No real unit eliminates all load.
Double-counting outdoor air
Be careful not to count the same airflow in multiple categories.
Ignoring latent load
A major issue in humid climates.
Ventilation and equipment selection
The Manual J load should include the ventilation effect.
Then Manual S verifies whether the selected equipment can meet:
- Sensible cooling
- Latent cooling
- Heating load
In some humid high-performance homes, dedicated dehumidification may be worth evaluating because the latent load and cooling load do not always align neatly.
Bottom line
Mechanical ventilation improves indoor air quality, but outdoor air has a heating and cooling cost.
Manual J should model that cost explicitly.
Tight homes do not eliminate outdoor-air load. They make the difference between uncontrolled infiltration and controlled ventilation more important.
Load Calc Guru lets you account for building leakage and ventilation separately so the load reflects how the home actually gets outdoor air.