Manual J includes mechanical ventilation load.
That does not mean Manual J tells you how much ventilation air the home should receive.
These are two different design questions.
Ventilation design answers "how much outdoor air?"
The required or selected outdoor-air rate comes from the applicable:
- Ventilation standard
- Building code
- Program requirement
- Project design
A common residential reference is ASHRAE Standard 62.2, but the exact adopted requirement varies by jurisdiction and program.
Do not invent a ventilation rate inside the load calculation.
Manual J answers "what load does that air create?"
Once the ventilation airflow is known, Manual J accounts for the heating and cooling consequences of bringing that outdoor air into the building.
In winter, the system may need to heat it.
In summer, the system may need to:
- Cool it
- Remove moisture from it
The load depends on climate and whether recovery equipment tempers the air.
Infiltration is not ventilation
This distinction is critical.
Infiltration
Uncontrolled leakage through the envelope.
Ventilation
Intentional outdoor air.
A house can be:
- Leaky with no designed ventilation
- Tight with substantial mechanical ventilation
- Somewhere in between
Manual J needs both represented according to its method.
Why tighter homes can have a meaningful ventilation percentage
As envelope loads fall, a fixed ventilation airflow can represent a larger share of total heating/cooling load.
A high-performance home may have:
- Excellent walls
- Excellent windows
- Low infiltration
- Small total cooling load
The ventilation air does not disappear.
That makes outdoor-air design especially important in efficient houses.
ERVs and HRVs reduce load
Heat/energy recovery ventilation transfers energy between outgoing and incoming air streams.
HRV
Primarily transfers sensible heat.
ERV
Transfers sensible heat and some moisture.
The incoming air is partially tempered before the HVAC system handles it.
But recovery is not 100%.
Manual J should account for the actual ventilation strategy.
Ventilation airflow can affect latent load substantially
In humid climates, outdoor air carries water vapor.
Even a modest continuous ventilation rate can create significant latent load in a very efficient house.
That can influence:
- AC latent performance
- Dehumidifier selection
- ERV strategy
- Controls
See How to Size a Whole-House Dehumidifier.
Exhaust-only ventilation changes pressure
An exhaust fan removes air.
Replacement air enters somewhere.
Depending on the building, that can increase infiltration through the envelope.
Balanced ventilation works differently.
Model the ventilation system consistently rather than double-counting outdoor air.
Fresh-air ducts tied to the return need control logic
Some systems bring outdoor air into the HVAC return.
Questions include:
- How many CFM?
- When does the damper open?
- Does the blower run?
- Is the air filtered?
- Is it preconditioned?
- What happens in humid weather?
The load calculation assumes a design airflow.
The installed controls have to actually deliver the intended ventilation.
Do not use Manual J as a ventilation-code calculator
If the project needs code-compliant ventilation, determine that airflow with the adopted requirement.
Then input the resulting design into Manual J.
That keeps the design chain clear:
- Ventilation requirement
- Ventilation-system design
- Manual J load effect
- Manual S equipment selection
Existing homes are different from new construction
A replacement contractor may encounter an existing fresh-air system with unknown airflow.
Do not assume it is delivering the label value.
Inspect:
- Duct size
- Damper
- Controller
- Filter
- Balancing device
- Measured airflow if available
The existing ventilation design may need evaluation separately from the HVAC replacement.
Bottom line
Manual J does not determine how much ventilation a home needs.
It calculates the heating and cooling load created by the ventilation airflow that the design requires.
Keep those two tasks separate, then make sure the load calculation and installed ventilation system use consistent assumptions.
Technical references
- ACCA Manual J — Residential Load Calculation
- DOE Building Science Education — Energy Recovery Ventilation