Air Leakage, ACH, and HVAC Load: What Manual J Accounts For

You can buy the most efficient windows and the thickest insulation, but if your house is full of holes, you're losing the battle.

Air leakage hvac load accounts for the unconditioned air that sneaks into your home through cracks, gaps, and vents. In older homes, this "Infiltration" can make up 30-40% of the heating and cooling bill.

What is Infiltration?

Infiltration is outside air entering the house uncontrolled. It happens due to two forces:

1. Wind Pressure: Wind blowing against the house forces air into cracks on one side and sucks it out the other.
2. Stack Effect: In winter, warm air rises and escapes through the attic (exfiltration), sucking cold air in from the basement/crawlspace (infiltration) to replace it.

Understanding ACH (Air Changes per Hour)

Engineers measure ach hvac calculation factors to quantify leakiness.

Definition: 1 ACH means the entire volume of air in your house is replaced by outside air once every hour.

• Old/Leaky House: 0.75 to 1.5 ACH (The air changes 18-24 times a day!).
• Average Home: 0.40 to 0.50 ACH.
• Tight/New Home: 0.20 to 0.35 ACH.

Latent Load: The Humidity Nightmare

Infiltration cooling load isn't just about temperature (Sensible Load). In humid climates, that outside air brings gallons of water vapor with it.

Your air conditioner has to work double-duty: first to remove the humidity (Latent), then to cool the air. If you have high air leakage, your AC might spend 50% of its energy just fighting the humidity coming through the cracks.

Duct Leakage: The Silent Killer

Duct leakage hvac load is often calculated separately, but it's related.
If your ducts are in a hot attic (140°F) and they leak even 10% of the air, you are:

1. Pumping expensive cold air into the attic.
2. Sucking dusty, hot attic insulation fiberglass into your home (Negative pressure).

This is why manual j infiltration calculations ask heavily about duct location. Keeping ducts inside the "conditioned space" (like dropped ceilings) is vastly more efficient than attics.

Air Sealing > Bigger AC

If your home is leaky, the solution isn't to buy a bigger unit to "overpower" the drafts. The solution is air sealing hvac sizing reduction.

Spending $500 on caulk, foam, and weatherstripping can often lower your load requirement by 0.5 Tons—saving you $1,500 on the equipment cost.

How to Measure It Properly

To get a truly accurate number, professionals use a "Blower Door Test." They put a big fan in your door, depressurize the house, and measure exactly how much air leaks in.

Read Next: When You Should Hire a Pro for a Blower Door/Manual J test →