Manual J Load Calculation for Apartments and Condos
Why your 3rd-floor unit needs less AC than the penthouse, even if they are the exact same size.
Sizing a system for a single-family home is straightforward: it has 4 walls and a roof exposed to the weather.
Sizing for manual j for apartments is a 3D geometry puzzle. Your neighbors might be your best insulation.
The "Shared Wall" Benefit
In condo hvac sizing, we have to account for "Partition Walls."
If your living room wall connects to your neighbor's living room, and you both keep your homes at 75°F, there is ZERO heat transfer. That wall effectively has infinite insulation. You don't need to cool it or heat it.
Compare this to a corner unit or a top-floor unit, which has more exposure to the outside world.
The Load Hierarchy
- The Middle Unit: Surrounded by neighbors on top, bottom, left, and right. Lowest Load. Might only need 1 Ton for 1,200 sq ft.
- The Corner Unit: Exposed on two sides. Medium Load.
- The Top Floor (Penthouse): Exposed roof load + exposed walls. Highest Load.
Critical Factors for Apartments
Duct Leakage
In apartments, ducts are often in "conditioned space" (drop ceilings), meaning duct losses are near zero. This drastically reduces the required equipment size.
Infiltration
Apartments are often tighter than houses, but smell transfer is an issue. Positive pressure is key.
The Danger of Oversizing in Condos
Developers often buy HVAC units in bulk. They might buy 100 2.5-ton units and put one in every apartment.
For the top-floor corner unit, 2.5 tons might be perfect. For the middle-floor unit, 2.5 tons might be double the necessary size.
This leads to the notorious "Cold & Clammy" apartment syndrome, where the AC blasts you with cold air for 5 minutes and then shuts off, leaving the air humid and sticky.
Apartment Dweller?
Use our calculator and select "Apartment/Condo" mode to see how shared walls lower your load.
Calculate Condo Load