- Home
- Sizing
How big should your new system be? It depends on the building.
Equipment size should come from a load calculation — a room-by-room account of how much heat the building gains in summer and loses in winter. Square footage alone can't tell you that.
One house, before and after.
Each bar is a house's summer heat gain, split by where it comes from. The second bar is the same house after the attic was insulated, the ducts sealed and the west windows shaded. A shorter bar means a smaller system can do the job.
As built Illustrative
After attic insulation, duct sealing and window shading Illustrative
- Roof & attic
- Windows & sun
- Walls
- Air leaks
- Ducts in the attic
- People
- Lights & appliances
Illustrative proportions for a single-storey house, not a measurement. Every building's split is different — that's the point of calculating it. In words: the biggest shares usually come from the sun through windows and heat through the roof and attic; air leaks and attic ducts follow; walls, people and appliances make up the rest. Insulating, sealing and shading shrink the first four.
The building, room by room.
- Windows
- Size, glass type, and which way each one faces. West glass in a Waco summer counts for a lot.
- Roof & attic
- Insulation depth, roof colour, and whether the ducts run through the attic.
- Walls & floors
- Construction and insulation, slab or pier-and-beam.
- Air tightness
- How much outside air gets in around doors, windows and penetrations.
- People & use
- How many live or work there, kitchens, equipment, and when rooms are used.
For homes, the industry method is ACCA Manual J, with Manual S for picking the equipment and Manual D for the ducts. Commercial buildings use the equivalent commercial load methods.
What oversizing and undersizing feel like.
Runs all afternoon
Can't keep up on the hottest days; the house drifts warmer through the evening.
Long, steady cycles
Runs long enough to pull humidity out of the air as well as heat. Even temperatures, quieter operation.
Short, loud bursts
Cools the air quickly and shuts off before it dries it — cold, clammy rooms, more wear from constant starts.
Replacing a system?
Don't assume the old size was right. Houses change — new windows, more insulation, an extra room — and so does the load.
New tenant, new load
An office that becomes a gym or a restaurant needs a different system. We recalculate before replacing.
Get it sized first.
Every Eagle Rock replacement estimate starts with the building.