Sizing rules
Get sizing right at the sales appointment and you eliminate change orders. Get it wrong and the install team has to come back.
The rule of thumb
Section titled “The rule of thumb”Square footage × 20 = BTU needed
For a 450 sq ft room: 450 × 20 = 9,000 BTU → use a 9K. For a 600 sq ft room: 600 × 20 = 12,000 BTU → use a 12K.
Default sizes by room type
Section titled “Default sizes by room type”These are the defaults you should be reaching for at the sales appointment. When in doubt, upsize. Going from a 9K to a 12K, or a 12K to an 18K, costs only a few hundred dollars more — and undersizing in winter is the #1 reason for callbacks.
| Room type | Default unit |
|---|---|
| Bedroom | 9K |
| Master bedroom / suite | 12K |
| Living room or dining room | 12K – 18K (depending on square footage) |
| Open concept (living + dining + kitchen, no walls between) | 18K – 24K |
| Large open space (~400 sq ft+) | 24K |
So a typical NYC two-bedroom with an open kitchen/living/dining: 9K + 9K (bedrooms) + 24K (open concept).
When to oversize
Section titled “When to oversize”On decommissioning jobs, lean toward the larger size or the extra unit by default. The whole job depends on covering full-home BTU for Con Edison.
Open-concept rooms (living + dining + kitchen)
Section titled “Open-concept rooms (living + dining + kitchen)”Rooms that flow into each other need more capacity than a single unit comfortably delivers.
- Two units preferred for any open-concept living/dining/kitchen
- A single 24K can cover one large open area if square footage allows
- On decommissioning, lean toward the extra unit even if a single 24K technically fits
Single-zone vs multi-zone size availability
Section titled “Single-zone vs multi-zone size availability”Single-zone systems start at 9K — there’s no 7K single-zone outdoor condenser. The 7K (multi-zone-head only when available) is currently out of stock until further notice, so upsize any tiny rooms to 9K. See Mini splits — sizes & zones.
Central air sizing
Section titled “Central air sizing”| Job type | Default tonnage |
|---|---|
| Decommissioning (heat pump central) | 5 tons to cover full-home BTU for Con Edison |
| Non-decommissioning replacement | Stay at existing tonnage (e.g., replace 3.5 ton with 3 ton if appropriate) |
See Central air for the full breakdown.
Discipline notes
Section titled “Discipline notes”Underselling on unit count is a red flag. Selling a 9K in every room without thinking about size or open-concept areas creates problems. Reps should oversize before underselling.