Assumptions
- 15 CFM per tube. 1 tube per register.
- 15 CFM supply per person, minimum.
- 30 CFM exhaust per bathroom and kitchen.
- 15 CFM exhaust for laundry & toilet rooms.
- Supply and exhaust should be balanced.
The total CFM should land near 50% of the ERV's maximum CFM to keep boost capacity and efficiency in reserve.
New to this? Start with one row per room. Bathrooms, kitchen and laundry get exhaust; bedrooms and living spaces get supply.
Project details
These print on the permit sheet and travel with the Excel file.
Rooms
Volume and tube count fill themselves in. Up to 19 rooms — the same as the workbook.
| Floor | Room | Areasq ft | Heightft | Volumecu ft | # of Tubes | Flow Rate (CFM) | Remove row | |
|---|---|---|---|---|---|---|---|---|
| Supply | Exhaust | |||||||
| Totals | — | — | — | — | — | |||
0 of 19 rows used
How this table is worked out
Volume
Each room's floor area multiplied by its ceiling height. Leave the height blank and an 8 ft ceiling is assumed so the total is never zero.
Air changes per hour
Total exhaust airflow divided by total volume, times 60. A result of 0.30 means a third of the air in the house is replaced every hour.
Tubes
Supply plus exhaust for the room, divided by the 15 CFM a single 3-inch tube carries. One tube feeds one register, so the tube count is also your register count.
Manifold size
Total supply divided by 15 CFM per port, rounded up to a size that is actually sold — 4, 6, 8, 10 or 12 ports.
Design CFM rate
Only valid when supply and exhaust match. An ERV moves air both ways through one core; an unbalanced system pressurises the house and loses efficiency.
ERV size
Under 130 CFM the MAX XTR 180 serves the house. From 130 to 300 CFM it is the 267. Above that, plan on two systems.
Local codes vary. Check the figures against the mechanical code adopted in your jurisdiction before submitting a permit application. Not sure? Send us your plans and we'll size it for you at no charge.