Duct Design has been a bit tricky. The information about designing a suitable duct system for ventilation only has been difficult for me to find and interpret. Most duct design documentation is for heating systems and the fan speed numbers don’t seem to apply to an ERV with a variable speed fan. I was able to adjust a duct design from another ERV example to illustrate the layout for the supply ducting for our system.
We are planning to limit the supply ducting to the remodeled area. Some of the air will blow back to the rear bedrooms from the living room while the returns will be located centrally in the pantry closet where the ERV will be located. Ventilation air will be released close to the ceiling and drawn in through stale air returns both above the ERV. The ERV comes with 6″ duct connections but to distribute air in our rather direct system, we will have to increase that to at least 8″. My first design was for 8″ supply and 6″ distribution but that won’t supply enough air except at the highest fan setting.
I learned this by asking the Ultimate Air folks what the static pressure should be in the system. That total static pressure determines how much air can be lost in the duct work itself. No fan can overcome a static pressure that is greater than its design capability. Although that seems intuitive, determining the fan’s capability at a certain duct length and diameter requires duct length tables.
The documentation for the RecoupAerator says not to exceed 7-8 inches of water column (in/wg) but that is the very highest resistance allowed to deliver air at the highest speed. For lower speed they recommended we keep the static pressure at 4 in/wg. So back to the drawing board.
Another confusion was the fan performance curves often published to determine sizing for a ventilation system. I could not understand the RecoupAerator curve as it was basically flat. Another question to their experts revealed that is because the fan is variable. It puts out steady air pressure all along the performance continuum.
The typical fan curve shows the delivery of air changes as the length of the duct and the static pressure i.e. resistance in the ducts increases. So the fan delivers fewer cubic feet per minute of air the higher the static pressure and the longer the duct work. But the RecoupAerator from Ultimate Air has a continuously variable fan that works harder as the duct length increases and the static pressure increases but only up to .8 in/wg at 200 cfm. If there is more resistance the fan cannot deliver 200 cfm. Also this curve doesn’t show the static pressure for lower speeds which works the same way. If you want to run this fan on low speed and deliver air through the ducts the static pressure can’t be more than about .4.
So although using .8 in/wg for the duct system works with 6″ ducting in the run I planned above, the system would only deliver enough air at 200 cfm or the highest fan speed.