What does your HVAC subcontractor say?

He says it’s fine. I see opinions elsewhere from pros who disagree.

Do the pros who disagree know as much about your project as the pro that says it's fine?

Hey mark, what’s the max CFM through a 4x12 toe kick? If you can’t a seer that you really aren’t being helpful. I’m not being rude but have noticed you post often but rarely provide actionable help.

"what’s the max CFM through a 4x12 toe kick?"

Zero*.

Before you jump on the bandwagon of a "pro" that does not know all the facts, make sure all the pertinent information is known by all those you are considering to rely on. Advice made on assumptions is many times bad and always risky. I rarely provide actionable help because adequate information is rarely provided as is the case here.

*If you want air to flow through your 'toe kick' put a vent in it, and ask the HVAC professional that knows the duct size(s) and path that feeds the vent .

The cross sectional area of 96 square inches offered by the 2 intake vents are fine for what you have.

if your 1200 cfm exhaust fan uses a 8" round exhaust duct it has 50 square inches of opening, and technically the makeup duct would only need to have the same cross sectional area as the exhaust duct. But because all ducts have a a certain amount of calculated resistance to air flow and the fact air will be pulled from sources of least resistance doubling the cross sectional area will give a air source of least resistance over other sources.

Opening a door will easily over ride the makeup ducts because it's a source of least resistance for air but the whole point is to make sure the house isn't under negative air pressure when the exhaust is running.

Just to provide more details, the hood is 1200 CFM and uses 10” ducting. It uses an inline fan and vents through the roof.

The makeup system is a Fantech MUAS 1200 - which uses 10” ducting into a filter box, fan and heater. It comes out in 10” over to the kitchen where it splits between the 2 toe kick vents.

I really like the HVAC contractor and I’m sure he’ll tell me to be quiet or offer another solution. I’m wondering if we don’t just cut in another floor vent across the room to bring additional air in. Not worried about pulling in cold air since it will be tempered.

You have a 1200 CFM exhaust through a 10" duct interlocked with a 1200 CFM makeup unit from a 10" duct branching out with two 4x12 ducts.. got it, The 10" duct has a cross sectional area of 78 square inches and the two 4x12 ducts have a cross sectional area of 96 sq. inches. You have no problem, it's sized correctly and adding another branch of 4x12 duct would be a waste of time and material. Your installer did it correctly and don't mess with it.

FYI, There is no answer to "how much CFM can a 4x12 duct handle". A duct that size could pass several thousand CFM with no problem, it only requires a correctly designed blower and the horse power to force the air volume through. This of course will drastically increase air velocity with the noise that goes with it.

It also requires a tolerance and ear plugs to listen to a screaming banshee roaring wind through a small duct, if you force too much CFM through it.

Without a specific grille chosen, use 70% to calculate the net free area of a metal grille.

What is 70% of a screaming banshee?