Install a double pole switch for the fan.
Use one pole for the exhaust feed and the other pole for the make up air.

If you use 2 separate circuits, I would use a double pole breaker.

Airflow switch in the vent output? You might ask an HVAC guy (in their forum) it's not so much an electrical problem.

Here is a link that might be useful: http://www.dwyer-inst.com/Product/AirVelocity/AirFlow/Switches/Model530

I installed a pressure switch, in the duct, near the roof exhauster (1500 cfm).

This switch controls when the make-up air in on.

I have the pressure switch set so that the MUA doesn't come on until it's about over 400 cfm.

Check Grainger under "duct pressure switches"

Thanks Geoffrey! Great idea! Question though- how did you get it 'tuned' to only kick-on at 400 cfm? Blower curve or a test of sorts? Also, has the pressure switch been reliable?

I guessed when it was about 400 cfm. Standing at the range, you should be able to sense when it begins to move a lot of air.

The switch I purchased, has a 1/4" compression fitting for copper tubing. The tubing is inserted into the duct.

The adjustment is a screw ( I believe it's simply weighted ). I set it up about 6 years ago, and it works well.

Not to get complicated...........

Since I had the kitchen gutted, the walls were easily acessible:

By the range, I added a 2 gang electricl box: For a MUA axial fan speed control, and a switch for the MUA duct heaters.

We have found you don't always want that much air blasting into the kitchen.

Duct heaters, we're in Minnesota. I found a double throw / off in the middle levitown decorator switch. This way we can get no heat, low, or high heat.

In my previous house I had a humidifier that was turned on by air flow in the ducts. There was a lightweight flapper mounted in the air plenum and as soon as the furnace cycled on it would move the flapper up and turn on the humidifier. There may be a similar type of switch you could use.

Bruce