Actually, it needs to be powered whenever the hood is turned on. I would ask if it needs a 24v transformer or not. Those are easy to obtain.

I would assume you have a variable speed hood? Mine is 1500 cfm. I certainly don't use the make up air when the fan is on low speed.

The damper has a 24 volt motor. When the damper is fully open, it has a set of contacts that turns on your makeup air fan.

I would put the makeup air on a switch ( a regular light switch) by the range. You can buy a 24 volt transformer at HD or Lowes. The transformer will mount in a 4" square box. Install the 24 volt transformer in the attic - the primary side of the transformer is 120v (wait to connect).

The secondary side is 24 volt - usually 2 screw terminals. Connect one of the thermostat wires to the secondary - this will go down to the switch - this is the 'switched 24v power'.

In the attic connect the switched wire to the damper, and connect the other secondary from the transformer to the other 24v connection on the damper.

It appears some terms were lost and/or confused in translation,here is what the hvac guy may have said if indeed the damper has a 24 volt motor. Tap into LOAD side of hood switch connecting b&w to b&w on a 24v transformer. Connect 24v transformer output to the wire running to damper. For this arrangement you should see what the va (watt) load the motor is and make certain the transformer is rated at least 150% of motor rating. I will repeat "the above is correct ONLY IF the danper motor is 24 Volt". You said somthing about relay and you notice i didn't include a relay with my instructions. SOOoo,make certain you are telling us exactly what he told you because a relay would lead me to believe the damper motor is 120 volt with a 24 volt control circuit which has transformer already installed awaiting a relay and 120 volt power supply. In that case the hanging wire would test 24v when hood is turned on.

You need to get the schematic from the damper. It's probably in the inside of the cover - or it came with a schematic - if the installer didn't throw it away. Otherwise go to the internet and get the schematic.

I would not wire the MUA to the hood power - as I've said you don't want the MUA on if you have the hood at a low cfm.

The hood unit is a LIN34 and it adjusts from 300-600cfm.
The line (hot) comes into a junction box on the hood fan and then the blower has a plug that connects to it.

As far as what the hvac guy said, what I posted in the original post is copy/paste from the instructions he left. He said relay but perhaps he meant transformer? I will contact them to ask if relay or transformer.

tx

I don't see an easy way to tap into the load side once the switch is enabled. I like the idea of a separate switch (wall) to turn on as needed but will have to see how easy to route power to it.

First I would get all the instructions that came with the MUA and the damper, etc. Hope the guy didn't throw them away.

It will depend upon you local codes but 300-400 cfm doesn't need make up air. Your 600 cfm would probably do with an open window.

I would put it on a simple SPST switch. You can turn it on and off at will.

On my 1500 cfm I have a pressure switch in the exhaust duct (Grainger - about $60). I set it to make 'the MUA circuit' when the exhaust fan was a little above medium speed.

I also installed a variable speed fan switch on the MUA.

With additional info, I can draw you a schematic, depending upon how you want it set up.

It appears the damper is 24v so that means needing a transformer? They said the damper is 6VA.

So, you said it is something I can pick up from HD and put in a box with a switch cover?

I'm thinking along same lines as Geoffrey "look at all the paperwork" then if you still have questions,post the info or email it to Geoffrey so he can draw a schematic for you.
If you wired your house you already know (A) the 6va damper should be fused one half to 1 amp and (B) if fuse holder isn't built into the damper,use an inline fuse similar to that used in automotive.
If I was going with a stand alone switch,I would supply it from load side of hood switch so damper couldn't be unintentionaly left open while hood is off. I don't understand why you are unable to locate and tap black wire between switch and plug.

@Geoffrey I responded before reading your post. I will ask about the paperwork because nothing was left at the house.

Code requires MUA above 400 so when on high, it would be required 'technically'.

Using a toggle switch and tucking it away on the hood is a bit cleaner than a wall switch ... and using a pressure sensing switch is even more intriguing.

I did wire the house but I'm not an electrician. I learned alot in the process but definitely seek the guidance/wisdom of you smart people.

tx

Makeup air should be interlocked with the hood, not activated by a separate switch. it is a safety issue. What if someone puts the hood on high and doesn't turn on the MUA, either because they forgot or they didn't know?

Here's the pressure switch I used (from Grainger):

This transformer can foot mount or mount on a box: