How are commercial exhaust hoods wired for control where there's one unit for exhaust and another for incoming fresh air. Would the on/off switch be wired for them to come on at the same time or switched seperately? BTW, the one I'm looking at has a 3/4 HP exhaust and a 1/3 HP return motor. Thanks in advance. Andy
Andy, I usually set a control cabinet or control the motor starters / relays with a separate 120v control circuit to pull in the starter coils at the same time with one switch. In my area the incoming air is usually heated depending on outside temp as well so there is another coil.
Here is the rub. Most Fire Dept Inspectors in my area want the makeup air shut off and the Exaust Hood to start in any alarm condition in the space. You can drive a separate relay using aux power and alarm contacts in the FACP or have the FACP drive a control module if it's addressable. I have a control drawing I could send as a JPEG file if you would like.
I thought the exhaust should not run without the make-up, because the exhaust alone can draw air down the chimney. I think the commercial make-up air units have an air proving switch, that locks out the exhaust.