Electronically controlled duct damper wiring question

mpasquarAugust 21, 2013

Hi garden web wiring folks,

Hoping someone can help. I'm installing a commercial-style kitchen vent hood with a powerful blower. The code requires a make-up air system that will allow fresh outside air to be let into the kitchen whenever the vent hood is on. So the typical installation is a separate inlet duct with an electronically controlled damper (in this case normally closed; open when energized). The damper runs on 24v... Transformer is included with the kit that I bought.

I'm stuck on how to tie damper 'open' with hood 'on'. The hood has an on/off switch and a variable speed control. But it is all contained in an enclosure/PCB device.... So no chance of splicing in downstream of the on/off switch. The only wiring I can get to is the output from the hood - which is a standard 14/2 wire that connects up the fan. The fan has no separate control - just the power coming from the hood. I'm guessing the fan speed is controlled by varying voltage.

Any ideas on how to link the hood output power wiring to the damper transformer? Should I just place an standard 14/2 box in series with the motor fan wiring and hook the transformer to that? Or is there a more elegant solution?

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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.

    Bookmark   August 22, 2013 at 6:30AM
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Ron Natalie

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

    Bookmark   August 22, 2013 at 8:00AM
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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"

    Bookmark   August 22, 2013 at 1:13PM
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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?

    Bookmark   August 22, 2013 at 3:06PM
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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.

    Bookmark   August 22, 2013 at 4:45PM
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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.


    Bookmark   August 25, 2013 at 8:04PM
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