ABOVE CEILING DISCONNECT FOR VAV BOXES

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Ragin Cajun

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Upstate S.C.
I used to spec BUSSMANN SSU5 for this but I can't find them in the Bussmann site.

What do you use to serve as a small 120V fused disconnect for VAV's? The VAV's we are using con'tr have a fan, only a small 30VA transformer integral with the VAV. We plan on feeding 5 - `10 VAV's off a single 20A circuit.

Thanks.

RC
 
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You could put a single 120/24vac power supply near the electrical panel and then feed 24 volts to each of the VAVs. We install 24/24 isolation Transformers in each VAV with a single fuse on the primary side of the Transformer. This fuse is really used more like a disconnect since most Transformers less than 50 VA have inherent protection. It does provide protection as well, it's just not really required.
 
Ragin Cajun said:
I used to spec BUSSMANN SSU5 for this but I can't find them in the Bussmann site.

What do you use to serve as a small 120V fused disconnect for VAV's? The VAV's we are using con'tr have a fan, only a small 30VA transformer integral with the VAV. We plan on feeding 5 - `10 VAV's off a single 20A circuit.

Thanks.

RC
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Why do you feel you need a fused disconnect? Or is it just preference? We use a standard 20A 1221 Hubbell switch all the time.
 
LarryFine said:
Okay, nobody has ever accused me of being shy, so I'll ask: What the heck is a VAV? :?
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VAV stands for Variable Air Volume as a general concept in HVAC, where the system supplies air into ducts at a constant temperature, and the temperature in individual rooms is controlled by letting in (and exchanging) more or less of that air. A VAV box is a motorized damper that makes that happen. What he is talking about is the way power is delivered to that VAV box, which is usually 120V (sometimes 277V) that then goes to a plug-in power supply that powers a 24V little reversing gear motor that moves the vanes inside the duct. The motors are actually cool little units, I have used the cheaper simpler ones (without all the direct digital control stuff) for other high torque-low speed needs.

Here's a somewhat simple one, they can get a lot more complex than this though.
c9ed2b03-2e22-4678-bf40-e636cca25624.jpg

Motor
vav motor.jpg
 
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I just use a single pole 20 amp rated switch. The only reason I use a 20 amp one is specs. A .35 cent hardware store 15 amp 120 volt switch would work fine in the same application, but spec is spec.
 
There are 2 types of VAV boxes in my buildings, one is a series unit, fed w/24v., only power needed is for the damper motor. The other is called a parallel box, these units have the same 24v. damper motor, but fed with 277v., which powers the reheat (duct heater) and a fan, which operates on heat only, with a separate non-powered damper.
 
VAV Specifics

VAV Specifics

macmikeman said:
I just use a single pole 20 amp rated switch. The only reason I use a 20 amp one is specs. A .35 cent hardware store 15 amp 120 volt switch would work fine in the same application, but spec is spec.
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I would also ask why the fused disconnect is needed.
I have worked on all manner of VAV boxes, also depending on the configuration known as Fan Powered Boxes. IN these cases I would assume the disconnect has to be motor rated. I detest fuses on fan drive gear, but its often seen on the electric heat side.
They come in all types of configurations. I have seen 115V-1, 240V-1, 240V-3,277V-1, 480V-1 etc.
The FPBs will mostly have fan drive motors of the typical squirrel cage type.
The dampers are historically pneumatically controlled but with DDC there is everything anymore.
In Exterior zones in Multi-Story they can contain resistance heat packs.
 
The VAV's we have are 120V with a 30 or 50VA transformer to run the damper motor. We plan on putting 5 - 10 on a circuit to save $$'s. The fuse is desired to avoid a single failure from shutting down the whole circuit. Code required a means of disconnect for servicing.

On this project, none have strip heat.

Yes we know these small transformers are generally self limiting, but this is a hospital and the owner wants to play it safe.

They are available at 24V but voltage drop is a killer.
 
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