VAV Thermal Overload Protection

[infinity]

I was looking at this installation on my job. There are hundreds of VAV's and someone has specified that each unit needs to be fed through a thermal switch with overload protection. So the other day I opened one up and found that the damper motor is actually 24 volts. The thermal switch with its integral overload feeds the 120/24 volt transformer in the unit. What could possibly be the reason for this besides bad engineering?

 

[Jraef]

Besides bad engineering?

I got nothing... so long as you include "lazy" in the definition of "bad". Too lazy to look at what it really is.
Most of those little actuator motors are "stall protected", meaning you can lock the rotor and they just sit there like that forever without burning up.

 

[infinity]

I asked around and someone told me the protection was so that the thermal switch (which has a small ~1 amp heater) would open if there was a fault on one of the VAV's and not trip the branch circuit OCPD.

 

[victor.cherkashi]

Sometimes we show the thermal switch on design drawings as worst case scenario (to cover cost if required) when we don't know specification of VAV box. Usually it stay as is on constuction set. It is cost saving opportunity for smart contractor.

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