CL (dnoyeB) Gilbert wrote:
I am adding an auxillery fan to my HVAC. Actually two; One for my
computer room where the computers keep it rather toasty, and one for my
bedroom where the large number of windows keep it rather 'outsidey.'

I want to start with the bedroom one which is already installed in the
duct. I have run the wiring to the furnace area. 3 wires; power and
ground. Wiring nailed into ceiling floor joists, etc.

The question is what to do now? I want to use a relay so the aux fan
comes on when the HVAC blower is on.

So I have a relay that I need to mount somehwere. Its coil will be on
the HVAC control which IIRC is 24vac. Its NO contact will connect from
a switch which will be powered by the main furnace switch next to the
furnace. This is lookin nasty though as it seems I need to install some
connectors on the wires and plug them in, or maybe find a relay with
screw terminals!?

I am assuming this relay needs to be mounted to something and in some
kind of enclosure? I intend to put it in a normal switch box mounted to
a floor joist in the ceiling.

I am assuming the wiring needs to be in metal pipes? Also does the
24vac need a seperate pipe from the 120vac wiring?

Is there a simpler way?

This is a pretty small fan which I believe is 1A load for 120VAC. I'm
an electrical engineer but not an electrician. I don't know the rules
but I'd like to follow them if I can learn them.


--
Respectfully,


CL Gilbert

As others have said, I would look first to rebalance the HVAC system,
etc. before using a booster fan.

If the final conclusion is to use a booster fan, one alternative to
adding a relay is to put an airflow sensor in the duct. That way, once
the heat or A/C comes on and starts blowing air through the ducts, the
sensors picks that up and will close a NO switch. All you have to do is
run 120 VAC to the sensor, and then he switched 120 VAC to the booster
fan. No electrical connections to the HVAC system are required.

My furnace has a relay on the circuit board for activating auxillary
equipment, such as a humidifier that I have for during the heating
season. Check the manual to see if you have this and if it will
support the load your fan will put on it.

If you have to add a relay, then see if you can add an electrical box
right on your HVAC system. I was able to do this on my system (just to
make some connections, no relay was required...) by just putting a
regular junction box and attaching with standard electrical fittings to
the electrical box that was part of the furnace. It already had
knockouts for just such a connection. I know nothing about this, but
if I was in your shoes I would call an HVAC place and I'm sure they
have some sort of relays for just these sort of purposes that they
would be willing to sell you.

Ken