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
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.
With all due respect:
Adding aux fans is the LAST thing you do.
If you have a cooling problem due to airflow, the fans may mask it, but they
will not cure it.
You can only get X-CFM down a duct of X-Diameter at the proper static
Velocity of the air has to be taken into consideration and if your aux fan
will not handle what you need to equal the proper airflow to deliver enough
BTUh back to the unit, then you have wasted time.
Normally, air flow issues are due to poor duct design, or proper duct design
based upon the original use of the home, or room, and if you have added a
heat load to it, then adding a fan to try to increase the air flow to the
room prob wont help, and even if it does, what have you done to the balance
of the system?
Well this is all I could think of. As I stated, the bedroom's
temperature is off in my opinion due to higher heat loss than other
rooms due to so many windows. It also has a vaulted ceiling.
The computer room stays HOT, even in the winter due to added heat
sources within the room. The rest of the house is pretty nice,
including even the basement. I can't keep the doors open since my Son
would enter the room and put his signature on it...
So increasing cooling in summer and heating in winder to the bedroom is
my objective. So I added a fan...For the computer room I could drill
holes in the wall, and run a radiator outside and cool my computers with
a water cooling system. But im hoping for something less obtrusive.
The master bedroom is the farthest room from the furnace, being on the
2nd floor and other side of the house. The computer room is on the 2nd
floor, but basically above the furnace.
Perhaps for the computer room I could put the fan on the return?
I don't know how hot your computer room gets.
In my home office there is a 2k VA Tripplite UPS, 4 tower PC's, 2 of
which run 24x7 along with ham radio gear that runs all the time and big
old 20 amp dc power supplies to run the ham gear. Add to that 3 CRT
style displays a laptop computer, battery chargers, printer, 2 routers,
a switch, a couple of police scanners, and a small Merlin PBX. Room size
is 9 x 15.
A/C in room next to it but no A/C duct in this room. A ceiling fan in
the computer / office seems to keep the air mixed up enough to be
comfortable even when it is 98 degrees outside.
Thats amazing. Im running an AMD XP3200+ and an Ati 9700Pro graphics
card. This computer is a heat box. Its the real heat generator and the
room is hot but bearable before I turn it on. Laser printer is there
too but the heat is not too much.
Im seriously considering the 'Reserator' silent water cooling system to
get the heat out from under the desk.
I should also mention this room is 1/2 over the garage, and the garage
gets HOT as well.
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.
Get an Aprilaire current relay #50 or #51. It clamps around the common
blower wire and senses the blower motor current. One switches 120
volts, the other switches 24 bolts, I don't remember which is which.
Each relay will carry 1 amp.
That being said, the best solution would be to run a load calculation
to determine how much air each room needs and resize your ducts to get
the right amount of air in every room. Note that is not a job for an
amateur. It would be the best solution to your problem.
Most "booster fans" are propeller fans. They do not produce the high
static pressures needed for duct systems and are not very effective.
You may not want to run it that way for the computer room in the winter
since it is too hot there anyway. In the summer run the computer room
fan 24/7. Just plug it in to a wall outlet.
The bedroom fan unit can be on a relay. The relay can be mounted
inside the furnace control cabinet for safety.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.