How to install a furnace booster fan on the cheap

Picture of How to install a furnace booster fan on the cheap

Our master bedroom is always either cold in the winter or hot in the summer. The fact that the builder messed up by installing just one register in the room and the room itself is right above the garage doesn't help either.

Using a digital thermometer I was able to determine that the temperature of the air coming in was half that of the hottest register in the house (obviously this is for the winter case), and the airflow was barely there.

My solution was to install a booster fan.

The process was not all that complicated and this job can be accomplished by anyone with some basic electrical skills and some tools you can buy at local dollar store.

Was it worth it? The answer is YES. Can it be done better? Yes but not by much. I'm sure some people out there who are familiar with how a furnace works will be able to come up with a better way in regards to how to control the booster fan, like using a pressure switch or choose other outputs on the controller furnace board. In my case, I just went with what worked for me and what I felt was safe from a functional point of view and liability point of view.

If any good recommendations are made here (and they are easy to implement in my current setup) I will try to include them in the overall design for other users' benefit so feel free to contribute.

I should also mention that this installation deals with electricity and electrical connection which means you might need a permit but most of all YOU NEED TO PUT SAFETY FIRST also, don’t forget your safety goggles.
Remove these adsRemove these ads by Signing Up
F22-Raptor (author) 4 months ago
I agree, Bill. When I did this, I couldn't figure out which terminal was the one for blower, hence the heat only option. I think in one of my comments I mentioned which terminal to use. Even if the booster fan comes on early, it is not a problem. It's not on for too long and it doesn't temperature. Maybe at some point when I have some spare time I'll do it right, that way, it will kick in when the AC is on as well.
Great write up! A couple of comments. As for picking up the relay control from the thermostat, your booster fan will come on when the tstat calls for heat. Not when the furnace finally warms up and is ready to deliver heat. Thats why the blower does not spin up right away. It takes time for the furnace to get hot. Otherwise the blower would be moving cold air. As for changing the fan speed with a dimmer, it might work to sme extent, but not properly, and will drastically reduce motor life. Motor speed on these is based on the frequency of the ac power coming in. Due to the laws of nature, reducing the voltage will just increase the current, and will overheat the motor windings and maybe your control relays and wiring.
csmillie1 year ago
Thanks for the great guide on installing the booster fan ( something I'm considering ). The only question I have is around your relay wiring. Did you consider using the R and G thermostat wires with a low voltage relay? It would seem to be easier to pickup the thermostat's request for the fan regardless of heating or cooling...

Also how to you find the noise? Did you need to insulate around it at all?
F22-Raptor (author)  csmillie1 year ago
I'm not sure how the thermostat interacts with the control board using those two wires. Maybe and HVAC guy/gal can help here. I was concerned that my adding a relay in that particular circuit might mess something up. If you look into it and it turns out it is a better option, please do share the info here. I know there is another output on the board that would power the relay regardless of setting (cold/heat) - I forget what it is called now. The other thing is that the thermostat, tells the board when the threshold has been reached/crossed but the board really turns the blower fan off a little while later to fully take advantage of any hot/cold air left in the system.
Personally, I'd still use the control board to turn on the buster fan.

The noise level is a little higher than what I want it to be but I got used to it. I am talking about the noise coming from the register. There is almost no noise where the buster fan is installed.
I can't tell is the register noise is high because of the increased airflow (I'm sure that is a factor too) or because of the noise coming from the motor through the ducts. I guess, if you want to eliminate any telegraphed noise, you could use some rubber connects (Home Depot sells them) to connect the motor to the ducts and avoid any motor noise or vibration.

One of these days, I'll measure the noise level and post it here. If I had to guess, I'd say it is twice as loud as an old fridge, not as loud as a regular dishwasher though.

I hope this info helps. Let us know what you find out about the R & G wires.
pjinst1 year ago
Just in time as I'm working on something similar. I've laid out my ducting with the booster fan and looking how to turn it on when heat/cool is called. So, just to clarify, the GREEN wire & WHITE (common) wire can actually energize a 24VAC relay to switch ON the booster fan? I've just inspected my thermostat's wiring and notice that the common wire is not connected (but present). Is common wire normally not connected with these digital thermostats?
dzip1 year ago
I bought exactly the same duct booster. The only problem I have with the unit is the noise. I'm considering installing a dimmer switch in the power run instead of a normal switch to see if there is a good trade off between noise and air flow. 250 cu ft per minute is a lot of air flow considering I'm heating a room of about 1200 cu foot. Thanks for the instructions.
F22-Raptor (author)  dzip1 year ago
That is a great idea. Be careful not to turn the dimmer all the way down. If the coils in the motor don't get enough juice to turn, you might short them.
seabirdbill2 years ago
Sorry for the delayed post. I finished wiring the fan using a 24VAC relay and the GRN (fan relay) and C(Common) leads from my thermostat. I did the actual wiring in the air handler since the duct and installed fan were located nearby. I installed the duct fan relay in a box near the fan. I pulled 120VAC from a nearby crawl space outlet. This serves a sun room that is at the end of the trunk. The thermostat gets it power from the 24v transformer in the air handler. The thermostat gets power from the transformer (the batteries are for programming backup and screen lights) and when the set temp calls for heat or cool a relay in the thermostat activates sending 24VAC via the GRN wire to the air handler fan relay. I connected the 24VAC duct relay to the GRN and C so that when the thermostat called for fan the duct fan would also activate. BTW, I have a single speed heat pump fan. This has been working great for me and the temperature in the sun room is tolerable. I got the 24 VAC relay from amazon. I included a scan of a thermostat schematic but it may not be readable. If you Google aprilaire 8346 thermostat wiring and look at page 9 of the pdf you will see what I tried to explain.
F22-Raptor (author)  seabirdbill2 years ago
Cool, thanks for the update.
seabirdbill2 years ago
Great instruction! Dont know if it was mentioned anywhere but I plan on installing the fan using a 24 VAC relay energized by the GRN fan relay from the thermostat switching the 120 VAC for the fan.
F22-Raptor (author)  seabirdbill2 years ago
Hmmm, I'm not sure how that is going to work out. I have a digital thermostat powered by 3 (I think) AA batteries. Energizing the replay from those batteries would drain them quickly.
I don't know enough about the thermostat and the control board from the furnace/AC communicate. From your comment, it sounds like there is power control on a 24V circuit coming from the thermostat.

Please post some details, it might simplify my solution and make the idea more appealing to people who are not comfortable "tapping" into the furnace/AC control board.
ewaldoh2 years ago
The reason for two speeds is that it is not difficult to get warm air to rise up to the upper rooms so a low speed will work normally.

Cold air from the AC is denser and not so easy to lift to upper rooms. Thus the higher fan speed
Spiff735 years ago
How greatly has that improved the temperature in your room?
F22-Raptor (author)  Spiff735 years ago
I checked the temperature last night with a digital thermometer (point and shot type) and it was ~106F/41C which is up about 30-40% from when I didn't have the fan. The air flow is almost 100% increase.
I compared the temperature to two other registers, one very close to the furnace and one a little closer to the furnace than the register in our bedroom. Here are the readings for the two:
- register very close to furnace: 122F/50C
- the other register 90F/32C

The last register (90F/32C) should be warmer than the one in our bedroom given that it is a shorter length and does not go through the space between the garage ceiling and bedroom floor.

I will update the Instructable with info re. performance.
I have the exact situation as you had with a cold bedroom above a garage.
I would like to add one of these booster fans to the heat duct as well as to the cold air return duct. ( I added a cold air return as there was none previously. However the pull is very weak).

Is it possible to connect both to the fan blower?
F22-Raptor (author)  vinnyd133 years ago
The short answer is no. The two air pathways are separate plus you can't change the direction on the motors on demand. When you mount them you make sure they pull or push based on your needs.
It sounds like you will have to use a separate blower for the cold air return but have both blowers turned on and off by the same relay.

I'm not sure you really need one for the cold air return. I suggest you talk to and HVAC guy and see what he says.
Maybe other people here can shed some light on this.
Yes, I was going to use two booster fans. One for the heat duct, one for the cold air return. I guess I could use a pressure switch for the heat duct fan, and wire the one in the cold air return to the main furnace blower.

The cold air return that I added barely pulls at all, so it needs something to increase suction. Thats why I was thinking of a booster fan, in reverse.
F22-Raptor (author)  vinnyd133 years ago
Make sure you use a relay. That way you keep the two circuits separated electrically and avoid liability to a certain extent.
(removed by author or community request)
Lucky for you I still had the receipt with me.

Here is what it says on it:

Relay DC 110V 1P2T 20A QT BRKT (not sure about the B in BRKT) $7.50

Please note that it says DC so you might want/ to look for (or need) an AC one.
HI F22-Raptor,

Where on earth, but more specifically in the GTA, can a mere mortal purchase the relay you used for this project? Or its equivalent?

Eastern Refrigeration won't sell it to me because I'm not an HVAC tradesman.
WWG Totaline won't sell it to me for the same reason.
Seriously, I'm standing there looking at it, cash in hand, sale. I couldn't even grease the guy. I'm assuming it's not some critical component in a nuclear device, so what's the problem I wonder.

Ebay sellers want US$30 just to ship it to me so I'm trying to avoid that route. Certainly someone in the GTA can supply this to a DIYer.

I've also tried Westburne-Ruddy, Nedco, and TES. No luck.

Thanks for any help. Other than the relay, I'm ready to go.

That is really strange. It's just a relay. When I got mine form Eastern Refrigeration I just walked in, told the guy I was looking for a relay and he got one for me.
Your best bet would be to go to Sayal ( and go to their relay section. I got 3 relays from the Matheson Blvd. and Dixie Rd. store for about $7-8 about 3 week ago. They have lots of them. Your biggest problem will be finding the one you want. They have one aisle just for relays 5V, 6V, 12V, 115V, 240V etc. They also have solid state relays but they are more expensive and for this kind of application it doesn't make sense to use them. While you are at Sayal look for quick connect terminals, shrink tubing etc. You should buy 2 relays (heat and cold) and be done with it. When I did mine I wasn't sure how the whole thing was going to work out. Next year I'll do the cold relay as well.
Let me know if you need help, I work 10 mins from Matheson and Dixie.
Sayal has a store in Vaughan as well (check out their locations on their site), that one is 10 mins from where I live.
 Just curious, but why did you use a relay?

Couldn't you just drive the booster fan from the HUM connection point? Isn't it only energized with 115V when the blower motor is on?

That's a great question. It's something I should've covered in the Instructable. There are two reasons:
1. My controller has separate outputs for Heat and Cold and from what I'd read it sounded like there are separate windings in the blower motor (2 different speeds I guess). Later on, this will allow me to install a second relay that will be energized by the Cold circuit, that way, my fan will kick in when the AC is on as well.
2. Liability, if my fan is totally separated from the control board via a relay, then I don't have to worry about it damaging the control board if something happens on the fan side.

Also, the relay option will allow other people to control fans that are not 115V, say 24V. Obviously they will have to use a separate transformer or use the one that's already in the furnace but from a control point of view, using the relay keeps things segregated and safe.