View Full Version : Multi-speed fan rewiring

Dec 7, 2006, 07:24 PM
Our house has somewhat high humidity in the winter and I have heard running the furnace fan 24/7 can be helpful to keep the moisture down.

Now, looking at what I have - the blower has a four speed motor. All wires are connected to the control unit. Is it as simple as wiring in a manual switch on the desired feed (ie. Low speed), or would that possibly cause problems with the control unit? I have no problem doing this, but am not familiar with how the control unit works. Is there a way to make the control unit default to being on low speed 24/7? It is currently only set up to run on one speed.

If it helps at all, the furnace is a Clare MEMC-75 and the control unit is a White Rodgers 50A50-215.

My T-stat does have an option to run the fan on auto or always on, but this runs the fan at its normal full speed - which is medium-high (3/4) according to the manual. I would like it to run on just low at all times (to reduce noise and power consumption), then kick on to its normal MED-HI when required.

Any help would be greatly appreciated.

Dec 7, 2006, 07:57 PM
Yes. Just identify the low speed lead and run a wire from the incoming power to a switch, and connect it to the other terminal.


It is not that simple. See posts # 3 & #4.

Dec 8, 2006, 05:50 AM
Yes. Just identify the low speed lead and run a wire from the incoming power to a switch, and connect it to the other terminal.

Thanks for the quick reply labman. So just to clarify for my own sake - I can tap of my main line source in the furnace, run a wire from there through a switch to the low speed fan wire (currently "parked" @ the control unit)?

Will this be fine for the motor to be receiving a lowspeed input at the same time as a med-hi input when the furnace actually kicks on? Or should I wire a relay in between to cut the power to the low speed any time the med-hi speed is on? (I would prefer not to bother if this isn't necessary just to keep it simple).

Dec 8, 2006, 10:55 AM
No! It is not that simple. You have to use a two pole relay and that won't be easy to wire so that the heat/cool and fan all work correctly. First of all it is set up to run at 2 fan speeds, heat and cool, fan on uses the cooling speed, probably High (black wire) On a call for heat, if the fan is set to on, the fan will stop the heat exchanger warms up then the fan comes on usually medium/low Blue or yellow wire. If you apply current to 2 windings at the same time you will burn up the motor. Have you thought of buying either a portable dehumidifier or a whole house humidifier? Are you on a basement?
Keep in mind fan settings are very important for your air and heat will work right. To fast on a/c it won't dehumidify good to low and the coil will freeze up. When heating if airflow is to low you will overheat the furnace, shorten the life of the heat exchanger and hve problems with limits and no heat issues. ERV's are also good in the wintertime to bring in dry fresh air from the cold outdoors.

Dec 8, 2006, 03:38 PM
Thanks for the clarification guys.

The one thing that would make it a touch easier to wire in with a relay and switch is that I would likely only ever use this in the winter so there wouldn't be an issue with the cooling (HI) speed. It would however be a manual switch though so you have to rely on remembering to turn it off (although the blower constantly going is a good reminder).

Yes, the fan runs HI for cooling, and MED-HI for heat. I have thought of a portable dehumidifier, but wasn't sure how well it would work in a two level house (w/basement) as all the windows gather a lot of condensation in the winter. A couple winters ago, 1/4" thick ice actually formed and had to be chipped off the inside. That only happened once though - normally it is just a fair amount of condensation.

It sounds like there is no easy way to do what I want to do, and that it isn't a common or standard practice then? Running the fan 24/7 has almost completely eliminated the problem, but I am sure I won't like my next power bill. I may have to look into an air exchanger or whole-house dehumidifier like you say.

Dec 8, 2006, 05:11 PM
Since you are getting condensation on the windows, perhaps you should be looking at upgrading the windows. Of course, the heating season is no time to be replacing windows or even adding storms. Your nose and wallet might both be better off with higher humidity and windows that steam up at that level of humidity. They will reduce your utility bills. The other solutions will raise them. Put your money into better windows or storms.

If running the fan helped the problem it would only do so by increasing the replacement of warm, moist inside air with cold, dry outside air. That could easily be accomplished by leaving a window or 2 cracked. Perhaps in the bath, kitchen, or laundry where most of the moisture comes from. A ventilator there could be a solution.

Dec 8, 2006, 05:33 PM
From what I understand and read/heard from others is that this is a fairly common problem with newer homes because the windows and the house in general being sealed up too good - the moisture inside the house simply cannot escape unlike older houses where there are a log of cracks/creavises for moisture to leave the home (typically older homes actually need to run a humidifier since it is too dry in the winter).

I am running the furnace fan, so it is the same inside air that is being circulated, not cold outside air. The reason I tried running the fan, is from what I have read it is the airflow that prevents the moisture from accumulating on the window (and also helps heat the home more evenly). There are exhaust fans in the bathroom and kitchen but I have left them on continually before with no noticed improvement (unless of course during cooking/showers where you would have them on anyways).

Please do correct me if I am on the wrong track with understanding the source of the moisture. A friend of mine's solution on a couple of houses he has lived in is an air exchanger - but @ around $1000 they aren't cheap.