Furance fan speed on Lennox HELP

**northeastguy78** · 12-10-2010, 01:22 PM

Hi folks,

I have a dilemma here and wonder any experts out here can shed some lights. I have a 2003 Lennox Furnace (model 48C-115-3), it's a 4 ton unit that sizes roughly 2100 sq.ft. single zone. I have a 2 story home with a basement. The furnace is located in the basement (finished) and seems to heat the house fairly well.

I noticed the fan speed board had the cool set to high (black wire) (this is normal as I know cooling needs to have fan speed set to high). However, on the board where it said "heat" it has the fan speed low (red) plugged into it. My furnace fan has 4 speed (goes from black, brown, yellow, red) black being highest speed and red being lowest.) So basically red wire is plugged to heating and brown, yellow are on park 1 and park 2.

I have always read the fan speed should be on low for heating and high for cooling. However, I open the Lennox manual and noticed it had a chart diagram for fan speed and read....for my unit model (48C-115-3) showed black wire (High) for cooling and brown wire (med-high) for heating. Now, the unit currently has RED (low) plugged onto heating... which shows the lowest speed. Should I leave it alone or switch it to factory recommendation at med-high?

I heard one thing but read another. Basically I want to find out if I could possibly damage anything by changing it to med-high. I also don't want the pressure to be too high since factory recommended med-high when it is currently set at low.

If I switch it to brown (med-high) or even say yelow (med-low), it would force more air into the upper floor but cooler heated air since the fan speed spins faster. Will that make the furnace cycle faster since it is pushing harder. When it is pushed harder, it will reach the desire temperture faster, but the objects may not necessary be warm fast enough, hence drop in temp faster and furnace cycles quicker.

I guess my main question is, what should I do? leave it on red (low) but the manual says it needs med-high (brown wire)

Please help.

thanks,

**jimbo** · 12-10-2010, 03:36 PM

Furnaces are equipped with multispeed fans so you can have the proper CFM ( cubic feet per minute ) for the square feet of space and the duct arrangement. The install manual for the furnace probably has some CFM / fan speed charts. The classic example is Phoenix, where you might need only a 45000 BTU gas furnace, probably low fan speed, for heat, but for A/C you might need a blower with the 4 ton capacity, running high speed. By the way, how many thousand BTU is the furnace?

If you are happy with the performance, I would not try to tweak it.

You could not damage anything by changing speed....after all it is the same system running on high speed for A/C. If you upset the balance, the efficiency might be affected, either postitively or negatively.

**Thatguy** · 12-10-2010, 06:03 PM

Just don't crack the heat exchanger.

**jadnashua** · 12-10-2010, 06:05 PM

If the temperature in the winter is even, you shouldn't need to do anything. Windchill can make you feel cooler, so the lower the fan speed that does the job saves energy and keeps you more comfortable.

**northeastguy78** · 12-10-2010, 09:36 PM

Originally Posted by jimbo

Furnaces are equipped with multispeed fans so you can have the proper CFM ( cubic feet per minute ) for the square feet of space and the duct arrangement. The install manual for the furnace probably has some CFM / fan speed charts. The classic example is Phoenix, where you might need only a 45000 BTU gas furnace, probably low fan speed, for heat, but for A/C you might need a blower with the 4 ton capacity, running high speed. By the way, how many thousand BTU is the furnace?

If you are happy with the performance, I would not try to tweak it.

You could not damage anything by changing speed....after all it is the same system running on high speed for A/C. If you upset the balance, the efficiency might be affected, either postitively or negatively.

Very well stated sir. I was just baffled how the diagram and the manual says Lennox furnace unit 48C-110-3 calls for the brown wire(med-high) and a RED wire (low) was plugged into heating. I moved into this house a year ago so I wasn't there when the unit was originally installed. So if I change to the red wire, it would make the unit run longer and thus make the room temperature more comfortable without damanging the furnace at all?

I won't by having low speed for heating makes the unit run better or worse....

Originally Posted by jadnashua

If the temperature in the winter is even, you shouldn't need to do anything. Windchill can make you feel cooler, so the lower the fan speed that does the job saves energy and keeps you more comfortable.

I can see by having a lower fan speed will draw the heat out to the vents slower hence more comfortable. But will that make the system run longer since it is push the heat out at a much slower pace?

**jimbo** · 12-11-2010, 05:42 AM

No free luncn!

There is the balance between efficiency, comfort, and user expectation! Early in the AM, or if you come home after it has been set back to a cooler temp all day, YOU may WANT and EXPECT to be able to warm the house up quickly! You often find that an HVAC contractor will use a higher fan speed for the heat, higher tonnage for the A/C , than the efficiency and manual J calcs would dictate, BECAUSE the customer will complain if they can't heat up or cool down quickly enough!

**jadnashua** · 12-11-2010, 03:42 PM

Some home thermostats have some interesting logic...many of the Honeywell ones keep track of how fast the temperature drops during a setback, and instead of just turning back on at the prescribed time (which you might make say 1/2-hour before getting up), rather, it learns when it must turn on to achieve your desired temperature setting at the prescribed time. So, on a mild morning, it might only turn on 10-minutes before your set time, but on a really cold day, it might start an hour before. Much smarter than just a dumb clock logic version.

I'm a big fan of multispeed (variable speed) air handlers and furnaces. These always start out with a slow fan speed, and only ramp the speed up when it determines it is not achieving the desired effect quickly enough. Works better in the summer with a/c and is more comfortable in the winter with heating. Very quiet and efficient.

**northeastguy78** · 12-11-2010, 09:27 PM

ok... so you guys recommend I keep the current setting? (red wire on heating) ?

Again, my furnace unit 48C-110-3 calls for med-low (yellow) wire on the diagram but somehow when they built this house several years ago (I only moved in a year ago) wired it in low for heating.

To switch wire or not to switch is the question....?

Also, closing vent downstairs (30%) draws more heat into second floor?