# Furnace fan speed



## Kyle (Sep 12, 2006)

Does changing the fan speed on a high effecient, variable speed gas furnace decrease it's efficiency?


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## bigMikeB (May 31, 2007)

It does more than that, the reason it's variable speed is to work in series with the heat or cool function that is running for optimum performance.


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## chris75 (Aug 25, 2007)

cold air is heavy so it needs a faster speed, warm air is lighter so a slow to medium speed will work.


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## bigMikeB (May 31, 2007)

chris75 said:


> cold air is heavy so it needs a faster speed, warm air is lighter so a slow to medium speed will work.


Too fast a speed for cooling and you don't get the desired cooling from the coil. Too slow a speed for heat and you trip out the high limit and cut down on the life of the heat exchanger. A variable speed furnace makes these speed changes through a micorprocessor automatically.


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## chris75 (Aug 25, 2007)

bigMikeB said:


> Too fast a speed for cooling and you don't get the desired cooling from the coil. Too slow a speed for heat and you trip out the high limit and cut down on the life of the heat exchanger. A variable speed furnace makes these speed changes through a micorprocessor automatically.


Thanks, I live in new england and a popular heating method is a hydro air, so no high limits, and I've never seen cooling on anything but high speed, but I like the idea of a variable speed, I can see the advantages of it.


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## Kyle (Sep 12, 2006)

I brings up another question that I have had. When I bought the furnace, it being a variable speed, I thought that the speed would automatically change as needed. But now it seems that the motor has three speeds - slow, medium, and fast. The blower is louder than the one I had which was 20 years old. The company is coming out next week to set the motor to a slower setting, but is that the right way to address the issue?


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## chris75 (Aug 25, 2007)

Kyle said:


> I brings up another question that I have had. When I bought the furnace, it being a variable speed, I thought that the speed would automatically change as needed. But now it seems that the motor has three speeds - slow, medium, and fast. The blower is louder than the one I had which was 20 years old. The company is coming out next week to set the motor to a slower setting, but is that the right way to address the issue?



What you have is a 3 speed motor, pretty common around here, they are set up with a heat and a cool relay, so two speeds can be used... but they are not variable...


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## Kyle (Sep 12, 2006)

That doesn't make me very happy. I was under the impression that I had a variable speed motor. Is there a significant difference? Can I keep the fan speed to medium without any issues? What is the purpose of three different fan speeds?


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## HvacWiz (Nov 24, 2006)

*High speed for cooling, low to medium speed for heat.*


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## chris75 (Aug 25, 2007)

Kyle said:


> That doesn't make me very happy. I was under the impression that I had a variable speed motor. Is there a significant difference? Can I keep the fan speed to medium without any issues? What is the purpose of three different fan speeds?



Around my parts they use high speed for AC, and medium speed for heat, i've never seen low speed used.. I'm not a duct guy, so I cant really tell you all the theory behind efficiency with a 3 speed motor and a variable motor...


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## duffers (Oct 11, 2007)

so if i understand this correctly, if you have a variable speed furnace, you need to manually switch it from summer to winter mode? The thermostat doesn't do this automatically for me?


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## chris75 (Aug 25, 2007)

duffers said:


> so if i understand this correctly, if you have a variable speed furnace, you need to manually switch it from summer to winter mode? The thermostat doesn't do this automatically for me?


A variable speed furnace is one that is variable, usually has senors inside and out to slow the blower down to 10,20,30,40,50% or whatever its setup to do,

sort of like the newer cars heating system, it slows the blower down after reaching the desired setpoint...


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## bigMikeB (May 31, 2007)

Kyle said:


> That doesn't make me very happy. I was under the impression that I had a variable speed motor. Is there a significant difference? Can I keep the fan speed to medium without any issues? What is the purpose of three different fan speeds?


 
There is a very significant difference in the price of a variable speed furnace and a normal furnace. All new basic furnaces have at least two blower speeds, most have three. The motor in a variable speed furnace is operated by a micro processor in the unit, that takes temps and pressures from various points in the unit and matches the speed accordingly. They do save a fair amount on utility bills over a standard unit. I hope you have a signed contract from the installer, so you have some recourse.


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