Fan Speed on forced air unit.


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Old 03-09-06, 07:03 PM
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Fan Speed on forced air unit.

Hello, I have a Tempstar Model FBF100F14G1, installed 2004, 80% forced air, gas fired unit. I am trying to find out if it is possible to slow down the fan speed. It blows real hard and hot, then cools down very quickly, making the furnace cycle on and off alot. I have taken pics of the diagrams inside the unit, and am VERY handy, so technical stuff usually doesn't get beyond me.Here is the link, http://photos.yahoo.com/ph//my_photos
Thanks in advance!! AL.
 
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Old 03-09-06, 08:30 PM
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Originally Posted by cajzmere
Hello, I have a Tempstar Model FBF100F14G1, installed 2004, 80% forced air, gas fired unit. I am trying to find out if it is possible to slow down the fan speed. It blows real hard and hot, then cools down very quickly, making the furnace cycle on and off alot. I have taken pics of the diagrams inside the unit, and am VERY handy, so technical stuff usually doesn't get beyond me.Here is the link, http://photos.yahoo.com/ph//my_photos
Thanks in advance!! AL.

The link to yahoo don't takes me to your photo area..

Fan could be slowed down, but you MUST take a temp reading by the air filter, then just above the A/C coil. and see what your temp rise is. By the burners, there should be a metal plate with the temp rise rating. i'd suggest it to be in between the min/max suggestion.

If it cycles on and off, it maybe over sized.

Also, what your make and model of t-stat?
 
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Old 03-09-06, 09:49 PM
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I think the Tempstar labels are in the blower compartment.
 
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Old 03-12-06, 09:16 AM
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My thermostat is a generic square box, no name on it. I don't have A/C. Is there a simple "move wire from A to B" to cut fan speed by 25 or 50%? It's a 100k btu for a 1650 sq. ft. house so I don't think it's oversized. Temp rise says 45-75 deg. fahrenheit. Thanks again.
 
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Old 03-12-06, 09:20 AM
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If pics of the diagrams inside the cabinet would help with an answer, I can e-mail them, don't know why the yahoo thing didn't work.
 
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Old 03-12-06, 09:23 AM
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Old 03-12-06, 01:52 PM
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Fan speed

Before messing with the fan speed you need to do as Jay said & measure those temperatures.
 
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Old 03-13-06, 10:41 AM
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Do I take those temps while furnace is running, or right after it has stopped? Also, I have no A/C, so do I take that temp at the plenum?, or in a close to the plenum vent? What am I looking for with these temps? I know I ask alot of questions, but I DO appreciate your help. Thanks.
 
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Old 03-13-06, 03:35 PM
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Let the furnace run about 10 to 15 min. then take a reading while it's running.. Do it in the plenum take a reading there (till the temp is no longer going up), then take a reading in the return at the air filter.

Take plenum temp - return temp, that is your temp rise.
 
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Old 03-20-06, 12:36 PM
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Hi, sorry to take so long with those temps, been a busy week. Anyways... I got 81 deg. F at filter inlet, 106 deg. F inside plenum, and 125 deg. F above plenum, in main trunk line. I took these readings with the thermostat cranked up all the way. If i use that formula, with plenum temp, i get 25 deg. rise. with trunk temp. i get 44 deg. rise. Next thing i do would be?........... Thanks again.
 
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Old 03-20-06, 01:49 PM
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Wink

You dont say where you are and what outside temp is?
It's a 100k btu for a 1650 sq. ft. house so I don't think it's oversized.

It blows real hard and hot, then cools down very quickly, making the furnace cycle on and off alot.

Unit to big for the home or duct work not right. Then work on the blower speed

My .02 cents

ED
 
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Old 03-21-06, 06:08 AM
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Temperature outside when I took temps. was approx. 28 deg. F . Rochester, NY (in between Buffalo and Syracuse). When I say blows hard, I mean really hard! Hard enough to make my vents whistle. When I do figuring for air space, i came up with needing roughly 87,000 BTU and asked my neighbors what they have in their homes, they said 85,000-100,000 BTU and my contractor father in law(not a HVAC guy) about what should be, he said 100,000 BTU. I'm pretty sure it's not oversized. Besides, the furnace is in place, has been there, is not going anywhere..... and ductwork came with the house. If it's wrong, it's been wrong for approx. 64 years, and who am I to mess with history. The unit was put in by a "friend" who has an HVAC license, and was able to help me put this unit in. He's the one who did the math for airspace. BTW the one I took out was a 1976 100,000 BTU furnace. Thanks in advance. AL.
 
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Old 03-21-06, 04:46 PM
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duct size

If the ducts are 64 years old, I'd be willing to wager, sight unseen, they are too small. You need to look at the wiring diagram (usually on the blower door) & trace the wires from the blower to the fan control. If the fan is already on it's lowest speed, (usually the red blower motor wire) there isn't a lot you can do. If it is on medium (blue) or high (black) you can slow it down one step & re-check the temperatures. I can't see the diagram well enough to read the wiring diagram.
 
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Old 03-21-06, 07:13 PM
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Thank you very much.
 
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Old 03-21-06, 08:31 PM
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Originally Posted by Ed Imeduc
You dont say where you are and what outside temp is?
It's a 100k btu for a 1650 sq. ft. house so I don't think it's oversized.
ED

I have a 1700 sq ft home, and I have a two stage 60,000/42,000(output of 56,000/37,000) Load calc came up about 54,000, and it cycles in and out of 2nd stage every now and then on our below zero degree days.

cajzmere, how did you come up with 87,000 BTU?

A 100,000 BTU furnace will move alot more air since it's a larger sized BTU.

On an avg, a 100k will move 875 cfm on low, and a 60k will move 600. I agree with Grady, that the ducts may be undersized, and that will cause a lot of air, but it has nothing to do with short cycling.
 
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Old 03-22-06, 09:10 AM
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I have ran into homes where the furnace is over sized. Then as it gets gold outside The furnace runs on limit control as it cant get the heat out. It will like blow hot then cold Air. Then cause of the over size furnace not be able to heat the home.
ED
 
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Old 03-22-06, 05:51 PM
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Thanks again. I didn't do the calculation for airspace, the guy who put in for me did. He's a stationary engineer, and he used a calculator with some kind of function to come up with that number. The unit that was replaced WAS a 100,00 btu model, but from 1976. It didn't blow near as hard as this one, but i figured that was because of the age of the thing (motors do wear out). I'm going to trace those wires and try to slow down the motor and see if that helps. If anyone has any ideas other than replacing the furnace with a smaller one....... or completely changing the ductwork........ I wouldn't want to do either option. I'm only in this house for maybe another year, so i'm not gonna kill myself or invest in another furnace. Again I appreciate the help. AL.
 
 

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