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Old 12-16-2008, 12:05 AM   #1
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HVAC unit moves too much air - can I slow the fan down?


Hi all,

I have two HVAC units in my home, one upstairs and one downstairs. The downstairs one seems to run just right. It's completely inaudible even though we leave the fan running 24/7 to circulate air. The air coming out of the vents seems to be just the right amount and you can't tell at all when it turns on or off. The temperature and lack of noise are very consistent.

The unit upstairs is a different story. It is mounted in the attic and the vents and returns are on the ceiling. We also leave this one on 24/7, the difference being that the air comes out of the vents like a wind tunnel.

All rooms have a constant air rush sound. In two rooms the beds are under the vents and it is actually uncomfortable to lay there because of the air blowing on you. Forget about closing the vents - that gives an extremely loud sound of rushing air (and a nice whistle.)

Besides the noise, the temperature swings are very rapid because so much air is dumping out of the vents. If you have a bedroom door closed and the heat turns on it becomes almost unbearably hot while it runs.

Was the upstairs unit just oversized, or is it possible that my fan speeds are incorrect? I've read that it should be running on low speed for heat, but it seems the fan only runs at one speed regardless if it is just fan, heat or A/C.

Any suggestions? We would really like it to run similar do our downstairs unit!

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Old 12-16-2008, 08:41 AM   #2
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HVAC unit moves too much air - can I slow the fan down?


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Originally Posted by UtahGuy View Post
Hi all,

I have two HVAC units in my home, one upstairs and one downstairs. The downstairs one seems to run just right. It's completely inaudible even though we leave the fan running 24/7 to circulate air. The air coming out of the vents seems to be just the right amount and you can't tell at all when it turns on or off. The temperature and lack of noise are very consistent.

The unit upstairs is a different story. It is mounted in the attic and the vents and returns are on the ceiling. We also leave this one on 24/7, the difference being that the air comes out of the vents like a wind tunnel.

All rooms have a constant air rush sound. In two rooms the beds are under the vents and it is actually uncomfortable to lay there because of the air blowing on you. Forget about closing the vents - that gives an extremely loud sound of rushing air (and a nice whistle.)

Besides the noise, the temperature swings are very rapid because so much air is dumping out of the vents. If you have a bedroom door closed and the heat turns on it becomes almost unbearably hot while it runs.

Was the upstairs unit just oversized, or is it possible that my fan speeds are incorrect? I've read that it should be running on low speed for heat, but it seems the fan only runs at one speed regardless if it is just fan, heat or A/C.

Any suggestions? We would really like it to run similar do our downstairs unit!
I am not a pro by any stretch of the imagination, but I can offer what I know about my furnace. The blower is belt driven from the motor, mounted above it. The pully on the MOTOR is adjustable. the pulley is 2 pieces, and the shaft is threaded. If I want to move less air (blower turning slower) than I widen the gap between the 2 halves of the pulley. First I loosen the tension on the belt, then remove the pulley on the blower, along with the belt. Then, on the pulley mounted to the motor There will be a set screw on one half of the pulley. Loosen the screw, turn the half of the pulley. when in the desired position, re tighten the set screw.


WARNING: OBVIOUS IINFORMATION AHEAD!
Make sure to completly kill the power to the furnace first. if that thing kicks on while your hands are in there, say bye bye to your digits.

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Old 12-16-2008, 08:47 AM   #3
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HVAC unit moves too much air - can I slow the fan down?


A lot of systems nowadays use small diameter high velocity ducting; by definition the air is going to shoot out of your vent openings.

You will have to make sure that the air conditioner evaporator coils don't freeze up after you have lowered the fan speed. An air conditioning system depends on a minimum air flow.
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Old 12-16-2008, 09:00 AM   #4
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HVAC unit moves too much air - can I slow the fan down?


I doubt the OPs AH has a belt drive. Don't see them in residental equipment any more with the exception of some older units.

Yes the fan speed can be changed. Depending on how your AH is wired when you chose "FAN ON" it may go directly into ac high speed.

I f it a modern typical set up "FAN ON" will use the lower heating speed.

There is a circuit board in your unit. Follow the wires and see where the the heat speed tap on the board is. Pull the wire lead off that terminal and see if it comes on in FAN ON> If it does not you can assume the fan is wired to run at heat speed (low) when in FAN ON position. If the fan does run with the wire lead off the heat tap on the board then it is running in hi speed ac mode.

Once you have determined which set up you have you can lower the speed by selecting the next speed down .

There is a wiring diagram that will tell you what colors the wires for all the speeds are.
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Old 12-16-2008, 09:34 AM   #5
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HVAC unit moves too much air - can I slow the fan down?


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I doubt the OPs AH has a belt drive. Don't see them in residental equipment any more with the exception of some older units.
that is how my furnace is set up and it is brand new, just bought it in september.
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Old 12-16-2008, 09:44 AM   #6
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HVAC unit moves too much air - can I slow the fan down?


What brand is it?

And is it a residential furnace or a package unit.? Some oil furnaces may still use them but the percentage is so small they aren't worth mentioning.

Point being that direct drive motors are the norm.
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Old 12-16-2008, 03:49 PM   #7
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HVAC unit moves too much air - can I slow the fan down?


OP.

What brand and model number is your furnace in the attic.

Good chance its oversized.
You may be able to slow down the blower some, but probably not as much as you would like it.
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Old 12-16-2008, 06:06 PM   #8
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HVAC unit moves too much air - can I slow the fan down?


Thanks for all of your replies. I'll get up in the attic tonight to see if it is a belt drive and what the manufacturer and model number is... All things I probably should have done before making this first post!

By the way, I'm just fine if the A/C runs on high speed so the coils don't ice. The biggest problem we have is the noise when it's just circulating and the rapid heating of the bedrooms.

I really think the panel is connected improperly, because I think it runs at the same speed regardless of the mode. Anyway, I'll head to the attic tonight. Thanks!

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