Author Topic: Adjusting HVAC blower speed?  (Read 206 times)

thenewguy

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Adjusting HVAC blower speed?
« on: August 08, 2018, 10:13:05 AM »
My biggest complaint about the house we bought about a year and a half ago (built 2002) has been that the HVAC (whether A/C, Heat, or just the fan) is very loud - to the point that if my wife and I are watching a movie after the kids are in bed, we have to turn it up loud enough to worry about waking them up when the AC kicks in. Additionally - if a bedroom door was cracked a few inches, it would slam shut when the HVAC came on (I was able to significantly improve this symptom by adding some return vents, but it's still there to a lesser degree).

This leads me to believe the air handlers are just plain pushing too much air through the ducts (all are rigid ducts - I understand flex ducts would dampen the noise a bit). It cools just fine, even on 110* summer days - in fact when our downstairs unit gets replaced in the next year or so, I plan to look into going down from a 3 ton to 2.5 or maybe even 2.

Until that time though, I understand that most if not all units will have multiple blower speeds - a setting for heat, a separate setting for cool, and another setting for just fan. Is that something that can be adjusted by a DIYer? I've come across some references to the temp differences between supply and return vents... ie, supply air should not be more than 15* cooler than return or there's a risk of the evaporator freezing, etc... Is there more to it than that? Could lowering blower speed sacrifice efficiency?

Thanks!

zolotiyeruki

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Re: Adjusting HVAC blower speed?
« Reply #1 on: August 08, 2018, 06:17:30 PM »
As I understand it, the blower speed is commonly lower for heart and higher for cooling. Your blower will most likely have multiple wires for various speeds, and you can try a lower speed for your AC. It's a simple matter of unplugging one wire and plugging in another.

Make sure to do it right, though. I'm not responsible if you burn down your house or fry the motor in the process.

px4shooter

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Re: Adjusting HVAC blower speed?
« Reply #2 on: August 08, 2018, 07:17:35 PM »
The coils need a certain amount of air blowing over them, in order to prevent them from freezing. If you lessen the airflow, the heat transfer will be reduced and the coils will freeze.

Pressure issues in rooms means there is less options for airflow. You can do connecting room vents. Keep in mind air and noise will both go through those ;)

You talk about noise. How is the airflow? Is the noise from the air or the blower?

thenewguy

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Re: Adjusting HVAC blower speed?
« Reply #3 on: August 09, 2018, 10:40:45 AM »
Thanks!

I looked up the stats on the air handler, and it looks like it only has two speeds, and only 3-5% difference between the two of them:

Edit: Pic didn't show up. These are the CFM ratings on High/Low at 0.1", 0.2", 0.3", 0.4", 0.5" H2O
High/Low 1,586/1,524 1,530/1,490 1,470/1,420 1,410/1,370 1,350/1,175

That being the case I'm starting to think it may not even be worth the hassle.

You talk about noise. How is the airflow? Is the noise from the air or the blower?

It's tough to say whether the noise is from the air or the blower. Airflow is strong - I'm assuming stronger than it should be for the size and quantity of ducts. The vents closest to the air handler are definitely the loudest. The vent in the kitchen for example seems to have moderately less air velocity than the vents in the family room, but it produces significantly less noise. I think that could be because they've got more air flowing through them because of shorter duct runs, but it may not be the whole story.

When I added the return vent to the master bedroom, I could feel a good bit of air coming out the other side (3 supply vents in the master suite), but it seems to be a much lower velocity than what comes out of the supply vents, and it makes no noise at all. I intentionally picked the largest return I could fit for just that reason.
« Last Edit: August 09, 2018, 10:43:04 AM by thenewguy »

Cadman

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Re: Adjusting HVAC blower speed?
« Reply #4 on: August 09, 2018, 10:51:42 AM »
Reminds me of a story. I had A/C added to my furnace/blower unit unit last year by a local reputable company. I had prepared the disconnect and had everything ready to go so that they were in and out in an hour or two. Where the tech struggled was setting the fan speed for cooling. Based on square feet and coil size, you config a number of DIP switches internally and you're set.

Things were fine for about a week, then one day, no A/C. Turns out the tech set the switches incorrectly and over-sped the motor, resulting in the rotor magnets literally delaminating(!) and freeing themselves from the shaft. Given the expense of new ECM's, I found a nice used unit on eBay for parts, made the repair, set the switches correctly, and we've been good ever since.

So the moral is, read up on your unit and make sure you understand what the correct settings should be. In this case, they could very well be set wrong.


zolotiyeruki

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Re: Adjusting HVAC blower speed?
« Reply #5 on: August 09, 2018, 11:31:52 AM »
Reminds me of a story. I had A/C added to my furnace/blower unit unit last year by a local reputable company. I had prepared the disconnect and had everything ready to go so that they were in and out in an hour or two. Where the tech struggled was setting the fan speed for cooling. Based on square feet and coil size, you config a number of DIP switches internally and you're set.
Interesting! In my unit, you set the A/C size by literally plugging in a different wire from the motor (tapped to a different point on the windings).