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Old 02-05-2011, 07:53 PM   #1
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Main Supply Duct Warping


Recently I had 2 HVAC professionals tell me to stop using Filtrete Ultra Allergy filters saying they are hard on the furnace. So, I switched to a lighter 30 day filter.

Since then, I've been hearing this knocking/popping/thumping sound when the furnace is on. And the airflow out of the vents is not a constant speed. After two weeks of sleuthing, I found the main supply duct, just above the furnace, is warping in and out as the blower runs and its echoing through the ducts.

This had not happened with the heavier filters and, sure enough, I take the light one out and put the heavier one in....the air flow is much slower but the warping stops.....but I don't want to use a filter that is going to cause problems in the long run. Also with the heavy filter, the air flow might be a tiny bit inconsistent, but barely noticable.

I was going to get some brackets and reinforce the sheet metal to prevent the warping. Is it possible the warping is causing the inconsistent air flow? I just had the AC coil cleaned, so i don't think it would be choking the supply. Besides, my furnace seems to prefer less air, not more...or is the filtrete filter only masking the problem with the lower air flow?

Any advice?

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Old 02-05-2011, 08:08 PM   #2
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Main Supply Duct Warping


Did they put the bends into the duct faces when it was originally installed, so as to not allow this behaviour?

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Old 02-05-2011, 08:22 PM   #3
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Main Supply Duct Warping


My home is over 50 years old. The furnace is 8 years old. There's about 18" of duct directly above the furnace with newer sheet metal and bends. That connects to the 50 year old portion that goes into the home's duct work. The 50 year old portion is what is warping and does not have bends.
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Old 02-05-2011, 10:51 PM   #4
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Main Supply Duct Warping


Are the high alergen filters 1" thick, if so they will do what they are supposed to do without causing any harm to your system. So put the good filters back in and rest easy.

The increased static caused by a tighter / better quality filter has minimal effect on overall air volume, and is no different than a cheap filter (static wise) left in a system for longer than it should be.

In fact the improved efficiency of the high alergen filter will save your system, as it removes smaller dust particles which the cheap filter let go through, thus the cheap filter lets dirt through to collect on the heat exchanger and "A" coil and supply ducting.

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Old 02-06-2011, 12:18 AM   #5
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Main Supply Duct Warping


Don't worry about the filters, go to home depot, in the duct section there will be 5' sections of "drive cleat". Get a couple of them and screw em to the duct at an angle / This should solve your oil can problem
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Old 02-06-2011, 04:17 AM   #6
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Main Supply Duct Warping


Your duct is oil canning. Its undersized.

And those HAVC guys are correct about those air filters.
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Old 02-06-2011, 08:11 AM   #7
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Yes, because it takes business away. There has been no scientific proof that the 3m Filtrete 3 month filters have caused any harm or problems on systems. The problem arises, when people do not check those 3 month filters every month, especially if they have pets in the house. I use them, and have to change them every month, because of the sheer amount of dog hair from our Golden that gathers on the pleats.
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Old 02-06-2011, 04:21 PM   #8
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Main Supply Duct Warping


Quote:
Originally Posted by gregzoll View Post
Yes, because it takes business away. There has been no scientific proof that the 3m Filtrete 3 month filters have caused any harm or problems on systems. The problem arises, when people do not check those 3 month filters every month, especially if they have pets in the house. I use them, and have to change them every month, because of the sheer amount of dog hair from our Golden that gathers on the pleats.
If the duct work is sized to use those filters. They are fine. Problem is no one sizes duct work to handle the resistance those filters have to air flow.

Check your furnaces temp rise using a regular air filter, and then with a new 3M allergen, and then after 30 days.
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Old 02-06-2011, 06:03 PM   #9
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Main Supply Duct Warping


I have, and it is the same, because the duct work was properly sized for my house. And as I stated before, I replace the 3 month filters aprx 30-45 days after replacing, because of the amount of dander, dog hair, and dust that they pull.
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Old 02-06-2011, 07:48 PM   #10
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Main Supply Duct Warping


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Originally Posted by gregzoll View Post
I have, and it is the same, because the duct work was properly sized for my house. And as I stated before, I replace the 3 month filters aprx 30-45 days after replacing, because of the amount of dander, dog hair, and dust that they pull.
Properly sized for your house means nothing, as far as you using the 3M allergen filters. The duct would have to be sized to use them.

So what was your temp rise under all 3 conditions?
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Old 02-06-2011, 08:13 PM   #11
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Main Supply Duct Warping


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Properly sized for your house means nothing, as far as you using the 3M allergen filters. The duct would have to be sized to use them.

So what was your temp rise under all 3 conditions?
Again, there is no scientific or noted proof that using 3m filters cause problems. Does not matter what the temp rise is, what matters, is the home owner inspecting the filters through out the month, and changing them before the 30 days, or 90 days. It is about making sure that the filter media is not clogged with dust or hair.

In an ideal condition, air filters should be changed every 15 days during SpringSummer/Fall, and left in no longer than 30 days during Winter, regardless what media. Using the cheap Blue filters, cause more damage on a HVAC system, along with the A-Coil.
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Old 02-06-2011, 08:21 PM   #12
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Main Supply Duct Warping


Quote:
Originally Posted by gregzoll View Post
Again, there is no scientific or noted proof that using 3m filters cause problems. Does not matter what the temp rise is, what matters, is the home owner inspecting the filters through out the month, and changing them before the 30 days, or 90 days. It is about making sure that the filter media is not clogged with dust or hair.

In an ideal condition, air filters should be changed every 15 days during SpringSummer/Fall, and left in no longer than 30 days during Winter, regardless what media. Using the cheap Blue filters, cause more damage on a HVAC system, along with the A-Coil.

Try again. Temp rise DOES matter. And it is excessive temp rise that causes heat exchangers to crack.

The cheap blue filters cause no damage to HVAC equipment. They do cause the evap coil of an A/C to need cleaning more often. But don't harm it.


Since you said it doesn't matter. I'll guess you either didn't really take it, or its so high you don't want to post it. Either way. Those filters do harm systems. I get lots of service because of them. Lots of failed high limits.
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Old 02-06-2011, 11:30 PM   #13
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Main Supply Duct Warping


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Originally Posted by beenthere View Post
Try again. Temp rise DOES matter. And it is excessive temp rise that causes heat exchangers to crack.

The cheap blue filters cause no damage to HVAC equipment. They do cause the evap coil of an A/C to need cleaning more often. But don't harm it.


Since you said it doesn't matter. I'll guess you either didn't really take it, or its so high you don't want to post it. Either way. Those filters do harm systems. I get lots of service because of them. Lots of failed high limits.
First, beenthere, I do have alot of respect for your knowledge in the HVAC field, you usually post some really good information, but, this one, at least part of it I don't buy.

Temp rise through a HE does matter I will not argue that, but I am sure the filters are usually not the sole cause of an increase in temp rise through the HE, I am sure that it may be a contributing factor, and one that is easily fixed as opposed to ducting the number of runs, just change the filter to a cheapy, instead of changing other "undersized" components, would help but still would not fix a system that is so poorly designed.

Typical fan curves, check pg I-2 of attached, show that an increase of static say from a nominal system sized correctly with a clean filter and a wet coil of .25", then add a high end filter and increase the static (exaggerated) to .4" clean, the average volume change would only be 100 cfm or less than 1% of design.

http://www1.eere.energy.gov/building...ndixi_0906.pdf

This would indicate that high end filters don't harm the temp rise, but do indeed protect equipment from dust impingement, collecting not just on the A coil fins, but also on the fins of the heat exchanger itself.

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Old 02-07-2011, 03:35 AM   #14
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Main Supply Duct Warping


You won't find a single stage furnace with an A/C coil running at a TESP(Total External Static Pressure) of .25". More like a .7"(often higher) with a standard air filter. What people think is proper sized duct work, generally isn't. Next, the manufactrurers of furnaces and air handlers. Don't make the filter frame large enough for an allergen filter that can move the amount of air that the furnace or A/C needs for proper operation.

So now look at that chart, and see how bad the air flow is when you go from .75" to 1" static(the .75" already isn't good). That would be with a clean allergen air filter. After a week or 2, the PD through the filter can easily increase by another .1". Which will drop air flow more, and increase temp rise that much more.

You can make a manometer, and test the PD of air filters in your own system. And also use thermometers to check the temp rise.

A bypass HEPA filter works much better then an allergen air filter, that all the air must pass through.
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Old 02-07-2011, 04:57 AM   #15
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Main Supply Duct Warping


Here is the fan data for a 80,000 BTU input, 64,000 BTU output 80% furnace at statics from .1" to 1" for the blower in both high, and medium high speed.

Static----high speed CFM--- Temp rise---med high speed CFM--- Temp rise
.1"-------1580--------------37.51-------1110------------------53.39
.2"-------1530--------------38.73-------1100------------------53.87
.3"-------1470--------------40.31-------1075------------------55.12
.4"-------1405--------------42.18-------1060------------------55.90
.5"-------1330--------------44.56-------1030------------------57.53
.6"-------1245--------------47.60-------980-------------------60.47
.7"-------1150--------------51.53-------920-------------------64.41
.8"-------1045--------------56.71-------835-------------------70.97
.9"-------890---------------66.58-------680-------------------87.15
1.0"------650---------------91.17-------520-------------------113.96


Basically, you can ignore any static under .5" for any furnace that has an A/C coil on it. Since the coil dry will have a PD/resistance to air flow of atleast .13" itself.

Using this furnace with an A/C unit. If the static with a hog hair air filter and the blower set to med high is .6" and moving 980 CFM. Then an allergen filter is installed in place of the hog hair. The air flow would drop to below 835. For this example though, we'll say it only dropped to 835. The air temp rise is now higher then allowed. Only by .97 degree. But the air filter is new. So in just 2 days, it will be more, and in 2 weeks it will be more yet, as the air flow drops more.

First thought for many people is if the blower on med high at .8" static is moving 835 CFM, they can switch to high speed which moves 1045 CFM at .8". Unfortunately they forget that in order to move that 1045 CFM, the resistance through the air filter will increase, making the total static rise to 1.25" to move 1045 CFM. The blower on high can't move 1045CFM against a 1.25" static though. And will top out around 890 CFM, leaving no room for the filter to get much dirt on it, nor for the filter to actually serve its purpose, because the high static will draw dirt and allergens through it.


In summer with the A/C coil wet, its filter rating would be equal to a MERV 6 to 8 air filter. So you don't need more then a MERV 8 air filter, to protect the equipment. Anything above MERV 8 is for you, and not the equipment.

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