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Old 06-16-2010, 08:12 AM   #1
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AC Temp drop over plenum run length


Is a 15 degree temp drop over the length of a plenum normal? Right at the furnance the air temp coming off the AC Coil (well about 2ft up stream from it) the temp is 50 degrees cooled.... run down the length of the plenum (about 40 ft down) the temp is now about ~68 degrees still cool but not cold compared to an outside temp of 80 degrees....

this is a 24x8 inch plenum that runs straight down the length of the house... all the rooms that are above the furnance (shortest distance from it) are cold... the ones on the far end do not cool much and have drastically less air flow too... the air speed is noticably different just by feel... the closest room the air blows hard and fast... the farthest room you can feel it coming out slow and gently.....

now is the temp difference due to the velocity and amount of air flow... or is the fact the plenum isn't insulated part of it? (its in a basement ceiling that is finished with drop down ceiling tile)

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Old 06-16-2010, 09:36 AM   #2
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AC Temp drop over plenum run length


yeah that's not normal. the key number is the temperature drop from the intake vent (measure the temp right at the filter) to the vents. some vents will obviously be a little cooler or warmer than others, depending on location and air flow.

i had a similar problem and it was the evap coil getting old and leaking and running low on freon. typically, even on a hot day in texas, where the outside temp is 95+ and the temp at the filter is about 78-80 degrees, the air coming out of the vents is still below 60 degrees. in other words, about a 20 degree temperature drop from intake to vents.

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Old 06-16-2010, 12:39 PM   #3
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AC Temp drop over plenum run length


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yeah that's not normal. the key number is the temperature drop from the intake vent (measure the temp right at the filter) to the vents. some vents will obviously be a little cooler or warmer than others, depending on location and air flow.

i had a similar problem and it was the evap coil getting old and leaking and running low on freon. typically, even on a hot day in texas, where the outside temp is 95+ and the temp at the filter is about 78-80 degrees, the air coming out of the vents is still below 60 degrees. in other words, about a 20 degree temperature drop from intake to vents.

I dont think the coil is bad its pretty new.. but is there anyway to tell the expceted temp drop created by the system you have? My heat rise is about +60 from filter to supply plenum when the heat is running.... but the AC temp drop is about -20 degrees...

but my real question though was about the vents on one side of the house its almost 20 degrees colder air then the other and how to mitigate that so its more uniform
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Old 06-16-2010, 01:38 PM   #4
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AC Temp drop over plenum run length


sounds like you have more of an airflow problem. check all your ductwork, make sure it's tight and there are no leaks anywhere. is the air flowing out the vents where it's warmer flowing also at a much slower speed?
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Old 06-16-2010, 01:40 PM   #5
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AC Temp drop over plenum run length


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sounds like you have more of an airflow problem. check all your ductwork, make sure it's tight and there are no leaks anywhere. is the air flowing out the vents where it's warmer flowing also at a much slower speed?
yes, the closest area to the furance is like a wind storm... its coming out a 6" duct but its still fast the other end is like a gental breeze
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Old 06-16-2010, 05:53 PM   #6
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AC Temp drop over plenum run length


i had a couple vents split off one main duct coming from the plenum that was like a windstorm also. cheap fix, but i put a piece of cardboard inside the plenum over a portion of the hole and kept moving it until the airflow was where i wanted it. that also increased the airflow to the other areas that weren't getting enough.

obviously, the main duct should have been a much smaller diameter, but cutting and inserting cardboard worked fine and took 5 minutes.
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Old 06-16-2010, 06:06 PM   #7
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AC Temp drop over plenum run length


You shouldn't have anywhere near 15 degrees difference from one end to the other running through a conditioned space. Worst case I ever had, the duct went through a 115 degree attic, only gained 7 degrees. Reducing the trunk size after every 250 cfm loss will keep the register velocity fairly constant from one end to the other.
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Old 06-16-2010, 07:55 PM   #8
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AC Temp drop over plenum run length


What size is your A/C?
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Old 06-18-2010, 07:28 AM   #9
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AC Temp drop over plenum run length


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What size is your A/C?
It's a 4 ton Lennox system

return opening box on furnance is 17x25 inch (I assume this is 1,400 - 2,000 CFM?)

the plenum going into the return is 24x8" with the boot being the same size as the filter box of course

the supply plenum is about 24x24 in size but splits into two 24x8 runs going to the left and right of the furnance (each side of the house)
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Old 06-18-2010, 07:37 AM   #10
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AC Temp drop over plenum run length


24X8 Return is way too small for 4 tons.
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Old 06-18-2010, 07:58 AM   #11
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AC Temp drop over plenum run length


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24X8 Return is way too small for 4 tons.

I'm actually making a list of stuff I want changed for my local HVAC tech's to look at, what would you suggest would be the size?
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Old 06-18-2010, 08:02 AM   #12
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AC Temp drop over plenum run length


A min of 22X10.

Can you make a drawing of your duct system with duct length and sizes. And post it.
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Old 06-18-2010, 08:24 AM   #13
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AC Temp drop over plenum run length


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A min of 22X10.

Can you make a drawing of your duct system with duct length and sizes. And post it.
will do
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Old 06-18-2010, 09:14 AM   #14
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AC Temp drop over plenum run length


not the easiest pic to read... only put in where it is plenum or the in joist carboard type returns (dark blue)




Simple diagram of air flow from the furance


And here is an actual photo of the furance


Notice the oversize filter because the boot they put in doesnt have a matching filter size anywhere me or my HVAC people could find to fit the bent up hole they cut into the boot to cram a filter in...... *rolls eyes*

Last edited by BlueBSH; 06-18-2010 at 09:24 AM.
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Old 06-18-2010, 01:05 PM   #15
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AC Temp drop over plenum run length


The supply section that is reduced to 12X8 should be larger. Probably 16X8.

The return ell boot that is attached to the furnace. Was meant to be attached to a return air filter bracket. Not directly to the furnace.

And the return drop, should be 25X10. Here is a PDF that will show you what you can have them do to make your return drop better. Which includes a better air filter set up. Go to page 7, look at #1221. You would want 1222. Since it has a larger filter.


I can't make out the sizes of your joist returns. Can you just post the numbers.

It looks like adding another return in the central area, would be a boost to your overall air flow problem also.

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