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Old 12-29-2008, 11:48 AM   #1
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HVAC - Airflow Issue


We recently moved to a new house built into a 2 story built in 1978. It was recently remodeled including new windows and a roof. It is approximately 3200 sq feet not counting the unfinished basement which we plan to finish in the future.

We installed a 5 Ton heat pump with 20kw backup heat tape along with a 5 Ton 2000cfm air handler (comfortmaker).

I was rushed to do the install sooner than I planned as the old system couldn't get the house cooler than 76 (running 24/7) and we had a newborn.

I did the installation of the A/H and ran the copper and set the HP and had a professional come in and braze the lines and charge the system. The system worked extremely well in cooling mode.

Now that winter is here, I need to fine tune the supply and return ducting as I feel the system is not performing at its peak.

I had a 2nd HVAC guy come out and check the sizing of the ducting and the line pressures, he stated it was all ok. I don't believe either of them.

I am prepared to call a third guy, but decided to check here first and see if my thinking is right or wrong.

The size of the return duct from the ceiling to the unit is 10x24. (1320 CFM)
The size of the return duct running 2/3 the length of the house (30') is 8x24. (930 CFM)
The size of the supply duct running 3/4 the length of the house (40') is 8x24. (930 CFM)
The max filter size is 20x16". Way too small.

I believe I need to increase the drop down from the return to a 30"x12" and build a shorter but wider return box for the A/H so that I can get at least a 20x25 filter in there.

Attached are several photos. The first two are the old system, the last ones are the new system.

I used this document for duct sizing CFM ratings.
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HVAC - Airflow Issue-img_1121.jpg   HVAC - Airflow Issue-img_1122.jpg   HVAC - Airflow Issue-img_1131.jpg   HVAC - Airflow Issue-img_1129.jpg   HVAC - Airflow Issue-img_1157.jpg  


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Old 12-29-2008, 11:52 AM   #2
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HVAC - Airflow Issue


There are 26 6" round take-offs from the main supply trunk line. The A/H sits almost in the center of the supply feed, but near the end of the return. I am open to taking the main trunks down and putting larger up....perhaps 24x10 for the supply (since it only needs to push 1000 CFM in each direction) and 26x10 for return, since the system sits at one end of the return duct.

Each room has a return in it, most rooms have two supply (except for bathrooms). All returns upstairs are up high on the wall, all supply are in the floors (1st and 2nd floor).

The reason I think I have an air-flow problem is that the highest CFM I have seen with a detector in the return is 1300 CFM. Changing the blower speed from med to high doesnt impact that number much at all. Cutting an additional hole in the return box in the basement did increase airflow a little.

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Old 12-29-2008, 03:29 PM   #3
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26 x 10 is not nearly big enough for a 5 ton system.

Your return trunk is too small, and your return drop is too small(increasing to 30 x 12 will help).
But, if you don't increase the return trunk, you won't get much benefit from it.

You'll probably need to add more return to the house, and then increase the size of the return trunk and drop.

Do both supply trunks supply the same amount of air. Not just the same number of supplies tapped into them.

With you air handler dead heading into the supply the way it does. It creates a high resistance to air flow.

Might want to make the return box only 12" high, and bring 2 drops into it, to get better air flow.
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Old 12-29-2008, 04:44 PM   #4
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That has got to be the least well planned out install i have ever seen.

Beenthere has stated the reason why with the exception you are probably over sized on the HP also.

Kinda ironic.
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Old 12-30-2008, 08:47 AM   #5
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Quote:
Originally Posted by beenthere View Post
26 x 10 is not nearly big enough for a 5 ton system.

Your return trunk is too small, and your return drop is too small(increasing to 30 x 12 will help).
But, if you don't increase the return trunk, you won't get much benefit from it.

You'll probably need to add more return to the house, and then increase the size of the return trunk and drop.

Do both supply trunks supply the same amount of air. Not just the same number of supplies tapped into them.

With you air handler dead heading into the supply the way it does. It creates a high resistance to air flow.

Might want to make the return box only 12" high, and bring 2 drops into it, to get better air flow.
Thanks for the feedback. I don't want to lose anymore headroom in the basement, so the deepest I can go on the trunks is about 10", however, I can go as wide as I need to.

I also figure one can't go too big on the trunks either.

So my thought was to upgrade both the supply and return trunk to 10x24 (1320 CFM each direction), make the return box much smaller so that there is more room above the top as you suggested and have the two return drop-downs made to incorporate larger filters. Making the return vents in the walls bigger, isnt a problem for the 1st floor, the 2nd floor I have the option to add a central return vent by utilizing the space that the exhaust pipe for the old furnace was in.
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Old 12-30-2008, 09:02 AM   #6
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Quote:
Originally Posted by hvaclover View Post
That has got to be the least well planned out install i have ever seen.

Beenthere has stated the reason why with the exception you are probably over sized on the HP also.

Kinda ironic.
I am not sure if you are poking me or the original designer. I had little choice at the time to do the upgrade. With a newborn (weeks old) and another young one with outside temps around 100, I had to push up the timeline to put the new system in. I knew I would have to redo the ductwork in the winter, which is where I am today. Yes, it is more work now, but keeping my family cool was a priority to me.

The house is a mansared style house, so the upstairs is subject to heat through the rafters due to the roof design (see photo)

It also bothers me that no less than *TWO* separate HVAC "professionals" have come out and all said, nah the duct sizing is fine, just reduce the blower speed to low if anything....well I can't run the blower on low with 20kw heat strips according to the manufacture.

I am curious why you think the 5 ton system is oversized. The current sq ft of conditioned space is 3,586. When we finish the basement, that will jump to between 4500 - 5000 (depending how much we finish)
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Old 12-30-2008, 02:23 PM   #7
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Don't get fixed on a 1320 CFM split.

How many CFMs does each side need to carry.
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Old 12-30-2008, 04:33 PM   #8
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Quote:
Originally Posted by AndrewF View Post
I am not sure if you are poking me or the original designer. I had little choice at the time to do the upgrade. With a newborn (weeks old) and another young one with outside temps around 100, I had to push up the timeline to put the new system in. I knew I would have to redo the ductwork in the winter, which is where I am today. Yes, it is more work now, but keeping my family cool was a priority to me.

The house is a mansared style house, so the upstairs is subject to heat through the rafters due to the roof design (see photo)

It also bothers me that no less than *TWO* separate HVAC "professionals" have come out and all said, nah the duct sizing is fine, just reduce the blower speed to low if anything....well I can't run the blower on low with 20kw heat strips according to the manufacture.

I am curious why you think the 5 ton system is oversized. The current sq ft of conditioned space is 3,586. When we finish the basement, that will jump to between 4500 - 5000 (depending how much we finish)

Because I have the same design temps where I live and the basement is not considered a full load as it is below grade and subject to little to no heat gain.
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Old 12-31-2008, 08:54 AM   #9
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Quote:
Originally Posted by beenthere View Post
Don't get fixed on a 1320 CFM split.

How many CFMs does each side need to carry.
I left the sketch at home that shows me how much airflow is on each side of the A/H. (# of 6" vents). I'll post back up once I have the sheet.

Thank you.
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Old 12-31-2008, 11:24 AM   #10
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A 6" supply to a bathroom, doesn't actually have teh same amount of air going through it as a 6" to a kitchen(at least it shouldn't).

You chould make a drawing and also determine how much air each room really needs.

Not unusual for one side of a house to need 20% more air then the other side.
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Old 01-02-2009, 11:51 AM   #11
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HVAC - Airflow Issue


Thanks. I'll be creating a new drawing tonight as apparently my other one is gone from the cleaning we did before having family over for the holidays.

There are no dampers on any of the ducts in the basement.

Is it worth paying a local guy to do a Load J? calculation or utilize an online service to do it?
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Old 01-02-2009, 04:22 PM   #12
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You can do your own.

http://hvaccomputer.com/talkref.asp
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Old 01-02-2009, 10:43 PM   #13
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I had seen that one before and was curious if it was reputable. Thanks for the recommendation. I have purchased it and will proceed with it and report back here what the results are.
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Old 01-03-2009, 11:46 AM   #14
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I completed the Manual J calculation this morning. I did it twice to make sure I did it right. I did have to make some guesstimates as to what type of insulation board is behind the brick.

Attached is the completed report.

In summary:

Total Heat Gain: 37,917 BTUH
Total Heat Loss: 61,166 BTUH
Sensible Gain: 34.694.

So while the system (5 ton) is plenty large for A/C, I think it is sized ok for heating, which is my main area of concern.


I could easily add the garage or the workshop portion of the garage into the A/C factors for summer if I needed to get longer runtimes to remove more humidity.

I'll be working on the sketch of the existing ductwork this evening...time to head out and work on the barn.
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File Type: pdf AndrewF- Manual J - Local Calculation.pdf (71.8 KB, 146 views)
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Old 01-03-2009, 11:54 AM   #15
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At least at first glance, it looks ok.
5000 sq ft.

Do you actually set your stat to 68 in the summer.

Its a code violation to have the garage on the same forsed or gravity air system that serves an occupied area.
CO poisoning potential.

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