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Old 03-14-2010, 11:54 AM   #16
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2nd stage, please


Bringing in fresh humid air is the bane of all air conditioners.

It adds load to the A/C, and increases the cooling bill.

It will throw that 2 stage into a long run time trying to remove the moisture from the fresh air. Specially, when his area is having those days when the dew point is in the 60s, which is around 5 to 6 weeks for his area. And cause it to go to second stage trying to get rid of the sensible heat.

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Old 03-14-2010, 02:21 PM   #17
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2nd stage, please


Beenthere, you mentioned that the trunks may be responsible for the lack of 2nd story air.


I seem to recall reading that a 5" duct would carry about 80 - 100 CFM (at .5" wc) and a 6" would be about 150 CFM. That equates to 11 x 100 (1100) + 7 x 150 (1050) = 2250.

So what would be the effect on supply static being that the old equipment was 4 ton? Would it not be lower than expected? Wouldn't a low(er) static favor better airflow. What about reduced trunk size?

I know there are many other considerations: turns, length of runs, etc., but just looking for generalities.



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Old 03-14-2010, 02:57 PM   #18
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2nd stage, please


If you have a .5" static pressure in your supply, you generally will have that much or more in your return. meaning your system Total static would be1". Your blower can't move set CFM at 1" static.

A 6" hard pipe will provide you with 80 to 100 CFM, depending on its length. A 5" around 65 CFM.
A .5" static in the supply duct is TOO much. And you VS blower will drop off its air flow.

Your old system may have been 4 ton. But no way to know how much air it was really moving. But it wasn't 1600 CFM.

Unfortunately, it was easier to scare techs into using a vacuum pump for R410A systems. then it is to convince them and salesman to check static of existing equipment before installing or selling VS blowers.
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Old 03-14-2010, 03:00 PM   #19
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2nd stage, please


Your trunk system. if its a reduing trunk. Is probably restricting air on the first section, and with each reduction the restriction is increased. So you have poor air flow to your second floor.

Your VS blower is able to deliver its best air flow at Total External Static Pressures of .5" or less.

Thats the return and supply pressure combined. Not just the supply.
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Old 03-14-2010, 03:06 PM   #20
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2nd stage, please


My thoughts are that he needs to get rid of the heat on the second floor. Maybe he needs more supply (not there), but until the stagnation thing is addressed, you are just spinning your wheels. As for bringing in fresh humid air, I would think he would be exhausting more than bringing in (negative pressure). Is the exhaust thing the ideal answer...maybe not, but, if he can't get some return from up there, it's the second best alternative to being comfortable.
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Old 03-14-2010, 03:10 PM   #21
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2nd stage, please


He need more conditioned air to the second floor.

Not more air brought into the house to condition.

Depending on his duct layout. A helper duct for his supply may be his answer.
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Old 03-14-2010, 03:19 PM   #22
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2nd stage, please


Bringing in 100 CFM of 85F 50%RH outside air while trying to maintain 70F and 50% RH inside, adds 3,974 BTUs an hour to the A/C load.
And obviously, at 50 CFM of outdoor air at the same conditions adds 1,987 BTUs.

Why add 1/4 ton of load to the house. Which will increase operating cost. When the rst of the world is trying to use less electric. And one of the reasons for getting a high efficiency is to use less electric.
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Old 03-14-2010, 04:57 PM   #23
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2nd stage, please


I agree with Beenthere. Perhaps another duct can be run to the second floor and hidden inside a closet or in a corner of a room and then painted or framed in.
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Old 03-14-2010, 07:05 PM   #24
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2nd stage, please


Quote:
Originally Posted by beenthere View Post
Your trunk system. if its a reduing trunk. Is probably restricting air on the first section, and with each reduction the restriction is increased. So you have poor air flow to your second floor.

Your VS blower is able to deliver its best air flow at Total External Static Pressures of .5" or less.

Thats the return and supply pressure combined. Not just the supply.
I believe I misspoke regarding the static. Too much information floating around.

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Old 03-14-2010, 07:08 PM   #25
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2nd stage, please


Quote:
Originally Posted by beenthere View Post
He need more conditioned air to the second floor.

Not more air brought into the house to condition.

Depending on his duct layout. A helper duct for his supply may be his answer.
Beenthere, would you expand on that please?


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Old 03-15-2010, 04:27 AM   #26
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2nd stage, please


A helper duct. Is a duct that is ran from your plenum to near the end of your supply trunk, and tapped back into the supply trunk.

What it does is it allows the end duct work to have the same amount of static pressure as the starting duct work. Lowers the resistance to air flow of all of the supply trunk.

Lets say your supply trunk starts out as a 8 X 24 and reduces a couple times over its length.
But it should have been a 10 X 28, and reduced down respectively.
This would mean its undersized. And perhaps only moving 1100CFM instead of 1600 CFM.

Adding a 10" helper. And tapping it back in near the end of the trunk(between the 3/4 mark and end) line will allow the end duct work to have more air, and increase all the air flow to all of the other section also.
It does part of the work of the main trunk line, hence the term helper duct.

If you can measure your duct work and post its sizes and lengths, might be able to tell you if a helper duct will work for you or not.

Or your supplies to the second floor are just to small or few, and you need an additional supply duct ran from your plenum to your second floor no matter what.

Can't tell for sure cause I can't see your house and duct work from my monitor.
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Old 03-15-2010, 06:17 AM   #27
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2nd stage, please


Quote:
Originally Posted by beenthere View Post
A helper duct. Is a duct that is ran from your plenum to near the end of your supply trunk, and tapped back into the supply trunk.

What it does is it allows the end duct work to have the same amount of static pressure as the starting duct work. Lowers the resistance to air flow of all of the supply trunk.

Lets say your supply trunk starts out as a 8 X 24 and reduces a couple times over its length.
But it should have been a 10 X 28, and reduced down respectively.
This would mean its undersized. And perhaps only moving 1100CFM instead of 1600 CFM.

Adding a 10" helper. And tapping it back in near the end of the trunk(between the 3/4 mark and end) line will allow the end duct work to have more air, and increase all the air flow to all of the other section also.
It does part of the work of the main trunk line, hence the term helper duct.

If you can measure your duct work and post its sizes and lengths, might be able to tell you if a helper duct will work for you or not.

Or your supplies to the second floor are just to small or few, and you need an additional supply duct ran from your plenum to your second floor no matter what.

Can't tell for sure cause I can't see your house and duct work from my monitor.
Have to leave, soon--will comment on this later.

Now, a quickie question: considering that the upstairs and downstairs returns are in the same line, would increasing the grill size of the upstairs, relative to the existing downstairs grills, improve the flow to upstairs?

Thanks.

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Old 03-15-2010, 06:26 AM   #28
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2nd stage, please


Depending on the grilles. It can.

Some return grilles are more resistive to air flow then others(fin spacing effects it).

An easy test, is to remove the second floor grille completely. See if you notice more air flow from the second floor supplies.

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