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Old 01-31-2010, 01:40 PM   #1
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Auxiliary in-line fans to help deliver heat to remote areas.


I've just installed two separate 8 inch metal ducts to supply 5 registers servicing two rooms and a closet, total 650 sq. ft. The ducts are insulated with 'the latest' insulation.

The problem is the temps of the new rooms and the air temp at the registers is approx 10 degrees cooler than the rest of the house.

The question is; Do those auxiliary in-line fans that wire either to a manual switch or to the furnace directly, actually work?

It seems that because the new addition is lower by a couple of feet than the rest of the house, and the new metal ducts have 90 degree elbows to compensate for this, there must be too much resistance in the duct to allow free flow as the other registers have. My thinking is the in-line fans would promote more heat to be drawn from the plentum. Seems logical to me.

Any feedback is appreciated.

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Old 01-31-2010, 03:33 PM   #2
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Auxiliary in-line fans to help deliver heat to remote areas.


How much additional return did you install so these rooms can have air.

Fantech makes fans that work.

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Old 01-31-2010, 03:50 PM   #3
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Auxiliary in-line fans to help deliver heat to remote areas.


You can either have them wired to the furnace blower, or have them wired to a duct heat sensor switch. Either way though you are robbing Peter to pay Paul. They will draw air flow away from other registers on the trunk.
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Old 01-31-2010, 04:11 PM   #4
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Auxiliary in-line fans to help deliver heat to remote areas.


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Originally Posted by beenthere View Post
How much additional return did you install so these rooms can have air.

Fantech makes fans that work.
Close to equal supply and return area. There are five supply registers and two larger returns.
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Old 01-31-2010, 04:13 PM   #5
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Auxiliary in-line fans to help deliver heat to remote areas.


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Originally Posted by NHMaster View Post
You can either have them wired to the furnace blower, or have them wired to a duct heat sensor switch. Either way though you are robbing Peter to pay Paul. They will draw air flow away from other registers on the trunk.

Exactly! However, we seem to have plenty of supply. I figure if I install a couple of these ditties I'll have to re-balance the system.
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Old 01-31-2010, 04:29 PM   #6
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Auxiliary in-line fans to help deliver heat to remote areas.


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Originally Posted by Alvis View Post
Close to equal supply and return area. There are five supply registers and two larger returns.
So you added 2 returns that are connected to 2-9" or 2-10 lines that run back to the furnaces return plenum?
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Old 01-31-2010, 11:59 PM   #7
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Auxiliary in-line fans to help deliver heat to remote areas.


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Originally Posted by beenthere View Post
So you added 2 returns that are connected to 2-9" or 2-10 lines that run back to the furnaces return plenum?
The two returns merge into one 12 inch trunk line that connects directly into the return plentum of which two other returns are connected. These other returns are located elsewhere in the original structure. Without the tools to absolutly be certain, there seems to be substantial draw in each room.

Thanks for asking. So, what are you thinking? I'll double check the duct measurements to be sure, and post if they're different.
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Old 02-01-2010, 12:20 AM   #8
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Auxiliary in-line fans to help deliver heat to remote areas.


What size return grille. And what size duct from the grilles to the 12"?

How long are the 8" pipes that feed the supplies you added.
And how long are those pipes.
How many els in each run.

Too long and too many els. And you'll need big fans that jack your electric bill up.
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Old 02-01-2010, 10:50 AM   #9
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Auxiliary in-line fans to help deliver heat to remote areas.


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Originally Posted by beenthere View Post
What size return grille. And what size duct from the grilles to the 12"?

How long are the 8" pipes that feed the supplies you added.
And how long are those pipes.
How many els in each run.

Too long and too many els. And you'll need big fans that jack your electric bill up.
The trunk of the return is actually 8"... not 12... my bad! The 2 return grates (airflow size) are 13.5 X 5.5". From the grates the returns are 6" in dia, with one running only a couple of feet to the trunk, the other, about 6'. There are a total of 5 90's and 2 45's in its 26' run.

ONE supply is 7", the other 9". The 7"er has 4 90's and 3 45s. Running about 25' to two average size registers. The niner has 3 45s and runs to three registars, one being in a walk-in closet and smaller. The main run is about 22' long with a Y that reduces to 6". They are each 5' long and supply two registers in one of the rooms. The closet supply Ys off the main trunk and is 4" and runs about 14' with two 90s.

A bit of a convuluted design, but it was necessary to work around existing and future plumbing and structure design to make it work out.

Thanks again for considering my options Beenthere.
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Old 02-01-2010, 11:12 AM   #10
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Auxiliary in-line fans to help deliver heat to remote areas.


Your short on return to that area for the supply you have.

Weather or not your other existing return is large enough to make up for it I don't know.

On e way to check. Is if you can have someone at a supply in that area. Run the fan so they can feel the air flow. Then you go down and remove the blower panel cover. And hold the blower door switch in. And have that person tell you if the air flow from that register increased or not.

The test can be somewhat misleading. but I believe you'll find that more return will get you more supply air to that area. And may be enough that you don't need to install inline fans.


Quote:
There are a total of 5 90's and 2 45's in its 26' run.
The equivalent length for the above quoted run is min of 76' to possibly 130'. Depending on configuration of the els.

EG: 2-90's put together to form an S have an equivalent length of 30'.
1-90 by itself is only 10' of equivalent length.
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Old 02-01-2010, 11:43 AM   #11
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Auxiliary in-line fans to help deliver heat to remote areas.


[quote=beenthere;392744]Your short on return to that area for the supply you have. Weather or not your other existing return is large enough to make up for it I don't know."

One of the existing return grates is (airflow size) 29" X 13" and is has a large (14"?) flex duct that is only several feet in length to the plentum. The other existing return grate is 11.5" square, and has an 18 foot 8" flex duct duct.

It seems odd that the supply airflow, which seems to be adaquate, looses so much temperature in those relatively short distances. The supplies that are in the new structure's crawlspace are insulated with a 6 rating air bubble wrap. However, under the old structure, which is considerably warmer yet unheated (wood furnace close by), those same supplies are not wrapped. The combined uninsulated section is approx 25' in length and are where most of the 90s are. I think the first thing I'll do (wednesday) is too wrap that uninsulated pipe and then take temps again at the supply grates to see if that helped.
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Old 02-01-2010, 11:57 AM   #12
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Auxiliary in-line fans to help deliver heat to remote areas.


That 14" flex only moves 790CFM at a .1" friction rate.
The 8" flex only moves 180CFM at a .1" friction rate.

The linear length is often misleading to most people. Even though its short. The connection to the return box the grille is in. may have an EL of 35' or more. The connection to the return plenum can be 65', or even 130' depending on its configuration.

So that 14" flex may only be moving 550 to 600 CFM.

Slow moving air in a duct will tend to cool off a lot before it gets to its supply register.

Try the test I said, and see if that doesn't increase your air flow.
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Old 02-01-2010, 12:15 PM   #13
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Auxiliary in-line fans to help deliver heat to remote areas.


Quote:
Originally Posted by beenthere View Post
That 14" flex only moves 790CFM at a .1" friction rate.
The 8" flex only moves 180CFM at a .1" friction rate.

The linear length is often misleading to most people. Even though its short. The connection to the return box the grille is in. may have an EL of 35' or more. The connection to the return plenum can be 65', or even 130' depending on its configuration.

So that 14" flex may only be moving 550 to 600 CFM.

Slow moving air in a duct will tend to cool off a lot before it gets to its supply register.

Try the test I said, and see if that doesn't increase your air flow.
Will do! I'll have help on Wednesday and we'll give er' ah' go. I'll post what we discover. Thanks again.
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Old 02-01-2010, 12:54 PM   #14
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Auxiliary in-line fans to help deliver heat to remote areas.


"On e way to check. Is if you can have someone at a supply in that area. Run the fan so they can feel the air flow. Then you go down and remove the blower panel cover. And hold the blower door switch in. And have that person tell you if the air flow from that register increased or not."

Should the air flow increase or decrease after removing the panel?
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Old 02-01-2010, 02:11 PM   #15
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Auxiliary in-line fans to help deliver heat to remote areas.


It should increase.

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