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residential duct design modifications a student 08-28-2006
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Posted by a student on August 28, 2006, 10:07 am
I have a LR which, on hot days, remains 5-10 deg hotter than the rest
of the house. The A/C is capable of cooling the LR, but that makes the
rest of the house uncomfortably cool. If you close the registers in an
effort to direct more air to the LR, it causes a lot of noise at the
registers as airflow is very strong everywhere except the LR.

The LR is twice as big as the next largest room in the house, and has
two registers. All other rooms have one. The LR registers are twice as
far from the blower as the rest of the registers. The air flow is
noticeably reduced at the LR registers when compared to other registers
in the house and the temperature at the LR registers is also 2-5
degrees higher.

The LR registers collectively share exclusive access the same trunk.
There are no significant leaks, but the duct path could be straigter
and some excess length could be trimmed (currently just compressed).
However, I do not believe straighting and fitting will sufficiently
improve the situation.

There is a header between the LR and the return which prevent efficient
return of hot air in the LR. There is a fan in the LR but it helps
little.

The LR is also the hottest room in the house due to windows, exterior
walls, and smaller attic space.

The A/C unit also appears to be oversized - typically on hot days the
cycle is only 8 minutes - on for 8, off for 8.

My thought at this point is to increase the size of the duct (trunk and
both branches) in order to increase airflow to the LR. Currently there
are four 8" trunks coming from the blower, each with two 6" branches. I
am proposing to increase the LR trunk to 12" and branches to 8".

Am I on the right track?

TIA,
chris


Posted by on August 28, 2006, 10:30 am

>I have a LR which, on hot days, remains 5-10 deg hotter than the rest
>of the house. The A/C is capable of cooling the LR, but that makes the
>rest of the house uncomfortably cool. If you close the registers in an
>effort to direct more air to the LR, it causes a lot of noise at the
>registers as airflow is very strong everywhere except the LR.
>
>The LR is twice as big as the next largest room in the house, and has
>two registers. All other rooms have one. The LR registers are twice as
>far from the blower as the rest of the registers. The air flow is
>noticeably reduced at the LR registers when compared to other registers
>in the house and the temperature at the LR registers is also 2-5
>degrees higher.
>
>The LR registers collectively share exclusive access the same trunk.
>There are no significant leaks, but the duct path could be straigter
>and some excess length could be trimmed (currently just compressed).
>However, I do not believe straighting and fitting will sufficiently
>improve the situation.
>
>There is a header between the LR and the return which prevent efficient
>return of hot air in the LR. There is a fan in the LR but it helps
>little.
>
>The LR is also the hottest room in the house due to windows, exterior
>walls, and smaller attic space.
>
>The A/C unit also appears to be oversized - typically on hot days the
>cycle is only 8 minutes - on for 8, off for 8.
>
>My thought at this point is to increase the size of the duct (trunk and
>both branches) in order to increase airflow to the LR. Currently there
>are four 8" trunks coming from the blower, each with two 6" branches. I
>am proposing to increase the LR trunk to 12" and branches to 8".
>
>Am I on the right track?
>
>TIA,
>chris


yup, you're on the right track. all you need is another 5-7 years of
hands on experience to arrive at a practical solution to the LR
insufficient airflow problem.

btw, what did the manual D say you needed for the LR duct sizing?

what? no manual-D? errrr well, keep rolling the dice....

Posted by Noon-Air on August 28, 2006, 10:42 am

>
>>I have a LR which, on hot days, remains 5-10 deg hotter than the rest
>>of the house. The A/C is capable of cooling the LR, but that makes the
>>rest of the house uncomfortably cool. If you close the registers in an
>>effort to direct more air to the LR, it causes a lot of noise at the
>>registers as airflow is very strong everywhere except the LR.
>>
>>The LR is twice as big as the next largest room in the house, and has
>>two registers. All other rooms have one. The LR registers are twice as
>>far from the blower as the rest of the registers. The air flow is
>>noticeably reduced at the LR registers when compared to other registers
>>in the house and the temperature at the LR registers is also 2-5
>>degrees higher.
>>
>>The LR registers collectively share exclusive access the same trunk.
>>There are no significant leaks, but the duct path could be straigter
>>and some excess length could be trimmed (currently just compressed).
>>However, I do not believe straighting and fitting will sufficiently
>>improve the situation.
>>
>>There is a header between the LR and the return which prevent efficient
>>return of hot air in the LR. There is a fan in the LR but it helps
>>little.
>>
>>The LR is also the hottest room in the house due to windows, exterior
>>walls, and smaller attic space.
>>
>>The A/C unit also appears to be oversized - typically on hot days the
>>cycle is only 8 minutes - on for 8, off for 8.
>>
>>My thought at this point is to increase the size of the duct (trunk and
>>both branches) in order to increase airflow to the LR. Currently there
>>are four 8" trunks coming from the blower, each with two 6" branches. I
>>am proposing to increase the LR trunk to 12" and branches to 8".
>>
>>Am I on the right track?
>>
>>TIA,
>>chris
>
>
> yup, you're on the right track. all you need is another 5-7 years of
> hands on experience to arrive at a practical solution to the LR
> insufficient airflow problem.
>
> btw, what did the manual D say you needed for the LR duct sizing?
>
> what? no manual-D? errrr well, keep rolling the dice....

manual J?? nawww..... the lowest bidder just uses a rule of thumb.

You can get it done, or you can get it done right.



Posted by a student on August 28, 2006, 4:18 pm

gofish@gonefishin.net wrote:
>
> >I have a LR which, on hot days, remains 5-10 deg hotter than the rest
> >of the house. The A/C is capable of cooling the LR, but that makes the
> >rest of the house uncomfortably cool. If you close the registers in an
> >effort to direct more air to the LR, it causes a lot of noise at the
> >registers as airflow is very strong everywhere except the LR.
> >
> >The LR is twice as big as the next largest room in the house, and has
> >two registers. All other rooms have one. The LR registers are twice as
> >far from the blower as the rest of the registers. The air flow is
> >noticeably reduced at the LR registers when compared to other registers
> >in the house and the temperature at the LR registers is also 2-5
> >degrees higher.
> >
> >The LR registers collectively share exclusive access the same trunk.
> >There are no significant leaks, but the duct path could be straigter
> >and some excess length could be trimmed (currently just compressed).
> >However, I do not believe straighting and fitting will sufficiently
> >improve the situation.
> >
> >There is a header between the LR and the return which prevent efficient
> >return of hot air in the LR. There is a fan in the LR but it helps
> >little.
> >
> >The LR is also the hottest room in the house due to windows, exterior
> >walls, and smaller attic space.
> >
> >The A/C unit also appears to be oversized - typically on hot days the
> >cycle is only 8 minutes - on for 8, off for 8.
> >
> >My thought at this point is to increase the size of the duct (trunk and
> >both branches) in order to increase airflow to the LR. Currently there
> >are four 8" trunks coming from the blower, each with two 6" branches. I
> >am proposing to increase the LR trunk to 12" and branches to 8".
> >
> >Am I on the right track?
> >
> >TIA,
> >chris
>
>
> yup, you're on the right track. all you need is another 5-7 years of
> hands on experience to arrive at a practical solution to the LR
> insufficient airflow problem.
>
> btw, what did the manual D say you needed for the LR duct sizing?
>
> what? no manual-D? errrr well, keep rolling the dice....

Yep. Or maybe a 5-7 minute conversation with someone who is willing
(and able) to give practical advice.


Posted by on August 28, 2006, 11:38 pm

>
>gofish@gonefishin.net wrote:
>>
>> >I have a LR which, on hot days, remains 5-10 deg hotter than the rest
>> >of the house. The A/C is capable of cooling the LR, but that makes the
>> >rest of the house uncomfortably cool. If you close the registers in an
>> >effort to direct more air to the LR, it causes a lot of noise at the
>> >registers as airflow is very strong everywhere except the LR.
>> >
>> >The LR is twice as big as the next largest room in the house, and has
>> >two registers. All other rooms have one. The LR registers are twice as
>> >far from the blower as the rest of the registers. The air flow is
>> >noticeably reduced at the LR registers when compared to other registers
>> >in the house and the temperature at the LR registers is also 2-5
>> >degrees higher.
>> >
>> >The LR registers collectively share exclusive access the same trunk.
>> >There are no significant leaks, but the duct path could be straigter
>> >and some excess length could be trimmed (currently just compressed).
>> >However, I do not believe straighting and fitting will sufficiently
>> >improve the situation.
>> >
>> >There is a header between the LR and the return which prevent efficient
>> >return of hot air in the LR. There is a fan in the LR but it helps
>> >little.
>> >
>> >The LR is also the hottest room in the house due to windows, exterior
>> >walls, and smaller attic space.
>> >
>> >The A/C unit also appears to be oversized - typically on hot days the
>> >cycle is only 8 minutes - on for 8, off for 8.
>> >
>> >My thought at this point is to increase the size of the duct (trunk and
>> >both branches) in order to increase airflow to the LR. Currently there
>> >are four 8" trunks coming from the blower, each with two 6" branches. I
>> >am proposing to increase the LR trunk to 12" and branches to 8".
>> >
>> >Am I on the right track?
>> >
>> >TIA,
>> >chris
>>
>>
>> yup, you're on the right track. all you need is another 5-7 years of
>> hands on experience to arrive at a practical solution to the LR
>> insufficient airflow problem.
>>
>> btw, what did the manual D say you needed for the LR duct sizing?
>>
>> what? no manual-D? errrr well, keep rolling the dice....
>
>Yep. Or maybe a 5-7 minute conversation with someone who is willing
>(and able) to give practical advice.

Oh I'm willing to give you practical advice, the huge question is tho,
are you willing to comprehend, understand & accept it?

In order to help size your ducts, you're going to have to provide us
with the Blower Curve chart for your particular furnace/air handler,
as well as the T.E.S.P. fo the air distribution system. For
simplicity sakes, post Make, Model & Serial # of furnace or air
handler. Also post your measured TESP, how and where you measured it.

From your past studies you will recollect that a blower will deliver a
specific amount of air based on the TESP of the air distribution
system.

What if your TESP is off the charts? What do you think that will do
for airflow? It doesnt matter if you have a 20" duct, the fan still
will not deliver the required airflow.

It's entirely feasible your entire duct system has to be replaced.

It doesnt make a bit of difference how big your LR is, where its
located, how many walls or how many windows it has. Regardless of
all of its physical characteristics, a person can determine how many
btu's are required to keep the room at 75 when its 95 outside. This
is called a load calculation. AKA Manual J.

How can you begin to size the duct if you're clueless on how many
btu's are required for the LR??

OK so now you've did a room by room load calc, and have determined if
your ac system has been properly sized, (or not). You did this by
comparing the load calc data to the engineering performance data for
your particular ac unit, AT DESIGN CONDITIONS.

OK ac unit is ok, blower is ok & within the fan curve chart, TESP is
within manufacturers range, now do the manual-D, duct sizing!!!

Here's the kicker.....ya cant do an accurate manual-d until you know
your TESP, and you can only measure your TESP once all the air
distribution is installed !!! Of course it is entirely possible to
calculate your TESP. if only you knew how, and had all the required
literature & info.....

ok, wife says the moose steaks are off the barbie, so it's hasta-la
bye bye for now.....

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