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Old 03-08-2009, 12:17 AM   #1
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how many trunk line and feeder lines


First off, due to a 20K pay cut, funds are limited and financing is not an option, so I can't even consider replacing my units. I know it would pay for itself in the long run, and I will eventually replace the units but until then, I can only do what I can afford to do with cash.

Living in Texas, we have two 23 year old heat pumps/AC, a 4 ton and a 2.5 ton. Both are blowing cold but the duct work is in shambles and we are wasting money. All was installed by the original owner, who was by no means an expert and according to my neighbor, he was seldom sober . He even ran duct work to the garage, which of course we removed the first year we were here. The large unit runs all day long in the summer and never cycles off until about midnight.

We know that when we replace the units, we will also replace all the duct work so we are thinking that to conserve what little money we currently have and to reduce our elec bills, we will start on the duct work this year. We know we are losing air in the ducts in the attic. The duct work is not taunt, with excessive bends, curves and even a loop. It crosses from one end of the house to the other end and at one time there was a trunk line that lead from one plenum to the other plenum. Go figure!

In case you need the info, the large unit cools about 1200 sq ft and the small unit cools about 750 sq ft. This is a ranch style house with the living/kitchen/dining on the south and all the bedrooms on the northside of the house. The living room has a cathedral ceiling. We have 6" exterior walls so that is a bonus.

The "blue print" I have drawn for the big unit has 5 trunks and 2 of those trunks are feeding 2 vents. I could play with this some more and reduce the main trunks and add more feeders, which has the potential for reducing the total length of duct needed. If I can convince my former-builder hubby, I will be using metal ducts for the main trunks and then flex duct to the vents. Of course, all of these will be insulated.

So on to my question. Is there a standard/maximum number for how many trunk lines from the plenum and how many vent lines from the trunk lines?

Thanks for your input.


Last edited by Grandma Lewis; 03-08-2009 at 12:28 AM.
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Old 03-08-2009, 07:51 AM   #2
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how many trunk line and feeder lines


There is no max number per say.

But first.
You have 6.5 tons, for less then 2000 sq ft.

Start with some easy things first to check.

Are all switches and receps sealed, Did you replace or repair all worn door seals, have you caulked around all windows.
If you have recessed lighting, has your hubby, done up in the attic and checked to see if they are sealed.
Air leaks around those lights, causes high cooling bills.

Next, how much insulation is in the attic.

Improve all those things, and your cooling load will drop, and your electric bill with it.

Then you can start to redo the duct work.

If you want to do it right.
First a load calc to see what size A/C your house really needs, and to determine how much air in CFM each room/area needs.

Then, a duct layout to determine what FR to use on the ductulator.
That .1" FR, is NOT static pressure.

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Old 03-08-2009, 09:32 AM   #3
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how many trunk line and feeder lines


Thank you beenthere. We've done all that. Well except the recessed lighting since we dont' have any. The attic could probably use some more insulation because it has settled some over the years. I am adamant about keeping the windows caulked because of the drafts in the winter.

We've replaced most of the windows and all are doubled paned.

I can feel cold air coming from the ducts while I'm up in the attic. Yes, I go up in the attic when necessary. I am smaller and can move around easier. I can see that there are rips in the duct work. Time has taken it's toll up there. We live in the country so I'm sure there have been critters up there over time.

I'm sure we could save some money by just reworking the existing duct work, removing the excess and re-wrapping and sealing but I'm old school and believe we should replace it with metal ducts and limit the amount of flex duct.

Thirty years ago, while six months pregnant, I helped my ex install a central heat/AC in our house and after that experience, one doesn't forget how it should be done. I'm not looking forward to doing it again but i know I can drop these utility bills with a job done right.

We got an estimate for replacing the unit a few years ago, but the estimate came in at 10k. However, the salesman didn't give us load calc. From whom would we get that?
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Old 03-08-2009, 05:23 PM   #4
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how many trunk line and feeder lines


Although any contractor that does an estimate, should do a load calc.
Many don't.

And, unless you pay them for it, its their property.

You can do your own however.

Its a 50 dollar fee, but worth the money.

HVAC CALC
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Old 03-08-2009, 06:07 PM   #5
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how many trunk line and feeder lines


I got a problem with a dometic a/c unit on the roof of a 5th wheel trailor.for some reason the compressor stays on after the temp is reached.It will freeze up the coil and then kick off in overload.This unit consist of a board with relays to turn on the fan and compressor.I replaced the board and t-stat twice and it fixes the problem for a week or two.It then goes back to the compressor staying on.It cools good and the t-stat works like it should.Im just not sure whats making the compressor stay on.there is no contactor,only the relays on the board.these units electrical is alittle differant then a home a/c.I need to find someone who is filmilair with this stlye unit and maybe encountered the same problem.the compressor will not shut of with the t-stat,only the fan will.If I reset the breaker,the system will work rite,but only for a couple of weeks or days.I have also install a surge protector on this unit thinking it might help protect it,only to find it still acting up.
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