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Old 04-03-2010, 05:41 AM   #16
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Zoning question


High static presure doesn't harm PSC blower motors in itself. They actually use less electric when they are working against higher static then they are designed for. Since they are moving less air, they aren't working as hard.

Installing a damper in your 10" to restrict air flow through it, will increase over all supply static, and lower overall air flow, on your system. Which is already not moving as much air as it should.

Using sheet 14" metal(hard pipe) will increase air flow to the sections of your house that the 14" serves.

Those saddle taps are what you can use, or you can buy wyes.

Don't strangle an already choking system. Increase your supply sizes on your 14".

Sun rooms often take 2, 3, or 4 times as much cooling as any other room in the house. Increasing both of its supplies from 6" flex to a 7" will give you another 60 to 70 CFM. Almost the same as adding a third 6" flex supply.

By increasing all of the other supplies on the 14", you will decrease the static pressure to all supplies, including the 10" flex for the master suite. Which will automatically decrease some of the air to your master suite.

A 20X20 return air filter grille is good for 640 CFM at a velocity of300 FPM(recommended velocity). 800 CFM if you want it to be at 500FPM. The higher the FPM through an air filter, the less effective the air filter is.

So your still short on return for your system.

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Old 04-03-2010, 09:21 AM   #17
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Zoning question


Spyder

This subject/thread is a little touchy, so I am a bit reluctant to post. I figured you would have additional rooms with temperature differences. The sizing of your duct system had a lot of red flags. Allocating 35 percent of the air flow to the bedroom and having six inch ducts throughout the house, for example. My suggestion wasn't to just add supply lines. My suggestion was to add supply lines if needed and increase the duct size to rooms that needed it. Basically, increase the air flow to WHERE it is needed. Your ultimate goal is the get the correct air flow going into the correct rooms. Your goal is not to just get proper cfm levels to the house. You can increase all of your ducts proportionally if you want to as suggested by others, but a temperature difference will still exist between your sunroom and the rest of your main house. Just increasing your ducts proportionally to achieve proper cfm levels is not advisable, if the air distribution isn't balanced. For example, let's say your original contractor installed a 6" supply into a small closet. If you increase your ducts proportionally, you would be adding more air flow to a room that already has too much to begin with. I'm not sure why there is dissension in regards to this. Maybe, I'm not explaining it well enough. Here is another example:

Let's say your vehicle needed to have the tires balanced. Your mechanic states that you need 10 oz of weight, in total (all four tires), added to the tires. How should the weight be distributed?

A) Add all 10 oz to one tire
B) Add the weight proportionally to all four tires
C) Add the weight to the tires that required the weight for a balanced tire

Correct answer is C

The weight needs to be added in the correct quantity to the correct tire. Maybe the above example will help clarify what I have been trying to express.

Now back to the original problem. It is true that the air flow from the master suite will decrease by increasing the duct sizes proportionally to the main house, but it may not be enough to even out the temperature. You can leave the damper fully open, if it is not required. The damper just allows for you to make adjustments to the air flow of the master suite without having to resort to removing or decreasing the size of the ducts. It will give you more control over the air flow to that area. I don't see how someone can know how much air flow is required to your master suite with the information given. I guess my point in general is that you have quite a few problems to address. It is going to take more than just matching your duct sizes to the cfm output of a system. Some of the things you are inquiring about are fairly expensive. If you are going to go through the trouble and expense of replacing ducts with metal, zone systems, total duct replacement, etc., you should have a local contractor determine the required air flow to each room and not just the house. If your main concern is just to have the correct cfm level, you should just remove your existing main duct and add an 18" main duct going directly into the middle of your house. I'm sure this will decrease air flow to your master suite. If your main concern is comfort level, you will do a little more to ensure even and proper air distribution throughout the house.

Last edited by pomelo; 04-03-2010 at 10:02 AM.
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Old 04-03-2010, 10:01 AM   #18
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Zoning question


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Originally Posted by spyder24 View Post

Nonetheless, I do appreciate your suggestions. One thing I realized today, I told you incorrectly what one of my returns is. The one with the 14" flex is actually connected to a 20x20 grill. How I got 14x20, I have no idea. Does that make any difference at all or do you still recommend that I bump the flex duct up to 16"?

Increase register size, and increase duct size to 16"

If you have the time, and desire to do a test of your systems total air flow. Let me know. And I'll tell you how to make a simple device. That will be accurate enough to show you how bad your duct system is. As far as high static pressure.
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Old 04-03-2010, 12:58 PM   #19
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Zoning question


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Originally Posted by beenthere View Post
Increase register size, and increase duct size to 16"

If you have the time, and desire to do a test of your systems total air flow. Let me know. And I'll tell you how to make a simple device. That will be accurate enough to show you how bad your duct system is. As far as high static pressure.
That would be great. I am very curious as to what the static pressure really is. How can I do this?
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Old 04-03-2010, 01:35 PM   #20
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You need a small piece of wood. Plywood, a 1X6, anything like that. About 12 to 18" long.
A length of 1/4 or 3/8" OD clear vinyl hose(6' or so).
A small piece of either copper, or aluminum tubing that will fit into the hose (smaller tubing is better, which means smaller tubing is better).
It works best if you can make a 90 bend in it ( the metal tube ). The opening should be going in the direction of air flow when inserted into the duct. So that the air doesn't blow directly into it.

On the wood, about center of it height wise. Mark off 1/10" increments ( ya, hard to see 1/10" ) starting with 0 (zero) at the center. Do this for 1 to 2" in both directions.
Then take the hose and fasten it in a U shape to the board. Leaving several foot free on one of the ends to work with.

Then insert the copper or aluminum pipe/tube into the free end of the hose.

Next, set the board upright on a level area and fill the tube with water to the zero line.
This can be done in the kitchen, garage, outside, where ever practical.

Then, drill a small hole in the plenum of the supply and return of the air handler. Just big enough to fit the metal tube into.
Make sure the water is at the zero, and then run your A/C.

Start at the supply, insert probe, and see how much the water moves. And record it.
Next do the same with the return.

If the water moves .3" on the side you have marked. It also moved .3" on the side that isn't marked. So that would be a reading of .6".

Same with the return. But you will see it draw the water down. But if it moves down .25 on the marked side, it still moved up .25 on the unmarked side. So it moved .5", this would then be your return static pressure.

Record and add both readings together ( the supply and the return ). This is your total static pressure. The force that the blower is working against in order to move air.

Look to see what blower speed tap your blower is set on.

If you still have the install manual for your air handler. There will be a page that tells you how much air it is moving working against that static pressure, at whatever blower speed tap.

If you don't have the install manual. Post the full model number of the air handler. Both static pressure readings. And the blower speed tap that is being used.

Even if you have the install manual, post both static readings.

What you will have made, is a simple water manometer.

You can measure the pressure in each of your distribution boxes. And see how much pressure you are losing.
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Old 04-03-2010, 04:47 PM   #21
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Zoning question


Thanks beenthere, I will try that this week. Two questions for you:

1. The original blower motor in my unit was a 1/3rd HP motor and was recently replaced with a Emerson Rescue 1/2 HP. I believe the new motor produces higher CFMs than the old one. Will the chart in my install manual still be accurate?

2. How do I plug the holes the return and supply plenum once complete with the test?
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Old 04-03-2010, 05:10 PM   #22
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Zoning question


Quote:
Originally Posted by beenthere View Post
High static presure doesn't harm PSC blower motors in itself. They actually use less electric when they are working against higher static then they are designed for. Since they are moving less air, they aren't working as hard.

Installing a damper in your 10" to restrict air flow through it, will increase over all supply static, and lower overall air flow, on your system. Which is already not moving as much air as it should.

Using sheet 14" metal(hard pipe) will increase air flow to the sections of your house that the 14" serves.

Those saddle taps are what you can use, or you can buy wyes.

Don't strangle an already choking system. Increase your supply sizes on your 14".

Sun rooms often take 2, 3, or 4 times as much cooling as any other room in the house. Increasing both of its supplies from 6" flex to a 7" will give you another 60 to 70 CFM. Almost the same as adding a third 6" flex supply.

By increasing all of the other supplies on the 14", you will decrease the static pressure to all supplies, including the 10" flex for the master suite. Which will automatically decrease some of the air to your master suite.

A 20X20 return air filter grille is good for 640 CFM at a velocity of300 FPM(recommended velocity). 800 CFM if you want it to be at 500FPM. The higher the FPM through an air filter, the less effective the air filter is.

So your still short on return for your system.
Beenthere, when you say wyes, are you referring to something like this?

http://americanhvacparts.com/Merchan...e=duct-teewyes

Also, since the trunkline goes about 30 some feet, I hear it needs to reduce in size. Is there some rule of thumb you use for reducing the trunk size every soo many feet?
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Old 04-03-2010, 06:08 PM   #23
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Zoning question


Yes, a Wye like that.

If you don't have a place that caries knock out buttons(like you use to plug an unused opening in an electrical box) that small. You can use caulk, and foil tape.

It will be slightly different. Not enough that it will throw off the measurement by a lot. At least not at high static.

Rescue motors are generally set up to match the HP of the old motor. So may be no difference at all.


PS: Take those static pressure readings with the air filters in, and also with the air filters out.
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Old 04-03-2010, 07:12 PM   #24
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Forgot your reducing trunk question.

If longer then 24 foot, yes the trunk should reduce.

Technically. You would want to reduce when ever your velocity drops too low. When that is varies slightly with each application.

Basically, on your house. If you use 14" hard pipe, you would want to reduce to 12" after you drop off 250 to 300 CFM(equivalent of 4-6" flex supplies, or 3-7" flex supplies). So that your velocity doesn't drop below 600FPM. And then reduce to 10" after 300 CFM is taken off of the 12".

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