Making the best of the Duct sytem I've got


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Old 07-22-06, 07:05 AM
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Making the best of the Duct sytem I've got

I have a two story house that has an upper and lower HVAC system. The upstairs system is a three-ton heat pump with an eight year old air handler and a three year old condensing unit. The air handler and duct work are in the attic.

When we bought this house late last summer I found that the upper system could not maintain a tístat set point of 78F on a 93F day. It would lose about seven degrees by mid afternoon with the system running constantly (which I would expect on a hot day, but also expect it to maintain the set point).

As I checked things out I found that the evaporator coil to be very dirty and cleaning it allowed the system to maintain the set point. It will now keep it to within one degree at the tístat on a 95F day.

Now on to the duct work. Of the four bedrooms on this level, two have good airflow from the supply registers and the temp is maintained at the set point. The other two bedrooms have minimal airflow from the supply registers (I havenít found any obstructions) and are about five or more degrees hotter in the late afternoon. Additionally, these rooms are the farthest from the air handler. The square footage of each of the four rooms is about the same but the hot rooms are on the west side, have much higher ceilings and more windows, thus there is more heat gain.

It looks like the duct work is supposed to be an extended plenum design. The air handler is at one end of the duct trunk and the rooms closest to it are the comfortable rooms. The supply system has a total of 15 supply registers with each one being fed with a six inch flex duct coming off the trunk. Each bedroom has three supply registers Ė even though two rooms have a larger volume area to be conditioned, and there are four other supply registers in bathrooms and such.

The return is a single 14 inch flex duct with a 20x20 return grill and filter located in the ceiling of the hall about in the middle of the upper level. The tístat is located there too. According to information I have found, this duct will only return 700 CFM to the system. If so, the system is returning 700 CFM for 15 six-inch supply ducts or a total supply of 1,500 CFM (1,500 for a three ton system seems to be the upper limits of its supply CFM Ė probably closer to 1,300).

My ultimate plan is to completely redo the duct system with all metal and two zones while also upgrading the HVAC equipment on this upper level. But, in the meantime I hope to be able to make it work better with minimal cost.

My thought was to add an additional 16 inch return duct. This would then provide the system with capacity to return 1,600 CFM, or slightly more than the system should need. I would put this new return closest to the hot rooms with the hopes of pulling some of the hot air out of those rooms. Plus, with more air being returned to the system I hope to get more airflow from the supply registers in these hot rooms.

I appreciate any and all comments and suggestions.

Mark
 
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Old 07-22-06, 12:36 PM
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I think that you have a pretty good handle on the problem and the fix. Certainly your return-air duct is woefully undersized and if it is readily accessable I would suggest that a properly sized main be installed to the central RA grille and that individual return-air ducts be run from each of the rooms that have problems. A rough rule-of-thumb is that individual RA ducts should have a cross-sectional area 1-1/4 to 1-1/2 times the total area of all supply ducts that the RA duct will be serving. Be sure that all supply and return ducts are adequately insulated.

Since the problem rooms have a larger volume (higher ceiling) AND a higher heat load (west side and large windows) you need to provide a higher volume of air to these rooms. Additional branch ducts and registers is the preferred way but I think you would get a fair amount of relief by using balancing dampers in the branches to the rooms that do not have problems. These dampers should ideally be placed where the branch leaves the main duct. Try closing down on these branch dampers which will restrict the airflow into the non-problem rooms slightly. This in turn will raise the main duct pressure and force more air out through the branch ducts at the end of the run and into the problem rooms. It will mean that the rooms that are now okay will take slightly longer to cool but it will make the other rooms more bearable.
 
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Old 07-22-06, 03:03 PM
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furd,

Thanks for your response.

I have thought about the individual damper approach but was not sure how easy it would be to retro-fit them. I am currently closing two of the three supply registers in both of the rooms that are okay (a total of four supply registers or 400 CFM) to try and force more air to the hot rooms. Canít say that it has done much to help though. I suspect the dampers will do a better job of cutting the air off to those supply registers, more so than merely closing the register (seems like there is a lot of leakage).

The two hot rooms have a total of eight supply registers (counting the walk-in closet and master bath) with each register served with a six inch flex duct. That is a total of 800 CFM. With the 16 inch return I will have 900 CFM or 1.125 times the supply. A little lower than the optimal but still more than 1 to 1. I guess alternatively I could put a 10 inch in each room for a total of 1000 CFM which would provide a ratio of 1.25.

Thanks again for all comments.

Mark
 
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Old 07-29-06, 09:39 AM
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I have been re-thinking my approach to this problem after furd suggested that putting the correct sized central return duct might be a better approach. I think he is right in that trying to put in two new returns from the hot bedrooms will take up valuable limited attic space. I still need to re-do the supply duct so I may need that space.

Therefore, I am thinking of putting a new central return of 20 inches. Am I correct in my calculations that it should be able to support 2,000 CFM? And will a 20x25 return grill be the correct size?

Thanks again for the help.

Mark
 
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Old 07-29-06, 01:55 PM
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First for 3 ton you need 1200 cfm 400cfm per ton. You sure dont want to go to metal duct in the attic.
The supply system has a total of 15 supply registers with each one being fed with a six inch flex duct coming off the trunk. Each bedroom has three supply registers
Thats 1500cfm that can get out..
Not there cant see it. But that 14" cold air has to go into a box like on the back of the unit. If not build one. Then run a12" flex from the cold air BOX also down a ways and branch off with 5 6" flex to the rooms for a cold air return in each room there .A 6"X12" grill will do it. Dont forget that you will have to put a filter in the unit . Most have a space for a slip in filter. Now with the 14" ==700 cfm and the 12" 500 CFM we now have the 1200 CFM that that poor 3 ton needs for air.
If the trunk line is the same in size all the way down it then the closing down some of a register some place can be done ok.
ED just my .02 cents
 
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Old 07-30-06, 07:00 AM
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Ed,

I appreciate your thoughts and comments. I have considered the individual room return air approach, and, in a perfect world, I would do it. Unfortunately, I have limited space in the attic that makes it prohibitive and getting up there to replace a filter regularly is a major effort. Not conducive to making sure the filter is always changed regularly.

That is why I thought I could just put returns in the two hot rooms but those would have to be so large (10 to 12 inches for each room) that again I have a space issue. So, I am back to upping the central return to a size that is sufficient to provide for the supply with a ratio of around 1.5 return to supply. There is enogh room to replace the exisitng 14 inch return with a single 20 inch central return in the limit space that I have.

If I go with the 20 inch central return I think it can provide 2,000 CFM. Is that correct? If so, then that gives me a 1.67 return to supply ratio based on 1,200 CFM supply. If I it is 1,500 CFM - since I have 15 supply ducts at 6 inches, then the ratio would be 1.33. Either way it should be about right. Correct??

Then I need to know if a 20 x 25 return grill is the correct size?

Thanks again for your comments.

Mark
 
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Old 07-30-06, 07:27 AM
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Then I need to know if a 20 x 25 return grill is the correct size?
Now for a good duct to a 3 ton unit is 80sq " that 240sq " + 15% for a filter grill = 276sq" . That 20X25= 500sq" . "So your good to go" A 16" round should do it
just a rule of thumb

Also you say bed rooms. Are door closed a lot???
Do the door clear the floor or rugs by about 2" to let the air outof the room when the door is closed?????

ED:
 
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Old 07-30-06, 08:59 AM
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Ed,

Again, I appreciate your comments.

The bedroom doors are mostly kept open so a central return should work. The doors do have an undercut of about 1.25 to 1.5 inches so for the few times they are closed (and only for short periods when they are) I think the undercut is sufficient. If not, then I can increase the undercut.

You suggest that a 16 inch return may be enough, but I thought I would go bigger to provide for more air flow and to keep air noise down. Just want to confirm - does 16 inch provide 1,200 CFM, 18 inch 1,600 CFM and 20 inch 2,000 CFM?

Based on your calculation the 20 x 20 (400 sq.) grill I currently have should be enough and I shouldnít really have to increase it to the 20 x 25 setup. Or would the current grill increase the air noise at the return too much?

Again many thanks,

Mark
 
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Old 07-30-06, 09:17 AM
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80sq" per ton works good 60sq" per ton is tight
This is the duct work.. Yes 18" round would be better 1350 cfm
Now on the bed room doors. with the unit fan on or AC on . Open a door about 1" if the blower pulls it close then cut some off the door Or this is what we look for .

ED have fun
 

Last edited by Ed Imeduc; 07-30-06 at 09:36 AM.
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Old 07-30-06, 10:03 AM
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Ed,

Okay, sounds like the return grill I have at 400 sq should be sufficient. That makes it even easier to upgrade the return duct.

Right now the doors donít move at all when closed down to just one inch open. Iíll see how they do once I increase the size of the return.

So an 18 inch will only flow 1,350 CFM? I must be calculating this incorrectly. Do you have an easy formula to calculate CFM?

Thanks again,

Mark
 
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Old 07-30-06, 02:14 PM
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Dont have the duct cal . here with me go try this
http://www.udarrell.com/proper_cfm_b..._SIZING_CHARTS

This will give you lots to play with but as I said 80sq" per ton is a very good job and 60sq" is a tight job.

ED
 
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Old 07-30-06, 04:08 PM
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Ed,

Thanks for the web site - I'll check it out. And thanks for all the help too.

Mark
 
 

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