What size duct?

Old 12-29-07, 09:31 AM
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What size duct?

I have a home with an existing Trane xe80 furnace. My back 2 bedrooms have virtually no air pressure AND they blow barely warm air. I was going to replace the ductwork with insulated ducting and make it larger so that it actually blows air back there, but I'm not sure what size ducting to use.

The furnace is about 40 feet away from the registers and on the same level. Any advice would be great!
Old 12-29-07, 05:17 PM
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Only way to know is to do a heat load on your home.
Old 12-29-07, 09:20 PM
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Not an easy task to give you good recommendations without a better picture of what you have there...say a sketch/floor-plan showing the ducts, or pictures.

I would check for leaks...I mean "air-leaks". I know of a neighbor where the HVAC installers neglected to put an end-cap to one of the trunks. In my own house I went around patching cracks and holes on just about every take-off (where branch ducts connect to the trunk) and this increased the overall airflow by 22%...then I proceeded to balance the system and insulate trunk and branches...Marilyn Monroe would now not dare to walk by any of the vents in my house
Old 12-30-07, 06:20 AM
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Thumbs up Welcome to our forums knottshawk!

Checking for air leaks is a very good idea and what airman is suggesting is that you take a step back and do a complete calculation on how to properly distribute the air from your furnace.

It is possible to increase the size of the ducts going to the problem rooms but it may not help.
If the ductwork going to other areas is too large or your blower can not deliver more air any attempts fix the problem by just adding a bit of duct may be futile.

Hvac-calc is one software program that is not to expensive if you wanted to do a heating/cooling calculation yourself.
Another option is to get estimates from reliable sheet metal contractors but be wary of any that take a quick walk-through to come up with a complex plan.
Old 01-28-08, 12:19 PM
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I have been the foreman for a ducting specialist for 3 years now and while I don’t have any qualms about what has already been said, there are a number of possibilities that come to mind regarding you problem. There is a lot of info that is needed before anyone can give some good advice, but I have had numerous clients with this exact problem. Some of the problems and solutions have been:

Dirty/clogged ductwork = get it cleaned professionally (power vacuum truck only. Portable equipment will often leave behind large pieces). Fallen/broken/smashed ductwork = this is probably the most common issue. If metal, it needs to be put back together and secured properly. If flex, often times smashed flex looks fine, but it will have minimal air flow. Replace all smashed lines and hang properly so that it will not get smashed again. Improperly sized/divided system = obviously if you have a 6” line from the furnace that is later divided into two or more 6” lines they will not have proper flow. This is harder to spot when dealing with larger lines and will probably take a professional to fix.

These are only a few possibilities. Hope it helps.
Old 01-30-08, 12:24 PM
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Have you checked out what speed the blower is set at?? Could be you can kick it up one speed.
Old 01-31-08, 07:18 AM
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Air Duct Dilemma

With a rough sketch or numeric description of your existing ductwork and a model number of your furnace most of these guys would probably be able to give you some very helpful suggestions, while you are looking at the ductwork take some tape with you (some screws and a cordless drill would also be helpful) patch any holes you can find look for dampers that might be closed where your branch lines take off from the main trunk or plenum box put the fan switch an your thermostat to on instead of auto and if you feel comfortable you can shut off the power to the furnace and check fan speed, several wires from your indoor fan will be plugged into a pc board one for nuetral one for high speed and depending on your model one for medium or medium high and medium low and low the one that is controlling your fan speed in low will be plugged onto a terminal that says heat one or two others will be plugged onto terminals saying park these are currently not being used and one will be on a terminal saying cool.
Say your heat terminal has the low speed wire on it you can take the medium or medium high wire off of park and fasten it to the heat terminal and take the wire you just removed and put it on the open park terminal this will increase your air volume but also decrease your supply air temperature in heating so you need to be a little careful.
The best thing to do is thoroughly evaluate your ductwork, system capacity and the load requirements of your environment all my other suggestions are just band aid adjustments with not enough information.
I hope that helps

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