Central Air duct question

[drdoct]

I realize this is hearth.com, but I'm always amazed at the amount of knowledge here so, why not? I live in a 1950's home that had central air retrofitted sometime before I bought the home. I've got a 3 ton unit and a 1500 sq ft home (which should be good). The problem is that the monkey that put the air in really rigged it. It was done with 7" ducts (which I can't find at fix-it stores) and only 1 of them is rigid. The problem is that when they ran the flex duct they held it up with little strips of cloth and after 20 years those strips gave way and the duct deteriorated. Now the only room that gets cold (or hot) is the one that's rigid ducted. What I want to do is run my own duct and insulate it using something from a box store. All they have is 6" and 8". Which do y'all think I should use? Everything now is so inefficient and even the rigid duct really needs sealed and re insulated. After that I've got to come up with something that moves the filter to a serviceable location. Right now it's right at the unit which is in the crawl space right in the middle of the house (gotta crawl in to change it).

So I guess the question is... 6" or 8"??? Has anyone done this before? What'd y'all use to seal the seams with?

 

[Whitebread]

Go with 6" if you're doing rigid duct. Spiral core flex flows 25% less air than the equivalent rigid duct. Use duct sealant to seal the duct seams. Buy a 1 gallon tub and use a chip brush to apply. RCD and Versa Grip are two brands that come to mind. Let it dry overnight, then insulate. Fasten insulation with staples, then use metal foil tape over seams.

 

[drdoct]

Thanks Whitebread, just what I was lookin for!

 

[Whitebread]

Regarding the filter, it depends on the return layout.

If you only have one or two returns in central areas of the house, just put in filter grills. The return grill has a removable louver front, and the filter fits into a recess behind the louvers.

If you have a small return pulling from each room (better for air exchange but harder to modify) then it'll be difficult to make the filter more accessible. In that case I'd add a 4" pleated filter housing to the return plenum at the unit. You'll still have to crawl to the air handler to change it, but you should be able to get about 1 year of service per filter unless you have a lot of pets or live in a very dusty area, in which case you should still get at least 6 months. 4" filters are more expensive ($10-20 each) but probably worthwhile in your situation.

 

[drdoct]

It's got twin returns... a 12" square and a 7X14 or so. I'm going to put them at the grills like you suggested. Think it's going to be a pretty straightforward job.

 

[Whitebread]

I don't have a ductulator nearby and it's been a couple years since I've sized a return, but that seems really small for a 3 ton unit. That might be part of your problem is you're starving your blower and not getting good flow through the air handler.

 

[Dune]

Or you could go to an HVAC supply store, buy 7'' flex-duct and not have to change the fittings at each end.

 

[imiller1974]

Will a 7" flex flow about the same as a 6" rigid??

 

[drdoct]

I'll see whitebread.... I'll check out the numbers off the interwebs. I could go back with flex, but after seeing all of it and how it deteriorates after a while, I'm just gonna hard pipe it all. I priced it out today at HD & Lowes and it's gonna be pretty cheap. I've done lots of duct work for dust collectors and flour mill spouts (4") so it shouldn't be too hard using adjustable elbows then brushing the goop in every seam.

 

[Whitebread]

Hard pipe will definitely last longer. Flex has become popular because it's quicker/easier to run. Both work fine when sized properly.

Generally speaking, the wholesale distributors don't like selling to the public. Those that do make you pay full list price, which is quite high. Not really worth it if you are comfortable running rigid and can buy it at a box store.

 

[gzecc]

Thats funny. I have the same size house and same size unit. Is yours a ranch. I did my own duct install after getting some quotes from AC companies. They all wanted to do the flex. There is no comparison to a solid trunk line. I installed a T trunk with about 5-6' flex running to all registers. It was alot of work, but it will outlast the house!
Definetly the better solution.

 

[drdoct]

Yep a ranch. Went under and measured it all up the other day. Maybe this week will get around to doing 1/2 of it. I'm going to do one side at a time. OH, and seal up the plenum that's leaking around the tape joints.

 

[gzecc]

Drdoct, what do you mean you went under? My air handler is in the attic space. Yours isn't?