Help with central air!!

[jagermeister5]

Hopefully someone can help me out. I'm stepping out of familiar territory (civil, water resources) into the unfamiliar (HVAC).

I had severl AC contractors over to my house to give estimates on installing AC. I received all kinds of conflicting info. I was told that my existing duct system could handle a 2 1/2 ton ac unit. I was also told that my existing duct system COULD NOT handle a 2 1/2 ton system.

Does anyone have a link to charts or graphs so I could try to determine myself whether or not I have adequate ducts.

I'm looking for something that would tell me the minumum duct size and # of runs for a 2 1/2 ton AC unit. Also maybe minimum air return duct and grate sizes.

Any help would greatly be appreciated.

Thanks!!

 

[imok2]

You need 1 supply main of 14" round or 12"x14" and a return of 16" round or 14'x 16"
You will have approx 1000 CFM at 0.5 static at 2 1/2 tons
For the rooms I would use 6" round for up to 220 cfm And 8" for 225 to 400 cfm. these are just rules of thumb.

 

[ChasBean1]

What's moving air through your ducts right now? What is the cfm rating of the existing fan? What is the pressure across the existing fan? If you don't have a manometer, you can make one out of flexible tubing (just measure the difference in heights of the water columns)...

 

[RACandH]

General rule of thumb is 400 cfm per ton. For residential you can usually run with slightly less than 400 cfm per ton.

Gas furnace? Measure the temperature rise accross the furnace heat exchanger once running for 5 minutes & stablized. Observe the nameplate for the Btuh input. Find out the rated efficiency of the furnace.

Use: Btuh = cfm x 1.1 x rise

For the BTUH use the nameplate input x efficiency (0.00) If you don't know the efficiency, post make and model.

This will give you the actual cfm delivered by the furnace within plus or minus 10%.

Same with an electric furnace except measure the energy input with an amprobe meter and assume 100% efficiency.

 

[IRstuff]

It may be that what the installers are referring to is how noisy the ducts will be, and not necessarily whether the ducts have sufficient capacity.

This will be particularly noticeable if they get their way and instally a system that's oversized by a ton, as they are wont to do.

As a general rule, older houses tend to have less ducting that a brand-new house, so retro's to existing houses will tend to wind up with a noisier system, unless the ducting is re-sized. But, that's a personal choice.

Additionally, you may find that putting in a 1st rate insulation will be paid back in lower electricity bills during winter and summer.

The decreased rate of heat exchange with better insulation may allow you to slightly undersize the A/C.

TTFN

 

[jagermeister5]

Thanks for the replies.

Well I measured my suppply (duct leaving the furnace). It’s 17”x11”. In the crawl space I have four ducts coming from the main supply, each of 6”, going to the downstairs vents. I assume the upstairs duct work is the same size, 6”. Upstairs I counted 8 supply vents. All the supplies transition from 6” duct to 2”x14” outlets.

The main return line (going into the furnace) is 18”x11”. I have 3 returns, all upstairs. Two of the returns have a 2”x14” intake that transitions to a 6” duct. The third return is just the 6” duct. All three returns have a 13”x5” grill on them.

My current furnace is a Lennox Model G12Q3E-82-6. I’ve been told by the majority of the contractors that this furnace is fine for ac. Two contractors told me I need a new furnace. I didn’t find much info on this type of furnace on the Lennox site.

Going with RACandH’s calcs.

Listed temp. rise= 70-100 or 30 degree’s
Btuh = 82,000
Assumed efficiency of 50%

cfm=1240

From this calc it seems like the furnace fill definitely push 2.5 tons. 2.5x400cfm=1000 cfm required.

I didn’t see an actual cfm rating on the fan. Only that it’s 1/3 horepower.

So I think I have no problem with the supply ducts. But I'm still a little worried about the return duct.

Any more insight?

Thanks again for the help!!

 

[ChasBean1]

You could duct in a portion of outside air (maybe 15-20% by volume) into the return plenum. This would reduce the return flow requirement, allow more turnover of the air within the house, and maintain the house under slightly positive pressure to prevent infiltration and moisture issues in unwanted areas.

 

[RACandH]

jagermeister 5

Sorry, maybe I didn't explain properly.

The "listed" temperature rise is the furnace design operating parameters and is found on the nameplate. You will need to measure the actual rise with two thermometers. One in the supply and one in the return. The difference is the actual rise. Your actual rise should be between 70 to 100*F for the furnace to be operating within it's design parameters. Use this measured value NOT the nameplate rating. Your furnace annual operating efficiency (AFUE) may be as low as 50%, but the "spot" efficiency is more likely around 85% Take all your readings when temps stabalize - after a few minutes run time and note both supply and return temps simultaneously for greatest accuracy.

Again, use the sensible heat formula

BTUH = cfm x 1.1 x rise
(82,000 x .85) = cfm x 1.1 x rise

I guarantee you this will give you your cfm MORE accuratly than running around and counting ductwork and takeoff's. The various sizes of your ductwork fail to prove anything. The addition of an 'A' coil / evaporator coil in your ductwork will generally reduce the cfm by about 10%.