House: 750 sq ft (not a typo LOL)
Heat Pump: Gibson 2.5 ton R-22 (yep, older)
Duct work in atic: 5 inch flexible
Filter at air handler: 20" x 20" x 1" washable
Supply side number of room vents total in house: 6
Location: Lower Alabama

I have 6 room vents (5" flexible duct work) coming off a gang box in the attic above the air handler. I'd like to close one of the vents off completely at the gang box and go down to 5 room vents throughout the house.

There are 2 room vents in my small kitchen and I only need 1. While the fan in the air handler is capable of being jumpered to 3 different speeds, the fan is set to the default (lowest speed). Don't think fan speed would be a factor but thought I'd mention it. I like the lowest speed. It's quieter.

Just wondering if going from 6 room vents down to 5 room vents would place undue stress on the fan in the air handler or other mechanicals in the system? Or if there are other considerations I might not be aware of? Thanks !!!

Not an engineer, but the general answer is yes - the increased resistance (higher static pressure) will place some stress on the fan/blower motor. Don't know whether it's enough to shorten it's life.

In my case, a higher static pressure has caused 2 damper motors to wear out prematurely. I have dampers in my ductwork and the motors open and close them to direct the air upstairs or downstairs. Kind of like in your car - you can have the air blowing out at the floor, the dash, or both. Because the blower is pushing too much air, the damper motors have to work harder to close the dampers against the higher pressure.

I've also read that the a/c compressor could could be harmed by high static pressure. As static pressure increases, air flow decreases. When the airflow is too low, the system may send liquid refrigerant into the compressor instead of gas. Over time, this damages the compressor and shortens its lifespan.

Kind of thoughtful answer I was looking for. I have the one vent blocked at the gang box since this afternoon. Think I'll crawl back up in the attic and unblock it.

(6) - 5" flex supplies for a 2.5 Ton heat pump....
Was the system installed by Bubba Gump and the three Stooges?

2.5 Ton, 350-400 cfm per ton = 875-1000 cfm

(6) - 5" flex, 50 cfm per run = 300 cfm

I'd be looking at increasing ALL the existing runs to 8", then you can cut one back!

quote:

Originally posted by Excam_Man:
(6) - 5" flex supplies for a 2.5 Ton heat pump....
Was the system installed by Bubba Gump and the three Stooges?

2.5 Ton, 350-400 cfm per ton = 875-1000 cfm

(6) - 5" flex, 50 cfm per run = 300 cfm

I'd be looking at increasing ALL the existing runs to 8", then you can cut one back!

Thank you for the math !!! Much appreciated. 1945 house was gutted and restored in 2005. I bought the house in 2006.

House never had a central air and heat before 2005. It was a completely new install by ... Bubba Gump LOL.

Honestly, the system has worked fine for 15yrs so far. Had to top the system off twice. First time was because I had a heating and cooling company come out for maintenance and they talked me into removing the "A" coil for cleaning. Fast forward about 5yrs and I had to replace the "A" coil because the previous cleaning agents were too harsh and led to premature failure of the "A" coil and leaks at the "A" coil. Mechanicals haven't let me down ... yet.

EDIT: Here's the stickers off the Air Handler, A-coil and Heat Pump ...

@Excam_Man ... After 15yrs I just noticed this evening, or it just registered, that the (alternate) vent diffuser in the kitchen I blocked/unblocked is smaller than the other 5 diffusers in the house.

The smaller vent near the edge of the kitchen is pointed at a small "room" (used to be a small back porch) just large enough for my washer on one side and dryer on the other ... and back door.

So it could be that the vent I was fooling with today is the only 5" flex run. The others with larger vent diffusers MAY be 8" flex, dunno. Not crawling back up there tonight LOL. Wouldn't be shocked to find they're also 5" either though. Just not curious enough to go back up there right now.

Honestly the only reason I was fooling with it today was because while in the attic to remove an FM antenna, I noticed that one 5" flex line was just about detached from the gang box where silver tape had failed. The attic was being cooled and heated too LOL. I hadn't even noticed air was no longer coming out of that one smaller vent in the kitchen or any diminished output at all the other vents. My electric bill hadn't changed noticeably either. Still averaging only about $100 a month and I'm not even conservative with usage. Keep it 77 degrees indoors year round, heating and cooling. Went to Lowes this afternoon and picked up some silver tape and a 5" elbow to reduce stress on that run at the gang box.

Get ya some 24-36" zip ties.

Put the inner spring duct liner on the fitting and zip tie it.
Then pull the insulation and outer cover over the fitting and add another zip tie.

Sealing it up completely would include using mastic.

Thanks Excam_Man. Much appreciated

Well, I couldn't rest tonight knowing the job on the one 5" run in the attic wasn't done right. Checked while up there and see the other 5 runs are 8" as they should be. Well I think they're 8". Didn't take anything apart to measure. They're bigger than the one 5" run anyway.

Found some zip-ties out in the shed. Zipped the inner spring duct liner and put the silver tape around it. Zipped the insulation and outer cover and put silver tape on it too. EVEN insulated the 5" elbow I'd installed going into the gang box. Used insulation material the original installers left in the attic 15yrs ago. Knew it would come in handy LOL. Got to thinking exposed metal with hot and cold air going through in the attic, would likely form condensation, so insulated the short elbow. Should have been something I did to begin with I guess.

That run will never give me a problem again. It'll last longer than the system. Thanks for talking me through it guys. I don't look at heating and cooling everyday and miss the details if not careful or thoughtful.

When it comes time for replacenent. Have someone calculate the size of the unit needed for your hone. 2.5 tons sounds like it's way over-sized for a 750 sq foot home. Here in South Florida generally a ton is needed per 500sq foot of home (generally depends on many factors like insulation/windows, etc.)