undersized supply ducts

[eric1978]

I'm curious about what people mean when they say that supply ducts are undersized. I think I understand static pressure and all the analogies about blood pressure and blowing through straws when running.

So let me ask this: if TESP at the air handler is too high (hypothetically, say, 1" instead of the rated .5"), would it help to add more registers or increase the size of the registers in each room? Or do you actually need to add more supply ducts?

In my pea-sized brain, creating more openings in the supply duct should decrease resistance. It seems obvious. On the other hand, when people talk about supply ducts being undersized, wouldn't cutting another hole in them actually decrease the square footage of the ducts? Would this exacerbate the problem?

Disclaimer: I'm not going to make major modifications to my home HVAC system on my own. I realize these things are more complex than they seem, and that any answers to this post will likely not include all the information needed to make an informed decision. Also: I don't own any sheet metal sheers*, so there's that.

*Edit: yet

[vstech]

ok... if your tesp is 1"... you do not know how undersized your ducts are because 1" is the maximum the blower can produce...

at a certain pressure, a pipe can only flow so much air.

all systems need a certain flow of air to transfer the heat to where you want it.

if you have ducts that will only flow 1/2 of the required air the equipment needs, the blower will spin faster and faster until it's efficiency is maxed out, and it will stall. the equipment will not have sufficient air to move the heat and either the furnace limit will pop, or the air conditioning compressor will flood with liquid...

it's got to be the CORRECT amount of air for the equipment to work right.


supply ducts are not one long duct with holes in it.

a blower should blow into a large plenum, and small ducts branch off the large plenum.

first the plenum has to be large enough to flow the required air.

then the branch ducts need to be large enough to flow the air the room needs. some rooms need more branch ducts in them to get enough air to condition the room

so... if you have a large plenum with only 4 small branches in it, it will not flow enough air out the 4 branches. making the branch ducts larger will flow more air into the rooms... or cutting more branches into the plenum will flow more air...

understand?

[BNME8EZ]

The fix for duct that is to small will vary depending on what the problem is. if the main trunk is to small cutting more holes will not help. If the main trunk is large enough but it doesn't have enough branches then more or bigger branches will. One thing to remember is it is not just size but length also, and not just any length but Total Equivalent Length. So what do I mean by this? A 6" pipe will move 100 cfm 100 feet. Pretty simple. But a #13 boot is equal to 80' and a take off is equal to 50' or more, then add in 15' of pipe and you are well over 100'. That 6" run won't deliver 100 CFM in this case so a bigger run would be the fix. If the run is flex instead of pipe then there is another derating factor that must be figured in.

[eric1978]

Thanks, this all makes sense. That's why I leave it to the pro's, but it's still something I'm interested in and always like to learn something new.

Regardless, it sounds like an expensive PITA to fix. I wish the people who renovated my 120 year old home had left the ductwork exposed. There's no access above or below (used to be apartments), so tearing out drywall is the only way to do any duct work. Oh well. It's sized right enough to be comfortable, I just hate it when things aren't perfect.