lostinLA
New Member
Hi all -
I'm working on adding central air and heat to a 1920's bungalow and am wondering if I'm trying a bit too hard to do all the math, given the modest size of the project.
I've spent weeks trying to educate myself and so far, I've run room by room load calcs for the house and confirmed what my senses already told me: that certain rooms needed a lot more or less Btus that you'd think, based on area alone. From that, I was able to size the equipment and calculate the corresponding air supply each room should receive.
But knowing what each room needs in terms of conditioned air is one thing; figuring out how to get it there is driving me nuts!
I called several of those online duct-design companies, and it seems that none of them do a real Manual-D for you; they just apply rules-of thumb based on your square footage.
So I've been trying it on my own and can figure out the branches and trunks based on nothing but desired CFM and FPM, but don't understand the whole friction loss thing. I'm worried about is system balancing, but don't know what to do about it.
I could do a simple extended plenum with two trunks (each under 24') supplying equal-sized branches running equal lengths to deliver similar air volume, but I'd have to use floor registers and would rather not. (And even then, I don't know that I trust the "rule" that says you don't need to reduce the plenum/trunk if the run is under 24 feet; if you feed 5 branches off that trunk without reducing, it still seems to me that the last one will suffer for it.)
I'd really like to run risers up from under the house to a small space outside of the upstairs kneewall and feed ceiling registers from there. But even though it's still a relatively short run to get up there, I don't know how to calculate and compensate for all the transitions it would require.
So, what I'm wondering is:
A) Can anyone explain friction loss and balancing thereby in a way that an idiot like me can understand it? In particular:
-How do you compute it?
-Do you measure each section separately, or the whole path from fan to vent, or for each section of same-sized duct?
-Do you count recuders? How?
-And what do you do with the numbers?
-
B) Do I really need to worry so much about this? Can I just size a given trunk based on the CFMs needed to supply its branches, then oversize the it by an inch or so to compensate for friction, finally adding dampers at each branch to control delivery to the registers?
Any advice would be appreciated. I tend to overanalyze this kind of stuff and I think I may be making myself nuts unnecessarily.
Thanks!
David
I'm working on adding central air and heat to a 1920's bungalow and am wondering if I'm trying a bit too hard to do all the math, given the modest size of the project.
I've spent weeks trying to educate myself and so far, I've run room by room load calcs for the house and confirmed what my senses already told me: that certain rooms needed a lot more or less Btus that you'd think, based on area alone. From that, I was able to size the equipment and calculate the corresponding air supply each room should receive.
But knowing what each room needs in terms of conditioned air is one thing; figuring out how to get it there is driving me nuts!
I called several of those online duct-design companies, and it seems that none of them do a real Manual-D for you; they just apply rules-of thumb based on your square footage.
So I've been trying it on my own and can figure out the branches and trunks based on nothing but desired CFM and FPM, but don't understand the whole friction loss thing. I'm worried about is system balancing, but don't know what to do about it.
I could do a simple extended plenum with two trunks (each under 24') supplying equal-sized branches running equal lengths to deliver similar air volume, but I'd have to use floor registers and would rather not. (And even then, I don't know that I trust the "rule" that says you don't need to reduce the plenum/trunk if the run is under 24 feet; if you feed 5 branches off that trunk without reducing, it still seems to me that the last one will suffer for it.)
I'd really like to run risers up from under the house to a small space outside of the upstairs kneewall and feed ceiling registers from there. But even though it's still a relatively short run to get up there, I don't know how to calculate and compensate for all the transitions it would require.
So, what I'm wondering is:
A) Can anyone explain friction loss and balancing thereby in a way that an idiot like me can understand it? In particular:
-How do you compute it?
-Do you measure each section separately, or the whole path from fan to vent, or for each section of same-sized duct?
-Do you count recuders? How?
-And what do you do with the numbers?
-
B) Do I really need to worry so much about this? Can I just size a given trunk based on the CFMs needed to supply its branches, then oversize the it by an inch or so to compensate for friction, finally adding dampers at each branch to control delivery to the registers?
Any advice would be appreciated. I tend to overanalyze this kind of stuff and I think I may be making myself nuts unnecessarily.
Thanks!
David