Friction loss - how important is it? Please help!

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

You do not have a trivial problem here. I once built a house which required a complex hot air heating system. The general hired a self discribed "tin knocker" to spec and install the system and it was a disaster. After spending countless hours attempting to patch and balance the system I called a HVAC professional engineer in. He came and spent 2 hours making measurements. In the end he couldn't come up with a reasonable solution and refused to even bill me.

Don't go there yourself. Get a qualified engineer in on the front end. The sooner the better, you can't do this stuff by the seat of the pants and the backside cure is very expensive. Unqualified installers typically overcompensate for their lack of engineering ability by oversizing the system, reasoning that too much is better than too small. The result is a poorly performing system that short cycles and overshoots the set point. On the cooling side the result is a system that short cycles and can't control humidity properly.

I cant repeat it loudly enough, Get a Professional engineer, PRONTO!

John in Herndon

In a 24 ft run there will be little "friction" loss but there is another factor that you should consider.

If you run a constant size main line with drops off of it the pressure can be greater at the end of the line than it is at the beginning. That is all related to something called the Bernoulli effect. That is why, if you look at a long run in something like a gymnasium, you will see the size of the main reduced just after each point where an outlet is located. It probably won't matter much if you have the means to adjust the vents.

The most important thing you want to do is make sure the ducts are large enough. That is especially important to achieve the rated efficiency of higher SEER units. http://ths.gardenweb.com/forums/load/hvac/msg0210535931895.html?12=

The amount of electrical power you need depends on temperature differences. Temperature difference are lowest when you have high air flow. Low velocities from large ducts makes for quieter systems. High air flow and low velocities make other things less significant.

Your A/C contractor should do a calculation for you to tell you what you need for duct areas to meet the SEER ratings of the system that you are going to be paying for. If the ducts are too small you will be paying for a system that won't deliver the performance that you are paying for.

http://www.advancedenergy.org/build...y/heating_and_cooling/seer_facts_bulletin.pdf

http://www.galvestonairconditioning.com/Articles/DOEChangesMininumSEERRatings.html

I would work on getting a reliable contractor that knows what he is talking about, and I would check out what he tells me. I would probably talk to 3 contractors and would be inclined to go with the one that recommends the largest ducts, unless the price is totally out of line. Since the contractor should be doing the engineering I would not spend money on a separate engineer for the size house you have.

lostinLA said:

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

Click to expand...

A) Pressure loss in the duct due to turbulence, balancing assures that the CFM requirement of a space is met...the air is balanced for the space neither too much nor too little only that which is required to condition the space

-Get a ductulator!
-Measure each section separately
-Consider your transition part of the next size duct
-You won't have to worry about the numbers, just design the system with the information provided by the ductulator

B)
-No
-The ductulator will properly calculate the duct size given the desired static pressure and CFM's; If you oversize them, what is the point in being precise with your calculation?
-If your ducting is properly sized you won't need dampers.

Have you tried your local supply houses? Some of them will figure your load calculation, and design your duct layout if you purchase your ducting from them. They can do it quickly by computer. You may want to check with them.