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two 45° vs one 90° elbow flow rate

Member Posts: 253
semi-silly question... regarding water flow rate for 1/2" copper pipe what is the difference if any of doing two 45° elbows separated by less than 2 inches of straight pipe versus using one 90° elbow? Is there any difference of 1/2" pipe vs 3/4" as well as pressure.... imagining in the context of shower faucet flow rate based on ~100 psi pressure from 1/2" pipe and hydronic baseboard heat pressure and flow rate in 3/4" pipe.

• Member Posts: 19,966
Two 45s -- or a long radius 90 -- have less flow resistance than a regular 90. I'd have to dig out my manuals to find out how much less, but... less.

Now the second half there I'm not sure I understand the question. The head or friction loss in a pipe is very close to being linearly related to the square of the velocity of flow, but has nothing to do with the pressure itself. Another way to look at it is a given CHANGE in pressure will produce a given flow, but the absolute pressure at the two ends of the pipe is not relevant -- only the difference.

Going back to the 1/2 inch vs. 3/4 inch. For a given flow rate, the 1/2 inch will have about twice the pressure drop of the 3/4 inch.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 253
my question was off the cuff, killing time, pondering installation of a mixing valve and how easily not paying attention to installation might cause a reduction of flow such as to a shower head. Was looking for a somewhat qualitative, practical answer, putting things into perspective. The 3/4" hydronic piping, being less than 3gpm max and often operates around 1-2 gpm? And for a [good] shower, being on city water with pressure of 100 psi, operates about 2-3 gpm typically these days?

without getting mathy I suspect the simple answer is just use a long sweep 90° elbow for the hot & cold to get into the mixing valve then its mixed output is straight out along the original path. The hot & cold 1/2" pipe run parallel to each other, for where I want to install a mixing valve, where I think I can fit it using long radius 90° elbow.
• Member Posts: 19,966
Ah. Well, I'd probably use long sweep elbows if I had the room. But otherwise, so long as the two flow paths are at least similar it should work just fine.

The pressure drop in your domestic water is almost entirely in the fixtures. Both the valves, which tend to have astonishingly small openings and consequent large pressure drops, but also in modern fixtures (like a low flow shower head!) in the shower head itself. Unless your piping looks like pretzel, the loss in the piping, while not negligible, is minor relatively speaking.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
• Member Posts: 17,543
Flow rates from plumbing fixtures are typically at max pressure of 80 psi. 80 psi is plumbing code allowed maximum, also manufacturers max pressure for most faucets, toilets, etc.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 12,664
Ther is not enough difference in flow resistance between 2 45s versus 1 90 to make any difference that will amount to anything
• Member Posts: 2,702
Hi, I was involved in flow testing a bunch of shower valves and @Jamie Hall nailed it... that's where your biggest restriction is by far. You could probably run 3/8" tube and not notice any difference.

Yours, Larry
• Member Posts: 6,688
I pay attention to trying to be more efficient and direct in the main lines from the service to the water heater and the fixture groups and from the water heater out to the fixture groups to try to keep the pressure better balanced as loads change but within a bathroom or kitchen it makes little difference.
• Member Posts: 17,543
Most residential mixing valves are a 3-3.5 Cv. So you need to determine the maximum gpm flow rate to determine the pressure drop just in the mix valve
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
• Member Posts: 1,906
two 45° vs one 90° elbow? Former is far superior. Especially for waste pipes.
• Member Posts: 4,288
I have always wanted to know if four 22-1/2° elbows are better that just bending the darn pipe with a tubing bender.

And was this fitting designed by an electrical diagram engineer?

And why did the chicken cross the road anyway?
Edward Young
Retired HVAC Contractor from So. Jersey.
Services first oil burner at age 16
P/T trainer for EH-CC.org

• Member Posts: 13,562
I would expect bending the tubing is better than fittings, period.

You can do a wider sweep and there's no rough joints inside the pipe and no joints to leak.

Seems like a win win win win.
Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
• Member Posts: 4,288
I still would like to know who designed that other fitting
At least your reply was not too long WINded.

Edward Young
Retired HVAC Contractor from So. Jersey.
Services first oil burner at age 16
P/T trainer for EH-CC.org

• Member Posts: 838
I have always wanted to know if four 22-1/2° elbows are better that just bending the darn pipe with a tubing bender. And was this fitting designed by an electrical diagram engineer? And why did the chicken cross the road anyway?
I’m pretty sure @JUGHNE used one of these on his dual steamer project at the local k-12 school. If I recall right it had been sitting in a box for many years until a need arose.
• Member Posts: 838
edited May 29

Re two 45 versus single ninety, I use 45 on duct collection pickup line because it gives me more flexibility to work around obstructions while maximizing airflow.   Sweep 90s perform very well, but often I don’t lay out the piping before hand and the 45s give me more options.

This picture is of my shop during setup after relocating from smaller space. I used 4” schedule 20 PVC for dust collection lines.  Several of the blue bins that @mattmia2 recommended in a different thread sitting empty waiting for someone to get around to organizing and labeling.

• Member Posts: 10,044
Great looking shop Pat.
You have seem mine.....now I feel bad.
Did you run a bare ground wire inside the dust vac piping?

Yes, I still have one 3/4" copper cross over.
They were auction "bargains" that I might have paid scrap value at the time.

As far as using 2 45's, I use them for DWV piping to avoid 90's.
Also in venting 90% furnaces/boilers.
Even though the chart might say 2 45's are equal to 1 90 on friction loss, it is obvious that the 2 45's with pipe between constitute a long sweep 90.

I imagine going to a water park that has the 3' tube slide and jumping in at the top.

Going down it amounts to long sweeps or 45's.
Now think about making that trip with only short radius 90's.

There will be some painful changes of direction.
• Member Posts: 838
edited May 30
JUGHNE said:
Great looking shop Pat. You have seem mine.....now I feel bad. Did you run a bare ground wire inside the dust vac piping?
Haha, your shop has a lot of history captured in the salvaged parts.  I made sure to have a full height basement as part of a two story addition to our “new” 1920’s house so I could sneak a large shop into plans.  The new shop is 14’x28 plus another 13x18 wood storage / finish area. With the larger area I added a 14” bandsaw, 13” surface planer along with 3’x8’ bench so it’s starting to feel small again!

I worked out of 17’x11’ shop for 20+ years.  With my large table saw and the miter saw bench it was a tight fit and I had to share it with the cat!

Time to end this thread jacking so we can return to normal programming!

• Member Posts: 6,688
See my table saw serves as a table a lot more than it serves as a saw.
• Member Posts: 838
edited May 30
Re the copper wire in the PVC ducts, I wrapped a grounded braid around the lines in the old shop and will be adding to this one shortly.