Bernoulli vs. the pool guy

Simbilis

LifeTime Supporter
Aug 13, 2010
74
One of the builders I've been talking with is absolutely adamant that using 2" instead of 1 1/2" pipe is a horrible idea. He says that only commercial installations use 2" pipe. He warns that using it for my pool will cause the pressure at the returns to be too low. Also, none of the normal pool fittings will fit 2" pipe, so many things will have to be custom and my hydraulics will in all other ways be completely fubar'd. He also says that running pipe back to the pad for each connection is a bad idea. He warns that since they won't be able to use buried tees to make the return pipe lengths roughly equal, I'm in for pressure balance problems at the returns.

Even setting aside everything I've read here, none of the hydraulics advice he's giving makes sense. From the dim mists of time, my fluid dynamics lectures whisper to me that increasing pipe diameter should decrease flow velocity, reduce work done to overcome friction losses, and actually increase pressure at the returns a bit.

Is there something special about pool fittings that makes my book learnin' useless? Will using all 2" pipe really cause massive fitting incompatibilities and ruin my pools hydraulics? I'm pretty sure the answers are no and no, but I hate disagreeing with professionals outside my specialty.

Thanks,

Sim
 

solarboy

Well-known member
Aug 1, 2010
337
Europe
You are right to be suspect. Larger pipes mean less work for the pump which means greater flow per watt which saves you money. Smaller pipes save him money. He has a point in using buried tees on the return lines. However you can usually balance the returns with differing orifice sizes. On the suction lines however he is trying to save money; it's much better to have separate lines with their own valves.
I'm not sure about in the US but 2" (50mm) is standard here, 63mm for larger pools.
 

teapot

In The Industry
Jul 25, 2009
574
London and France
I am with you Sim, my understanding of fluid dynamics is the same as yours. My pipe runs are a bit long so returns are in 2" with burried "t"'s and 63mm (2.5") for my skimmer. I turnover my water in 4 hours but do not filter at too higher speed so I get good filtration.
 

Melt In The Sun

TFP Expert
LifeTime Supporter
Oct 29, 2009
3,899
Tucson, AZ
I would just say to him "I will have 2" pipe in my pool. Will you shut your yapper and build it?"

He is wrong. Using 2" pipe will absolutely result in a more efficient system. It's true that you will probably have to step down to 1.5" at the return fittings, but that is hardly "custom", it's very common. Pumps, filters, heaters, etc come in both 1.5" and 2" sizes.

If I were building I'd probably get even bigger pipe, at least 2.5". You could suggest 8" pipe and see if his head explodes..... :lol:
 

Simbilis

LifeTime Supporter
Aug 13, 2010
74
I'll definitely insist, and he has no problem with that. He was honestly trying to stop me from making what he saw as a huge mistake. "It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so."

Thanks for the info about fittings and equipment. I'll make sure it's all spec'd for 2".

2.5" flex is about a 60% price premium over 2", which isn't much in context. I don't know if it's worth taking people that far out of their comfort zone, though.
 

lbridges

LifeTime Supporter
Nov 12, 2009
323
Indialantic, FL
My guess is he has mis-remembered something he was told/learned when he first started out - less restriction in the return than you have on the suction side can cause a pump to run noisy. For companies that don't have a firm grasp on the dynamics of fluid flow it's generally easier to spec 2" suction and 1.5" return - but using that philosophy you could spec 2.5" suction and 2" return for not a whole lot of difference in $$$$ (compared to the pool as a whole).
 

Simbilis

LifeTime Supporter
Aug 13, 2010
74
That could explain it. Let's see, with 3 2" lines suction side (2 skimmers, 1 combined main drains) vs. 3 returns, it would be nearly a tie. Extra stair jets might be a problem. What causes the pump noise? Would a cartridge filter and the reducers on the returns side be enough extra restriction?
 

Heckpools

LifeTime Supporter
Feb 8, 2009
318
New Jersey shore
To me, Every plumbing line should be by itself with its own valve at the filter. If 5 years from now your losing water and its one of your return lines, how will you know which one is the probelm if they are all tied together under ground???
 

Heckpools

LifeTime Supporter
Feb 8, 2009
318
New Jersey shore
guys built pools in the 70s and 80's with 1.5" black poly pipe. They had 1 skimmer and 1 return. The pools were crystal clear and the customers enjoyed it... the end

I guess dudes will always talk about their pipe size and whats better :shock:
 

Simbilis

LifeTime Supporter
Aug 13, 2010
74
Heckpools said:
To me, Every plumbing line should be by itself with its own valve at the filter. If 5 years from now your losing water and its one of your return lines, how will you know which one is the probelm if they are all tied together under ground???
Yeah, I said that. He said "but that can never happen!" :shock:
 

Simbilis

LifeTime Supporter
Aug 13, 2010
74
DBfan187 said:
time to look for more competent builder
I swear, I'm ready to buy a DIY kit and only sub out the things that are too hard to learn to do for just one pool. Excavation (maybe), concrete for the beam and deck, pool bottom - what else is really hard to get right? Are there any stopping points where it could sit for a while without falling apart? I can see you can't just leave the excavation sitting, but once the structure is installed, the foundation beam is poured, and the sides are backfilled and compacted, it seems like it'd be safe to let it sit over winter.
 

4JawChuck

Well-known member
Jun 13, 2010
223
Winnipeg, Manitoba, Canada
There are good reasons to spec 2" pipe but not every person needs this size. Your installer is likely trying to save you money on something you really don't need and he likely doesn't carry a lot of 2" stuff so he would have to pick up everything special just for your job...not to mention adding a reducer at every return in the pool which doubles the amount of joints in the system which is where most leaks occur.

I have often read the 2" pipe spec here on the forum and as a person who works with plumbing I question the need. Don't forget doubling the joints is not the greatest idea in any plumbing situation. The water speed through the pipes is not so high that doubling the cross sectional flow is going to get you something you wouldn't have with the smaller pipe. Its up to you but for the typical install its a waste of money if you ask me...now if your plumbing runs are 300 ft away then going to a larger size and then tee'ing out to 1.5" is not a bad idea or if you have 6 returns and a huge pump it might be worth the investment.

I often run into customers that want to use smaller pipe than required and some that want too big a pipe, I usually have to explain why smaller is a bad idea but if the customer wants the larger sizes I just do it while explaining why doubling the joints with reducers is a bad idea...99% get it and the other 1% want what they want no matter what I say. Those customers have a caveat in their sign off form so I don't get bit later on when they have an issue.

Pool plumbing is so low tech (to me anyway) that getting carried away with custom piping is kind of ridiculous, but then again in medicine I sometimes get specs that call for welded plastic pipe from hospitals...you want to know how much extra that costs! In the field I work in arguing with the customer is fruitless so everyone gets what they want if they are willing to pay the price.

Its your money, if your willing to pay extra for a feature you don't need who is the installer to argue with you...explain yes...argue no.
 

lbridges

LifeTime Supporter
Nov 12, 2009
323
Indialantic, FL
Simbilis said:
...What causes the pump noise? Would a cartridge filter and the reducers on the returns side be enough extra restriction?
1. It's cavitation - the sound is produced from the collapse of air bubbles. People who find this on their pool can "prove" it out by using a valve to partially close off a return line - if the sound went away it was cavitation due to pressure differences between pump inlet and outlet.

2. I think probably not by themselves, but I've just taken fluid dynamics classes and know very, very, very little about real world pool plumbing design. There are a lot of variables, enough that a plumbing designer would have to look at the plans to make an estimate.
 

4JawChuck

Well-known member
Jun 13, 2010
223
Winnipeg, Manitoba, Canada
I just re-read the OP and thought I should mention, if your returns are 1" orifice size...how would a 2" pipe offer less restriction over a 1.5"? Have you done the flow restriction calculation for 2" versus 1.5" at the typical flow rates a pool centrifugal pump delivers?

Its pretty miniscule...

I seriously doubt the added cost of the larger pipe will ever pay for itself over the life of the pool in reduced pumping losses when you consider the orifice is the restriction...not the pipe. 2" pipe to the return split off junction is common, thats how my pool is plumbed and it only travels 20ft...seems to be SOP in my country.
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,082
San Rafael, CA USA
4JawChuck said:
I just re-read the OP and thought I should mention, if your returns are 1" orifice size...how would a 2" pipe offer less restriction over a 1.5"? Have you done the flow restriction calculation for 2" versus 1.5" at the typical flow rates a pool centrifugal pump delivers?

Its pretty miniscule...

I seriously doubt the added cost of the larger pipe will ever pay for itself over the life of the pool in reduced pumping losses when you consider the orifice is the restriction...not the pipe. 2" pipe to the return split off junction is common, thats how my pool is plumbed and it only travels 20ft...seems to be SOP in my country.
The resistance of fluid to flow is not based on the narrowest restriction, per se. There is resistance to flow in pipe runs and there is more resistance in narrower pipe. This is in addition to the resistance from a narrow return which simply represents some equivalent length of pipe. If the flow rates are low, then the resistance from the pipe may be fairly low, but at higher flow rates this can make a difference.

My own pool is piped the way you describe with 2" from the pump to a split near the pool into three separate 1.5" lines, one to each return. There are two separate 1.5" suction lines all the way to valves near the pump. I very much wish I had 2" pipe where 1.5" was used (especially for the suction lines) and 2.5" where 2" was used. Here's the difference it would have made with 90 GPM (around what my older 1 HP single-speed pump did when solar was off) making some simplifying assumptions below:

[EDIT] I've added the suction lines I didn't account for the first time and separated out solar on vs. off at different flow rates (60 GPM on, 90 GPM off) and changed from using my spreadsheet to using standard tables here for head loss in pipe (my spreadsheet is more optimistic). [END-EDIT]
[EDIT] I corrected the head loss for the split lines only counting once -- I originally correctly split the flow, but the head loss is that from only one line assuming they are of roughly the same length so the same loss in each line. [END-EDIT]

SOLAR OFF @ 90 GPM
3 returns 1.5" pipe 30 feet (avg.) 30 GPM each ---> 30*(5.5/100) = 1.6 feet of head
1 combo line 2" pipe 40 feet 90 GPM ---> 1*40*(12.4/100) = 5.0 feet of head
2 separate lines from floor drains and skimmer 1.5" pipe 70 feet 45 GPM each ---> 70*(11.7/100) = 8.2 feet of head
TOTAL = 14.8 feet of head from straight piping alone

3 returns 2" pipe 30 feet (avg.) 30 GPM each ---> 30*(1.6/100) = 0.5 feet of head
NOTE: an abrupt contraction 2" to 1.5" reducer would add only 0.06 feet of head at 30 GPM
1 combo line 2.5" pipe 40 feet 90 GPM ---> 1*40*(5.2/100) = 2.1 feet of head
2 separate lines from floor drains and skimmer 2.0" pipe 70 feet 45 GPM each ---> 70*(3.4/100) = 2.4 feet of head
TOTAL = 5 feet of head from straight piping alone

SOLAR ON @ 60 GPM
3 returns 1.5" pipe 30 feet (avg.) 20 GPM each ---> 30*(2.6/100) = 0.8 feet of head
1 combo line 2" pipe 40 feet 60 GPM ---> 1*40*(5.8/100) = 2.3 feet of head
2 lines to/from solar 2" pipe 50 feet 60 GPM each ---> 50*(5.8/100) = 2.9 feet of head
2 separate lines from floor drains and skimmer 1.5" pipe 70 feet 30 GPM each ---> 70*(5.5/100) = 3.9 feet of head
TOTAL = 9.9 feet of head from straight piping alone (not counting roof piping, solar panels, and their 2" headers)

3 returns 2" pipe 30 feet (avg.) 20 GPM each ---> 30*(0.8/100) = 0.2 feet of head
1 combo line 2.5" pipe 40 feet 60 GPM ---> 1*40*(2.5/100) = 1.0 feet of head
2 lines to/from solar 2.5" pipe 50 feet 60 GPM each ---> 50*(2.5/100) = 1.3 feet of head
2 separate lines from floor drains and skimmer 2.0" pipe 70 feet 30 GPM each ---> 70*(1.6/100) = 1.1 feet of head
TOTAL = 3.6 feet of head from straight piping alone (not counting roof piping, solar panels and their 2" headers)

In reality, my solar runs are longer and the piping on the roof for the solar panels as well as the 12 panels themselves (with 2" headers) is substantial so is much more than shown above.

You can see how the resistance is cut by around 1/3rd by going from 1.5" to 2" pipe for each return but in absolute terms there is greater savings going from 2" to 2.5" for the combo line. Now in reality the above gain would only be achieved at constant flow rates so from a variable speed/flow pump. With a fixed speed pump, the lower head would have the flow rate rise, but with enough savings from piping one could downsize the pump to get down to a similar flow rate but at much greater energy savings. I could very likely have been able to go down to a 3/4 HP pump, for example.

So yes, if one just uses larger piping without also getting a lower HP pump (or lowering the RPM of their variable speed pump), then one won't get much benefit.

Heckpools said:
guys built pools in the 70s and 80's with 1.5" black poly pipe. They had 1 skimmer and 1 return. The pools were crystal clear and the customers enjoyed it... the end
No one is talking about using larger pipe diameters to get better circulation. It's mostly to get better energy savings by not having as large a pump or as fast a speed on a variable speed pump. People didn't care that much about energy saving in the 70s and 80s at least with regard to pool pumps. Why waste energy on friction in pipe? Use the energy as much as possible for circulation -- not to improve it, but to not use as much energy for the same flow rate.
 

Simbilis

LifeTime Supporter
Aug 13, 2010
74
Why would a builder not carry a lot of 2"? Not carrying much 2 1/2" I can understand.

It's also a little bit about increased circulation capacity. Using a SWCG and a variable speed pump, I'll have the option to increase the flow rate to get more chlorine generation for pool parties, maintenance screwups, etc.

Actually, it's about a lot of things, and it's a balancing act. Just taking whatever the builder is comfortable building means learning very little and being at risk from whatever they know that's wrong or out-of-date. Blindly demanding that they build it exactly to my spec means being at risk of using bad data or not understanding correct information. I'm hoping for a middle course, which is why all the questions ;)
 

X-PertPool

TFP Expert
In The Industry
Jun 12, 2009
1,385
Exeter, PA
Do you still see energy savings by using 2" pipe instead of 1.5" pipe on return lines when it eventually gets reduced to the directional "eyeball" fitting?
 

teapot

In The Industry
Jul 25, 2009
574
London and France
Yes, as it's only reduced for the last few inches which increases the jet of the return. As Chemgeek brilliantly posted, there is less friction down the whole of the pipe. My system runs beautifully at 7 PSI so saving a fair amount on the pump motor electricity, helped by the glass media too.
 

bk406

Well-known member
Dec 3, 2009
2,690
Central Massachusetts
Simbilis said:
Using a SWCG and a variable speed pump, I'll have the option to increase the flow rate to get more chlorine generation for pool parties, maintenance screwups, etc.
It doesnt work that way. Increased/decreased flow rate doesn't change chlorine production. A SWCG has a minimum flow rate that it can operate under. If the flow rate is less than that amount, it shuts down. Anything over that, it works. Chlorine production is determined by how long the unit is on during a given hour.
 

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