Pool plumbing question (1.5" to 2")

Thanks in advance for any feedback. I am looking for some confirmation of my analysis. I am an engineer, so I believe I understand the physics. However, I am not a plumber, nor am I a pool builder. I still feel my analysis is sound, but it conflicts with some pool builders. Hopefully, I won't screw up describing it. :)

While investigating whether my pool plumbing setup could support the new heat pump I just purchased, I noticed something concerning. I have a 32,000 gallon pool with a 60 sq inch DE filter and 2 HP motor. Unfortunately, the entire pool is plumbed with 1.5" piping. After researching, the theoretical "limit" on 1.5" PVC is 42 GPM. This means my pump is probably strained against the plumbing (thereby wasting electricity) and more importantly my turnover rate is suboptimal (probably 11 to 12 hours instead of 8 to 10).

After speaking to some pool builders, they suggest that it is impossible to fix without ripping out all the underground piping. This is where I totally disagree. Since all the intake pipes and return pipes are joined above ground, I believe a well designed and installed 2" solution would give the exact same benefit of 2" throughout. Here's why:

Since the theoretical limit of 1.5" is 42 GPM, each 1.5" will be limited by something close to that. However, I have 4 intake 1.5" (drains and 3 skimmers) and 3 return 1.5" (pool jets, stair jets, and swim out jets). Since each 1.5" intake can supply around 42 GPM, theoretically you can add each one for a 168 GPM flowrate. However, since these would be joined into a 2" junction, the limit would go back to the theoretical 73 GPM of a 2" pipe (as the slowest piping determines the limit). Therefore, whether I replaced all the piping to 2" or just the above ground piping to 2", by the laws of physics and simple engineering principles, my max flow rate would be exactly the same (again at or close to 73 GPM).

More importantly, for any pool that uses 2" for intake and return lines underground, they are probably wasting their money as they can't push more water than the 2" pipe that they all lead into (meaning into and out of the pump and filter). I have a couple of engineers that have checked my analysis and agree with me, but I haven't had any pool builders agree yet. After having a lengthy conversation with 1 pool builder, he did say that I am starting to make him rethink his beliefs for the last 25 years, but he wasn't ready to commit.

This website seems to back up my analysis (http://www.poolplaza.com/pool-pump-sizing-2.shtml). However, I would love to hear from some plumbers and pool builders that support my analysis. Or point out why it is wrong. If you need more details or I did a poor job of explaining, let me know.

Thanks.

Hi Viking;

Since I'm both a plumber (held a master's license for over 30 years) and an engineer (at least by training), I think I can answer. :D

If your all your lines are joined above ground, you absolutely can solve your flow problem by connecting them in a 2" or 3" header. Just avoid any bullhead-T connections, where you have two flow streams colliding head-on.

A couple of corrections, though.

+ For virtually all centrifugal pumps, the max load occurs in the low pressure / high flow zone, not in the high pressure / low flow zone of the curve.

+ High efficiency pool circulation is achieved with balanced piping that is oversized, compared to what's standard and matched to a low max head pump. In practice this combo is available via a slightly oversized pump that has a two speed motor. Typical centrifugal characteristics commonly result in 1/2 of the high speed (3600 rpm) flow at low speed (1800 rpm) , but only 1/6 of the load.

+ Some elements of your hydraulic analysis aren't quite right, but as they say, "you're close enough for government work".

+ I've never met a pool builder with even a vague understanding of how to calculate flows in a pool system. I know there are some -- I've just never met one.
[ In fact, most of the engineers I've seen involved in projects that included pools apparently could not do so, either. Given that it's all a matter of applying the empirical Hazen-Williams formula (which you can load into a spread sheet), and combining it with data supplied by most pipe and gear manufacturers, it's obviously tech that's well within reach of any engineer. It's just that most engineers seem to work in environments where they design only by the supplied cookbook . . . and there's no "Handbook of Swimming Pool Design" worthy of the name. ]

Good luck!

Ben

There are a couple things that I don't like about the way PoolPlaza does for pump sizing. As I mentioned in this post, there is no fundamental limit to flow rates in pipes and they seem to imply there is one. Again, what they are showing is simply a recomendation. Second, they also imply that head loss is constant with flow rate which is not correct either.

But if you are interested, I have quite a bit of experience in doing full head calcs on swimming pool plumbing so if you want, I can estimate the performance improvement with just the pad changes. I just need some more details about the pool and pad layout plus equipment models.

Number of suctions runs pool to pad and lengths
Number of return runs pad to pool and lengths
Number return eyeballs and diameter
Equipment layout (values, fittings, lengths, equipment, etc)
Pad height above the pool water
Filter pressure when clean
Pump model #
Is there a heater?

The underground plumbing I will guess at but I have a pretty good idea what is put underground based upon other pools and the filter pressure will help calibrate the model.

Hopefully I can provide the appropriate info:

Number of suctions runs pool to pad and lengths: 4
1 drain run that branches inground to 2 drains (1.5" pipe) (roughly 30 ft run)
3 skimmer runs (1.5" pipe) (30ft, 32ft and 60ft)

Number of return runs pad to pool: 3
Swimout (1.5" pipe, 25 ft) - 4 jets installed
Steps (1.5" pipe, 60ft) - 2 jets installed
Return eyeballs (1.5" pipe, 40ft)

Number return eyeballs and diameter: 3 (all 1.5")

Equipment layout (values, fittings, lengths, equipment etc): ALL 1.5" pipe with 1.5"/2" reducing adapters where necessary) - lengths between fittings is pretty small (anywhere from 2" to 6") except between pump and return which which goes through a T-Cell chorine generator and is roughly 3 feet long with 4 elbows.
Suction: tee-fitting leading to a 4-way leading to the pump
Return: elbow leading to a tee leading to another tee leading to an elbow for the last return
Filter: Hayward D.E. 6020
Multi-valve: Variflo SP710XR50

Pad height above the pool water: 18 to 21"

Filter pressure when clean: 20 psi (from memory)

Pump model #: Hayward 2HP - K48M2N111

Is there a heater? Not yet, but that is what I was in the process of installing: Heatpump AquaCal SQ155

Hope that covers everything. If not, let me know what additional details I could supply.

I'd like to understand what benefit I would get if I were to upgrade the pad pipe to 2" including the multi-valve to 2". Also, if there is a better way to upgrade the suction and return pad fittings (as I was thinking I could just replace each existing fitting with a 2" version and reduce before the existing 1.5" ball valve.).

Lastly, I'd like to know how much I am losing by having 1.5" suction and return lines underground (instead of 2").

Thanks in advance.

Quote:

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Originally Posted by mas985

There are a couple things that I don't like about the way PoolPlaza does for pump sizing. As I mentioned in this post, there is no fundamental limit to flow rates in pipes and they seem to imply there is one. Again, what they are showing is simply a recomendation. Second, they also imply that head loss is constant with flow rate which is not correct either.

But if you are interested, I have quite a bit of experience in doing full head calcs on swimming pool plumbing so if you want, I can estimate the performance improvement with just the pad changes. I just need some more details about the pool and pad layout plus equipment models.

Number of suctions runs pool to pad and lengths
Number of return runs pad to pool and lengths
Number return eyeballs and diameter
Equipment layout (values, fittings, lengths, equipment, etc)
Pad height above the pool water
Filter pressure when clean
Pump model #
Is there a heater?

The underground plumbing I will guess at but I have a pretty good idea what is put underground based upon other pools and the filter pressure will help calibrate the model.

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I seem to be getting a much lower filter PSI than 20 PSI so if you could confirm the filter pressure it might help reconcile the difference.

Also are you sure the return eyeballs are 1.5" or just the pipe feeding the eyeballs? 1" eyeballs are more common and generally use 1.5" pipe to feed them.

Also do you normally run with all suction lines fully open and all return lines fully open? For example with the steps and swimout jets, are you redirecting more water to those than the pool returns or is everything have equal flow?

But just given the number of suction and return pipes that you have, 1.5" is plenty. The parallel runs, if all fully on, will have about the same head loss as a single 2.5" pipe so there is not much to gain from that. However, this also means that most of your head loss in is the pad plumbing so you will be able to achieve quite a bit of gain with just changing the pad plumbing. However, I would also change the valves because they tend to have more head loss than just fittings.

Unfortunately, I am not opening the pool until next Wed, so I can't give you an actual. You are probably right about the lower reading. Probably between 15 and 20.

Also, you are right about the eyeballs, I misunderstood. So, 1.5" to feed and probably .75 to 1" (not sure).

Yes, I normally run them fully open. Only occassionally do I run the swimout jets more when sitting. For suction, all open except when vacuuming.

Again, thank you very much for the assistance. As you can tell, I am new to pools in general. I did a bunch of homework before purchasing, but I see I could have done more (would have pushed for better plumbing and a variable speed pump). Oh well.

Quote:

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Originally Posted by mas985

I seem to be getting a much lower filter PSI than 20 PSI so if you could confirm the filter pressure it might help reconcile the difference.

Also are you sure the return eyeballs are 1.5" or just the pipe feeding the eyeballs? 1" eyeballs are more common and generally use 1.5" pipe to feed them.

Also do you normally run with all suction lines fully open and all return lines fully open? For example with the steps and swimout jets, are you redirecting more water to those than the pool returns or is everything have equal flow?

But just given the number of suction and return pipes that you have, 1.5" is plenty. The parallel runs, if all fully on, will have about the same head loss as a single 2.5" pipe so there is not much to gain from that. However, this also means that most of your head loss in is the pad plumbing so you will be able to achieve quite a bit of gain with just changing the pad plumbing. However, I would also change the valves because they tend to have more head loss than just fittings.

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Once you open the pool, you can confirm the filter pressure but with what you have told me and a target of around 17 PSI, here is what I get from the hydraulics model:

Current setup

75 GPM @ 54' head, 1800 watts, 2.52 Gallons/watt-hr

Changing only the pad plumbing to 2"

87 GPM @ 45' head, 1920 watts, 2.71 Gallons/watt-hr

Changing all plumbing to 2"

90 GPM @ 43' head, 1960 watts, 2.76 Gallons/watt-hr

Gallons/watt-hr is the primary metric I use for efficiency so changing the pad plumbing gives you about 7.5% more efficiency. Replacing the suction and return lines only about 2% more. But this assumes that all of the fittings and valves are changed with the pipe.

Thanks Mark for all your analysis. Quick question, since I only run my pool for 4 months out of the year, do you think there would be considerable payback by upgrading pad piping.

Also, what size pump would you recommend with existing piping? What size pump with the pad piping upgraded to 2"? How much benefit of a 2 speed pump? Variable speed?

Thanks.

For cost, one turnover per day would require about 7 hours @ 1.8 kw with your current pump and plumbing but only 6 hours @1.9kw with a pad change. So a monthly difference of 36 kwh or 144 kwh per year savings. Multiply that by your utility rate and you can get the savings per year. Even at the higher energy rates, it is probably not worth the trouble.

For pump sizing, the smallest SuperPump is a 1 HP uprated pump two sizes down from what you have and would provide about 60 GPM on your plumbing or about 2.7 turns per day. Fixing the pad plumbing would bump that up to 68 GPM or over 3 turns per day which is plenty.

Another option would be to just replace the motor with a two speed motor. If you ran on low most of the time (2 hours high, 10 hours on low), you could save about 550 kwh per year. Still not a lot but could pay for the motor in a few of years.

If you normally run 2 turns per day just double the energy savings.

I know this is an older post but I'm considering a similar upgrade and trying to determine if it is worth it. Pool is 16'x32' roughly 20K gallons I believe. I have a new Pentair Superflo VS variable speed pump Model 342001. All of my eyeball fittings are the flexible nozzles that change diameter as the pump speed increases.

The pool has two 1.5" suctions lines (one from the bottom drains and one from the skimmer) that come above ground to a hard tee then go into the pump with 1.5" piping. The entire system is 1.5" piping then it splits off to two 1.5" zones of return jets (one that has 2 jets at the steps and one that has 4 jets at other locations).

I'm an engineer type and I would have thought the improvement going from 1.5" to 2" for the above ground piping would be dramatic. Those tees seem like a serious choke point.

If you are an engineer type, you might be better off using some of the free online pipe and flow calculators to do your own rough hydraulic analysis. There are a bunch of them out there.

But, it's hard to do, with residential gear: the piping is no problem, but the supplied equipment flow data range from poor to unavailable.

I've done such calculations many times, but for large commercial pools. When I tried, this spring, to do them for some idealized residential pool / pump / filter combos, I was stymied by the lack of meaningful data.

I suspect some of this is deliberate: the VS pumps are often way, way too big for the piping or filters with which they are coupled. And when they are occasionally, even by accident, run at full volume, they are bursting pipes and valves, splitting filters, destroying cartridges, and back washing sand out all over the ground.

Obviously, that's not something for which the pump makers want to stand up, and take the credit!