Bernoulli vs. the pool guy

[chem geek]

I don't have the full system details with me, but a rough estimate of head loss savings and resulting cost savings is the following assuming 50 GPM (easier to look up in the table than 48 GPM) when the solar is on and I'm being VERY conservative (i.e. under-estimating) on the pipe run lengths, not counting elbows, etc.:

To/From Solar: (1.8-0.8) PSI/100 feet @ 150 feet round-trip --> 1.5 PSI
Solar on Roof: (1.8-0.8) PSI/100 feet @ 120 feet --> 1.2 PSI
Combo Line to Returns: (1.8-0.8) PSI/100 feet @ 50 feet --> 0.5 PSI
Suction Lines: (1.7-0.5) PSI/100 feet @ 80 feet --> 1.0 PSI
Return Lines: (0.7-0.2) PSI/100 feet @ 50 feet --> 0.2 PSI
Eyeballs: (2.10-0.66)/2.31 PSI --> 0.6 PSI

TOTAL HEAD LOSS REDUCTION: 5.0 PSI (11.6 feet)

My solar on has 48 GPM, 3000 RPM, 1530 Watts, 24 PSI (pressure-side only, but the gauge may not be accurate) with a calculated head from pump curves of 68.6 feet (29.7 PSI) which would give an implied suction head of around 6 PSI. The skimmer/drain is less than 1 PSI, the pump strainer inlet around 0.3 PSI so with suction piping adds up to 2.3 so clearly there is significant under-estimation not counting elbows, pump strainer basket, true pipe lengths, etc. Nevertheless, let's continue on with the calculation.

If I use the new 29.7-5 = 24.7 PSI, then I get an implied RPM (using the same 48 GPM since that's what the solar system needs) of 2755. That may not sound like much, but it translates into an energy savings of around 270 Watts or a savings of around 18% because the energy formula at fixed GPM has the RPM in two factors either cubed or squared. Remember that the above calculations were very conservative so it's very likely I would have had greater savings -- my best guess would be somewhere in the 300-500 Watts range. Let's use the unrealistic conservative low-end of 300 Watts. The solar is on in my system an average of 5 hours a day (hotter months it's 4 hours, cooler months it's 6 hours) every day for a 7-month swim season and I won't count the the solar off savings for both the swim and off seasons which would be lower. At 40 cents per kilowatt-hour marginal rate this is (300/1000)*5*7*30*0.40 = $126 per season.

Again, this is very conservative and would more likely be closer to $200 per season, plus (assuming the conservative 300 Watts savings) using at least 1.5 KWh/day (45 KWh/month) less energy which is more ecologically conscious. Notice that the 1.5" return/inlet lines are the least problematic since there are 3 or them to split the flow rate and probably would have been kept as is.

Looking at the above, I see that I can get another 0.8 PSI savings above what is shown by going to 3" pipe to/from solar which would have been easy to do for that one to/from dual run (and to the roof). I don't know if I could have stomached 3" pipe for the long return on the roof for another 0.6 PSI savings -- would have been a tough aesthetics decision at that point. If I were to go from 2" to 3" pipe for the to/from solar and the solar return roof portion, I could get over 4 PSI savings from where I am today and wouldn't need to tear up any hardscape, only dig up pipe through a garden walkway. It'll never pay for itself at this point and would have been so much easier to do during installation, but it's something I'll think about, especially if I do any sort of significant upgrade/change to the solar system.

I understand it might save you a bit of money. I guess my point is that I really dont like it when the first thing a few people do is jump on someones pool build that has, or is proposing 1.5 inch pipe. For most, its fine. I just have observed time after time that builders and new posters get barrage of "2 inch is better, get 2 inch or find another builder". IMO, such blanket statements are irresponsible, and makes new members nervous about their new pool when the reality is they have nothing to worry about.

I agree with you. I don't think people should assume that 2" pipe is going to save them a lot nor should PBs ignore using 2.5" or 3" pipe that might be better for pipes carrying the entire flow rate if runs are long, especially when solar systems are involved. It should also be noted that if one has larger pipe, it does allow for higher flow rates without having costs go through the roof. This may not be normal everyday running, but if someone wanted large flow rates for better skimming during some parts of the day or to do intentional strong aeration with returns pointed up, for example, they could do so without blowing up energy costs. In my system right now, the maximum flow rate is around 80 GPM with solar off and with my IntelliFlo it's at 2500 Watts at that point (interestingly, my older pump could get roughly to the same point at around 2000 Watts so the IntelliFlo is probably not as efficient at high flow rates with high head).

Richard

 

[Simbilis]

I guess my point is that I really dont like it when the first thing a few people do is jump on someones pool build that has, or is proposing 1.5 inch pipe. For most, its fine. I just have observed time after time that builders and new posters get barrage of "2 inch is better, get 2 inch or find another builder". IMO, such blanket statements are irresponsible, and makes new members nervous about their new pool when the reality is they have nothing to worry about.

I see this very differently. I'd much rather be aware of issues that knowledgeable people disagree on. In fact, I came here and asked questions not to create an ironclad list of non-negotiable demands, but to be as informed as possible when dealing with builders.

My pool is a huge purchase, more than I've spent on anything except my house. Nothing could make me more nervous about it than being ignorant. Discovering that smart people disagree about some facet of the technology actually makes me less nervous because (a) I have the chance to listen and learn and (b) I know that at least two smart people have bought pools

Sim

 

[bk406]

I see this very differently. I'd much rather be aware of issues that knowledgeable people disagree on.

I dont disagree with you, really. I've been here longer than you have though, and I've seen the barrage of the " 2 inch is better" under any circumstances and the advice to change builders w/o any of the thought thats gone on in this thread. Thats all i was trying to get across. I think too many times new folks get a little rattled, thats all. You are an exception in that you seem more informed than some.

 

[Isaac-1]

Have we really been through 4+ pages of disagreement over investing an extra $20-30 to upsize pipes?

 

[4JawChuck]

Have we really been through 4+ pages of disagreement over investing an extra $20-30 to upsize pipes?

Actually I think the real question should be;

"With my new extra large more efficient plumbing, can I run my pump 5 minutes less or 20 minutes less to get exactly one turnover per day?"

...cause really, thats what we are talking about here....recirculating water...ultra efficiently.

Its the only calculation I haven't seen yet....but I have a feeling

...its next. :|

 

[chem geek]

Read my post again. There isn't any change in the time for one turnover because the pump is set to run at the same flow rate as before. The difference is that with larger piping it takes a lower RPM and lower energy rate (Watts) for that same turnover and in my case would probably save around $200 per year. That's the difference with variable speed/flow pumps -- the larger piping doesn't have to result in a smaller amount of energy and cost savings but with a higher flow rate so faster turnover. Instead, you can slow the pump down to get even more energy savings for the SAME flow rate and turnover rate.

[EDIT] In my case, the 48 GPM flow rate for my 16,000 gallon pool implies a 5.55 hour turnover time so my having solar on for 5 hours is not unreasonable in terms of getting one turnover and in practice I run the pump for 8 hours with the remaining solar off time at 26 GPM plus 2 hours at 15 GPM for the pool sweep so I usually get 1.3 turnovers per day on average but I don't really have a choice given the requirement for solar and the limited time when the sun is strong enough to heat the panels and my wanting reasonable panel efficiency. If there were no solar at all during the day, then this would be an 89% turnover, but in practice this doesn't happen very often and it's not a problem when it does (i.e. my water remains crystal clear). [END-EDIT]

Of course, if you get enough efficiency from larger piping you may be able to use a smaller fixed speed pump, but a variable speed pump gives more flexibility and also has much lower operating cost when running at lower speeds (even compared to a 2-speed pump). It does, of course, cost more initially.

And yes, spending a fairly small amount extra for larger piping during installation is a heck of a lot cheaper than having to change it later. As shown here pricing for white PVC Schedule 40 pipe varies from $0.84/foot for 1.5", $1.11/foot for 2", $2.12/foot for 2.5", $2.29/foot for 3". Going from 1.5" to 2" is $27 per 100 feet while going from 2" to 3" is $118 per 100 feet. This link gives pricing of $0.76/foot for 1.5", $1.01/foot for 2", $2.14/foot for 3" (all sold in 20 foot sections).

 

[mas985]

Using the OPs situation as an example again and assuming a more realistic target of 42 GPM for a 12 hr turnover in both cases:

1.5" Suction/Pad:
42 GPM
1480 RPM
13.3' Head
280 Watts
3.33 kwh/turn
9 Gallons per Watt-Hr

2.0" Suction/Pad:
42 GPM
1210 RPM
7.4' Head
199 Watts
2.37 kwh/turn
12.7 Gallons per Watt-Hr

With 2" plumbing, the OP would save 960 watts/turn.

 

[chem geek]

Mark,

Can you run your model again assuming 2" at the pad and only changing the suction lines from 1.5" to 2"? With the multiple suction lines splitting the flow rate, I'm surprised to see much difference so suspect that it's the changes at the pad that are the primary source for that. I don't think Sim's PB is proposing 1.5" at the pad, only for the return/inlet and suction lines.

Richard

 

[mas985]

Definitely it is the pad. Most of the head loss is there. With only 2" on the suction, I could only lower the RPM slightly to get back to 42 GPM.

2" on suction only:
42 GPM
1440 RPM
12.4' Head
266 Watts
3.18 kwh/turn
9.4 Gallons per Watt-Hr

1.5" Suction/Pad:
42 GPM
1480 RPM
13.3' Head
280 Watts
3.33 kwh/turn
9 Gallons per Watt-Hr

You save only 14 watts with the 2" suction.

 

[4JawChuck]

Gents you should temper those calcs with the caveat that they do not reflect actual measured conditions, they are sample calculations and do not reflect reality. The factor of error in any of these calculations can be easily off as much as 20% off from measured conditions so a calculated savings of 14 watts isn't even worth looking at and could easily be a loss not a savings. An experienced engineer would not take any of this as serious or even factual, we have a technical term for it in the industry..."Bernoulli illusions of grandeur". Its great if your trying to sell something, not so great if your trying to achieve a measured end result.

Just wanted to make sure we are still talking about recirculating water through a filter to keep a pool clean and that the recirculation takes X amount of time to accomplish...right?

 

[chem geek]

...a calculated savings of 14 watts isn't even worth looking at...
:
Just wanted to make sure we are still talking about recirculating water through a filter to keep a pool clean and that the recirculation takes X amount of time to accomplish...right?

What Mark's calculations showed (and is why I asked him the question) was that in Sim's case with separate pipe lines, going from 1.5" to 2" has minimal benefit, but going from 1.5" to 2" at the pad does have benefit. That's all and is something I think you would agree with since you never said anything about using 1.5" at the pad. Mark was not saying that 14 Watts was a real (as in significant) benefit, but rather that changing the piping to the pool from 1.5" to 2" in Sim's case isn't of importance, but having 2" at the pad is important.

The task of a good PB isn't just to make sure that water recirculates through a filter keeping the pool clean in X amount of time, but to do so with reasonable operating costs. My PB cost me over $700 a year (around 1750 KWh) by not using a variable speed pump (or at least a 2-speed pump at the time) and another $200 or so a year (around 500 KWh) by not using larger piping, such as 3" where he used 2" for the solar and the main line before the split to the returns and 2" where he used 1.5" for the two suction lines (especially for the skimmer line since that would give more flow through the skimmer) and using 1" eyeballs where he used 3/4". My $700+ a year is a real measured cost savings; the $200 is an estimate, but the $126 I calculated is extraordinarily conservative with any errors being estimating way too low the savings that would be achieved (for the reasons I gave in the post).

As has been said before, 1.5" by itself is not "bad", but it depends on how the 1.5" pipe is used. If it's after splitting the flow rate down to go to three or more returns/inlets or from three or more skimmers and floor drains, then the benefits of 2" are very low unless the flow rates are very high and/or pipe lines very long. The calculations done so far roughly indicate that 1.5" when the split is 3 or more is not a problem, but when it's only a split of 2 it can be a problem (2" would be better) and when it's taking the full flow rate 1.5" is usually a problem (i.e. more costly). Even 2" taking the whole flow rate isn't great unless the pipe run is short.

"Bernoulli illusions of grandeur"

I'll bet that this idea comes from the fact that with FIXED SPEED pumps, increasing the pipe size doesn't result in as much savings as a simplistic calculation would indicate because one must account for the fact that the flow rate will increase (something Mark's spreadsheet can calculate if one knows the pump curve). Unless this allows one to go down to a lower HP pump, there may not be large enough energy savings to be worth it and one could just end up with faster turnovers which wasn't the goal. However, with VARIABLE SPEED pumps, the situation is very different and the calculated savings from pipe head loss tables is directly applicable because one adjusts the variable speed pump to produce the same desired flow rate. So the lower head loss translates only to energy savings and not to any change in flow rate or turnover rate.

So the thinking of PBs used to operating with fixed speed pumps needs to change when dealing with variable speed pumps in terms of the benefits of using larger piping in parts of the system where it carries all or half of the flow rate.

 

[bk406]

With 2" plumbing, the OP would save 960 watts/turn.

I think teapot mentioned something about saving energy. Form everything i can find on the net, it takes ~ 0.04 gallons of gas to produce 1 kW of power. So for one turnover a day for a 6 month swim season, thats a savings of 8 gallons of gas for the season. I burn more than that on a one way trip to NYC from Boston.

 

[Ohm_Boy]

Fascinating tome.
I think that the bottom line for me would be that if I were building a pool, when my PB tried to upsell me on a bigger pump, I think I'd prefer to move the extra money into plumbing instead.

 

[svenpup]

...a calculated savings of 14 watts isn't even worth looking at and could easily be a loss not a savings...

So you think going to a larger diameter pipe would somehow cause more loss? Get real.

...An experienced engineer would not take any of this as serious or even factual...

Yea...engineers use their gut rather than equations. Why do we even go to school?

 

[mas985]

An experienced engineer would not take any of this as serious or even factual

I beg to differ. As an engineer with many years of experience building and using computer models for a variety of applications, I do take it very seriousously and I have spent a lot of time and effort on calibrating this hydraulic model with actual measurements from a variety of pools, see this thread.

Also, engineers use models all the time to predict performance before production or prototyping. Computer time is much cheaper then prototyping and/or finding out there is a flaw in the design after production. This is no different than in pool plumbing design where by the way, prototyping is not even an economic possibility and you certainly don't want to find out major flaws in the design after the pool has been complete. Most PBs don't bother with hydraulic designs but simply rely upon their experience which is why bad habits persist.

The hydraulics industry has been using Darcy-Weisbach models for designing water distribution systems close to 200 years so I think it is safe to say they are fairly acurate otherwise they would have changed to something else. I have found that the accuracy of hydraulic modeling depends primarily on how much information the pool owner knows about the plumbing, lengths, fittings, etc rather than the model itself. The more information that is known, the better the results. However, you would be very surprised on how close I can get without knowing all that much about the plumbing. Most plumbing seems to be designed in similar ways and just knowing pipe size, run lengths, equipment lists and pump model is usually enough to get close enough for the PO to understand the trade offs involved.

 

[chem geek]

I think that the bottom line for me would be that if I were building a pool, when my PB tried to upsell me on a bigger pump, I think I'd prefer to move the extra money into plumbing instead.

In general the priority should be to get a variable speed/flow pump (or at least a 2-speed pump) since this will usually lower operating costs (and save energy) more than larger piping. Then, you can look at larger piping in areas of the system with full or half flow through the pipe. If your electricity costs are low, then the ROI may be long. In my case, however, I went from around $1400 to $700 a year by changing from a fixed speed pump (plus booster pump for the cleaner) to a variable speed/flow pump and I could have gone from $700 to maybe $500 a year by having larger piping.

 

[4JawChuck]

An experienced engineer would not take any of this as serious or even factual

I beg to differ. As an engineer with many years of experience building and using computer models for a variety of applications, I do take it very seriousously and I have spent a lot of time and effort on calibrating this hydraulic model with actual measurements from a variety of pools, see this thread.

Also, engineers use models all the time to predict performance before production or prototyping. Computer time is much cheaper then prototyping and/or finding out there is a flaw in the design after production. This is no different than in pool plumbing design where by the way, prototyping is not even an economic possibility and you certainly don't want to find out major flaws in the design after the pool has been complete. Most PBs don't bother with hydraulic designs but simply rely upon their experience which is why bad habits persist.

The hydraulics industry has been using Darcy-Weisbach models for designing water distribution systems close to 200 years so I think it is safe to say they are fairly acurate otherwise they would have changed to something else. I have found that the accuracy of hydraulic modeling depends primarily on how much information the pool owner knows about the plumbing, lengths, fittings, etc rather than the model itself. The more information that is known, the better the results. However, you would be very surprised on how close I can get without knowing all that much about the plumbing. Most plumbing seems to be designed in similar ways and just knowing pipe size, run lengths, equipment lists and pump model is usually enough to get close enough for the PO to understand the trade offs involved.

I know a dead horse when I see one, been doing this kind of thing for a long time. I don't do computer modelling, I build them out of stainless steel and plastic and glass for the pharmaceutical and medical industry...for real. I'm the white hat engineer you hand the drawing to when your done on the computer and the one who makes the revision requests when it doesn't work. I'm sure if we were working together on a project we would get along fine and do great things together, you might even learn a thing or two from me along the way and me from you also.

However this is the internet and not reality and I sincerly hope you accept my apology if I have offended in any way...not my intent. My goal was only to inject some real world experience into the discussion but since this is entirely theoretical and calculation based its obviously not required. If you ever get into a situation where the theoretical model doesn't fit the measured reality give me a call, its what I specialize in and why they pay me the big bucks. I have more revisions to designs under my belt than anyone in the company and why my user name is 4JawChuck, it was given to me because I have a firmer grip on the situation than most.

I sincerley hope the OP's project goes well, post pics when your done.
Chuck.

 

[ncpool]

This thread could use a summary, as there is a lot of good info, but there is a lot to go through. I will give it a shot:

-If you have an existing pool with 1.5" plumbing, don't stress. If you are spending time wishing you had 2" to save energy costs, your time would be better spent evaluating the size and efficiency of your pump...take your filter flow into consideration as well.
-If you are building a new pool, spec 2" plumbing. It can't hurt anything, and you will get better flow rates, which *may* save you by allowing your pump to run at lower rpms. Note this may require a vari-speed pump to get max savings.

Thoughts?

 

[chem geek]

It's not so simple to just say use 2" vs. 1.5" and in many pools that's not where you would first upsize.

First would be to get a variable speed/flow pump if one can afford it and if not at least get a 2-speed pump. Also, get an oversized filter and clean it before the pressure rises very much (i.e. don't wait until it rises as far as the manufacturer recommendation). Note that cartridge filters generally have much lower head loss than sand filters (but there are other pros/cons between filter types I won't get into here).

The recommended size of pipe depends on the flow rate through that pipe and its length, but generally the following is a reasonable guide. The splits are usually the same as the number of returns/inlets on the pressure side and, separately, the number of skimmers and floor drain pairs on the suction side.

Typical 15,000+ Gallon Pools
Splits / Flow Rate .... Size of Pipe
.. 3+ ... 1/3rd- ......... 1.5"
... 2 .... 1/2 ............... 2.0"
... 1 .... full ............... 2.5" (min. 2" at pad)

Long Pipe Runs (especially with solar heating)
Splits / Flow Rate .... Size of Pipe
.. 5+ ... 1/5th- ......... 1.5"
. 3-4 . 1/3-1/4 ........ 2.0"
... 2 .... 1/2 ............... 2.5"
... 1 .... full ............... 3.0" (min. 2" at pad)

An additional recommendation, especially if the pool has only has one or two returns/inlets, is to use 1" eyeballs instead of 3/4". If you will be reconfiguring to use only one pipe for suction, say for draining the pool, then using a 2" or larger line would be better. Note that if your electric rates are low and your pump is very close to the deep end of your pool and you don't have long lines to solar, then the savings from bigger piping will be very small. Also note that smaller pools can use lower flow rates to achieve one turnover in a reasonable time (and have smaller solar systems, if present) so the savings will be small in these situations as well.

DETAILS:
The above recommendations are mostly based on the relative head loss for different splits of flow rate (using 90 GPM and 60 GPM total as examples):

1/3rd flow (30 GPM) through 1.5": 2.4 PSI/100 feet
1/2 flow (45 GPM) through 2": 1.5 PSI/100 feet
full flow (90 GPM) through 2.5": 2.3 PSI/100 feet

1/3rd flow (20 GPM) through 1.5": 1.1 PSI/100 feet
1/2 flow (30 GPM) through 2": 0.7 PSI/100 feet
full flow (60 GPM) through 2.5": 1.1 PSI/100 feet

If one were to upsize even higher, there is additional savings, but it's generally not worth it unless the pipe length is very long:

1/5th flow (18 GPM) through 1.5": 0.9 PSI/100 feet
1/3rd flow (30 GPM) through 2": 0.7 PSI/100 feet
1/2 flow (45 GPM) through 2.5": 0.6 PSI/100 feet
full flow (90 GPM) through 3.0": 0.8 PSI/100 feet

1/5th flow (12 GPM) through 1.5": 0.5 PSI/100 feet
1/3rd flow (20 GPM) through 2": 0.3 PSI/100 feet
1/2 flow (30 GPM) through 2.5": 0.3 PSI/100 feet
full flow (60 GPM) through 3.0": 0.4 PSI/100 feet

The above is consistent with my own pool where the 1.5" lines for the three returns is OK but is not good for the two suction lines and is not good for the 2" line from the pump to the split and is worst for the long 2" lines to/from the solar and the solar return on the roof.

 

[chem geek]

On the Jandy website they have several interesting calculators including one for Total Dynamic Head (TDH). Unfortunately, it uses the total system GPM for both the suction and return pipes without allowing one to split that flow. Interestingly, they do split the flow in Step #7 when calculating exit loss through return eyeballs (the implied K value for eyeballs is 1.45). So maybe such mistakes have contributed to the perception that doing the calculations doesn't come up with what one sees in reality -- this spreadsheet will significantly overestimate TDH due to not splitting the GPM for individual return and suction lines.