Could use some advice on sizing a pump for an older pool.

[turt1e]

Well, my parents old 2HP Hayward superpump finally bit the dust a week ago. Of course it happened just as I was in the process of shocking the pool back from its dark green swamp state. This lead me to an impulse purchase of a Pentair Superflo 1.5HP pump at my local pool supply place. The sales guy there thought I would be OK with a 1HP pump, but when I told him I had an Autopilot SWCG he recommended I buy the 1.5HP instead. He thought the 1HP may not be able to provide the flow rate the Autopilot might need. I haven't installed the pump yet so I could still go back and swap out for the 1HP unit if needed.

Which leads me to where I need some advice. My parents 23,000 gallon pool is an old in ground with 1.5 inch plumbing through out. It has separate bottom drain and skimmer lines on the suction side and two returns; one at the deep end and another line at the shallow end steps that splits off to two eyeball adjustable returns.

At the pad there is a Triton TR-60 Sand filter and of course the Autopilot SWG manifold, which is two inch pipe. All other pipe at the pad is 1.5 inch. The Autopilot has a by-pass type manifold which has a check valve that allows excess flow, over 20 gpm, to by-pass the cell. This is why I'm thinking I could probably get away with the smaller pump after all.

I've attached a picture I drew up in Sketchup that illustrates the pool and line runs. Blue colored pipes are return, and of course red are the suction side. I've also attached a pic of the pad and the pump curves for the Pentair Super Flo line of pumps. Yes, the pad plumbing needs some serious work, and is one area I'm thinking I could use 2 inch pipes to make the system more efficiet.

So I could use some advice on how to approach this pump replacement. I've read through the Hydraulics 101 sticky and best I can come up with is that I've got in the area of 35+ feet of head if I go by the sand filters pressure reading of 12psi with the old 2HP Hayward pump. But if I do a rough estimate based on pipe run and bends it shoots up to 60+! I know it's not much to go on but what do you guys think, could I get away with the 1HP pump? It is only a $70 price difference between the two, but I'm sure my parents would appreciate the lower utility bills if it would work.

Thanks,

Paul

 

[duraleigh]

Hi, Paul,

Very nice drawing and an excellent picture of your equipment....very clear post and easy to read.

I think you're on the right track with a 1hp pump. Your pool size and filter would match it's output pretty darn good.

A rule of thumb is a pump that will turn your pool water 2-3 times in a 24 hour period. You've got that just about right if you assume about 40+ TDH in your system.

Importantly, I believe that filter is rated at 60gpm.......that's about perfect as you do not want the pump to push more water thru the filter than it's designed flow rate.

In short, the 1hp looks like the solution to me. There are many strong advocates here on the forum of the intelliflo pump but I have no first hand experience so I'll let them chime in about it.

I would have expected your filter psi to be a little higher with your current system. Those guages are frequently faulty. Do you know if that 12 psi has been the norm in times past?

 

[spishex]

At 60 GPM (pretty much the max your filter and plumbing can do) you'd get a 6 hour turnover which would be just about perfect for you. That would be 40 ft on the 1 HP vs. 55 ft on the 1.5HP. Given how straightforward your pool is, I think you could get into the 40 ft range but I'd definitely streamline the pad and use 1" eyeballs for the returns. When you do replumb the equipment:

-Go with 2" PVC around the equipment.
-Your pump is DYING (dead, I guess) in that setup. Get rid of the three 90s just after the outlet, and you need to have a length of straight pipe at least 4x the diameter of your pipe size going into the pump on the suction side. So that spot where the valve goes right into it? You need 6" of PVC in between, or 8" if you go with 2" pipe.

 

[mas985]

I noticed that you have a three valve on the suction side. Is it set for both lines on? If not, that might explain the 12 PSI filter pressure. I get about 11 PSI @ 62 GPM with a 2 HP pump using my head model but only when I assume that there is one 1 1/2" suction line and the skimmer. Otherwise the filter pressure should be much higher.

Assuming I modeled your plumbing correctly (i.e. skimmer only), I think you could go with a 3/4 HP pump. That should deliver close to 47 GPM @ 39' of head. Going with 2" pad plumbing would help a little bit and reduce head to 37' and increase flow to 50 GPM. If you have both suction lines open, then the flow rates would be higher. This would still give you an 8 hour turnover but lower energy costs further.

Note too that if you went with a 2 speed pumps, the flow rates would halve on low speed. If memory serves, the autopilot has a 15 GPM minimum flow rate so it should be ok even on low speed but you might want to confirm that. This would save even more energy even after doubling run time.

Let me know if the assumption on the suction line is correct so It can make adjustments to the model.

 

[turt1e]

I would have expected your filter psi to be a little higher with your current system. Those guages are frequently faulty. Do you know if that 12 psi has been the norm in times past?

duraleigh, thanks for your input. The filter gauge is pretty consistent about staying in the 11 to 12 psi range after a backwash. When it climbs to about 15 to 16 psi I usually will backwash. Although I also am suspect on the accuracy of that gauge as it's probably from the early 1990s!

 

[turt1e]

At 60 GPM (pretty much the max your filter and plumbing can do) you'd get a 6 hour turnover which would be just about perfect for you. That would be 40 ft on the 1 HP vs. 55 ft on the 1.5HP. Given how straightforward your pool is, I think you could get into the 40 ft range but I'd definitely streamline the pad and use 1" eyeballs for the returns. When you do replumb the equipment:

-Go with 2" PVC around the equipment.
-Your pump is DYING (dead, I guess) in that setup. Get rid of the three 90s just after the outlet, and you need to have a length of straight pipe at least 4x the diameter of your pipe size going into the pump on the suction side. So that spot where the valve goes right into it? You need 6" of PVC in between, or 8" if you go with 2" pipe.

spishex, What is the significance of the "40ft on the 1 HP vs 55 ft on the 1.5HP" statement? Is the that flow rate of the water through sched 40 piping?

I definetly want to streamline the equipment pad plumbing so your little nuggets of info like needing 6" to 8" of piping between the pump and valve is much appreciated. Thank you.

 

[turt1e]

I noticed that you have a three valve on the suction side. Is it set for both lines on? If not, that might explain the 12 PSI filter pressure. I get about 11 PSI @ 62 GPM with a 2 HP pump using my head model but only when I assume that there is one 1 1/2" suction line and the skimmer. Otherwise the filter pressure should be much higher.

And that is partly why I'm suspect of that filter gauge. Normally I would leave the three valve cracked slightly to allow some flow from the bottom drain to mix with the skimmer. Of course doing so slightly closes the skimmer also, but this seemed to keep the filter from drawing in air which seemed to be a problem if set for just one line. The best I could tell was that it was sucking in air through the old Hayward Superpumps strainer basket. Any time I would turn the pump off I would notice water shooting out around the strainer baskets lid. I don't think the strainer lid has a leak because if you notice in the attached picture the filter gauge still shows 11 psi even though the pump motor is obviously missing from the pump. Also I should state that I don't think I ever noticed the filter gauge change more that 1 psi when I had the 3 valve set to only one suction line versus both.

* Edit: Actually I think we can rule out the filter gauge all together now. I just went out and relieved the pressure from the filter and the gauge didn't move!! Obviously it's broke so now I don't even have that to go on.

Assuming I modeled your plumbing correctly (i.e. skimmer only), I think you could go with a 3/4 HP pump. That should deliver close to 47 GPM @ 39' of head. Going with 2" pad plumbing would help a little bit and reduce head to 37' and increase flow to 50 GPM. If you have both suction lines open, then the flow rates would be higher. This would still give you an 8 hour turnover but lower energy costs further.

You mention "skimmer only" but the pool also has a bottom drain, does that change anything? Or is the assumption that the bottom drain usually stays closed? I personally do not like that bottom drain being on when bathers are in the pool so I've always told my parents to just leave it off and only use the skimmer. But I would imagine if both were on it would improve the efficiency of the plumbin, or is that a wrong assumption? Also, in going with 2 inch plumbing on the pad, should the filter multi-valve be a 2 inch type. I'm thinking the one they have has 1.5inch connections.

Note too that if you went with a 2 speed pumps, the flow rates would halve on low speed. If memory serves, the autopilot has a 15 GPM minimum flow rate so it should be ok even on low speed but you might want to confirm that. This would save even more energy even after doubling run time.

Let me know if the assumption on the suction line is correct so It can make adjustments to the model.

You know, I thought about a 2 speed pump, but the guy at the store told me that in his experiance they tended to need servicing more often so that kind of scared me off of them. Could just be that he didn't have any in stock though now that I think about it. :grrrr: I'm in a bit of a catch 22 here I think. I kind of need to install a pump to get better flow data, but if I install this pump I won't be able to return it. If instead I go back and get the 1HP or even a 3/4 HP pump and hook it up and find out it's not enough for some reason, then I'm stuck as well. Decisions... decisions...

I really appreciate all the help guys!

 

[spishex]

At 60 GPM (pretty much the max your filter and plumbing can do) you'd get a 6 hour turnover which would be just about perfect for you. That would be 40 ft on the 1 HP vs. 55 ft on the 1.5HP. Given how straightforward your pool is, I think you could get into the 40 ft range but I'd definitely streamline the pad and use 1" eyeballs for the returns. When you do replumb the equipment:

-Go with 2" PVC around the equipment.
-Your pump is DYING (dead, I guess) in that setup. Get rid of the three 90s just after the outlet, and you need to have a length of straight pipe at least 4x the diameter of your pipe size going into the pump on the suction side. So that spot where the valve goes right into it? You need 6" of PVC in between, or 8" if you go with 2" pipe.

spishex, What is the significance of the "40ft on the 1 HP vs 55 ft on the 1.5HP" statement? Is the that flow rate of the water through sched 40 piping?

I definetly want to streamline the equipment pad plumbing so your little nuggets of info like needing 6" to 8" of piping between the pump and valve is much appreciated. Thank you.

Just looking at the curve you'd achieve 60 GPM with either a 1 HP @ 40ft of head or a 1.5 HP @ 55 ft. For your system you'll need to be somewhere between 45-60 GPM.

 

[mas985]

turt1e,

I had originally given you numbers for the case where the main drain was closed since from the filter pressure is seemed like that was the normal operating condition.

The reason the pump was sucking in air is probably because the main drain was off. With a 2 HP pump and 1 1/2" plumbing, that will starve the pump of water and the pump will start to suck air from the valves and the pump basket. This also explains why the filter pressure was only 12 PSI and the air in the pump basket. If both the main drain and skimmer are 100% open the filter pressure would increase to 18 PSI. For most efficient operation, this valve should be set such that both lines are always open.

If you open the main drain and leave the skimmer open as well, you could expect the 3/4 HP Superflo pump to give about 54 GPM at 32' of head. Changing the pad plumbing to 2" whould increase the flow rate to 58 GPM at 29' of head.

So to compare pumps and assuming you go with 2" plumbing on the pad:

3/4 HP 58 GPM @ 29' of head
1 HP 66 GPM @ 37' of head
1 1/2 HP 73 GPM @ 46' of head

If the filter rating is 60 GPM, I wouldn't put in more than a 3/4 HP pump. That should have more the enough flow rate for your setup. The two speed is optional but I think that should work as well.

 

[turt1e]

Just looking at the curve you'd achieve 60 GPM with either a 1 HP @ 40ft of head or a 1.5 HP @ 55 ft. For your system you'll need to be somewhere between 45-60 GPM.

Ahh... now I got it! Thanks

 

[turt1e]

turt1e,

I had originally given you numbers for the case where the main drain was closed since from the filter pressure is seemed like that was the normal operating condition.

The reason the pump was sucking in air is probably because the main drain was off. With a 2 HP pump and 1 1/2" plumbing, that will starve the pump of water and the pump will start to suck air from the valves and the pump basket. This also explains why the filter pressure was only 12 PSI and the air in the pump basket. If both the main drain and skimmer are 100% open the filter pressure would increase to 18 PSI. For most efficient operation, this valve should be set such that both lines are always open.

If you open the main drain and leave the skimmer open as well, you could expect the 3/4 HP Superflo pump to give about 54 GPM at 32' of head. Changing the pad plumbing to 2" whould increase the flow rate to 58 GPM at 29' of head.

So to compare pumps and assuming you go with 2" plumbing on the pad:

3/4 HP 58 GPM @ 29' of head
1 HP 66 GPM @ 37' of head
1 1/2 HP 73 GPM @ 46' of head

If the filter rating is 60 GPM, I wouldn't put in more than a 3/4 HP pump. That should have more the enough flow rate for your setup. The two speed is optional but I think that should work as well.

Wow.. I never even thought I might be able to get away with a 3/4HP pump. When changing the plumbing at the pad to 2 inch, whould I also change out the MultiValve since it's a 1.5inch version? Here is the info I can still see from the top:

Pac.Fab SM10-3 Sand
Oper Pressure 50 PSI
Rated Flow 75 GPM

 

[turt1e]

Well, we finally got some nice weather here in Atlanta and my Dad and I finished installing the new pump. I did end up taking the 1.5 HP pump back and exchanging it for a 1HP Superflo. I tried to get a 3/4HP pump but they didn't stock that in the Superflo and my Dad was skeptical that a 3/4HP would cut it after 15+ years of running with the 2HP pump.

So after installing a new pressure gauge on the filter and a vacuum gauge on the pump, I get 11psi on the filter and 7 In. Hg of vacuum with all valves open. Now when i start closing valves in different combinations the numbers change a good bit and I can't make sense of it. So I've attached a spreadsheet with different valve setting combinations in hopes someone here could make sense of it. The "air bubbles" column just means I saw air in the filter and pump strainer basket for that combination.

*EDIT Just realized I missed two combinations of valve settings on my original spreadsheet attachment so I've updated the attachment to include those *

 

[mas985]

It looks like it is behaving as it should.

Everything open is where you will get the lowest total head loss and the largest flow rates.

Total Head (ft) = Filter PSI * 2.31 + Vacuum (in HG) * 1.13

When a suction port is shut off, the vacuum should increase and filter pressure decrease. In this case, shutting off a suction path increases head loss/vacuum on the suction side which reduces flow rate. Lower flow rate on the return side will reduce filter pressure. This will sometimes cause air to be sucked in via a valve or pump lid which you saw in three of the cases with the main drain off.

When a return port is shut off, the opposite happens, vacuum will decrease and pressure increase. In this case, the head loss of the return is going to increase because there are less paths to travel. More head loss = less flow rate and less flow rate in the suction will lower the vacuum.

Hopefully, that makes sense. You can use the total head calculation to then find the flow rate for each of the cases. The first case should have the highest flow rate.