Insufficient pressure. Poor circulation pattern

[edshire]

Pool was originally constructed with two returns. One is at the bottom of deep end of pool. It is a return and not a drain. The other return is at the shallow end of pool. There are two wall skimmers. One is very weak because years ago when a leak was discovered in it I replaced the line by running a new one on the outside of the concrete thereby adding 30 to 40 feet of additional length.
A few years ago I added some additional valves to the system with the idea of sometime being able to use that bottom of the pool return as a drain. I thought that this would make it easier for me to clean the pool when I opened it.

I now have the valves set as originally configure with the bottom as a return. But I can't use it. If I open the wall return and the bottom return the filter pressure drops. If I try closing the wall return and using only the return on the bottom of the pool then there is also low pressure at the filter. It is only when the bottom return is closed and the wall return is open that I get the greatest pressure at the return. So I am stuck with only one return usable and really only one effective skimmer working. Leaves me with poor circulation. I don't know for sure if I am getting sufficient water flow. I assume that when the pressure at the filter is highest that that is when I am getting the greatest flow. I have considered getting a more powerful pump. Can someone explain why the pressure at the filter drops when I open both returns. Do you thing a more powerful pump would work. I currently have Pentair 1 1/2 hp pump.

 

[JasonLion]

Welcome to TFP!

There is nothing wrong with lower filter pressure, that is actually a good sign, as it means your system is running more efficiently than before. Assuming a fixed pump speed, lower filter pressure means more total water moved.

A 2 HP pump is rather large. Unless you have a spa it is at least twice as large as what you need. If you do have a spa, or other large water feature, you didn't mention then you might need the 2 HP.

High pressure reflects a large amount of resistance to flow. If the water is flowing freely, the pressure would (in an ideal world) be zero. As the pressure increases, you are losing more and more water movement to friction, so the total water moved goes down.

 

[Richard320]

You will have lower pressure because you have less resistance. That's a good thing. Blow through a straw. Now blow through two straws. Easier, right? Same thing.

Garden hose. Wide open, not a lot of pressure, but a lot of volume. Put a jet nozzle on it and you get a lot of pressure but not a lot of volume.

The more flow (less restriction) you have on the suction side, the higher the pressure in the filter because the pump can move more water easier. The more flow you have on the pressure side, the lower the pressure in the filter because it can move more water easier.

Sounds to me like it's working exactly the way physics says it should.

 

[edshire]

I erred when stating that it was 2hp. It is 1 1/2 hp pump. As I said in my original query one of my skimmers being very far from the pump in terms of total pipe length is very weak. I also have lots of 90' turns. Do you thing it would be advisable to upgrade to a more powerful pump. Thanks

 

[edshire]

Using the formulas for vacuum and pump pressure I came up with the following. If I open bottom return and side return and open both skimmers I got a TDH of around 40. When I closed that bottom return and closed the distant skimmer I came up with a TDH of aroung 45. Both these readings do not even register on the pump curve in the Pentair pump curve. I'm almost certainly not getting sufficient water turnover. My idea it to get a 2HP motor to replace the 1.5 motor that is there now. Do you think that this might work? The plumbing is all 1 1/2" pvc. Would I risk damaging the pvc plumbing?

 

[JasonLion]

You are focusing on the wrong things, working off of incorrect information, and coming to the wrong conclusions.

Even a 3/4 HP two speed running on low speed for four hours a day is more than enough circulation for a typical residential pool. Residential pools are not limited by circulation, they need very very little circulation. The main limitation is clearing surface debris. You need enough water movement to carry debris to the skimmer, hopefully at a couple of different times of day so surface debris doesn't float there long enough to sink to the bottom.

Moving to a higher HP motor will use more electricity, and thus cost more to run, without proving any improvement of any kind at all. Getting a larger pump wouldn't damage the plumbing, but there is some chance it would damage the filter.

 

[mas985]

Can someone explain why the pressure at the filter drops when I open both returns.

When you have two returns open, the flow rate is split between the two paths which results in lower return head loss and lower filter pressure.

However, 45 feet of head would be very low for a 2 HP pump. Do you know the model of the pump? How exactly did you measure TDH? I just want to make sure we are all on the same page.

 

[edshire]

One skimmer is at a long distance from the pump. It is virtually useless. It draws very little surface debris. The only skimmer basket that I usually have to empty is the one closer to the pump
To measure the head I did the following:
I put vacuum gauge under the basket and the pressure gauge under the impeller. The vac gauge read 8. I multiplied it by 1.3. The pressure gauge read 16. I multiplied it by 2.31. I added the two results and came up 45 TDH. That is done with the distant skimmer closed and the bottom return closed.
When I did the same, with those two open, I came up with total head of only 40.25
The current pump has a 1.5 hp motor. I am considering replacing the motor with a 2hp motor.
The pump is 1.5 hp Pentair superflo.
Thanks
Ed

 

[mas985]

With that pump and 46' of head, the flow rate should be about 73 GPM so there is really no issue with any of that. Replacing only the motor will do nothing. You would also need to change the impeller but the change in flow rate would go up to only 78 GPM with a 2 HP impeller & motor which is not a big change.

But I am still not sure what you are trying to accomplish. If you want to improve the flow of one skimmer, you need to sacrifice the flow of the other skimmer and MD. Is the second skimmer necessary? Is that where the wind blows? Just because you have it doesn't mean you need to use it.

 

[edshire]

I spoke to someone yesterday who sells these pump and I was told that the 1 1/2 hp Pentair superflow pump actually had an effective power of only 1 hp. That is why it is cheaper than the whisperflow model. The manual that comes with it shows a chart with minimum TDH of 55 producing 75 gpm. It doesn't even show what the gpm would be for TDH of 40-45 would be. I do know that I have been running the pump 24 hours a day for over a week and it is still not clear. Before doing this I cleaned most of the gook off the bottom of the pool by vacuuming it to waste. I believe that I can live with only one skimmer which effectively clears the debris off the surface. But with only suction side intake I just don't think that I am pulling enough water through the system. I am considering running a new suction line over the top of the pool deck that is closer to the pump and totally disconnecting that far side skimmer. What do you think?
Thanks
Ed

 

[mas985]

You want to look at the performance head curve on this page:

http://www.pentairpool.com/products/pumps-inground-superflo-high-performance-pumps-165.htm

Line H is your pump.

I do know that I have been running the pump 24 hours a day for over a week and it is still not clear.

If your chemistry is not correct, no amount of pump run time will help. You are focusing on the wrong thing. A cloudy pool is due to algae not a lack of run time. You probably need to SLAM the pool.

Pool School - SLAM - Shock Level And Maintain