Recent increase in hydraulic head?

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flex727

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Apr 2, 2010
17
While I don't have specific numbers from a year ago, it seems to me that my pump is working much harder right now that it did in previous years. I have a variable speed drive, and according to the display, the most flow I can get out of it is 50 GPM at 3450RPM, using about 2010 watts. When I reduce pump speed to 20 GPM, RPM goes to 2375 at 695 watts. I recognize that the GPM is a calculated value, and perhaps it's wrong, but the power requirements seem very high.

Of course, hydraulic head is very pool specific, but I have 1-1/2" piping with 5 pool returns and two water features - a spa and a slide. The slide is off, but the spa is fully on (water returns up from the center bottom thru 1-1/2" pipe, not thru the spa jets). All the returns appear to be working properly. The pump is located less than 30' from the pool and there is no elevation to overcome (no solar panels, etc). On the other side, I have two skimmers and a standard double head drain at the deep end - all are fully on. I do have gas heater (Pentair 400K BTU/Hr, I think). I did have some chemistry balance problems last year before I discovered this website and the BBB method and allowed the pH & TA to rise too much. My CH has always been very high (typically 400-450), but tap water is around 250, so lowering it is very difficult. Considering this, could I have had some scaling that has clogged the heat exchanger in the heater? How would I check this, and if it IS the problem, is it repairable?
 
JohnT said:
How old is your cartridge? It could be the culprit.
Click to expand...
Filter pressure is running about 17 at max pump speed and only 10 at the lower speed I mentioned. The filters were 3 years old and very stained and worn, so I went ahead and replaced them. The above psi values (and other data in the first post) are after replacement. Replacing the filters dropped the psi about 2-3 from prior values, so it doesn't look like there was really much of an issue there, but it's nice to have fresh filters.
 
The ratio of GPM should be about the same as the ratio of RPM so there is clearly something not quite right. Also, only 50 GPM at 3450 RPM is very low and would indicate a head loss of about 90' of head! 20 GPM at 2375 RPM would indicate a head loss of 46' of head! The wattage levels you are seeing also reflect very high head loss and are consistent with the GPM/RPM levels

However, with the lowish filter pressure, it might indicate that the suction head loss has increased and/or there is an impeller clog. Have you checked the impeller?
 
First check in the pump basket and feel in the hole that leads to the impeller. If that feels clear, then you may need to take apart the pump to check the impeller closer. But before that, make sure all your suction lines are clear but with multiple lines, I would assume that at least one is. Did the pool recently open and did you end up vacuuming out a lot of crud? If so, then it is a good possibility that the impeller has collected some of it.

After making sure that it couldn't be anything else, then you might look at inspecting the impeller. If you are handy and comfortable with the tools, taking apart the pump is not all that hard to do, I have done it several times. Check over the preoceedure they have in the manual (pg 43 & 44) and make sure you are comfortable with the process.
 
Thanks, mas. Before I get too deep into the impeller issue, here are a few facts:

The pool is 3 years old. Several months ago the pump was making a lot of noise and I had a Pentair guy come out to check it. He found some damage to the impeller and replaced it. There was no indication what the cause was, but I think the issue I'm concerned about in this thread started before the replacement and was not solved by it. As of now, the noise I had before has not returned.

I will perform the check on the impeller that you suggest, but I'm thinking there may be two likelier causes - either a partially blocked heater core due to scaling or partially blocked drain and/or skimmer lines.

Does anyone know if blockage downstream from the filter would show up on the filter pressure gauge? It seems to me that it would. If so, that would rule out the heater. If have some blockage in the lines coming from the drain or skimmers, how can I check that? And if they are partially blocked, how can they be cleared? I know that neither the bottom drain nor the skimmers are fully blocked because I can see them operating.
 
Blockage downstream shows up has higher PSI which is why I suggested that it is probably an issue on the suction side. One way to clear a suction side bockage is to put a hose in the pump suction port (toward the skimmers), wrap it with a wet rag and try to flush out stuff backwards through the lines. If you have a valve for each port, then do one at a time.

However, given the tech was working on the impeller, I suspect he messed something up. Either put the wrong impeller on or forgot the impeller ring/seal or perhaps something else.
 
Okay, I did a little experimentation today looking at the suction side. I have 2 skimmers and 1 main drain and I can isolate any of them through the valving setup. If I make an assumption that there is a clog, it would almost have to be in one of the individual pipes, and if I isolate that path, I should see a significant difference in flow or power requirement for the pump. Unfortunately I couldn't detect any difference in either the flow or power requirements no matter which path was selected or isolated. And since there is clearly not a pressure issue downstream, I think I need to focus on the pump impeller. The hard part is going to be convincing Pentair that they need to take responsibility.

Anyone see anything I'm missing or a flaw in my reasoning?
 

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It is looking more like the impeller is cause and especially since that is what they tried to "fix". There is one more thing that you could check and that would be the line between the pump and filter. This should not clog under most circumstances but might be worth a check if it is easy. Are there any valves, check or otherwise, between the pump and filter?
 
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