Pump Motor Recommendation & TDH Question

[Finbar127]

Hi Everyone,

My thirteen-years-old 1 1/2 HP Hayward Super Pump developed a slight seal leak and the casing is all rusted out so I decided to replace the motor. I have no clue if my pump was properly sized when the pool was built . Using gauges I took the following vacuum/pressure readings at the pump:

Vacuum: 8
Pressure: 18

Using those figures I calculated a TDH of 50.62 and a GPM of approximately 52. However the pressure reading on my DE filter (with a fresh DE charge) is 15 which would put the numbers at 43.69 TDH and 60 GPM. I verified both the pump and filter pressure readings by swapping pressure gauges between the two ports. My questions are:

1) Which pressure reading would I use, the pump or the filter?

2) Would the seal leak affect the pressure reading? (I'm assuming it would but I don't know how much)

3) Would the 1hp motor be more appropriate for my installation?

4) If I went with the 1 HP motor would I have to change the impeller?

Thanks
John

 

[mas985]

Don't bother with head calcs, you don't need to do that.

The only reason to have a large pump is either a spa or wate rfeature that requires a lot of flow rate. So the smallest pump would do.

1&2 don't matter

3) Yes but even smaller would be ok. I have a 1/2 HP on a 20k pool. My spa is run off a separate pump.

4) Yes, you would need to change the impeller.

 

[Finbar127]

Don't bother with head calcs, you don't need to do that.

The only reason to have a large pump is either a spa or wate rfeature that requires a lot of flow rate. So the smallest pump would do.

1&2 don't matter

3) Yes but even smaller would be ok. I have a 1/2 HP on a 20k pool. My spa is run off a separate pump.

4) Yes, you would need to change the impeller.

Thanks for the response. My main concern is flow rate. My Heat Siphon heat pump requires a minimum of 25 GPM to operate. If I go with the smaller motor will there still be enough flow for the heat pump to operate?

 

[mas985]

Ok, I now understand and that is a good reason. With your measurements and assuming 1.5" pluming and 1.5" multiport, your flow rate with your current pump should be around 66 GPM and low speed would be 33 GPM. So that would be ok. On exactly the same plumbing, the 1 HP Superpump would have a full speed flow rate of 59 GPM and a low speed of 29 GPM so still ok. If you have 2" plumbing, then the flow rate would be better than that.

 

[Finbar127]

I do have 1.5 plumbing and a 1.5 multiport. I also have a Pool Pilot Digital SWCG which also has a minimum flow rate but it is lower than the heat pump.

How did you arrive at the GPM numbers? Are they based on the TDH calculations? I'm just trying to understand the process and wondering why they differ from my estimates.

So a 2 speed 1hp motor would be the best choice?

 

[mas985]

I used the spreadsheets in my signature but I just realized that I forgot to include the multi-port. With that, here are the new numbers:

1 1/2 HP - 54 GPM on high and 27 GPM on low

1 HP - 49 GPM on high and 24 GPM on low

But you really should run the heat pump only on high speed to keep efficiency up. Given there is some error in the calculations, I probably wouldn't use either pump on low speed when running the HP. But if you plan on only using high speed when the HP is on, then the 1 HP would work fine.

 

[Finbar127]

I used your spreadsheets and decided to order the 1 HP A.O. Smith high efficiency motor UCT1102, the 1 HP impeller SPX2607C, and a GOKIT3SALT. After I hook everything up I will run some more tests and report the results.

Thanks for your help.

 

[Finbar127]

The new motor came today. Changing out the motor and changing the impeller, seals and gaskets went smoothly. Everything is running fine so far. Vacuum reading is 6 and pressure at the filter is 13. Using a calculator and the Hayward pump curve I end up with 37 TDH and about 57 GPM. Your spreedsheet gives me 45 TDH and 57 GPM. Close enough for government work. I'm amazed that there is only a couple of GPMs difference using the smaller motor and impeller. The only difference I've noticed so far is the pump takes about 30 seconds longer to prime.

Thanks again for your help.

 

[mas985]

Your spreedsheet gives me 45 TDH and 57 GPM.

Remember that includes the offset for the filter gauge height and the plumbing between the pump and filter. Which are not included in the pressure measurement or the suction measurement you made. So you can't just convert the suction and pressure measurements without including those extra components. If you had measured the pump pressure directly at the pump, it would be about 3.5 PSI higher. So 45' should be more accurate.

 

[Finbar127]

Sounds right. I didn't take the pressure reading at the pump this time but I did with the old motor. The PSI at the pump was 3 PSI higher.

I just noticed the pump is wet at the base. It is not leaking like before but it definitely looks like water is seeping out. I was very careful reassembling the pump so I don't know what happened. The back plate and seal plate did not look damaged and I'm positive I installed the seals and gaskets properly. Bummer! I will let it run for a few hours and keep an eye on it. Worse case scenario I will take it apart again tomorrow and see what is up.

 

[Hayward Pool]

Nice seeing a pool owner using a pressure and vacuum gauge to determine TDH. Not often is this done. Taking both readings at the pump will gave you a more accurate reading.

 

[Finbar127]

I fixed the leak. The pump appeared to be leaking where the pump assembly attaches to the pump housing. I disassembled the pump yesterday morning and checked the seals/gaskets. I also confirmed that none of the pump parts were cracked or damaged. I reassembled taking extra care when inserting the assembly into the pump housing and slowly tightening the four housing bolts in a 1,3,2,4 pattern (about 1/8 turn at a time). That seems to have done the trick. I think i was a little too aggressive attaching the assembly to the housing the first time perhaps causing the housing gasket to not seat properly.