Is my pump oversized? 1.5 HP / 7500Gals

[MountainStone]

This is my first pool, and the equipment is a bit of a mystery to me insofar as why it was chosen and sized. Reading other threads, it seems like my little pool might have a really big pump motor. Is a 1.5 HP too big for a 7500-gallon inground pool?

The info I can discern from the data plates is as follows:

Pump housing: Sta Rite 'Model' MPRA6F-148L 'Code' 1C04T

Filter Housing: STA-RITE "POSI-CLEAR" CARTRIDGE FILTER PXC150
Max Working Pressure: 50 psi
Max GPM: 150

Motor: A.O.Smith
Total HP = 1.65
USQ1152
THERMALLY PROTECTED CET-50-ABM
MOTOR MOD: C48L2PA105C5
VOLTS: 115/230
AMPS: <blank>
RPM: 3450
MAX LOAD AMPS: 19.2/9.6
INSUL CLASS: B
AMB: 52 C
TYPE: UAC
HP: 1-1/2
PH: 1
CODE: J
FR: 48Y
HZ: 60
SF: 1.1
TIME RATING: CONT.

I have photos of the data plates if that would help. The motor was replaced by the seller of this house as part of the purchase agreement (motor was missing when we toured the house). I replaced the very old PXC150 cartridge filter last week with a new PXC175 cartridge filter, as that was the size/model cross-referenced when I looked up replacements online.

I have no other features installed on my pool; no SWCG, no spa, no heater, no make-up water connection, no water features. My plumbing is simple: One skimmer, pump, filter, return to two eyeball inlets. The T-valve between pump and filter has a capped stub, I assume for pumping to waste. All visible plumbing is 2". The pump sits on grade, about 12" above the surface of the pool water. The pump is 15' from the skimmer, and the longest return line is probably about 50' if they ran it the longest route. Startup pressure with the new filter was 15 psi (down from 30 psi with the old filter). As you may imagine, flow from the 2 inlets is very strong. I also notice the case of the motor gets really hot while in operation; you can touch it but not for more than a second or two. The vent ports on the bottom of the motor case aren't blocked; it's flowing hot air.

I suspect this large motor is turning over my pool in just a couple hours, though I have it running four hours per day. Should I get a new motor? Is my motor overdriving my filter? If I can keep this new one, how long should I be running it? Is there any way to estimate the GPM/GPH using the info I have included?

Thanks for any opinion/advice you care to share!

[cleancloths]

In a word it is huge. It can turn over the pool at full tilt in about an hour, once a day is enough.

[mas985]

I would agree, the PUMP is over sized, not just the motor. Your flow rate is probably around 85 GPM or about a 1.5 hour turnover. If you are looking to downsize the pump, you have a couple of options. Here are a few from cheapest to more expensive:

1 - Replace the Impeller and Diffusor with the 3/4 HP version
2 - Replace the motor with a two speed and run on low speed most of the time
3 - #1 plus replace the motor with a 3/4 HP two speed motor
4 - Replace the current pump with a 3/4 HP two speed pump
5 - Replace the current pump with a variable speed pump

Each of these options will save you energy costs with #5 saving you the most but then it costs the most up front as well.

Also, you didn't mention if you have solar or are planning to add solar at any point. That can make a difference when choosing a pump.

[waterbear]

Actually, your 1.5 HP pump is an "Uprated" model so it is really a 1 hp pump. (Full rated, max rated, and up rated is a game the manufacturers play to make their numbers look better and prices seem more competitive. However, pump performance curves are really what tell the story, not HP. Your brake HP is 1.65 which is the same brake HP that a full rated 1 HP pump would have. A 1 hp pump is not unreasonable on a 150-175 sq ft cart filter. The filter is oversized for a pool that small but oversizing carts is considered good practice since it gives you a very long mean time between cleanings. my pool is under 7k and has a 150 sq ft cart and a 1.5 uprated pump also.

A 2 speed pump would have been a a better idea when the pool was built and your cheapest upgrade would be to install a 2 speed replacement motor on your exisiting setup.

[PoolDoc]

Data taken from Pentair DynaPro page

Specs





DynaPro pump curve - use 'F' curve




StaRite PosiClear PXC150 -- 150 sft cartridge



Your current pump is the 'F' curve, and is probably running between 60 and 110 GPM.

Your filter (150sft) is optimal at 55 GPM or less (0.375 gpm/sft). I wouldn't put stock in the 175 sft rating: they achieve that by adding pleats, but the pleats that are close together tend to become clumps of pleats that act as one large thick and ineffective pleat. High flow with high pressure greatly increases the tendency toward pleat collapse.

The fact that your pump is running hot suggests a high load, which equals high volume. Given your low restriction piping, it's likely you are running on the right side of the curve, at maybe 100gpm. This wastes electricity and is hard on your filter.

I'm going to have to go with Mark on this one. Your best option for now is to replace your impeller with the smallest one you can get. This site -- http://www.poolplaza.com/C-Sta-Rite-Dyna-Pro-Parts.html -- shows a 1/2HP full rate / 3/4 HP uprate impeller that corresponds to the 'D' curve above. You should end up running 40 - 70 GPM, rather than your current 60 - 100 GPM. You'll save electricity; your motor will last longer; your cartridge will last longer and your filter will remove finer particles.

On your pool, even with the minimal impeller, you'll still only need to run 3 hours per day, to get a full turn-over. I'd recommend trying (2) 2 hour run periods.

You'll need to replace both the impeller and the seal. I'd get (2) seals, so you have a spare, and also all exposed rubber parts.

Later, you can look for an energy efficient 2 speed motor. I looked, but didn't find one.

[MountainStone]

Wow. You guys really know your business; thanks so much for the thoughtful analysis and advice. What a treasure this forum is due to folks such as yourselves.

I'll have to contemplate which solution works best for me, balancing ease with cost. My complete unfamiliarity with the pump makes me hesitant to tear into it, though I'm handy enough with tools. Is there a tutorial/DIY for replacing the impeller & diffuser somewhere, or is it simple enough to figure out just by disassembling it? Do I need torque specs, loctite, etc? Doc recommends replacing the exposed rubber parts, but I dunno what those are; is there an impeller kit that comes with those rubber parts?

Thanks again!

[PoolDoc]

If you want to do it, I'll walk you through it.