Replacing a motor on a pump, and getting the correct size for your pool?

[mitchryan912]

I've got a bad motor (smoking when running) and am ready to order up a new motor and stick it on the existing pump (see sig below.) It seems as if this would be a good opportunity to upgrade to a bigger motor, as there was once a 1.5 HP pump installed before we (possibly wrongly) put a new pump with a smaller (1 HP) motor in last year.

Just curious how one would calculate how much power a motor would need for a given size pool? I've got a 26,000 gallon in ground rectangular pool, with an outlet to the pool in the shallow end, a skimmer in the middle, and a return port in the deep end (all piped together with 1 1/2" PVC.)

[mas985]

Changing just the motor on a pump does not change the performance of the pump. It is the impeller and wet end that determines performance of the pump. The motor is just there to turn the impeller but must be sized correctly for the impeller. If you don't change the impeller, the motor size should not change.

But when it comes to circulation and filtering, even the smallest pump is usually sufficient for the largest pool and usually the best choice. Larger pumps require larger filters and just use more energy without much benefit. I have a 1/2 HP pump on a 20k pool and it is more than sufficient for cleaning the pool.

But lets start with why you think you need a larger pump.

[CarlD]

Ok, two threads on the same topic is a no-no. Don't do that.

[mitchryan912]

Sorry, posted the other topic in the wrong spot, and what I'm after has changed. I think this is probably the thread to use, and the other should be locked or deleted. My bad.

I assumed that since the previous owners of the house (my in-laws) had a 1.5 HP motor in there previously, that what we should have replaced the old pump with the same size. I don't know if that is a correct assumption, now that I've been doing some digging on the topic. It seems as if 1.5" PVC pipe is going to be the limiting factor, and that anything over 1 HP could well be doing more harm to the motor than good?

Does this sound correct, that 1.5" pipe is going to allow for the maximum flow rate of 44 gpm, while my pool probably needs a minimum flow of about 54 gpm (26,000 g/8 hrs, and then divided by 60 min to get the max flow rate.) From what I'm seeing, it sounds like a 1 HP motor would put me in the ballpark of 40 gpm, depending on which motor I get.

As far as the current motor, smoke was emanating from this hole near where the wire enter the back of the motor. I don't see any scorching on the wires, I've tested for voltage on both legs of the wiring (it's 230v), and the motor is set to 230v.

IMG_1366.jpg

[mitchryan912]

While we're at it, my pump runs 24/7. Does that change the calculation for how many gallons need to be filtered per 8 hours? We don't really use it that often to need it to be cleaned 3x per day... or does that mean we probably ought to consider putting in a timer to limit how much it runs?

[mitchryan912]

A 1HP IG pump should be fine, but not a 1HP AG pump.

Are they constructed differently for each type of pool? I guess I'm potentially not following what you mean by this statement.

[CarlD]

Smoking motor is not good.
AG pumps are designed to run with the pool water higher up so there is gravity pressure. An IG pump is able to pump from the surface and pump the water up.

Further, most AG pumps are "up-rated" while IG pumps tend to be "full-rated". To super-simplify, it means that an AG pump rated at 1.5HP may have the same flow rate as a 3/4HP IP pump.

Re-read mas985's post above.

[mitchryan912]

OK, re-reading mas985's post. I think I understand it a bit more now that I did before... and now I think I might just benefit from going to a SMALLER motor and impeller after looking at the spec sheet for my pump. To answer his original question though, I thought I might need a bigger motor because I was seeing debris just float by my skimmer, so I thought maybe there wasn't enough suction to properly skim the pool?

To the specs: http://www.waterwayplastics.com/prod...d-pumps/smf-2/

According to their flow rate chart, the 1 HP motor I've had might just be too big. 40 ft of head for the skimmer line = 75 gpm flow rate, and that's higher than the rating on my filter. FWIW, I've got 25 feet of head to the other suction line.

I guess I'd be looking more at the 3/4 HP pump now, based on the flow rate chart. This is all assuming that I'm calculation my GPM's correctly (54 gpm to turn over the pool in 8 hours, 60 gpm flow rate of the filter, 44 gpm per suction line x2) Based on all of that, would I be correct in choosing a 3/4 HP pump instead? If I were to get a replacement 3/4 HP motor, would I also need to get a different impeller as well?

Thanks for all the help on this, BTW. Last year I got a crash course on chemicals, and now I'm getting a crash course on the mechanical aspects of pool ownership!

[CarlD]

mas985 is far better at figuring the mathematics of head to gpm to filter rate than I am.

[mitchryan912]

Hopefully mas985 reads this and can give some suggestions. I'm not sure how long I can let the pool go without circulation, as it's been since late Friday night that it died. I'm keeping the chlorine high to make sure stagnant water doesn't help lead to any funkiness going on in the water. Obviously manually skimming much more often than I usually do as well.

Here's some info on my filter. The pressure gauge on it seems to be broken, as there's no pressure currently, and it's reading 30 psi. When the pump is running, it typically reads 55-60 psi, so I'm assuming it's actually about 25 psi most of the time. I'm not sure if that pressure reading matters, but I'd seen some sites that used that to calculate the head (I don't have a way to check the suction pressure before the pump, at least not yet...)