Sizing pump for best efficiency

[bzeller]

Hi, we're in the process of receiving pool proposals for what will work out to approximately a 35' x 16' pool with 7' x 7' spa, no other water features. It seems most PBs in our area recommend a VS pump of some sort. Many seem to be proposing 2 HP. I've read during the swim season you want to turn over your water twice. I've also read that you're much better off running a pump 24/7 vs. say 8 hrs a day for peak cost efficiency. What I'm wondering is, how do you really size the pump to insure you turn the pool over those 2 times at the most cost efficient pump speed? I don't want to pay for a 2 HP VS pump if a 1.5 HP VS will work just as well for my pool size with less up front cost.

Thanks,
Benjie

 

[mas985]

First, no pool really requires two turnovers unless it is really really dirty and many get away much less than one turnover per day so the best thing to do is run the pump as little as possible. But any pump will turnover your pool more than 2x per day so that isn't really an issue.

Next, spa jets require a lot of flow rate and pressure so you will need a big pump for that. If the PB designed the spa well, then perhaps you might be able to go a little smaller but you don't want weak jets.

But since you are considering a 2 HP VS and 1.5 HP VS, I assume these are the Jandy JEP 2.0 and JEP 1.5 correct? Those are the only ones that I know of that come in different sizes. Anyway for efficiency it really doesn't matter because they are both VS pumps. At the same flow rate, they will have about the same efficiency. The JEP 2.0 just gives you a bit more head room in case you need it for the spa jets so I would go with the larger pump. But is the price that different?

 

[bzeller]

Well, that was one pool builder quoting the JEP 2.0. I'm awaiting three other bids, but I'm guessing they'll come in with similar "standard" pump options.

I believe what you're saying is that it's best to run the pool pump as little as possible at the most cost efficient speed correct? I saw an example where an owner did the following calculation using a flow meter on his pool (24,000 gallons, raised spa, Pentair Intelliflow pump, 2 skimmers, Polaris sweep, saltwater chlorinator)...

What I learned is that minimum electrical usage occurs at minimum speed. At high speed (3450 RPM), one turnover takes 3.8 hours at a cost of $1.07. At minimum speed (1100 RPM), one turnover takes 15.9 hours and costs $0.22 (assuming $0.1 per KWH)

At 1250 RPM, flow is 30 GPM and power is 236 Watts
At 1500 RPM, flow is 39.9 GPM and power is 281 Watts
At 3,000 RPM, flow is 91.5 GPM and power is 1870 Watts
Notice that going from low to high speed, flow increases by 300% and power by 800%. That is why minimum electrical cost occurs at minimum speed.
Flow is per a Cole-Parmer S-05611-14 flowmeter
Power is per the Pentair pump


Given his calculation above, it would be best to size the pump so that you could run it at the lowest speed enabling say 1 - 2 turns a day but with enough head room to ratchet the power up for spa jets, correct?

I havent't really priced out 2HP vs. 1.5HP pumps. My guess is its probably not a huge delta given your response.

Benjie

 

[jblizzle]

Another cheaper option upfront would just be ~2HP 2-speed motor. Run on high for the spa and low for general circulation and filtering. If you have low power costs, it may take years to recoup the cost delta of a VS over the 2 speed ... but, possible rebates on the VS can make that option more attractive.

High on a 2-speed is the same as a single speed (not recommended), but on low speed it moves 1/2 the water for 1/4 the cost of electricity. Although the VS can run at even lower speeds for even less electrical use, but again, how long will it take to save the initial cost delta?

 

[mas985]

Given his calculation above, it would be best to size the pump so that you could run it at the lowest speed enabling say 1 - 2 turns a day but with enough head room to ratchet the power up for spa jets, correct?

More or less that is true. To have the most flexibility, you want a VS pump large enough to provide enough flow rate for the spa jets. The low end doesn't really matter since all VS pumps can produce very low flow rates at high efficiency.

But again, I get away with less than 1/2 turn per day so you really shouldn't need 1-2 turns per day. In fact, turnover really has no bearing on what a pool requires in terms of run time. Studies have shown that most pools only need 3-4 hours per day. A bit longer on lower speeds.

You might want to read this Pool School article:

pool-school/pump_run_time