Sizing a new pump & optimizing long term cost

[jkapp]

We are in our first full season with our pool. I'm quickly learning that our pool builder didn't understand the finer points of pool equipment. I am considering replacing our 1.5 HP SF 1.5 (Jandy Model SHPF1.5) single speed pump with either a two speed or a variable speed pump. I currently run our current pump for 8 hours per day (~1 turn) which gives me perfectly clear water but isn't long enough for my SWG to generate enough chlorine even with the SWG set to 100%. A lower speed pump will allow me to run the pump longer to generate more chlorine while lowering my electric bill. My spa jets run on a separate pump, and my Polaris runs off of a separate booster pump. Therefore, it will be rare for me to need to run the filter pump at high speed.

I just installed a flow meter to get idea of how much water I'm pushing through my system. With my filter pressure gauge half way between clean and dirty (which is about as dirty as I let it get), I'm moving 71 GPM. With my multiport on recirc, I move 92 GPM. I have three suction side feeds into the pump and three pressure side returns to the pool. All piping is 2".

Looking at the pump curve for my pump at this link, I think I have about 72 feet of head. With a flow rate of 71 GPM and 72 feet of head, from the formula in http://www.troublefreepool.com/hydraulics-101-have-you-lost-your-head-t915.html I think I have a plumbing factor of 0.0142 (between curves B and C).

My heater and SWG have a minimum flow rate requirement of 30 GPM. If I was to install a pump that gave me 30 GPM, I would turn over my pool water 1.4 times in 24 hours. I think that rate would work very well given that I only turn over the water a little more than once per day now with very good results.

I'd love to get a variable speed pump, but the upfront cost will make the payback very long. Therefore, I think I need to get a standard two-speed. Looking at the curves for the Whisperflo two speed pumps at low speed, the following pumps would give the following approximate flow rates:

WFDS4 (1HP - 1/8HP): 34 GPM
WFDS6 (1.5HP - 3/16HP): 37 GPM
WFDS8 (2HP - 1/4HP): 40 GPM

On to my questions:
Am I approaching this correctly?
Given all of the calculations above, am I cutting things too close if I got the WFDS4? I need a flow rate of 30 GPM to run my SWG and heater.

Thanks for your help!

 

[duraleigh]

Am I approaching this correctly?

Almost surely yes with one caveat.....What's your CYA level?

I'm sure you have that where it needs to be but, if not, you could solve your SWG issues.

 

[jkapp]

What's your CYA level?

70 PPM. I also switched over to bleach for a week to confirm that my SWG was creating the appropriate amount of chlorine for an 8 hour run time - which it was. I now face running my pump longer (or supplementing my SWG with bleach). My thought is to solve my chlorine production problem and save money over the long term by running a low speed pump continuously.

 

[learthur]

Consider going with the 37gpm choice as you have to account for error of measurement in your flow meter and also in the flow switch.

Oherwise it sounds like you have done everything correctly.

Would be interested in your FC levels to get an ideas how much keeping up your Swg is doing. Also if you don't maintain at least 7 ppm FC your FC consumption will be higher than if you did...(counterintuitive).

 

[jkapp]

Would be interested in your FC levels to get an ideas how much keeping up your Swg is doing. Also if you don't maintain at least 7 ppm FC your FC consumption will be higher than if you did...(counterintuitive).

It's funny that you say that, because it's something I couldn't explain. Originally, I was trying to keep my FC in the 3-4 range. With my pump running 8 hours/day and my SWG at 100%, I would loose about 0.5 PPM per day. Although my water was crystal clear, my CC was 0.5 or less, and my overnight FC loss was 0.5, I still wanted to boost up my FC to make sure there wasn't some latent algae issue. So brought my FC up to 7-8 PPM with bleach. I didn't touch my SWG run time and surprisingly my SWG was able to maintain 7-8 PPM for a week. I really would prefer to have the lower chlorine levels, so I let FC drift back down to 3-4 and I'm once again losing 0.5 PPM per day.

Long story short, I can confirm that there is lower FC demand at 7-8 PPM versus 3-4 PPM. I will likely bring my FC back up to 7-8 to make my pool as trouble free as possible, but I still think my ultimate solution is getting a two-speed pump and running it on low continuously. I can then make chlorine for 12 hours per day or longer if I need to.

 

[Brad S]

Are you sure your SWG is sized correctly for a 31,000 gallon pool?

 

[jkapp]

We are committed to Jandy electronics and it's the biggest one that they make - the Aquapure 1400.