pump size adequate for backwashing sand filter

bricktron

Active member
Dec 30, 2013
40
SF, CA, USA
#1
hi folks, i want to use a very small pump, run continually for my tub. i have a lower-threshhold flow switch to replace the one in the heater. however, not sure about the pump size yet. doesn't it take more pressure to backwash a sand filter than to clog it up in the first place? how much more pressure, and what does this mean for pump sizing?
 

jblizzle

Mod Squad
TFP Expert
LifeTime Supporter
May 19, 2010
41,197
Tucson, AZ
#3
:wave: Welcome to TFP!!!

I do not understand what type of spa and equipment setup you have. Please add your location (City, State or City, Country) to your profile and pool details to your signature as described HERE as it will help us help you.

Standalone hot tubs usually have a 2-speed pump and cartridge filter ... so I am guessing this is not what you have?

Are you not intending to have jet action any more?
 

bricktron

Active member
Dec 30, 2013
40
SF, CA, USA
#4
hi folks, thanks for responding. i am building a tub setup with no jets from scratch. the heater i have is a raypak gemini 3 from the '80s and am shopping for a sand filter and pump now.

rather than a 1/2 HP pump that runs occasionally, i've been looking into a much smaller circulation pump (1/10 HP for example) that would run continually. the reason is noise: the equipment is beneath our bedroom. location is SF, CA if it makes a difference. multispeed is definitely a desirable feature on the pump motor.

my question is: how much pump do i need to be able to backwash the sand filter? i've only used cartridges in the past.
 

Kiss4aFrog

LifeTime Supporter
TFP Guide
May 22, 2013
2,723
Hudson, WI
#5
Welcome to the forum Bricktron :wave:

You have three variables you would need to narrow down some to get an answer. There is a minimum HP you'll need to spin the pump but there is also a minimum of GPM the pump will need to be able to push through the filter to properly clean it and then there is the minimum GPM the filter requires. Since you are trying to go with the smallest motor you can, then you need to find out the requirement of the filter first, then a pump that meets or exceeds it's GPM requirements and a motor that will drive the pump.

Would be helpful if you added your info to your signature.
 

bricktron

Active member
Dec 30, 2013
40
SF, CA, USA
#6
all right, i put a little info in the signature though i had to keep it vague since i am still buying the equipment :)

i understand that filters are rated for minimum GPM for filtration. if there is a second minimum GPM for backwashing which is a multiple of the first, that's the number i need to target with the pump's high speed. i bet this all probably means using a small filter, like a 12" one, but i should get my hands on the manual for one to read its specs.

thanks folks!
 

JasonLion

TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
37,879
Silver Spring, MD
#7
15+ GPM/square foot of sand surface area is typically required for backwashing.

The best solution is a two speed pump. That will give you high speed to backwash the filter and low speed (1/8th the HP) for circulation.
 

jblizzle

Mod Squad
TFP Expert
LifeTime Supporter
May 19, 2010
41,197
Tucson, AZ
#9
Just to add, running the pump continuously is rarely if ever required. The easiest way to save money on electricity is to not run your pump as much. I would suggest something in the 1HP 2-speed range and run on low for heating/circulation and on high for jets and backwashing.
 

bricktron

Active member
Dec 30, 2013
40
SF, CA, USA
#10
you all are great, one speed for each mode seems like how it should work. props to JasonLion for providing a number. now i can see about how strong the pump should be!

jblizzle, i like the continual idea because it is the simples way to achieve rapid response time for temperature drops at any time, which really matters in night visits to a hot tub. my dad's tub pump is on a timer with manual override buttons (they are funny air switches), but using that is too complicated. the alternative is adding a low-voltage circuit linking a sensor right in the tub with a switching relay to turn on the pump, which is even more complicated, but as you say, it sure would save some power.

say i did want to continually turn over 500G three times a day, which means a rate just over 1 GPM, and had a round 14" filter of just over 1 ft^2 area. regular circulation would see 1 GPM/ft^2, and in order to get 15 GPM/ft^2 for backwashing i'd need a second speed (or a second pump) 15 times as powerful. that's nuts, so i can see that i'd have to get a way larger pump or use a way smaller filter. exactly what i need to know before buying things. cheers!
 

mas985

TFP Expert
LifeTime Supporter
May 3, 2007
12,128
Pleasanton, CA
#12
Why don't you just stick with a cartridge?

-Better filtering
-No backwashing
-Less water wasted (CA drought)
-Can be sized for very infrequent cleaning (I clean mine once per year)
 

bricktron

Active member
Dec 30, 2013
40
SF, CA, USA
#13
mas985, i have always disliked washing the cartridges, but you make a good case. does anyone put a multiport valve on a cartridge filter to facilitate filling and draining? also, would it just rip if i did gently backwash it?

gwegan, good call. continuous variability on a pump would be sweet. the PG&E rebates say they are just for pool installations but perhaps the claim could be made anyway.

thanks everyone for your advice!