This is a loaded question because you may have several different flow requirements on the very same pool. Lets say, for instance, that you have a 24,000 gallon pool with a waterfall and an attached spa. This pool has several different flow requirements:
50 GPM – Basic circulation (on an 8 hour cycle)
if you run the pool 8 hours per day, you need to “turn over” the water once per day. If you take 24,000 gallons divided by 8 hours, that means you need to circulate 3000 gallons per hour or 50 gallons per minute.
33 GPM – Basic circulation (on a 12 hour cycle)
if you run the pool 12 hours per day, you need to “turn over” the water once per day. If you take 24,000 gallons divided by 12 hours, that means you need to circulate 2000 gallons per hour or 33.3 gallons per minute.
70 GPM – Waterfall Operation
If you want to use the waterfall, it might require extra flow, depending on the size of the waterfall
120 GPM – Spa Operation
If you have an attached spa with 6-8 jets, you want as much flow as possible out of those jets.
The fact of the matter is that a 1.5 HP pump (or even a 1.0 HP pump) would be fine for this pool except for the fact that you have an attached spa. A 1.5 HP pump would give you a flow rate of approximately 90 gallons per minute which would circulate this pool just fine . . .
BUT . . .
Since you have an attached spa, you will probably want a 2.0 HP pump, simply based upon the flow requirements of the spa, even though you really do not need a 2.0 HP pump for your pool circulation.
The end result is that you end up running a 2.0 HP pump and paying the added energy costs every month even though most of the time your pool really only needs a 1.0 or 1.5 HP pump.
There is another way . . . today’s variable speed pool pumps allow you to calibrate the exact speed of the pump to the needs of the pool. For instance, you can program the pump to run at lower speeds for circulation and higher speeds for spa operation. The possibilities are almost limitless. Variable speed pumps typically save 75% in energy costs when programmed properly.
Here are some general flow ratings for different sizes of pipe, based on a maximum velocity of 7 feet per second.
1.5″ PVC pipe can carry a maximum flow rate of 45 gallons per minute
2.0″ PVC pipe can carry a maximum flow rate of 87 gallons per minute
2.5″ PVC pipe can carry a maximum flow rate of 105 gallons per minute
3.0″ PVC pipe can carry a maximum flow rate of 165 gallons per minute
Older pools often have smaller piping that cannot handle the flow rate that you would like to have. If so, you just have to size the pump for the piping that is in place.
If you tend to be more energy conscious, then you need to consider a variable speed pump. If it is not a major consideration, then you will probably want to go with a standard single speed pump.
We have put together a detailed discussion on the cost saving considerations of a variable speed pump. Click here to go to that page.
If it is time for a new pump, call the experts at PoolStop to evaluate the flow needs of your pool or spa and recommend the the very best pump for your pool. Today’s newer variable speed pumps pay for themselves in 1-2 years and are definitely worth your consideration.