There
is no guarantee that your pool builder sized the pump properly to
begin with.
In the Dallas area, for instance, the builders are notorious for
putting 2 horsepower pumps on everything because the next guy does.
Nobody wants to lose a sale because they sized the pump smaller.
Properly sizing the
pump involves several steps as described below. If you do
this, you will avoid wasting electricity and extra wear on the other
equipment.
Step
1: Determine the MINIMUM FLOW RATE.

This
factor calculates the minimum flow rate that must be
maintained in order to circulate all the water in
the pool through the filter within a certain number
of hours, otherwise known as the "turnover
time".
A good turnover
time for a home pool
is usually 810 hours. A heavily used pool
will need to have a shorter turnover rate.
Once you know the desired turnover rate, you can
determine your desired flow rate.
Use
the following equation to determine the desired
flow rate:
Pool
Size  number of gallons of water in the pool
Turnover
Time  time to move entire capacity of pool
through filter
Flow
Rate  desired flow rate in gallons per minute
Pool
Size
(US
Gallons) 
divided
by 
Turnover
Time 
divided
by 
60 
equals 
Minimum
Flow
Rate 
35,000 
divided
by 
8
hours 
divided
by 
60 
equals 
73
GPM 
35,000 
divided
by 
10
hours 
divided
by 
60 
equals 
58
GPM 
24,000 
divided
by 
8
hours 
divided
by 
60 
equals 
50
GPM 
24,000 
divided
by 
10
hours 
divided
by 
60 
equals 
40
GPM 
18,000 
divided
by 
8
hours 
divided
by 
60 
equals 
38
GPM 
18,000 
divided
by 
10
hours 
divided
by 
60 
equals 
30
GPM 
The
following chart is another way to help calculate the
desired flow rate.
If
you have a 25,000 gallon pool and want a turnover
rate of 8 hours, then you will need a pump with a
flow rate of 52 gallons per minute.
If
you have a 35,000 gallon pool and want a turnover
rate of 10 hours, you will need a pump with a flow
rate of 56 gallons per minute.
For
this situation, we will use a 24,000 gallon pool
with a turnover time of 8 hours, giving us a MINIMUM
FLOW RATE of 50 gallons/minute.


Step
2: Determine the MAXIMUM flow rate.

The
flow rate through any given pool system is limited by the
size of the piping and the equipment. This determines
the maximum flow rate through the system.
The
first thing to check is the pool system plumbing.
For instance, the maximum flow rate through a 1.5 inch PVC
pipe is 44 Gallons per minute. Even if you have a pump
that can move 100 gallons per minute, it will do very little
good if the piping will only allow 44 gallons per minute
through it. An oversized pump will actually strain
against the piping and can be damaged in the process.
Check
the PLUMBING FLOW RATE 
As a
general rule, the velocity of the water through
the pipe should not exceed 7 feet per second.
The chart below shows the maximum flow through
different sizes of pipe.
Count
the intake lines and you will see what the maximum
flow into the pump will be.
For
each 2.0 inch intake line, the
maximum flow into the pump will be 73 gallons
per minute (GPM).
For
each 1.5 inch intake line, the
maximum flow into the pump will be 42 gallons
per minute (GPM).
Make
sure the the lines going back to the pool will
also support the same level of flow.

In
the diagram to the right, the system is plumbed in
2.0 inch pipe. Since the system can be run
in either spa or pool mode, you have to calculate
the flow rate each way.
Spa
Mode  73 GPM (One 2.0" pipe)
Pool
Mode  146 GPM (Two 2.0" pipes)
The
maximum flow is the lower of the two, which
would mean a
MAXIMUM PLUMBING FLOW RATE of 73
gallons per minute (GPM).

Check
the FILTER FLOW RATE 
Another
limiting factor is the pool filter. Each filter has a
maximum flow rate. If you exceed that maximum flow
rate, the filter will not operate properly and can be
damaged as a result. It is very important to
The proper
filter flow rates are as follows:

Recommended
flow
rates for different sizes of SAND FILTERS

Tank
Diameter 
19" 
21" 
24" 
30" 
36" 
Surface
Area 
1.8
sq. ft. 
2.3
sq. ft. 
3.1
sq. ft. 
4.9
sq. ft. 
6.9
sq. ft. 
Max
Flow Rate 
40
GPM 
50
GPM 
60
GPM 
100
GPM 
140
GPM 
Recommended
flow
rates for different sizes of D.E. FILTERS

Surface
Area 
24
sq. ft. 
36
sq. ft. 
48
sq. ft. 
60
sq. ft. 
72
sq. ft. 
Max
Flow Rate 
48
GPM 
72
GPM 
96
GPM 
120
GPM 
144
GPM 
Best
Flow Rate 
36
GPM 
54
GPM 
72
GPM 
90
GPM 
108
GPM 
While
the manufacturers will specify the Max flow rate,
we strongly recommend staying with the Best flow
rate to avoid crushing grids due to high
pressure. 
Recommended
flow
rates for different sizes of CARTRIDGE FILTERS

Surface
Area 
100
sq. ft. 
200
sq. ft. 
300
sq. ft. 
400
sq. ft. 
500
sq. ft. 
Max
Flow Rate 
38
GPM 
75
GPM 
112
GPM 
150
GPM 
150
GPM 
Best
Flow Rate 
30
GPM 
50
GPM 
75
GPM 
100
GPM 
125
GPM 
While
the manufacturers will specify the Max flow rate,
we recommend choosing one within the Best flow
rate for best filtration and better dirt loading
capacity. 
In
this particular system, if you have a 48 square foot DE
filter (see yellow highlighted area above), your MAXIMUM
FILTER FLOW RATE would be 96 gallons per
minute. In a perfect world, we would try to keep the
flow rate down near 72 gallons per minute, but for now we
are dealing with the maximum flow rate.
Maximum
plumbing flow rate  73 Gallons per minute Maximum
filter flow rate  96 Gallons per minute The
overall MAXIMUM FLOW RATE is the the lesser of the two 
73 gallons per minute

Step
3: Choose an IDEAL FLOW RATING for your new pump

After
you have calculated the MINIMUM FLOW RATING required
to meet the turnover requirements of the pool, and after you
have calculated the MAXIMUM FLOW RATING based on the
limitations of the filter and piping, you are ready to
select the proper size pump.
Draw a simple
chart like the one below. Mark the MINIMUM FLOW RATING
and the MAXIMUM FLOW RATING. The space between the
minimum and maximum flow rate is the ideal range.
If you have a
pool with no attached spa, you can choose a pump on the
lower side of the ideal range. It will be sufficient
to properly filter the pool without wasting energy in the
process. In that case, we would look for a pump in the
50  60 GPM range.
If you have a
pool/spa combination, you need to consider the flow
requirements of the spa jets. This will often push you
towards a pump on the higher end of the acceptable
scale. In that case, we would look for a pump in the
75 GPM range.
If
you have a pool/spa combination, you might want to consider
a 2 speed pump. It can run on low speed while it
filters the pool, and then switch to high speed when using
the spa.

Step
4: Look at the POOL PUMP RATINGS to find the right
pump

Once
you have decided to buy a new pump, take some time
to research your options..
Don't just
assume that you need the same horsepower pump as you
now have.
The
sample pump chart on the right shows the pump curves
for different sized pumps.
Suppose
your system has 60 feet of head and you want a pump
that will deliver 50 gallons per minute.
We
have added a yellow line denoting 60 ft of head, and
a blue line denoting 50 GPM. Find where
the two lines cross and you will see the pump that
will deliver the desired rate of flow.
From
this chart, you can see that a 1 horsepower pump
(the green curve) will give you 52 GPM flow on a
system with 64 feet of head. We
have tried to give you the flow information on each pump so
that once you come up with a rate of flow, you can come up
with the proper pump, the first time.
Note:
Figuring out feet of head (below) is
difficult. Typically a inground pool will
have 5060 feet of head pressure and an above ground
pool will be somewhere around 30 feet of head.
These are only approximations.

What
about 'FEET OF HEAD' ? What does that mean?

The
total resistance to flow is measured in "feet of
head". As you can tell from looking at the pump
charts, the greater the resistance to flow (expressed in
feet of head), the lower the flow rate. The
greater the resistance to flow, the more powerful the pump
needs to be to overcome it.
The
resistance is measured in "feet of head".
The best way is to approximate the resistance is as follows:
1.
Measure the vacuum pressure at the pump and multiply
by 1.13.
A
reading of 12 inches mercury times 1.13 equals 13.5
feet of head.
2.
Measure the clean filter pressure and multiply by
2.31
A
reading of 22 times 2.31 equals 51 feet of
head.
3.
Add the two together to get total friction loss in
feet of head.
13.5
plus 51 equals roughly 64 feet of head.
Look
at our pump
head page, which gives sample calculations of
Total Dynamic Head


