- Pressure increase in a cleaned cartridge filter
- The real deal on pump and filter sizing
- Explaining service factors and uprated motors
- GFCI trip thresholds
- Pump squirting upon starting
I cleaned my cartridge filter this week but the pressure went back up after a few days. What’s the problem?
Scott P. Gleason:
A few things could be contributing to the quick pressure build-up.
For cartridges over two to three years in service, it is most
likely time for replacement. Over time, pleated filter media
accumulates particulate between its fibers. This cannot be removed
even with the best of cleanings. As more and more of these
particles become permanently trapped, the filter’s
dirt-holding capacity is reduced while its efficiency is increased.
The result is a quicker increase in the filter housing pressure and
shorter cycle times between cleanings.
For cartridges less than two to three years old, there could be a
build-up of calcium or minerals on the surface of the pleated
filter media. This is most likely blinding off a good portion of
the media’s open area. As particulate becomes trapped in the
remaining open areas, the pressure quickly rises in the filter
housing due to flow restriction. Typically, this build-up can be
resolved with a good, standard cleaning of the filter cartridge and
then an acid wash. Always follow the manufacturer’s
recommended procedure for performing an acid wash to avoid permanent damage.
Another possibility for a cartridge less than two to three years
old is that it has removed a non-water-soluble substance from the
water, which has adhered to the media; or a bio-film has developed
on the surface of the filter media. Unfortunately, in both of these
cases, the substance cannot be removed, so a new filter cartridge
will be needed, along with steps to remove the non-water soluble
substance most likely still present in the water, so the new
cartridge will not experience the same fate.
B. Champlin/T. Isberg/W. Kinard/ R. Raub/R. Thibault (Horizon):
If the pressure raises rapidly after cleaning, then the cartridge
has either not been thoroughly cleaned or there is still a large
amount of contaminants in the water.
After cleaning, there may be some contaminants left in the fabric
of the cartridge filter. It could be body oil, facial cosmetics,
shampoo, chemical residue or any number of substances brought in
from outside the pool or spa. The condition of the fill water or
addition of chemicals could also be contributing to an increase of
total dissolved solids or calcium content of the water. All of
these things can become impacted in the filter media and are very
difficult to clean with plain water. Sometimes this residual matter
can be hard to see in the cartridge — your indication is the
pressure reading of the filter compared to the baseline reading that you recorded.
After washing the cartridge down, try soaking it overnight in a
solution of trisodium phosphate (TSP) or other filter degreaser.
This should remove most of the oil-based substances from the media.
If, after reinstallation, the system pressure still does not return
to the baseline, there may still be a calcium substance in the
media, which will require a soak in an acid-based solution. Be sure
you have first cleaned the cartridge in a degreaser solution to
remove any oils that might be coating possible calcium
contaminants. You might also test the water at this time for total
dissolved solids, since a high level of TDS will increase the
frequency of filter cleanings.
What’s with these huge pumps? Over the past year I have noticed new 3/4-, 1-, even 1
1/2-horsepower pumps on 36-square-foot DE filters! The filter is usually trashed — crushed grids, cracked tank, broken tank bands. I’ve been in the business for a long time and have never put in a 36-square-foot with a 1 1/2-horsepower pump. What is the real deal on filter and pump sizing? Is it just the gallons per minute, or does filter pressure matter? And how does 25-plus psi day in and day out affect the lifespan of the filter?
Filters should be sized based on gallons per minute after the pump
has been sized for the pool. The pump flow rate should not exceed
the maximum flow rate recommended by the filter manufacturer.
Higher-than-normal filter pressure can force particulate into the
media, making the filter difficult to clean, thus shortening its
There were some schools of thought that one could use a larger-size
pump and then valve it down to achieve the correct flow rates.
However, with energy costs being high, the preferable method is to
size the pump correctly from the start. To operate most efficiently
you maximize flow (gpm) at the lowest possible system pressure (psi).
B. Aubrey/R. Denkewicz/S. Petty (Hayward):
Filters should be matched to the pump based on the design flow rate. It is usually better to select a larger filter than is needed, as it should prolong the time between cleanings.
Properly matching the pump and filter is usually easier for new
construction, given that the equipment pad is a clean slate. The
challenge for existing installations where a pump and filter are
mismatched is breaking the understandable consumer expectation that
you must replace like-for-like. Service techs and retailers will
need to continue to educate the consumer on the benefits of
right-sizing pumps and filters, such as possible reduced energy
consumption and prolonged equipment life.
A change in pressure occurs when the pump turns on and expands the
tank. When the pump turns off and it contracts to the original
shape, the pressure changes again. These changes cause mechanical
stress. This is normal. The materials are flexible. However, like a
copper wire, if you keep bending it back and forth it will
eventually break. This expansion and contraction can decrease the
life of the filter tank and O-rings. The higher the operating
pressure, the greater the expansion and contraction. To maximize
the life of the filter vessel, it is best to design a system with low operating pressure.
The following chart can be used as a guide, but it is always best
to check manufacturers’ recommendations when sizing your pump and filter.
|Filter and Pump sizing|
|GPM per sq. ft. of filter area|
To figure out the square footage of a sand filter, use this formula: (radius2 x 3.14)/144.
If you have a 36-inch diameter filter, the radius is 18 inches. 18 x 18 x 3.14 / 144 = 7.06. That filter would perform best with 105 to 141 gpm; 7.06 x 15 = 105 and 7.06 x 20 = 141.
Pumps that use variable-flow technology, which is different from
variable-speed, automatically adjust themselves to prevent the
crushing of grids or over-pressurizing the system. Variable-flow
pumps automatically sense the clean system pressure after
backwashing. They can be programmed so that when the system reaches
a certain pressure, the pump stops monitoring flow and starts
monitoring pressure. The pump then decreases its rpm, thereby
lowering the pressure so it doesn’t crush grids or elements,
or over-pressurize a filter when it gets dirty. It can also send a
signal when it’s time to clean the filter.
The short answer is that both gpm and filter pressure are relevant to matching pumps with filters.
The filter’s flow rating in gpm tells you how much water can
safely move through it in a given amount of time. The NSF flow
rating for DE filters is 1 to 2 gpm per square foot of filter area.
So a 36-square-foot DE filter will handle up to a maximum 72 gpm.
To try and move more than that through the filter with a large pump
will mean that filtering efficiency will go down due to particles
of dirt being driven into the media at higher velocities and
impacting the media. Once particles start impacting into the media,
the flow rating of the filter is essentially decreased, making the
situation worse and creating the potential for a higher-than
recommended pressure reading.
The pressure (psi) rating of the filter tells you how much
sustained pressure the filter body is capable of handling on a
consistent basis. To exceed that is to create a safety hazard that
must be corrected immediately.
Also, there is the matter of differential pressures across the
media. If the media is impacted with dirt, oil and other
contaminants due to excessive flow of water (high velocity), or if
the flow of water exceeds the maximum rating of the media, there
will be a much higher pressure reading on the upstream side of the
media than on the downstream side of the media. The media, clean or
dirty, is creating a block or dam in the water. The pressure
difference, if excessive, could eventually result in the implosion
or crushing of the DE grids, filter cartridges or collection
laterals. If something other than the filter media, such as a
complicated return plumbing system, is causing a high pressure
reading, then the pressure in the filter will be relatively equal
between the upstream and downstream sides of the media. This
situation is not as much of a concern as long as the volume of the
water moving through does not exceed the maximum flow rating of the
media and the pressure of the water is not in excess of the maximum
pressure rating of the filter.
So, you must look at the flow curves of the pumps, calculate the
total dynamic head of the system (and static head, if applicable),
and then use the pump that has an appropriate output at that amount
of dynamic head. If you find that the filter is a limitation to the
pump’s output, then downsize the pump or increase the size of
the filter, or install a bypass valve to route some excess water
around the filter. If there are other features such as a waterfall
that require additional water flow, then consider using valves and
controllers to provide adequate water to the feature while staying
within flow limits of the filter, or just use a separate pump for
the additional feature.
How do you explain something as complicated as service factors and uprated motors to a
consumer who is replacing a dead motor?
John Bent: Because of confusion created by uprated motors and service factors, installers and consumers need to look at the total or maximum horsepower (nameplate hp x service factor =
total horsepower) rating of replacement pumps and motors for the true horsepower rather than the nameplate horsepower.
To explain full-rate versus up-rate, one should introduce two
terms: Total Horsepower (Thp) and Service Factor (S.F.). Total
horsepower is the maximum amount of horsepower that the motor can
deliver without overloading, and can be derived by multiplying full
horsepower by service factor. In the event that an “uprated
motor” has a service factor of 1.0, the uprated horsepower
equals total horsepower.
Explain that pump curves are how we measure pump performance. Then
open a catalog and take them to the page of the pump you’re
replacing. Show them that the uprated 1-hp pump has the same pump
curve as a full-rated ¾-hp pump. It’s the same motor and
impeller, and therefore the same performance. So just make sure
that you are comparing uprated to uprated or full rated to full
rated, or talk about pump performance and help the customer see
that the numbers aren’t important.
Title 20 requirements, which some states are adopting, have
simplified this by requiring that pool pumps list “service
factor” horsepower, which is the horsepower times the service
factor. Service factor horsepower is listed in most
manufacturers’ catalogs as SFHP; this can be used to compare
like for like.
Also keep in mind that selling the customer the same kind of pump
that they had before may not be the best, safest, most
energy-efficient alternative. The market has historically oversized
pool pumps, which is why service factors on pumps were adjusted in
the first place. To correctly size the new pump for the whole
system, make sure that the new pump will not exceed the safe flow
rate of the drain cover, ensuring that the velocity in the branch
line between the drains does not exceed 6 fps if one drain is
blocked. Also match the pump to the filter, piping (especially if
it is copper), and other parts of the system.
Educate the customer on energy-efficient alternatives like variable
speed, or improving the safety of the system by upgrading to pumps
that include a safety vacuum release system. After going through
this analysis with the customer, they will be assured that you are
looking out for their safety and best interests.
The easiest way to explain this to a consumer is an uprated motor
has a lower service factor, meaning more total horsepower (sf x
rated hp = total hp) compared to a full-rate motor with a higher
service factor. In today’s economy, an energy-efficient motor
that is a full-rate motor will save you money.
How did the trip thresholds on GFCI’s get changed a few years ago? Even between the
different manufacturers, the trip-points are different. Is there an actual calibration that’s done by the manufacturers to make sure that these are tripping at 5-6 milliamps and not 2?
Most manufacturers of pumps for permanent pools are building product per UL 1081. The requirements for GFCI products are dictated per UL 943, and their use in pool and spa installations is typically driven by the National Electric Code and/or particular state/local requirements. Pool equipment manufacturers, therefore, are usually not involved in the specific design parameters for GFCIs (e.g. trip threshold).
Why do pumps always squirt a little water out of the lid when the pump starts and stops?
The occurrence is not uncommon in high back-pressure installations, particularly with pumps that have non-pressure testable lids or covers. This can be exacerbated by pressure spikes (or “water hammer”) that can occur when valves, including some used in in-floor cleaning systems, are rapidly opened or closed.