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SPA Jets Explained
Consider a simple spa jet. Spa Jets have 3 main connections – water in, air in, and a water and air mixture outlet. The water is pushed into the jet by pressure from the pump and flows through and comes out the front. This is how the air gets mixed into the pressurized water stream that we will be discussing.
How Spa Jets Work
The air gets mixed into the pressurized water by use of a venturi inside
the jet. The venturi effect happens at the tip of the orifice which is
a small hole inside the jet that all the incoming water gets pushed
through. To understand the venturi effect you have to first understand the way fluid acts inside a pipe. If we reduce the size of a pipe with fluid moving through it, the velocity (speed) of the water increases, but the pressure in the pipe decreases. So if we have spa water flowing through a 2” pipe and we reduce the size to 1.5” for part of the run, the water in the 1.5” section moves faster but at a lower pressure.
If you look inside a spa jet they all have a reduction in the internal plumbing size, we call this reduced size section the “orifice”. The most common size of the hole in the orifice is 5/16” of an inch. This means that we reduce the incoming water feed that was lets say 1” pipe, and we force all this water through a hole just over a 1/4” wide.
This causes a section of the water to travel at a higher velocity and at a much lower pressure. This low pressure at the tip of the jet orifice is lower than the pressure of the air in the atmosphere outside. Since the outside air is a higher pressure than the water at the tip of the orifice, air gets pushed into the water stream at the tip of the orifice and mixed into the water. This water and air mixture then flows out of the front giving us the jet action we are all familiar.
What goes wrong with Spa jets?
Most complaints about spa jets have to do with them no longer mixing
air properly. Customers will report that the jets are “broken” or not
working, they will also sometimes refer to the pump “not being as strong
as it used to be”, or even talk about the air controls or the air
blower being broken. Jet malfunctions usually lead to weak jets or jets with no air in them at all, but they can also lead to water shooting out of the air controls or a the jet sucking water in instead of blowing water out.
Almost all jet problems are centered around the orifice. I will list some of these troubles below:
Insufficient water input – if the pump is not delivering enough water to the jets they may not develop a low enough pressure inside to suck air from the outside world effectively. This can cause jets to be weak or have no air at all. The diameter of the hole in the orifice determines how much water a jet needs to work properly. If you have insufficient water input it can mean that you have a dirty filter cartridge, an obstruction in the pump impeller, or even an undersized pump for the amount of jets you have.
Too much water input – if the pump is over sized it can cause back pressure inside the jet after the orifice, this can cause the jet to not draw air correctly by applying pressure inside the jet where the low pressure is supposed to be formed. This back pressure is between the orifice and the eyeball at the front of the jet where the water and air mixture come out. Sometimes this problem can be relieved by increasing the size of the eyeball in the front. I have seen this problem most often in gunite spas.
Another problem with the older style gunite spa jets is the length can be an issue. With the orifice tees set back inside the wall the length of the pipe between the orifice and the front eyeball can vary greatly. The length of the pipe run itself can cause back pressure with long piping runs through walls it is often necessary up size the eyeballs, or even reduce the size of the orifice opening in the back of the jet and reduce the water flow. Modern design gunite jets move the orifice to the front and eliminate this issue.
Gasket/O-ring Problems – many modern jets have the orifice inside a removable jet internal. On these jets there is often a gasket or o-ring that separates the water pressure area from the low pressure air suction area. When this separation breaks down the high water pressure can leak over to the low pressure air side making the jet not draw air correctly or at all. If this leakage gets severe enough it can also cause water to come out of the air controls.
Plugged Orifice – a small obstruction can get lodged it the orifice obstructing the water flow. Running a screwdriver or other long, thin object through the orifice will often knock it loose, this works best with the jet pump running to help push the object out. If the the object is too large, you will sometimes need to remove the orifice and manually pull the object out of the jet.
Note that other jets may be affected from a single obstruction. The air line on the plugged jet is usually hooked to the air lines from other spa jets. The neighboring jets will still be drawing a suction on those air lines. Sometimes this will cause the surrounding jets to suck water through the air line backwards through the plugged jet, causing the plugged jet to be pulling water into the eyeball instead of outward. This usually only happens on inground spas where the air intake is far away, or for any reason, easier for one jet to pull water from another jet than to pull air from the air intake.
This Article is courtesy of Spa Parts Plus
Until Next Time....
