Pump Loses Prime While Vacuuming
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If you are having a problem with your swimming pool where you connect your hose and vacuum head to clean the pool but then your pool pump loses prime, this article is for you. First, before we can even begin to troubleshoot this problem, it is important to understand that every swimming pool is different. There are different kinds of pools, different ways to build pools, different geographic conditions to consider...there is a lot about pools which makes each one unique and this can make it a little difficult for a pool owner to search for repair and troubleshooting information like this.
You might have a new pool pump that you are running for the first time, or an old pump that is failing and about to give up completely. Your pool could be inground with 20 suction lines, or it could be above ground with one suction line. All of these will have a different approach to troubleshooting this problem of the pump losing prime when you try to vacuum the pool. When your car breaks down you probably just take it to a mechanic to have it fixed. A swimming pool really should be viewed the same way. However there are some basic troubleshooting steps you can take on your own before resorting to paying a professional to resolve the problem for you. Let's look at some of the more common reasons why your pump might be losing prime when you try to vacuum.
Added strain on pump - When you connect a vacuum hose to your pool (suction line) you are essentially making the suction line for the pool another 20-40' long depending on the length of your vacuum hose. In doing this you make it harder for the pump to draw water than before. While the pump should be able to handle this increase in resistance to suction, if you have a pump which is minimally adequate (too small / not powerful enough) then it is possible that the added suction line is simply too much / too far for your pump to draw water. This is pretty unlikely versus some of the more common problems. Perhaps the pump itself is not the problem, but a part on the pump is giving you problems when the pump has to work harder.
Dry Gaskets or O-rings - Every pool pump has a pump lid gasket as well as winterization ports. Also unions, commonly. All of these contain rubber gaskets and O-rings which help to seal the pump against air leaks which would prevent the pump from running or from running properly. If the gaskets are getting old or are drying out then they can begin to allow air into the system. The harder the system is pulling to get the water, the more air that will pull past the gaskets. It is possible that the increase in suction vacuum when you add the vac hose on is too much for the failing gaskets. Turn off the system and apply a silicone based gasket lubricant to all gaskets and O-rings on the suction side of the pump (including the pump lid gasket). Often this will resolve an intermittent / developing problem with leaks on the suction side of the pump. Loose winterizing plugs can also cause this problem so lubricate these O-rings also, and double check the port is fully closed. Valves in front of the pump with gaskets and O-rings should also be lubricated as they can cause the pump to be losing prime also.
Leaks in the vacuum hose - Probably the most common cause for your pool pump to lose prime when you connect the vacuum hose is that the vac hose has developed a hole in it somewhere. As the hose floats on the surface of the pool water, air is able to be drawn in through the hole and this would definitely be enough for your pump to struggle with holding a prime. The vacuum hoses are cheap and not made to last, and many people drag them around / over the pool deck which is aggressively textured to be slip resistant. The hard concrete deck easily wins versus the corrugated plastic vacuum hose. Inspect your vac hose length by length for any sign of damage, scrapes, abrasions or holed and replace either that one damaged section of pipe, or the entire vacuum hose.
Leaks in the suction line - Just as the pump can draw air from a small leak if you increase the difficulty / length of the suction line, so too can you find an intermittent problem with the (buried) pool suction lines when you connect your vacuum hose. If you have a small leak in the suction line somewhere then it will become worse if the pump has to work harder. So it could be you have a leak in the suction line all the time but it is really only noticeable when you try to vacuum the pool as the pump is running at high speed and the extra length of vacuum hose making the suction line effectively longer is making the pump work harder. A pressure test of the suction side plumbing is how you would verify the integrity of the suction side plumbing.
There are more things which could be causing the pump to lose prime when you plug in the vacuum hose. A common one would be that you failed to purge the air from the vacuum line before you plugged it into the suction port. If the vacuum line is full of air then you will force the pump to take a big gulp of air and this can cause it to lose prime. Be sure to bleed all air from the vacuum hose before you plug it into the suction line port. You can do this by submerging one end of the hose (on the vacuum head) and then feeding the hose down into the water hand over hand until you get to the other end of the hose. Alternatively you can submerge the vacuum head and take the other end of the hose and hold it over a return port. The return port will force water through the vacuum line and purge all of the air. Once the air is gone you can plug the vacuum line into the suction port without causing the pump to take a gulp of air.
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Swimming Pool Steve is an award winning, second generation swimming pool and spa specialist from Ontario Canada. With over 10,000,000 views on the Swimming Pool Steve YouTube Channel, winner of the 2018 Pool & Spa Industry Leadership award and author of hundreds of pool and spa articles both online and in print Steve is committed to helping pool and spa owners as well as pool and spa industry workers learn more about the technical side of building, renovating, repairing and maintaining all types of swimming pools and spas. Follow Swimming Pool Steve on Facebook, Twitter and YouTube.
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