Pump problems when vacuuming

Last year we purchased a new variable speed Pent air 3 hp motor. All was well until a few days ago. we backwashed it since, here in AZ we have tons of dust.
Anyway when my husband went to clean the pool, after a few seconds of connecting the hose to the skimmer the water slowed almost to a stop entering the pump we made sure all valves were open and there was no air in the sand filter. retried it and the same thing happened. the vacuum head practically sticks to the bottom of the ingound pool, the suction is so strong.
When the filter is running normally there is no problem and all is well...its only when we try to use the vacuum....Any one have any ideas of whats causing this? Thanks

3hp pump? That is a mighty powerful pump! What is the volume of your pool? How big is your sand filter?

I am going to let one of the pump guys address your question.

Welcome to the Pool Forum!

my pool is approx. 32,000 gallons...sand filter is huge also 50 lbs of sand. Pentair triton. Thanks

A 50 lb sand filter is tiny. I don't think that can possibly be right.

Your pump is gigantic and way too powerful and that is why it is sticking to the floor when you vacuum.

I'm sorry there are 10- 50 lbs bags so that would be a 500 lb sand filter. We have not had a problem since it was installed over a year ago. This just started last week. Our pump is a Pentair intelli flo variable speed 3 hp. Thanks

I don't usually talk about this, because it scares people to death, but when new members include their address, I look it up on Google Maps & Bing. (That's me and nobody else; none of the moderators have access to the address info.) If there's a pool at your address, that matches what you've posted, you get a higher membership privilege level than you would otherwise. (It's one of the ways I keep forum spammers out.)

Anyhow, all that to say, Google Maps has a particularly good image of your house and pool -- I can see the cleaning heads! And, it doesn't look like you have a main drain.

That's relevant, because with no main drain, your skimmer may be the ONLY way water can get back to your pump. Most vacuum heads can't handle the suction from a 3HP pump, without getting sucked down to the bottom of the pool.

So . . . try this: turn your pool pump down to about 1800 RPM, and start the vacuum. See if it works OK then. If it's still tool strong, turn it down to 1200 RPM. If that works, you're good to go. Sorta.

There's still another problem, which is safety. If there's only one water path back to your pump, the suction at the pipe end (bottom of the skimmer in this case) will be strong enough to rip the flesh off your arm. If your hand is calloused, you might just get a 'hand hickey' -- but it could also break a finger slamming your hand down against the opening.

You MUST EITHER make sure the pump has TWO water paths that are operating (1 skimmer + 1 main drain, or 2 skimmers, etc) OR you MUST make sure NOBODY ever gets hands or feet down that skimmer, when the pump is on.

They make suction relief safety devices for pools, but the ones I've seen are (a) very expensive, (b) not very reliable, & (c) a pain in the patootie for pool operations. But if you only have 1 water path, that may be your only option if you also have kids. A curious 4 year old, exploring the skimmer, could loose foot, putting it against the bottom outlet of the skimmer!

Also, when you vacuum, the suction on the hose is potentially great enough to cause injury.

Good luck!