Hydraulics Question- Detached Spa Overflow Line to Pool

[dag212]

I am building a pool with detached spa that will share equipment. Since this is a detached scenario a standard spillover into the pool is not possible so an underground 2” overflow line will serve this purpose. This 2” overflow line is fed by two 2x6 rectangular spa drains at the spa waterline. Each drain connects to a 2” vertical line that immediately drops about 6 in. or so into to a 2” manifold which then gradually slopes down, through a few turns and then outlets into the pool which is about 15ft away (Approximately 25 ft of pipe length total). The 2 in. outlet into the pool is currently 14 in. below the waterline. The pools water surface area is 1350 SqFt but 150 SqFt is a tanning shelf with only 8 in. of water depth. The spa water surface area is 43 SqFt. The waterline in the spa is 13.5 inches higher than that of the pool. I suspect the pool outlet is too deep so I am trying to get some guidance here before I am past the point of no return. I’m post shotcrete but my trenches are still open so correction will be a pain if necessary but possible. Any help will be greatly appreciated. I was originally planning dedicated spa equipment but due to inflation my budget is not going as far as it would have 2 years ago when I began planning this project.

 

[ajw22]

Really you have a typical spa with shared equipment just not the standard spillover connection.

Water flows downhill and will equalize with the pool water level. We see that with spas leaking back through the spa return into the pool. And the pool return outlet is below the waterline.

I think the spa outlet into the pool can be any depth and work fine. The key is the spa water level being above the pool water level which you have.

Let's see what @mas985 thinks.

 

[Jimrahbe]

Dag,

With a standard spa that spills over into the pool, it does not really matter how much water you send to the spa (when in the pool mode) as it will all spillover.

With your set up, you need to ensure that you can't send more water to the spa than can flow back to the pool through your 2" pipe. I think I would have used a much bigger pipe.

That said, I have no idea if a 2" pipe is too big or too small...

Thanks,

Jim R.

 

[mas985]

With that setup, the main issue is that there is the possibility the spa will overflow at higher flow rates. The drains with the connecting pipe to the pool is a gravity fed system will drain slower than what the pump can deliver to the spa when drawing from the pool (aka spillover mod). The entrance level into the pool is immaterial.

The spa might start to overflow at about 40 gpm per drain. Larger pipe would increase the critical flow rate so that is an option if possible.

What pump are you using?