Spa water level drops drastically when pump is off

[lichen]

We bought a house with a pool recently and I have been coming up to speed on how to properly use and maintain a swimming pool, but there's one thing that I'm not able to explain, and I'm not even real sure how big of a problem it is.

I have a pool with an attached spa that waterfalls into the pool. Typically I keep the skimmer/main drain on, with the pool and spa returns on for the waterfall effect. However, once the pump stops running, the water level will drop down up to 4 inches, or possibly more (I've only recently started actually measuring the water level drop). I have yet to see it go below the spa returns, but possibly I just haven't waited long enough. It does however drop below the tiles and the plaster is exposed. I should also note that there are dual drains in the spa (I'm guessing that's so it doesn't trap someone's toe via suction if there was a single drain?).

So, that being said, I haven't noticed any wet patches in the grass, and it doesn't seem like the overall volume of water in the pool is decreasing any more than what evaporation would account for.

Is this actually a problem, and if so, how big of a problem is it? I would appreciate any insight you guys could offer!

 

[jblizzle]

Sounds a lot like the check valve on the spa returns is leaking, so the water is drain backward through the spa returns after the pump shuts off.
Post up a picture of your equipment pad.

 

[lichen]

Black arrows indicate water flow direction. The red arrow is what I'm assuming is the backflow check valve? I guess I could test your theory by turning off the spa returns and seeing if the water level in the spa drops?

 

[jblizzle]

Correct, red arrow would be the check valve. You could test the theory by closing off the spa return valve one night and see if the water level still drops.

Actually, it could also be flowing back through that red ball valve. Hard to tell, but I think that is a chlorinator bypass that flows into the spa return line. ... seems like that should have been plumbed above the check valve. With that open you should be getting spill over from the spa even if the spa return valve is closed.

 

[lichen]

Oh yeah that's a good point. That is definitely a chlorinator bypass. After inheriting the pool I decided to keep the ball valve open since I wasn't even going to be using the chlorinator (BBB!). I figured by leaving it open would be a good thing as it would generally increase the flow of water to the spa, and by proxy the entire pool. I'll try closing it for a night and see how that goes.

On a slightly different note, I realized the chlorinator was actually installed backwards, judging by the flow indicator markings on the unit. How big of a deal do you think this is? It seems like it must have at least worked somewhat as I believe the previous owners were using it.

 

[jblizzle]

I would just close the bypass and leave the return valve set to both as you have in the picture. If the spa does not drain, then the check valve is fine and the ball valve was your problem.

Not sure about the chlorinator install ... I have never had one. I do not think there is a whole lot of resistance in it, so closing the bypass would be fine. I think the bypass would only have been opened when using the chlorinator and the spa ... so that the chlorine did not go WAY up while you were in the spa. Then I would imagine you were supposed to close the bypass when you were done with the spa.