What Is Water Transfer? A Pool Leak Professional Explains

Don’t Panic—That Dropping Spa Water Level Might Not Be a Leak!

You walk your dog in the backyard in the morning before the pool pump turns on and you notice the water in your spa is lower than it was yesterday. Uh oh—do you have a leak?

Good news: if your spa water drops overnight after the pool pump turns off, you’re probably not dealing with a leak at all.

At Aaron’s Leak Detection, we’ve helped thousands of Central Florida homeowners figure out what’s going on with their pool and spa. One of the most common misunderstandings we hear is about something called Water Transfer—and it’s usually not a problem.

Let’s walk through what it is, how it works, and how to tell the difference between normal water transfer and an actual leak.

What Is Water Transfer?

The Basics

Water Transfer happens when water moves from your spa back into your pool—not out of the system entirely.

It sounds confusing, but let’s break it down with an example:

• Let’s say, when your pump is running, your pool holds 90% of your total water and your spa holds 10%.
  
  
• When your pump is on, some pool water is sent up into the spa to create that relaxing spillover.
  

• Overnight, a small amount of the spa water may backflow past the checkvalve to the pool. Let's say 1%.
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• The next morning before the pump turns on, the pool has 91% of the total water while the spa now has 9%.
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• You still have 100% of the total volume of the system. That's not a leak.
  
  

But Wait—Isn’t There a Check Valve?

Yes, most spas are built with a check valve to prevent water from flowing back into the pool when the pump shuts off.

Here’s the key: check valves rarely seal perfectly, even in brand-new pools.

So while you may think the water should stay put in the spa overnight, it often doesn’t—and that’s normal.

Why Water Transfer Looks Like a Leak (But Usually Isn’t)

This is where it gets tricky for pool owners.

If you see your spa water drop by 2–3 inches overnight, you assume there’s a leak. But because the spa is small, a small amount of water transfer shows up fast.

Let’s break it down:

Example

• A 50-gallon shift from the spa to the pool might drop the spa water by 2.5 inches.
  
  
• That same 50 gallons added to the pool? It might raise the water level by ⅛ of an inch—almost impossible to see.
  
  

So, it feels like your spa is leaking, but in reality, all the water is still there. It just moved.

This is classic Water Transfer.

So... Is It a Leak or Not?

This is the question we get the most. Let’s help you figure it out.

A leak happens when your 100% total volume drops.

Water Transfer happens when your 100% total volume shifts, but remains 100%.

How to Know for Sure

Ask yourself these two questions:

1. Are you regularly adding water to your system?
   
   
   • If yes, you may have a leak.
     
     
2. Did you measure water loss from the pool over a few days?
   
   
   • Use a ruler to check the pool (not the spa) to avoid Water Transfer confusion.
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   • 💧Use our expert method to measure your water loss and see if you have a leak.
     
     

At Aaron’s Leak Detection, we recommend a ¼ inch of water loss per day as normal for most pools that aren't heated or running water features.

If you’re losing more than that, it's worth getting it checked.

Can You Have Both a Leak and Water Transfer?

Yes, you can!

Even if Water Transfer is happening (and it usually is), your system could still have a real leak somewhere else.

This is why it’s important to measure water loss from the pool, not the spa, and to have a professional check things out.

Why Your Spa’s Check Valve Doesn't "Fix" the Problem

Check valves are designed to stop this backflow, but rarely do they hold perfectly day after day.

This is especially the case as valves age, but it can even be true for brand-new valves, which is Water Transfer is totally normal.

The good news? This isn’t usually something that needs to be fixed. It just looks worse than it is because the visual of the water volume of the relatively smaller spa compared to the pool seems alarming.

How Aaron’s Leak Detection Can Help

If you’re unsure whether you have a leak or just Water Transfer, don’t worry—we’ve got you covered.

We’ve built our reputation as the go-to pool leak detection expert in Central Florida by offering:

• Accurate testing to find every leak in your system
  
  
• Clear answers—we explain what we find in simple, honest terms
  
  
• Transparent pricing—most repairs are already included in your leak detection cost
  
  

Whether it’s a true leak or just Water Transfer, we'll give you peace of mind and you'll know for sure.

Recap: What You Need to Know About Water Transfer

• Water Transfer is normal: Spa water moves to the pool when the pump is off.
  
  
• Check valves often leak a little: Even brand-new ones.
  
  
• Spa water drops fast: Because it’s a smaller space, small shifts look big.
  
  
• Pool level stays the same: Because it’s a large body of water.
  
  
• Measure water loss from the pool: Not the spa.
  
  
• ¼ inch per day is normal: More than that may mean a leak.
  
  
• You can have both: Water Transfer and a true leak can exist together.
  
  

Still Not Sure? Call a Pool Leak Professional Today

If you’re tired of guessing, give us a call.

At Aaron’s Leak Detection, we’re here to save you time, money, and stress by giving you clear answers and expert help. Whether it’s Water Transfer or a hidden leak, we’ll find the problem and help you fix it—fast.

📞 Call us: (407) 924-9888
📧 Email us: office@aaronsleakdetection.com
🌐 Visit: aaronsleakdetection.com

Let’s get your pool back to perfect.

Helpful Resources

Here are some great resources to help you figure out what’s going on with your pool or spa:

• Frequently Asked Questions About Pool Leak Detection and Repairs
  
  
• How to Measure Pool Water Loss
  
  
• Learn About Our Leak Detection Process
  
  
• Contact Aaron’s Leak Detection

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