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Overflow for Wet/Dry Filter or Sump

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This article explains the basic concepts of how an Overflow/Syphon box for a Wet/Dry filter works. It also gives you an idea of how mine is built. Note that I did not build my own because I had an old one on-hand. But it's basic acrylic construction. You need to be very precise on the outside box if you are building your own, because if that part leaks, your floor gets all wet!

 

Overflow theory of operation

The overflow box is a relatively simple concept. You use two boxes, one inside the tank, one outside the tank, and a syphon tube to move water from one to the other. But there are a few important things to be aware of.

 

It's very important that the Overflow not lose it's syphon if the power fails. If the syphon were to break, and then the power comes back on, the water will be pumped from the sump back to the tank, but no water will flow from the tank back to the sump. This means that the main tank would overflow, all over your floor!

 

My overflow box is set up so that it will maintain water above the bottoms of syphon tubes regardless of the tank water level. Here's how it works:

 

Overflow Diagram

 

Normal tank water level

When the pump is operating, the tank water level will be maintained at a constant level, just above the inlets on the inside overflow box.

Water will then flow thru the syphon tube (or tubes), to the outside box.

 

The outside box is divided into two compartments. The right compartment is where the syphon tubes are placed. The left compartment is where the water exits (thru a pipe in the bottom of the box. There is a divider seperating the two compartments. Water spills over the divider from the right side to the left side.

 

Once the water reaches the left side of the outer overflow box, it drains thru a prefilter (sponge, shown in yellow), and down the pipe, to a hose that is connected to the wet/dry.

 

Overflow when power is out

Low tank water level when the return-pump isn't pumping water

 

During a power failure, or during tank maintenance, the return pump will not be pumping water from the sump back to the tank. The water level in the tank will drop. The water will drain thru the overflow, and will also drain backwards down the return-pump hose.

 

NOTE: Normally the water level in the tank will drop to just below the overflow box inlets (The diagram is just exaggerated to show how the box maintains syphon). But if the return-pump hose ends deep in the tank, a syphon will form, and attempt to carry water from the tank, back down the return-pump hose, and into the sump going backwards thru the pump. To prevent this, either have the return hose end near the top of the tank, or if you want to have the water return deep in the tank, place a small hole in the return hose, near the top of the tank, just below the normal water level. Then, when the pump is off, water will drain down to the overflow level, and the hole inthe return hose will let in air, and break that reverse-syphon.

 

Once the tank water level drops below the overflow inlets and below the water return hose, the tank will stop draining. But even though the water level is below the overflow inlets, the syphon tubes will still maintain their syphon, ready to start moving water once power is restored.

 

You can see in the diagram, that the water drains down to the level of the divider in the outside box. The inside and outside boxes equalize at the same level, and no more water flows, but syphon is maintained.

 

FREQUENTLY ASKED QUESTIONS
I got a lot of emails asking me about the operation of the overflow box and wet/dry filter. Many of the questions are about maintaining the proper water level in the tank and/or sump. So I'll take care of a few issues right here.

 

How do I balance the flow of the overflow box and the flow of the pump?
This is the biggest mis-understanding. You don't need to do anything to balance the flow. In fact, the overflow box should always be able to flow MORE water than the pump. The only "balance" problem you could ever have is if the pump was pumping water faster than it could flow thru the overflow box back down to the sump. So, lets assume the overflow box can handle the flow rate of the pump.

 

Many emails ask if I use a valve on the overflow box to match the overflow's flow rate to the pump. You should NEVER do this on an overflow/sump system. Observant viewers may note that there is a picture or two on my website that shows a valve on the bottom of my overflow box. This is only used to cut off flow when I need to disconnect the overflow from the sump for major-cleaning purposes. It is left wide-open during operation. I previously used it to SLIGHTLY reduce the flow to try to eliminate noise, but that wasn't very effective.

 

So, let's say we have an overflow box that can flow 1000 gph, and our pump can only pump 500 gph. At first glance, it seems like we would end up with all the water in the sump, since the pump can't keep up with the overflow box. But this simply isn't the case. The pump doesn't need to "keep up". This is why we are using an overflow box, and not simply using a hose directly from the tank down to the sump. Only the water in the tank that rises above the lip of the overflow box can flow down to the sump. It is impossible for the overflow box to ever drop the water level any lower than the lip of the overflow box. If the pump is fast, then more water will flow over the lip, into the overflow box, and down to the sump. If the pump is slow, then less water will flow over the lip, into the overflow box, and down to the sump.

 

This only real problem with flow rates would be if the pump moves water faster than it can flow thru the overflow. Then the water level in the main tank would rise until it overflowed the tank, creating your own personal flood. This is why you should NEVER restrict the flow of the overflow box.

 

How do you prevent the sump from overflowing in a power outage?
As I stated above, Only the water in the tank that rises above the lip of the overflow box can flow down to the sump. It is impossible for the overflow box to ever drop the water level any lower than the lip of the overflow box. So, even with the power off, the water level in the tank will drop until the water level is just barely below the lip of the overflow box. You need to set up your sump and overflow box so that the sump can always hold that much water. In my 29g sump with the pump off, the sump should contain about 20 gallons of water. When the pump is running, the level in the main tank rises, and the level in the sump is around 10 gallons.

 

While it's true that the overflow box won't cause the sump to overflow, there is one situation that could if you don't take precautions. In a planted tank, we almost always want the pump-return output to be below the tank water level. Otherwise it creates a lot of surface agitation and releases any injected CO2. But with the end of the pump return hose below the water surface, if the pump is shut off, water will flow back down the return hose thru the pump and into the sump. Water will flow backwards until the tank level drops below the end of the return hose. If the return hose ends just below the water surface, then very little water will flow backwards. But, if your pump return output was at the very bottom of the tank, then ALL of your tank would drain backwards thru the pump. BIG FLOOD!.

You have a couple options to prevent this flood. Easiest/cheapest is just to position the pump return output just below the normal "pump on" water level. If you don't mind spending a few bucks, you can buy a check-valve that will allow water to flow from the pump to the tank, but block any backwards flow from the tank to the pump. They are available from www.DrsFosterSmith.com and www.ThatPetPlace.com .

 

Overflow for Wet/Dry Filter

This page shows the concepts of how an Overflow/Syphon box for a Wet/Dry filter works. It also gives you an idea of how mine is built. Note that I did not build my own because I had an old one on-hand. But it's basic acrylic construction. You need to be very precise on the outside box if you are building your own, because if that part leaks, your floor gets all wet!

 

Overflow theory of operation

The overflow box is a relatively simple concept. You use two boxes, one inside the tank, one outside the tank, and a syphon tube to move water from one to the other. But there are a few important things to be aware of.

 

It's very important that the Overflow not lose it's syphon if the power fails. If the syphon were to break, and then the power comes back on, the water will be pumped from the sump back to the tank, but no water will flow from the tank back to the sump. This means that the main tank would overflow, all over your floor!

 

My overflow box is set up so that it will maintain water above the bottoms of syphon tubes regardless of the tank water level. Here's how it works:

Normal tank water level

When the pump is operating, the tank water level will be maintained at a constant level, just above the inlets on the inside overflow box.

Water will then flow thru the syphon tube (or tubes), to the outside box.

 

The outside box is divided into two compartments. The right compartment is where the syphon tubes are placed. The left compartment is where the water exits (thru a pipe in the bottom of the box. There is a divider seperating the two compartments. Water spills over the divider from the right side to the left side.

 

Once the water reaches the left side of the outer overflow box, it drains thru a prefilter (sponge, shown in yellow), and down the pipe, to a hose that is connected to the wet/dry.

Low tank water level
when the return-pump isn't pumping water

During a power failure, or during tank maintenance, the return pump will not be pumping water from the sump back to the tank. The water level in the tank will drop. The water will drain thru the overflow, and will also drain backwards down the return-pump hose.

NOTE: Normally the water level in the tank will drop to just below the overflow box inlets (The diagram is just exaggerated to show how the box maintains syphon). But if the return-pump hose ends deep in the tank, a syphon will form, and attempt to carry water from the tank, back down the return-pump hose, and into the sump going backwards thru the pump. To prevent this, either have the return hose end near the top of the tank, or if you want to have the water return deep in the tank, place a small hole in the return hose, near the top of the tank, just below the normal water level. Then, when the pump is off, water will drain down to the overflow level, and the hole inthe return hose will let in air, and break that reverse-syphon.

 

Once the tank water level drops below the overflow inlets and below the water return hose, the tank will stop draining. But even though the water level is below the overflow inlets, the syphon tubes will still maintain their syphon, ready to start moving water once power is restored.

 

You can see in the diagram, that the water drains down to the level of the divider in the outside box. The inside and outside boxes equalize at the same level, and no more water flows, but syphon is maintained.

 

How do I start the syphon in the overflow box?
To start the syphon, you need to remove all the air from the U-Tube. Take the tube out of the overflow box, insert an airline tube more than 1/2 way into the U-tube, and then reinstall the U-tube. With the U-tube in place, slowly pull the airline out until the end is at the highest point of the U-tube.

 

You need to fill the outside overflow box with water (the part of the outside box that the U-tube sits in), and then turn on the pump so that the tank water level rises and fills the prefilter box. Now suck on the airline tube to remove all the air from the U-tube. As you do this, the U-Tube will fill with water. Once all the air is removed from the U-tube and it is filled with water, pull the airline out. Your overflow box is now ready to go!

 

Basic Construction

This diagram shows the overflow box that goes inside the tank. The inlet notches should be much thinner than shown, and they should extend around the sides of the box as well. The notches are intended to keep large objects from entering the overflow. Even small notches won't keep small fish out of the overflow. I've heard to tips for keeping fish out of the overflow. The first recommendation was to use some nylon window-screen material to cover the inlets. Another option is to use panty-hose. The pantyhose idea works well. With either of these options, you need to make sure that too much junk doesn't collect and block the inlets.

This is a simple diagram of the outside overflow box. The hole on the left side is where you connect your plumbing hardware for the hose to the wet/dry.

The two boxes are connected by an acylic U shaped connector, shown in red in the diagram. In my box, the connector is glued to the outside box using acrylic adhesive, and is connected to the inside box using two nylon bolts and nylon wing-nuts. Drill holes in the connector part, and cut slots in the inside box, to allow you to adjust the height of the inside box. The level of the inside box will determine the water level in your tank.

Now on to the plumbing hardware:

In my case, the hole in the bottom of the outside box is 1" is diameter, and I used 1" PVC pipe fittings.

On the inside of the box, there is a 1" PCV bulkhead fitting. It has threads on the end facing down. Then inserted up thru the bottom of the box, is a 1" male-to-male coupler. It is threaded on both ends. On both the inside and outside of the box, you should use rubber o-rings to provide a water-tight seal. (not shown)

connected to the bottom of the male-to-male coupler is a 1" Ball Valve. This isn't 100% necessary, but the valve can be used to turn off the flow of water exiting the overflow box for maintenance purposes. The valve is threaded on both ends.

 

Connected to the bottom of the valve is a simple coupler that is threaded on the top, and accepts 1" PCV piping on the bottom. I've got a short bit of PVC pipe (about 4 inches worth) attached to this coupler, and then a length of blue 1" flexible hose clamped over the pvc pipe. That hose runs down to the wet/dry.

 

One part I didn't discuss is the overflow syphon tubes. I don't have any ideas on how you could build your own. You can buy them in several sizes from most good fish stores. The diameter of the tubes, and the number of tubes depends on how much waterflow you want. The larger the return-pump, the higher the flow. The higher the flow, the more tubes and/or the larger the tube needs to be. I am personally using 2 3/4" tubes on my 75g. I'm using a Rio 2100 pump, which claims about 400 GPH.


To start the syphon, you need to remove all the air from the U-Tube.

 

Take the tube out of the overflow box, insert an airline tube more than 1/2 way into the U-tube, and then reinstall the U-tube. With the U-tube in place, slowly pull the airline out until the end is at the highest point of the U-tube.

You need to fill the outside overflow box with water (the part of the outside box that the U-tube sits in), and then turn on the pump so that the tank water level rises and fills the prefilter box. Now suck on the airline tube to remove all the air from the U-tube. As you do this, the U-Tube will fill with water. Once all the air is removed from the U-tube and it is filled with water, pull the airline out. Your overflow box is now ready to go!

 

Basic Construction

 

Overflow box


This diagram shows the overflow box that goes inside the tank. The inlet notches should be much thinner than shown, and they should extend around the sides of the box as well. The notches are intended to keep large objects from entering the overflow. Even small notches won't keep small fish out of the overflow. I've heard to tips for keeping fish out of the overflow. The first recommendation was to use some nylon window-screen material to cover the inlets. Another option is to use panty-hose. The pantyhose idea works well. With either of these options, you need to make sure that too much junk doesn't collect and block the inlets.

 

Overview cross section

This is a simple diagram of the outside overflow box.

The hole on the left side is where you connect your plumbing hardware for the hose to the wet/dry.

 

Overflow U tube

The two boxes are connected by an acylic U shaped connector, shown in red in the diagram. In my box, the connector is glued to the outside box using acrylic adhesive, and is connected to the inside box using two nylon bolts and nylon wing-nuts. Drill holes in the connector part, and cut slots in the inside box, to allow you to adjust the height of the inside box. The level of the inside box will determine the water level in your tank.

 

Now on to the plumbing hardware:

 

Overflow Bulkhead and Pipes

In my case, the hole in the bottom of the outside box is 1" is diameter, and I used 1" PVC pipe fittings.

On the inside of the box, there is a 1" PCV bulkhead fitting. It has threads on the end facing down. Then inserted up thru the bottom of the box, is a 1" male-to-male coupler. It is threaded on both ends. On both the inside and outside of the box, you should use rubber o-rings to provide a water-tight seal. (not shown)

 

Overflow pipes

A 1" Ball Valve is connected to the bottom of the male-to-male coupler. This isn't 100% necessary, but the valve can be used to turn off the flow of water exiting the overflow box for maintenance purposes. The valve is threaded on both ends.

 

Connected to the bottom of the valve is a simple coupler that is threaded on the top, and accepts 1" PCV piping on the bottom. I've got a short bit of PVC pipe (about 4 inches worth) attached to this coupler, and then a length of blue 1" flexible hose clamped over the pvc pipe. That hose runs down to the wet/dry.

 

One part I didn't discuss is the overflow syphon tubes. I don't have any ideas on how you could build your own. You can buy them in several sizes from most good fish stores. The diameter of the tubes, and the number of tubes depends on how much waterflow you want. The larger the return-pump, the higher the flow. The higher the flow, the more tubes and/or the larger the tube needs to be. I am personally using 2 3/4" tubes on my 75g. I'm using a Rio 2100 pump, which claims about 400 GPH.


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