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FAQ - Overflow to Extetnal Filter

May I use the float switch into the biofilter to control the water level in the main tank?
Float switch could be a good choice in filter connected with the main tank without overflow system. However, it may not be able to work properly (also float valve in a lavatory flush tank get jammed time to time, mainly if there is a hard water). If it happens in the filter, the water will be on the carpet :-). Also, the frequent pump switch off and on is not good for the pump. The pump can get jammed after it is switched off by float switch and it may not be able to work after the switch is on.

How an overflow works?
The principle of an overflow function is to ensure water drain to filter under the tank. This isn't an issue, everybody knows it and do it during the tank vacuuming. One short suck and the water is flowing over an aquarium edge through the hose even it is above the tank water level...atmospheric pressure forces the liquid through the hose. The return pump drives the water back to aquarium. That's OK but we need to asure that in case of the blackout or pump failure water will not continue to drain from aquarium. The flow must be stopped. It also isn't an issue. The more difficult thing is to ensure the water starts to drain from aquarium to filter after the pump is on again. Everybody knows this...when you move the hose out of water while vacuuming the gravel, the air is sucked and syphon is broken even the hose is put back under the water. The same is going on in aquarium if the pump will go on and the water will drain from the filter...aquarium will flood as the water coming from filter has no chance to drain back to the filter. The amount of overflowed water will not be big because the return pump would draw water only from the last chamber of filter and then it would fail. On the other hand, in case of the blackout the water will flow from aquarium to filter through syphon (like during vacuuming the gravel) and out of the filter until the water level reaches the pipe input. The input could be close under the normal water level in aquarium and the amount of water overlowing from aquarium would be small.

So, it wouldn't work. That's the reason why the small box (internal overflow box) is put into aquarium. Its upper edge matches an aquarium water level. The box can be really small in order not to occupy the big area in aquarium. It can be made of pexiglass or glass. This internal overflow box hangs on the aquarium edge. From the outside of an aquarium hangs another box. Bothe boxes are connected by reverted U-shaped tube. Both U-tube ends are sumberged in these boxes. Now, we have coupled-tank system. In the coupled-tank system, the water levels are balanced regardless of their volume. U-shaped tube must be filled with water all the time in order the pressure and syphon will be not disrupted. This is the first part of the system.

An outer box is drilled and pipe is inserted in the hole. The hole can be drilled in the bottom or in side of the box. It doesn't matter. Important thing is that the pipe must lead up and its input end in the outer box should be at or close under the water level in aquarium. This pipe or hose drain the water to the filter uder an aquarium. Water returns from filter back to aquarium by pump through the pipe or hose without any break. Regulation valve can be connected to the return pipe to control the water flow back to aquarium.

Now, how it works? Fill an aquarium, internal overflow box, U-shaped hose, outer box and filter with water. Switch on the pump and the water flows from filter to aquarium. In aquarium, the water level rises, reaches the upper edge of an internal overflow box, the water level in an internal box rises, flowing through U-shaped hose, so water will not overflow from an aquarium. Water flows through U-shaped hose into the outer box and it drains through the pipe connected to the esternal box the water flows to the filter. The cycle is then closed and works.

Case 1 - Blackout or pump failure:
The pump is off. The water is flowing from aquarium to filter but only until the water level in aquarium decreases under the upper edge of an internal overflow box. Water levels in an internal and external boxes are then balanced (coupled-tank system principle) and stay connected with U-shaped hose still filled with water as its ends are submerged. The water level in the outer box is, however, under the input into the pipe connected to the outer box and it sucks an air. The syphon is broken and the water flow to filter stops. In the filter, tehre has to be some reserve volume to receive some water after the blackout. It is solved by the dividers hight in the filter which don't reach the upper edge of the filter but are a few centimetres under the top of the filter. OK, the pump is off, water drained into the filter, water flows is stopped because no water comes from the outer box and the pipe in this box sucked an air.

Case 2 - The pump is on again:
Filter full of water is drained by pump, the water level in aquarium rises, the water level then rises also in an internal box, water flows through the U-shaped hose to the outer box (because in the U-shaped hose was still water under atmospheric pressure and this is the most important part of the system and most difficult to achieve for long time). The water level in the outer box rises together with the rising level in an aquarium (level balancing) and finally reaches the pipe input in the outer box and flows to filter. And so on.

The biggest problem is:
- to determine the U-shaped hose diameter correctly. It has to be large enough and it is hard to predetermine it. It is possible only to try but the biggest, the best. It is never too big. The water balancing speed in the boxes depends on the U-shaped hose diameter. When it is too small, water fow from aquarium is slow.
- to ensure that the water is all the time in the U-shaped hose and NO AIR. An air will accumulate in it sooner or later as it is dissolved in the water. The microbubbles accumulates and after few months, there is some air which slows down the water flow through this hose. Therefore, drill small hole at the top point of the reverted U-hose and glue small pipe. It serves for sucking an air out of the U-hose with your mouth. I recommend to use a transparent U-hose to see if there is some air.
- to determine the diameter of the drain pipe leading from the outer box to filter. If the pipe diameter is too large, the system will be loud because is able to drain the water from the outer box easily and sucks an air like when you drain the basin. You can hear the gurgling sound. If the diameter is too small, the water will be not able to drain properly and you need to throttle the pump to slow down the water flow.

For further study I recommend the next web sites with pics:
- overflow construction and animation
- overflow boxes for sale
- DIY overflows from plexiglass
- pipe overflow 1
- pipe overflow 2
- pipe overflow 3

Copyright © 2003 - 2012 Robert Toman



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