Why Doesn't My DIY Overflow Box Skim?
- Thread starter ddolinoy
- Start date
I don't know what my flow rate is. All I know is that I can adjust it from a raging waterfall (since I have a 2 inch bukhead connector attached to it) to a slow trickle and I still see skum on the surface. My overflow is 7 inches wide. It seems that the water flows into it while not disturbing the surface. I would have thought that impossible, but there it is. Is it because my boxes (inside the tank and outside the tank) are the same depth or would teeth on the inside box edge help?
Dave
Dave
I noticed that the more flow I have (closing the bypass valve on my return pump), the deeper the water in the tank. Conversely, if I reduce the flow, the edge of the box (on the inside) is much closer to the water level since the water in the tank is not as deep. Either way, I have micro bubbles floating on the surface and they aren't moving toward the box. I also have the overflow box in a corner of the tank. I hope that doesn't have anything to do with it.
Dave
Dave
Richocet83
New member
Can you use a PH in the DT to break the surface and cause some agitation?
NanoReefWanabe
Active member
to me it sounds like your box is too small to effectively skim the surface, adding teeth to the box will only further reduce the size of your box...
you need a larger box...how big is the tank? what are the dimensions of the box?
increasing the tanks flow overwhelms the overflow weir hence the increase in weir height (amount of water on top of the weir) and lowering the flow then allows the overflow to work a little better...
to be sure we need pics...
you also need a powerhead pointed to the surface to help break the surface tension...if you have an overflow box then i assure you you are skimming the surface of the tank, however if the water isnt moving fast enough or the weir height is too high combined high surface tension, only the water below the surface will go down the drain and the surface will stay stagnant, kinda like a thin sheet of ice on the surface.
you need a larger box...how big is the tank? what are the dimensions of the box?
increasing the tanks flow overwhelms the overflow weir hence the increase in weir height (amount of water on top of the weir) and lowering the flow then allows the overflow to work a little better...
to be sure we need pics...
you also need a powerhead pointed to the surface to help break the surface tension...if you have an overflow box then i assure you you are skimming the surface of the tank, however if the water isnt moving fast enough or the weir height is too high combined high surface tension, only the water below the surface will go down the drain and the surface will stay stagnant, kinda like a thin sheet of ice on the surface.
Airywhitesoul
New member
The theory is that the longer the edge to an overflow the better since the volume of water entering the overflow is the same but the amount of surface area skimmed is increased (since water column height is reduced). Thus the coast to coast overflow is presented as an ideal. I went through all of these calculations when designing my current system. Bean animal goes into great detail on overflow design and flow calculations. Since you have already set your linear length, I don't think that changing the flow will affect surface skimming above a certain point. You might need to add a supplemental trough into your existing overflow.
Thank you for your opinions. NanoReefWanabe described my situation exactly. It is like a sheet of ice that refuses to move into the overflow box. After looking at it a little more I noticed what the problem was. The water wasn't falling into the overflow box. I had the flow rate too high for the box to handle. Consequently, the water level was the same both inside and outside of the box. For it to skim, the water level must be lower inside the box so the surface skum can fall into the box. Airywhitesoul also hit it on the head when he said that I need another overflow box. The box is definitely the bottleneck in my system. I made my own weir and sump out of acrylic. I'm going to make another one. Is there any limit to the width of a weir? Is there any limit to the thickness of the siphon in the weir? I made mine 1/2 inch thick. Could I make one 3/4" or 1" thick? Thanks.
Dave
Dave
Airywhitesoul
New member
I'm going to recommend bean animal's silent overflow design for the siphon. I built one on a 55g with the siphon/open/emergency and it functions amazingly well after the weir and is completely quiet. The wider you make the weir, the better it will work since it will have less Head height of the water running over the weir for the same volume thus skimming more surface water. However the bigger you make it, the more intrusive it becomes in the tank. It's a balance.
Salty,
No question is stupid when it comes to my tank! Yes I do have a protein skimmer and it is working well. I think the problem is that the water has to fall into the overflow box.
I had the flow rate too high, so the water level remained the same inside and outside the box. Consequently, the scum just sat on the surface. Now that I've lowered the flow rate, the skum is almost gone. I think my weir would work better if it wasn't in the corner of my tank. The first chance I get, I'm going to make a bigger skimmer and add it to the one I'm already using. This way I'll be able to increase the flow rate and still have the water fall into the box. Thanks.
Dave
No question is stupid when it comes to my tank! Yes I do have a protein skimmer and it is working well. I think the problem is that the water has to fall into the overflow box.
I had the flow rate too high, so the water level remained the same inside and outside the box. Consequently, the scum just sat on the surface. Now that I've lowered the flow rate, the skum is almost gone. I think my weir would work better if it wasn't in the corner of my tank. The first chance I get, I'm going to make a bigger skimmer and add it to the one I'm already using. This way I'll be able to increase the flow rate and still have the water fall into the box. Thanks.
Dave
gweston
New member
The level within the overflow box itself should be lower than the tank level... so the water 'falls' into the overflow box to then drain through a bulkhead, or go through a siphon tube down into a sump. This falling is what skims the surface and keeps the surface scum from just floating back out.
It won't skim really if the flow through the box is not fast enough... if your return rate overpowers the drain rate, you will eventually overflow the display tank. So, It could be the drain/siphon isn't flowing fast enough, so you don't get the 'falling' affect in the overflow box.
Teeth aren't required... my overflows in my new system don't have teeth. The teeth can potentially help keep fish or other things from zooming over the edge of the overflow. Not 100% effective but can help. No teeth I'd think would result in better surface skimming.
I have built a bean style overflow myself. A super design. Extremely quiet if done right. But.. it took me 2 years to finish my upgrade project due to all the do it yourself I committed myself to.
Its worth looking into, but as with any DIY, best to weigh cost(in time spent)/benefit. Its certainly fun if you like to build things!
It won't skim really if the flow through the box is not fast enough... if your return rate overpowers the drain rate, you will eventually overflow the display tank. So, It could be the drain/siphon isn't flowing fast enough, so you don't get the 'falling' affect in the overflow box.
Teeth aren't required... my overflows in my new system don't have teeth. The teeth can potentially help keep fish or other things from zooming over the edge of the overflow. Not 100% effective but can help. No teeth I'd think would result in better surface skimming.
I have built a bean style overflow myself. A super design. Extremely quiet if done right. But.. it took me 2 years to finish my upgrade project due to all the do it yourself I committed myself to.
Its worth looking into, but as with any DIY, best to weigh cost(in time spent)/benefit. Its certainly fun if you like to build things!gweston,
Can you share with me where I can see a bean design?
The way I designed my overflow box is to have the water fall 1/2 inch into the box on the inside of the tank. Do you think that is the problem? If I had it fall 1 inch or more into the box, would it work better? However, I would then be concerned about the siphon having to climb higher to get over the side of the tank. What do you think?
Again, I'd like to see other designs, especially the bean design. Thanks.
Dave
Can you share with me where I can see a bean design?
The way I designed my overflow box is to have the water fall 1/2 inch into the box on the inside of the tank. Do you think that is the problem? If I had it fall 1 inch or more into the box, would it work better? However, I would then be concerned about the siphon having to climb higher to get over the side of the tank. What do you think?
Again, I'd like to see other designs, especially the bean design. Thanks.
Dave
Keep in mind, there are two things going on here, skimming and draining. The size of the plumbing and how it functions, e.g. open air, full siphon, etc., determine the capacity of the drain. If the return pump has a higher flow rate than the capacity of the drain, the tank will overflow, period. The size/shape of the overflow box in the tank has nothing to do with this as long as it is not restricting flow to the drain(s). The overflow box controls how much skimming occurs. It is the length of edge on the overflow that determines its capacity. The capacity of the overflow box should be higher than that of the return pump to minimize the thickness of the water layer spilling over the edge and maximize the skimming action.
There should be a difference in height between the edge of the overflow box and the drain(s) inside the box to cause the water to fall and allow skimming to occur. If the drain plumbing can handle the pump, the water will leave the tank as it is pumped in. If the overflow box is properly sized skimming will occur as the water moves from the display tank to the drain. Imagine the system with the pump off, the water level in the display tank will be at the top of the over flow box. The water level in the overflow box will be at the bottom of the drain hole(s). If you turn on the pump for a moment, water moves into the display tank, spills over the edge of the overflow box and down the drain.
If there is a long edge on the overflow box, the layer falling over can be very thin and still move a lot of water because of its length, lots of skimming. As the rate of flow increases, the thickness of the layer has to increase to move the extra water thus reducing the skimming effect as more water is entering the overflow box from below the surface “skin”. I think this is the problem you are having, not enough edge length. The location of the box and the amount of fall are negligible factors. According to the calculator, a 7” overflow can handle at most 450 GPH. It is hard to do more than speculate what the true issue is without more information about your setup. If you don’t know what your turnover is, what pump are you using?
There should be a difference in height between the edge of the overflow box and the drain(s) inside the box to cause the water to fall and allow skimming to occur. If the drain plumbing can handle the pump, the water will leave the tank as it is pumped in. If the overflow box is properly sized skimming will occur as the water moves from the display tank to the drain. Imagine the system with the pump off, the water level in the display tank will be at the top of the over flow box. The water level in the overflow box will be at the bottom of the drain hole(s). If you turn on the pump for a moment, water moves into the display tank, spills over the edge of the overflow box and down the drain.
If there is a long edge on the overflow box, the layer falling over can be very thin and still move a lot of water because of its length, lots of skimming. As the rate of flow increases, the thickness of the layer has to increase to move the extra water thus reducing the skimming effect as more water is entering the overflow box from below the surface “skin”. I think this is the problem you are having, not enough edge length. The location of the box and the amount of fall are negligible factors. According to the calculator, a 7” overflow can handle at most 450 GPH. It is hard to do more than speculate what the true issue is without more information about your setup. If you don’t know what your turnover is, what pump are you using?
Ronic
New member
http://reefcentral.com/forums/showthread.php?t=1310585
on the first post he goes over everything.
on the first post he goes over everything.
Airywhitesoul
New member
Awesome... That's the page. And just in case you get it in you head you don't need the emergency drain. You do! It kicks in occasionally when you restart your tank or somehow you change the steady state. It's protected my floors on a number of occasions. The calculator for the length of your weir is on the homepage of reefcentral on the left side in the calculators. Good luck! I made my drains only 1", so it doesn't have to be 1.5" like everyone claims. Just adjust your flow rate appropriately.
Airywhitesoul
New member
:wavehand:Awesome... That's the page. And just in case you get it in you head you don't need the emergency drain. You do! It kicks in occasionally when you restart your tank or somehow you change the steady state. It's protected my floors on a number of occasions. The calculator for the length of your weir is on the homepage of reefcentral on the left side in the calculators. Good luck! I made my drains only 1", so it doesn't have to be 1.5" like everyone claims. Just adjust your flow rate appropriately.
