If theirs a blockage in the downpipe a sump will run dry but you only have enough water in the sump so if it happens it won't overflow the main tank. However if you have an auto top-up unit connected to the sump & this happens then you can have serious flooding cause the float switch thinks it needs more water & carrys on pumping fresh water in. (talking from experience here!!!!!)
Weirs or a standpipe (or both) control the amount of water that goes down too the sump. Soon as the water hits the top of the weir/standpipe it stops flowing as it has know where to go anymore.
When you build your tank & fill with water block off the water to the sump & fill the main tank to a level you are comfortable with as the absolute maximum. (i.e. inch from top lip or so).
Then open up the tap & let the water flow down into the sump (no pumps on) till it hits the top of the weir & no more will flow. Then mark the water levels in the sump with a marker pen & these will be your guide levels when you do water changes & top ups etc. These levels are you maximums, if you excedd these expect a flood if the power cuts or standpipe blocks ( as in your fears!!)
p.s. standpipes are on;y used in bottom drilled tanks (like most). if you have a rear drilled tank or an overflow box the water only goes down to the pipe levels obviously. You still build mini-triangular weirs though to stop things entering the pipe!!
Weirs or a standpipe (or both) control the amount of water that goes down too the sump. Soon as the water hits the top of the weir/standpipe it stops flowing as it has know where to go anymore.
When you build your tank & fill with water block off the water to the sump & fill the main tank to a level you are comfortable with as the absolute maximum. (i.e. inch from top lip or so).
Then open up the tap & let the water flow down into the sump (no pumps on) till it hits the top of the weir & no more will flow. Then mark the water levels in the sump with a marker pen & these will be your guide levels when you do water changes & top ups etc. These levels are you maximums, if you excedd these expect a flood if the power cuts or standpipe blocks ( as in your fears!!)
p.s. standpipes are on;y used in bottom drilled tanks (like most). if you have a rear drilled tank or an overflow box the water only goes down to the pipe levels obviously. You still build mini-triangular weirs though to stop things entering the pipe!!
