This is a repost of part of another thread going on now, I think that the topic deserves it's own heading so I'm beginning anew.
Many people choose to fill the canister with water before reconnecting, in the hopes that excluding that air will make start up easier. This does not solve the problem for typically a bubble will be trapped below the valves and the bubble rising in the inlet line breaks the siphon or get trapped in the impeller and airlocks the filter. Most importantly, this method ignores the physics involved.
The whole thing to priming is to be sure the air is expelled the right way, through the outlet side of the filter, not up the inlet which will break your siphon. How much air is expelled is not important, as long as it moves the right way. That can be done, as was said in the other thread, by connecting the waterfilled inlet lines to the canister and letting air out the outlet side of the filter until water has filled the filter and runs out the outlet (have a towel or bucket handy.)
It can also be done by having the outlet tube completely empty of water when the unit is reconnected, but be sure the in-tank outlet/spraybar is above (or alt least very near) the level of water in the tank so air can escape. The weight of the water in the inlet tubes will assure you that no air can move up the inlet line, water will flow down that tube, air up the other. If both tubes are dry, perhaps you just cleaned the tubes, connect the inlet taps, suck on the filter outlet tap just to get water flowing into the filter then connect the outlet tubes before the filter is full, all the inlet side is full of water which is heavy, so air must go up the outlet. When rushing water stops you turn on the filter. If the inlet tube is still full, perhaps you bled water from the lines to clean them and the outlet is empty but the inlet is full, then simply connect all tubes, open outlet taps to allow air flow, then open inlet taps to begin water flow. Again, when the water stops rushing, turn on the filter.
I cut the &#%$@% out of my hand rocking a Fluval 303 trying to get it going following that bad advice to first fill the filter with water. I spent an hour on the stupid thing, running late for an appointment by then, and while rocking the filter to try to get the air out my knuckle hit the sharp glass edge of the lower tank. It bled a lot, a real gusher. So I finally had to sit and think about why it was so difficult. Looking that physics of the thing, it becomes obvious -- you gotta get the air to go up the outlet. Either bleed it out, past the impeller by letting air, then water, escape the canister from the outlet side, or have the entire oulet line full of air so that it all has to go up with water pushing it from the inlet.
So, for units that have the integrated taps but no self priming, you might need to bleed all water from the outlet line -- I have begun doing this as part of my procedure, it flushes a lot of crud from the lines that otherwise can flood into the tank and it gives me some tank water in my bucket to rinse media in.
I find that the instructions on the canisters I have show pictures of the outlet lines (spray bars) above the water level of the tank when starting the filter, but they don't state that is necessary for air to get out of the system. Even the self-priming units can have difficulty if the outlet is below water when starting up if air needs to get out of the system.
If you are having trouble starting a canister -- look to the path the air is taking. It is the key.
As you mentioned, this priming pain must be very dependent on the model used. Do you have to do this every time you clean the filter?
I can't really share much, since my only experience with canisters is with XP's, but once you have them primed, you never have to prime them again, unless you disconnect the hoses themselves from the unit for some reason.
With those models, it doesn't really matter if you fill them up or not. As soon as you close the quick disconnect, water flows into the canister through the inlet, air goes out the outlet. I fill mine up, because I don't want the scare the fish too much with the sudden bubble bath from the spray bar (which is located on the bottom!)
With the much older Fluval 303, this was constant problem. Even after a simple cleaning of the filter, with the tubes both full of water, when reconnected, the unit would not start. If I opened the inlet valve first, the air would rise up the inlet, killing the siphon. If I opened the outlet valve first, there was still a trapped bubble in the inlet line, ready to kill the siphon or airlock the motor.
The Classic was often able to clear trapped air without rocking because the setup of the inlet and outlet insures that new water pushes all air out to the top, but understanding the path the air takes makes getting it restarted foolproof.
The Eccos are perhaps immune to this, but you have to disconnect the power and close the taps in the order specified or else the handle gets "fiddly" (as the British say -- I like that term!) do that correctly and it works like a charm -- mess up and you'll be digging at it with a screwdriver to try to get the handle lock to move.
The Pro2 seems to be more affected by having the in-tank outlet below the water level -- I think the priming button only has so much head that it can handle, if the outlet is also under water it has to push twice as much water to get the siphon going if it was interrupted.
The Pro has no priming button but does have the integrated taps and I have only cleaned it once so far, so I'm not sure if it is subject to any of these issues. When I cleaned it, I cleared the output line so it started right up.
So, yes, it will depend on the model, but that is why I have had to stop and think about the reasons behind the issues that each filter seems to have.
And clearing the tubes of junk by letting water out seems to be a good practice.
I can see that for many of the newer filters, this post will be irrelevant. But for the guys out there who are snapping up those old filters at garage sales or auctions, or buying used tanks with used filters, or are looking at the less expensive filters without the priming aides, maybe this can help. At least maybe I can save you a busted knuckle!
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