Auto-water change/top-off idea


Junior Poster
Jan 24, 2005
Portland, OR
I put to paper a design inspired by Tom B.'s comments on automated water changing.

The basic idea is that a normally closed solenoid on a timer opens to drain while another in combination with a float valve opens to fill. The solenoid on the fill side is redundant but prevents a stuck-open float valve from overfilling the sump. It would also keep the water change reservoir from emptying into the sump if the water level dropped during a maintenance procedure, although a ball valve would be installed (not shown) and could be used for that purpose.

In conjunction with the water change process, another reservoir would contain pure water (RO or RO/DI) for evaporation replacement. This would use a normally open solenoid and a float valve. You would want this solenoid closed when a water change is being performed.

I see two flaws. You might find more. The first is if I allow the water change reservoir to be depleted, that is, not have enough capacity to cover the scheduled change. In this case, the timer would open the solenoids and water would empty, but there might not be enough new water to replenish the changed water. Assuming that my normal operating water level in the sump is sufficient to cover the amount of water drained, this would not be catastrophic. However, it would be possible to put it together in a way for this to be a big problem. Let's say I have a 50 gallon water change reservoir, but my operating volume in the sump is only 25 gallons. If I set the timer for a period of time necessary to drain 50 gallons, and I have 25 or less gallons in the reservoir, I'll empty the sump and burn out the return pump.

The second problem is if the drain solenoid sticks open, of if the timer fails to shut off once it starts a water change session. In this case, the sump would drain completely and I would burn out the return pump.

I have a couple of ideas about using float switches and maybe a powerhead to empty instead of gravity-drain, to build in some failsafe. Before I go down those paths, anyone have any comments, suggestions, observations?


Tom Barr

Staff member
Jan 23, 2005
Re: Auto-water change/top-off idea

Well, instead of using the reservior, use the tap water line.

When the water in the sump goes down, it fills up till the FS stops it and closes the valve.

To solve the other issue, use a pump to add the water to the tank and also too drain the tank(you'd need two such pumps).

As long as the drain level is avove the sump, or you have an anti siphon air break, then it will never run dry should the solenoid fail. :gw
You place the solenoid and the pump on the same timer or forego the solenoid for the drain since it's now pump dependent.

For the refill, the float switched mechanical refill is pretty reliable.

If you still feel weird about that, having the refill lower than the sump level and using a pump to refill will work with an antisiphon valve.
But you still have a similar issue, pick your poison.

The other rather simple thing, you can use two devices such a flat switches or solenoids in series, since they will respond to the same thing, and be on the same timer etc, they would complement eachother should one fail.

On that note, a simple bilge pump and a small 2-6" waterproofed lip around the sump is not that hard to install. Anytime the the lip area gets flooded, the bilge pump drains it to the drain.

So there are 3 or so levels of safety.

And that is a good thing.

Tom Barr