Auto Top Off: Solenoid or pump?


New member
So I'm going to be using a float switch to control the auto top off system.
My top off water is above the sump.

I'm wondering if there's any advantage to using a solenoid and gravity to top off instead of a small pump? (other than the fact that I'd have to buy a solenoid whereas I already have a small pump)

My primary concern is safety; I live in an apartment(my landlady lives downstairs), I have to avoid floods at all costs. (floods=0, spills=many)

Also where do you buy your float switches?
I've been considering but leaning more towards an already made double switch at


New member
I'm using a solenoid plumbed directly off the DI product line, controlled by a DIY ATO unit. You can use a pump, but be careful of the elevation of the storage container vs. the level of the sump. Once the pump starts pumping water over the side of one container into a lower container (sump), a syphon will continue to flow even after the pump shuts off unless you use something to let air into the pump line and break the syphon.


In Memoriam
Anyone know where we can get a 3/4in or 1in solenoid and what it costs?

I have had syphon probs with my current system... but what happends is backsyphon.. once pump stops the topping off process, if the hose is below the water line, i sometimes get back syphoning and water goes from sump back to my top off container... note to self, place top off line above sump water line to prevent backsyphoning ;)



New member
I'll post a picture later today, but what I've used in the past is the little venturi nozzle from a Rio 800 at the high point of the pump hose. I attached about 1" of silicone airline tubing to the little nipple and to that I plugged in an airline checkvalve backwards (so water won't squirt out, but air can be sucked in) to break the syphon of the powerhead.

[\/] <-check valve
_____ ->water flow


New member
ah, thats just when i did 5 min ago except that I used a little male male airline hose adapter at the high point seems to work fine.


New member
A couple of tips:

a) It is safer to top off from a reservoir than directly from your RO/DI. If the float fails or the solenoid stuck open you can have a flood not counting the salinity drop. A reservoir makes possible to limit the maximum amout of water to top off if the system fails.
The second issue is that when the RO/DI starts the first water produced is contaminated so if it only operates for short periods there will be contaminants added all the time.
In both cases (powerhead or solenoid; direct or reservoir) additional safety can be implemented by installing a mechanical float above the max level float switch. If the switch fails the mechanical float will shut the flow although not the solenoid or the pump. Do not use the mech float if you are using a peristaltic pump as it can be damaged or the line burst open by the pressure. Powerhead will not have that issue.

b) Floats usning low voltage level switches and relay are safer than 120V direct float switches. A short circuit to the water can be a serious disaster if using 120V

c) If using a reservoir, Keep your reservoir below the sump level and the tubing outlet out of the water, this will be enough to prevent syphon back into the reservoir. If your space does not allow it, use a peristaltic pump so there is no possibility of syphoning.

d) If using a solenoid use one that is normally closed (energize to open) to prevent flow when power is lost.

e) Use two floats a lower one to turn the system on and a higher one to turn the system off. This will alow for adjustment of the permisible level change. If you use only one float the pump or solenoid will be working too often with very small changes in level or with just surface waves or ripples. You may add a third float to your reservoir to prevent the pump from turning on if the reservoir is empty.

f) If using Kalk for the top off, a peristaltic pump is recomended rather than a powerhead because it can be regulated to replace the evaporation at a slowr rate preventing large PH impact. If the pump does not have an integrated timer you can connect it to a battery operated timer and the timer to the float so you can set it up in a way to add Kalk only at night when the PH is the lowest

g) So in summary design it assuming it will fail.

Hope this info helps.


New member
Here's the actual breaker I formerly used (fits 1/2" i.d. tubing):


All good points jdiek. Assume it will fail, the house will flood, wife will divorce you (or worse yet, make you quit the hobby... :D ), etc.

I don't trust float switches, wired to a DC transformer or 120 VAC! They've failed on me in the past and redundancy only means it takes twice as long to bite you in the butt.

I'm sure you've seen the threads on the low air pressure switch type of ATO, and that's what I'm using. The switch is 20A @ 120V, and (knock wood) hasn't failed in years. I've DIY'd several for friends and they all work like a charm.


They've evolved over time to include 120V outlets or water solenoids, as well as indicator lamps that tell you when and what it's doing at a glance.


My personal favorite is the solenoid version running directly off my DI unit. My tap water is 37ppm of TDS so I don't RO. Sediment and Carbon pre-filters and a mixed bed resin canister right here baby.


The solenoid cycles frequently enough that the water in the line from the filters doesn't stagnate, and only runs 2.5 quarts between high and low levels.

I wouldn't bet on the fact that it's entirely fool proof, but I sure trust it more than that hoaky taped up mess with butt splices in 120V lamp cord you can order on-line.

If I were to recommend a comercial unit (but then it wouldn't be DIY) I'd go with the SpectraPure unit: