For an atmospheric tank connected to a municipal water system, there would generally be a good bit more pressure available than was required to reach the top of the tank. So yes, the tank will fill regardless of inlet location, as a general rule. If you're on top of the mountain or have abnormally low pressure or something, or the tank is 100' high, that may be different.

Quite often, inlets are installed above the high water level as a sure-fire backflow prevention. A lot of fire-protection tanks store non-potable water, and it is desirable to make sure it never flows the other way into the system. This is more commonly seen where there is a custody transfer, either city water going into a private tank, or water wholesaler pumping into a municipal tank. And this arrangement may be required by local water rules. It may be required by NFPA 22, come to think of it.

I will be running 3 inch pvc from the roof to the tank. No pressure involved, just run-off. I'm just wondering if the water will back up in the pipe to the top of the gutter from where it's coming? Or will the tank just fill without worrying about the former?
Thanks!

Casey,

It helps if you provide some decent information.

will the tank fill all the way up even if the inlet is not at the top? We don't know as we don't know where the water is coming from or what pressure it is at?

Do I need a check valve? Always a good idea to stop the water going backwards if the incoming pipe leaks or breaks or can reverse flow.

There is a 3-inch pipe coming from the roof that will feed the tank. Do you mean the roof of the tank??

I'm just wondering if the water will back up in the pipe to the top of the gutter from where it's coming? If this is coming fromthe roof of the tank you're filling then yes it will. And then when the tank is full where does the water go?
What happens if you empty the tank and it doesn't rain?
Fire systems need a guaranteed water supply / storage. Rain water won't cut it, plus it will slowly fill your tank with gunge (dirt, bird shit, leaves etc)

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Quote (Casey2

.... I am installing a fire suppression water storage tank. the inlet is going to come in a few feet from the top. When filling the tank, will the tank fill all the way up even if the inlet is not at the top? Do I need a check valve?)

The tank will fill all the way if the pressure is satisfactory... Regarding the check valve.. better to use float valve to close the inlet when the WL reaches max. level..Just write FLOAT VALVE and see the outcomes.. one of them;

https://www.hawle.de/en/products/infos/product-gro...

But still you will need overflow pipe . I will suggest you to look AWWA M 42 ( Steel Water-Storage Tanks ) to see the piping set ups..

You did not mention the shape , size of the tank..

The common practice is, inlet and outlet from the bottom .

Good luck...

I have already installed the lines from the house out to where the tank is going to be. The lines are coming in lower than I originally anticipated so I either need to lower the 2500 gallon plastic tank or bring the line in at the bottom and let it fill to the top that way. I will initially fill the tank from my well as we have not had any rain in quite a while and there are massive fires in the area. So, from my reading, it looks like I can fill from the bottom.

thanks!

Ah - A glorified water butt!

Your key issue is dirt and gunge in the bottom of the inlet line. Even if it goes in at the bottom, make sure you include the ability to flush it, i.e. an isolation valve and a tee with a valve.

But yes, you can go in at the bottom, but then remember that the line right up to the level of the top of the tank could be full of water so make sure any supports and the pipe allow for that. Most drain systems are not designed to flow full of water.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

Thank you on all counts. I will take your advice...