Airlock forming in water pipe from the overhead tank

[RaviBanthia]

Hi folks,

I have a problem with an airlock forming every time the overhead tank becomes empty.

Even when I fill the overhead tank to the brim, the water in the household outlets comes at a very low speed.

By experimenting, I found that there are two ways to fix the problem each time there is an airlock:

1) Go to the roof of my house and open a tap in the main pipe that distributes water to the whole house.

2) Flush a commode (on the ground floor) that has a manual valve with a large water outflow.

There is an air vent (T-type) installed in the pipe just after the water tank, but the airlock is still forming.

Once, I have removed the airlock using one of the two methods I have listed above, water flows in all the pipes in the house at a decent speed.

My overhead tank is unfortunately not easily accessible (as the stairs to that part were not built).

So I am trying to understand the reason for the airlock forming before calling a plumber.

Also, I am in a town that does not seem to have expert plumbers (or maybe in my 11 year stay, I haven’t found the right person).

It would be immensely helpful if I could get some pointers to I can request the plumber to locate and resolve the problem.

Thank you.

 

[breplum]

It is exciting to hear from people all over the world, so, thanks for reaching out.
Most all of the regulars on this site, at this point in time, are based in the United States and Canada, where we don't have water systems like you describe.


Nevertheless I can't help but think your dilemma through a bit:
You need air venting from the top of the tank to prevent the air lock.
I don't comprehend how your air vent after the tank can prevent air lock. There needs to be a top of tank sourced air vent.
Your local water tank supply shops ought to have suggested resources, since anything we have would be different.

I have to imagine that the tanks in your area have to normally have a vent already.

 

[RaviBanthia]

Hi Breplum,

Thank you for your reply.

I am attaching an image to indicate the type of air vent I am talking about.

I am referring to item 2 in the image.

I do not have item 1, but even when the tank had no (plastic) cover at the top, I still had the same airlock problem. So the item 1 type of vent will not help.

The exit pipe from the tank is perhaps inclined slightly upward (rather than level or downward).

I wonder in the junction of the outlet pipe and the first vertically downward pipe has an airlock forming.

Also, the air vent (item 2) could possibly be clogged; the outlet pipe too could have a minor clogging.

I am trying to narrow down where and why the airlock is forming.

Edited to add:

If there was vacuum forming over the water in the overhead tank, water would not flow out of the tank at a decent speed ever.

 

[wpns]

I have a problem with an airlock forming every time the overhead tank becomes empty.

[...]
/*
The exit pipe from the tank is perhaps inclined slightly upward (rather than level or downward).
*/

There's your problem right there. You don't need (or want) the vent labelled #2 above, as it's just going to let air into the outlet pipe when the supply tank gets low enough.

But when your tank runs dry, you are going to get water into the system, no avoiding it.

First, prevent your tank from running dry. Float valve on the inlet (what is your supply?) or something to keep a minimum level in the tank. I assume it's a rainwater cistern?

Second, if you _can_, remove the upwards slant to the outlet pipe.

I'm also confused by:

/*
Go to the roof of my house and open a tap in the main pipe that distributes water to the whole house.
*/

Why is that valve not already open?

 

[RaviBanthia]

Hi Wpns,

Thank you for your reply.

“You don't need (or want) the vent labeled #2 above, as it's just going to let air into the outlet pipe when the supply tank gets low enough.”

Yes, that is true.

But any air that has collected in the outlet (provided it is perfectly level) gets pushed out of that pipe when the tank is refilled.

“First, prevent your tank from running dry. Float valve on the inlet (what is your supply?) or something to keep a minimum level in the tank. I assume it's a rainwater cistern?”

I do have a float valve in the inlet pipe.

We do not have a 24-hour water supply.

We get water once a day for one and a half to two hours, but the pressure on the supply water is good and water fills the overhead tank without the aid of a pump. (I also have an underground tank and a pump on the ground floor in case the tank empties out.)

“Second, if you _can_, remove the upwards slant to the outlet pipe.”

Yes, I have bought a spirit level, and I intend to have the slant (if any) checked out.

“I'm also confused by:

“Go to the roof of my house and open a tap in the main pipe that distributes water to the whole house.

“Why is that valve not already open?”

That is a water outlet tap on the roof (in case I need water when on the roof/flat terrace above the house).

It is not a valve that controls the water supply from the overhead tank to the rest of the house.

When there is an airlock, opening that tap (for a few seconds) removes the airlock and restores the normal flow of water.

 

[wpns]

Maybe an “air release valve” at the high point of the oulet pipe?

 

[RaviBanthia]

Yes, that seems to be the solution.

The outlet pipe is perhaps slanting upward and letting an airlock form.

So, I can try to change the slope of that pipe to at least horizontal (preferably downward).

Failing that, I will move the air vent to that junction.

Thank you!

 

[wpns]

move the air vent to that junction

If that's easy to do and will solve the problem, then that's probably OK.

 

[RaviBanthia]

I will update you when I get the problem resolved.

Thank you for your advice.

 

[RaviBanthia]

Sorry for the late update.

The problem turned out to be that the outlet pipe from the tank was sagging several inches (not visible from the rooftop).

Once I had the sagging reduced by inserting a brick and some pieces of broken tiles, the airlock stopped forming.

To make the outlet pipe perfectly horizontal or sloping downwards, I would have had to either raise the water tank or have the down-coming pipe shortened; both would have required involving mechanics.

Just reducing the sag solved the problem.