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Most water storage tanks need a way for air to move in and out. This opening is often called a breather, vent, or tank vent.
A tank breather prevents unwanted pressure or vacuum from building inside the tank as water enters or leaves. For rainwater tanks, the vent should also be designed to keep insects, debris, and small animals out.
Why Water Tanks Need to Breathe
The air space above the water level changes whenever the amount of water in the tank changes.
When water flows into a tank, it pushes air out. When water is pumped or drained out, replacement air must enter.
Without enough airflow, the tank can develop:
- Positive pressure while filling
- A vacuum while emptying
- Slower water flow
- Pump performance problems
- Tank wall movement or deformation
- Stress around lids, fittings, and seams
A rigid tank is not meant to act like a sealed pressure vessel unless it was specifically designed for that job.
For normal atmospheric rainwater storage, the tank should usually be vented.
What Is a Tank Breather?
A tank breather is simply an air passage between the inside of the tank and the outside air.
It may be a dedicated vent fitting, a screened opening near the top of the tank, or part of another fitting designed to allow airflow.
The important point is that the vent must let enough air move through it for the tank's maximum filling and emptying rates.
A tiny opening may technically allow air through but still be too restrictive when a large pump or fast inlet moves water quickly.
What Happens If a Tank Has No Vent?
A completely sealed tank can develop pressure changes surprisingly quickly.
When the tank is filling
Incoming water reduces the space available for air. If that air cannot escape, pressure rises inside the tank.
Depending on the tank and fittings, that pressure may cause:
- The tank walls to bulge
- The lid to lift or leak
- Water to back up through the inlet
- Fittings or seals to leak
- Extra stress on the tank structure
When the tank is emptying
Water leaving the tank creates more air space. If outside air cannot enter fast enough, a partial vacuum can form.
That can cause:
- Reduced outlet flow
- Poor pump performance
- Tank walls pulling inward
- Tank deformation
- Stress on fittings and seams
Thin-walled plastic tanks can be especially sensitive to vacuum conditions.
Do Rainwater Tanks Need Breathers?
Yes, most above-ground rainwater tanks need ventilation.
A typical rainwater tank may receive water quickly during heavy rain and discharge water quickly when a pump starts. Both conditions require air movement.
The vent should normally be near the top of the tank and above the normal maximum water level.
It should also be protected so the opening does not become an easy entrance for mosquitoes, insects, leaves, rodents, or other contamination.
In many systems, an overflow arrangement may provide some airflow. However, you should not assume an overflow automatically provides enough ventilation. Its design, pipe routing, screens, traps, and flow restrictions all matter.
Can the Overflow Act as the Tank Vent?
Sometimes, but not always.
A large, open overflow connection may allow air to move as the tank fills and empties. In other systems, the overflow may be too restricted to serve as the only vent.
Problems can occur when the overflow has:
- Fine screens that become clogged
- Long pipe runs
- Water traps
- Check valves
- Undersized fittings
- Screens covered with insects or debris
- Pipe ends that become submerged
A dedicated breather can provide more predictable airflow.
Check the tank manufacturer's installation instructions before relying on the overflow as the only vent.
How Large Should a Tank Breather Be?
There is no single vent size that works for every tank.
Vent capacity should match the fastest rate at which water can enter or leave the tank.
For example, a small rain barrel connected to a garden hose moves water much more slowly than a large cistern supplied by several roof downspouts or emptied by a high-flow pump.
Important factors include:
- Maximum inlet flow
- Maximum pump flow
- Gravity outlet flow
- Tank construction
- Vent screen restriction
- Vent pipe length
- Number of fittings or bends
The vent should not become the narrowest path in a system that moves large amounts of water.
For larger tanks or pump systems, follow the tank manufacturer's vent requirements rather than choosing a vent size by guesswork.
Should a Tank Breather Have a Screen?
For outdoor rainwater tanks, a screened vent is usually a good idea.
The screen helps keep unwanted material and pests from entering the tank while still allowing air to pass.
The screen must remain open enough to provide adequate airflow. Very fine mesh collects dust, spider webs, insects, and debris more easily.
A blocked screen can turn a properly vented tank into a poorly vented one.
Inspect the vent during normal tank maintenance and clean it when needed.
Where Should the Breather Be Installed?
The vent normally belongs near the highest part of the tank's air space.
It should remain above the normal water level so water does not block airflow.
A good location should also reduce the chance that:
- Rain enters directly through the opening
- Overflowing water covers the vent
- Leaves collect against the screen
- Insects can bypass the screen
- The fitting is easily damaged
Follow the manufacturer's approved vent locations whenever the tank has molded vent ports or specific installation requirements.
Avoid drilling new openings in a tank unless the manufacturer allows it. Poorly placed holes can weaken the tank, create leaks, or affect warranty coverage.
Do IBC Totes Need Breathers?
An IBC tote used as a non-pressurized water storage container also needs air to replace the volume of water entering or leaving.
The original top closure should not simply be treated as a pressure-tight lid while water is being rapidly pumped out.
Some IBC caps and lids include venting features, while others do not. IBC containers also vary widely in their original intended use.
Understanding the effects when you don’t have a vent pipe helps evaluate this decision within the complete collection path.
If you are repurposing an IBC tote for rainwater, confirm that the container is suitable for that use and that its venting arrangement can handle your expected inlet and outlet flow.
Do not use a container with an unknown history for drinking-water storage.
Do Small Rain Barrels Need Vents?
Yes, although the airflow requirement is usually modest.
Many rain barrels are naturally vented through their lid, overflow fitting, inlet screen, or downspout connection.
You may notice poor ventilation if:
- Water drains unusually slowly
- The barrel sides pull inward
- Air repeatedly gulps through the outlet
- The lid moves when water enters
- A pump loses flow even though water remains in the barrel
If you modify a barrel to make the lid more tightly sealed, make sure you do not accidentally remove its only air path.
What About Underground Cisterns?
Underground cisterns also need proper ventilation, but their installation requires more care.
Vent routing may need to account for:
- Surface water
- Soil and insects
- Flooding
- Pipe termination location
- Tank loading
- Access for inspection
- Local plumbing or building requirements
Large underground tanks can also involve excavation and confined-space hazards.
Never enter a cistern or underground tank unless it has been designed for safe entry and proper confined-space procedures are being followed. Tank cleaning or repair that requires entry should generally be handled by qualified professionals.
Do Pressurized Water Tanks Need Breathers?
Pressurized tanks are different.
A well pressure tank, expansion tank, hydropneumatic tank, or similar pressure vessel is designed to operate under pressure. It should not be modified by adding an atmospheric vent unless the manufacturer specifically requires one.
The breather advice in this article applies mainly to atmospheric storage tanks such as:
- Rainwater tanks
- Rain barrels
- IBC totes
- Non-pressurized cisterns
- Gravity storage tanks
Always identify what type of tank you have before changing its venting.
Breathers and Water Quality
A vent is necessary for airflow, but it is also a potential path into the tank.
An unprotected opening can allow:
- Mosquitoes
- Flies
- Dust
- Leaves
- Small animals
- Windblown debris
Outdoor vents should therefore balance airflow with contamination control.
If the stored rainwater will be used for drinking, the vent is only one part of a much larger water-quality system. Roof runoff is not automatically potable, meaning safe to drink.
Drinking-water systems require suitable collection surfaces, debris control, appropriate treatment stages, maintenance, and current laboratory testing. Local requirements may also apply.
Keep the Breather Clear
Tank vents are easy to forget because they have no moving parts.
Include them in routine tank inspections.
Check for:
- Spider webs
- Dead insects
- Leaves
- Dust buildup
- Algae or biological growth
- Damaged screens
- Cracked fittings
- Nesting insects
- Ice in freezing weather
A vent that looked large enough when installed may become restrictive after months of debris buildup.
Breathers in Freezing Climates
Cold weather adds another concern.
Moist air moving through a vent can sometimes form condensation or ice. Snow and freezing rain can also block poorly positioned openings.
Do not seal the tank for winter just to keep cold air out. A tank that is still being filled or emptied needs a working air path.
Winter preparation may involve draining parts of the system, protecting fittings, or changing how the tank is used. The correct approach depends on the tank, plumbing layout, and climate.
Signs Your Tank May Not Be Venting Properly
Check the tank vent if you notice:
- Tank walls bowing inward during pumping
- Tank walls bulging during filling
- Slow outlet flow
- A pump that struggles despite adequate water
- Gurgling or repeated air noises
- A lid lifting during heavy rain
- Water backing up through the inlet
- Fittings leaking only during filling
- The tank changing shape as the water level changes
Stop using the system if the tank is visibly deforming. Find and correct the airflow restriction before continuing to fill or empty it.
Permanent deformation, cracked fittings, or structural damage may require inspection or replacement.
A Simple Rule for Tank Venting
For a normal non-pressurized water tank, air must be able to move about as quickly as water moves.
That means the vent should be:
- Large enough for the expected flow
- Located above the normal water level
- Protected from insects and debris
- Kept clear during maintenance
- Installed according to the tank manufacturer's instructions
Do not intentionally seal an atmospheric rainwater tank unless the system was specifically engineered to operate that way.
Frequently Asked Questions
Does every water tank need a breather?
Most non-pressurized storage tanks need some form of ventilation. Pressurized vessels are different and should only be vented according to their manufacturer's design.
Can I leave the tank lid slightly open instead?
That may allow air movement, but it can also allow insects, debris, and small animals into the tank. A properly designed screened vent is usually a better solution for outdoor rainwater storage.
Can a blocked tank vent damage a plastic tank?
Yes. Restricted airflow can cause pressure or vacuum conditions that may deform some plastic tanks or place extra stress on fittings and seams.
Does a rain barrel need a separate vent?
Not always. Its inlet, lid, or overflow may already provide enough airflow. Make sure at least one air path remains open when modifying the barrel.
Can I use the overflow pipe as the vent?
Possibly. It depends on the overflow size, routing, screen, and whether the pipe can move enough air. Check the tank manufacturer's requirements before relying on it alone.
Should a tank vent be screened?
Outdoor rainwater tank vents generally benefit from insect and debris protection. The screen must also be inspected because clogging can reduce airflow.
Why does my tank collapse inward when the pump runs?
A restricted or blocked vent is one possible cause. The pump may be removing water faster than replacement air can enter. Stop pumping if the tank is deforming and inspect the vent and tank before continuing.
Can I completely seal a rainwater tank to keep contamination out?
A normal atmospheric tank still needs ventilation as the water level changes. Instead of sealing the air path, use a protected vent designed to allow airflow while limiting entry of insects and debris.
