If you have noticed water coming from your hot water system, it is natural to wonder whether something is wrong.
Maybe there is a small drip from a pipe near the unit. Maybe water is running onto the ground. Maybe the system seems to release water at certain times of the day. Or maybe there is a constant flow that does not look normal.
The confusing part is that some water discharge from a hot water system can be normal.
But not all of it.
A hot water system is designed to heat and store water safely. As water heats, it expands. That pressure needs somewhere to go. This is why hot water systems are fitted with safety valves that allow water to discharge under certain conditions.
The key question is this:
Is your hot water system doing what it is designed to do, or is it showing signs of a fault?
Here is what homeowners should know.
Why hot water systems release water
Storage hot water systems hold water inside a tank and heat it for later use.
When water is heated, it expands. This expansion increases pressure inside the system. To manage this safely, hot water systems use valves that release water when needed.
The two common valves homeowners may hear about are:
- Temperature pressure relief valve
- Expansion control valve
Many people simply refer to the drain line from these valves as the “overflow pipe”.
That wording is common, but it is not always technically precise. The water is not overflowing in the same way a sink overflows. It is usually being discharged through a safety valve.
A small amount of water from this area during heating may be normal.
A constant leak, heavy discharge or water running all day is different.
What does the relief valve do?
The temperature pressure relief valve is a safety device.
Its job is to help protect the hot water system from excessive temperature or pressure.
If pressure inside the tank rises too much, the valve can release water through the drain line. This helps prevent unsafe pressure building inside the system.
That is why you should never block, cap or plug the discharge pipe.
If water is coming from the relief valve, the answer is not to stop the water from escaping. The answer is to work out why it is discharging.
When a small amount of water can be normal
A small amount of water coming from the relief valve or expansion control valve during the heating cycle can be normal.
This can happen because water expands as it heats.
For many homeowners, this may look like occasional dripping or a small amount of water from the drain line.
This is usually most noticeable after the system has been heating, especially with storage hot water systems.
If it is only a minor amount and only happens during heating, it may simply be the system releasing pressure as designed.
When water from the hot water system is not normal
There are times when water discharge should not be ignored.
You should arrange a plumber to inspect the system if:
- Water is constantly running from the drain line
- The discharge continues when the system is not heating
- There is more than a small amount of water each day
- Water is pooling around the hot water system
- The valve is dripping continuously
- The system is losing hot water faster than usual
- The water bill has increased unexpectedly
- The ground around the unit is always wet
- The hot water system is making unusual noises
- The water is too hot, not hot enough or inconsistent
- There are signs of corrosion around the tank or fittings
A leak from the tank itself is also different to discharge from a valve.
If the tank is leaking, the system may need urgent attention.
Common causes of hot water system leaks or discharge
Water around a hot water system can come from several places.
Normal expansion during heating
A small amount of discharge during heating may be normal.
This is the system releasing pressure as water expands.
Faulty relief valve
A relief valve can wear out, fail to seal properly or become affected by debris.
If the valve does not close correctly, it may continue dripping or running.
Excessive water pressure
High incoming water pressure can place extra stress on the hot water system and its valves.
If pressure is too high, the relief valve or expansion control valve may discharge more often.
Faulty expansion control valve
Some systems have an expansion control valve fitted on the cold water supply side.
If this valve fails or is affected by pressure, it may discharge more than expected.
Temperature problems
If the system is overheating, pressure can increase and the valve may release water.
Hot water that is suddenly too hot should be checked.
Sediment or debris
Sediment, scale or debris can affect valves and the way they seal.
This can lead to dripping or irregular discharge.
Tank corrosion
If water is coming from the tank body, seams or base of the unit, the issue may be corrosion or internal tank failure.
This is more serious than normal valve discharge.
Loose or leaking fittings
Sometimes the issue is not the relief valve or the tank.
Leaks can also occur from fittings, pipe connections, unions, valves or flexible connectors around the system.
How to tell where the water is coming from
Before assuming the whole hot water system has failed, look carefully at where the water appears to be coming from.
Do not touch anything hot or unsafe.
Instead, visually check:
- Is water coming from a small drain line?
- Is water coming from the valve area?
- Is water leaking from the pipework?
- Is water pooling under the base of the tank?
- Is there corrosion around the cylinder?
- Is the discharge constant or occasional?
- Does it happen only after heating?
- Is the surrounding ground wet all the time?
This information helps a plumber diagnose the issue faster.
A photo or short video can also be useful, especially if the discharge happens intermittently.
Why you should not ignore a leaking hot water system
Even if the leak looks small, it can still matter.
A leaking hot water system can waste water, increase energy use, damage surrounding areas and indicate a fault with a valve, pressure issue or ageing unit.
If the system is installed near the house, in a garage, in a cupboard, beside a wall or near timber framing, small leaks can eventually cause damage.
Common problems caused by ignored leaks include:
- Wet concrete or paving
- Damp walls
- Cabinetry damage
- Rust and corrosion
- Mould or musty smells
- Higher water bills
- Reduced hot water performance
- Emergency replacement if the system fails suddenly
A small discharge issue is usually easier to deal with early than a failed hot water system later.
Why temperature matters
Hot water systems need to balance two safety issues.
The water needs to be hot enough inside the storage system to reduce bacterial risk. But water delivered to showers, baths and basins also needs to be limited to reduce the risk of scalding.
This is why storage hot water systems are generally heated to at least 60°C, while temperature control devices are used to limit the water delivered to personal hygiene fixtures.
If your water is suddenly too hot, too cold or fluctuating, the issue may involve the thermostat, tempering valve, heating component, gas control, booster, mixing valve or another part of the system.
It is not always as simple as turning the temperature up or down.
Is the hot water system actually leaking, or is it a valve issue?
This is an important distinction.
If water is coming from the relief valve drain line, the tank itself may not be leaking.
The issue may be:
- Normal expansion
- Valve wear
- High pressure
- Overheating
- Faulty valve operation
- Expansion control issue
f water is coming from the tank body, base or rusted areas, the system may be failing.
That is why it is worth having the source checked before assuming the unit needs replacement.
Sometimes a valve can be replaced.
Sometimes pressure needs to be corrected.
Sometimes the system is at the end of its life.
The right answer depends on where the water is coming from and why.
Should you operate the relief valve yourself?
Some hot water system manufacturers provide maintenance instructions for operating relief valves at set intervals.
However, if you are unsure, the valve is old, the system is leaking heavily, or you are not confident, do not force anything.
Old valves can sometimes fail to reseal after being disturbed.
If you are not sure what you are doing, it is safer to have a licensed plumber inspect it.
What to do if water is constantly running
If water is constantly running from your hot water system, treat it as something that needs attention.
You can:
- Check whether the water is coming from the drain line, valve, pipework or tank.
- Take a photo or video of the issue.
- Check whether the water is hot or cold, without touching anything unsafe.
- Note whether it is constant or intermittent.
- Avoid blocking or capping the discharge pipe.
- Contact a licensed plumber for inspection.
If there is a major leak, water is entering the home, or you cannot control the flow, turn off the water supply if you can do so safely and arrange urgent plumbing help.
What a plumber will check
A plumber may inspect:
- The temperature pressure relief valve
- Expansion control valve
- Cold water inlet valve
- Tempering valve
- Incoming water pressure
- Pipework and fittings
- Tank condition
- Signs of corrosion
- Hot water temperature
- Discharge rate
- Age and condition of the unit
- Whether repair or replacement makes more sense
The goal is to work out whether the issue is normal operation, a valve fault, a pressure issue, a temperature issue or a failing hot water system.
When repair may be enough
Repair may be possible if the issue is limited to a valve, fitting or pressure problem.
For example, a plumber may be able to replace a faulty relief valve, address a leaking fitting or check whether pressure control is needed.
This depends on the age, condition and type of system.
If the unit is otherwise in good condition, repair may be the sensible option.
When replacement may be the better option
Replacement may be more practical if the hot water system is old, corroded, leaking from the tank, unreliable or repeatedly needing repair.
A system that leaks from the tank body is often near the end of its life.
If replacement is needed, it can also be a good time to consider whether the same type of system still suits the home.
Household size, hot water usage, energy costs, available space, existing connections and budget all matter.
For some homes, a like-for-like replacement may be the simplest option.
For others, it may be worth considering a heat pump, gas continuous flow, electric storage, solar hot water or another option depending on the property.
The mistake homeowners make
The biggest mistake is assuming all water near the hot water system means the same thing.
It does not.
A small amount of discharge during heating may be normal.
A constantly running relief valve may indicate a valve or pressure problem.
A leak from a fitting may need repair.
A leak from the tank may mean the system is failing.
A temperature issue may involve the thermostat, tempering valve, element, gas control or another component.
The visible symptom is only the starting point.
The source matters.
Need your hot water system checked?
If your hot water system is leaking, constantly discharging water, running out too quickly, making unusual noises or showing signs of corrosion, RJT Plumbing & Gas can inspect the system and identify what is happening.
The issue may be a normal discharge, a valve problem, a pressure issue, a leaking fitting or a sign the system is reaching the end of its life.
The best step is to check the source before guessing.
Find out whether it is normal, repairable or time to replace.
Frequently Asked Questions
Why is my hot water system leaking from the overflow pipe?
Water from the overflow pipe is usually discharge from a relief valve or expansion control valve. A small amount during heating may be normal, but constant running or heavy discharge should be checked by a plumber.
Is it normal for a hot water system to release water?
Yes, a small amount of water can be normal during the heating cycle because water expands as it heats. If the discharge is continuous or more than expected, there may be a valve, pressure or temperature issue.
Should I block the hot water system overflow pipe?
No. Never block, cap or plug the discharge pipe. It is connected to a safety valve and must be able to release water when needed.
Why is water pooling around my hot water system?
Water pooling around the system may come from a valve, pipe fitting, tank leak or discharge line. The source should be checked to determine whether it is normal operation or a fault.
Can a leaking hot water system be repaired?
Sometimes. If the problem is a valve, fitting or pressure issue, repair may be possible. If the tank itself is leaking or badly corroded, replacement may be the better option.
Why is my hot water suddenly too hot?
Suddenly hotter water may indicate a thermostat, tempering valve or control issue. Because hot water can cause scalding injuries, sudden temperature changes should be checked.
When should I call a plumber for a hot water leak?
Call a plumber if water is constantly running, pooling around the system, coming from the tank, damaging nearby areas, or if the hot water system is noisy, corroded, unreliable or producing inconsistent temperatures.
