When hot water is drawn from the hot tap it appears to be milky. After a few minutes it clears. What causes this?

Milky water can at times be a common complaint. What causes milky water and why does it occur?
All Water supplies contain varying amounts of dissolved air. The air dissolves in the water when it is stored in open dams and reservoirs.

Air is more soluble in cold water than it is in hot water. Ordinary tap water contains more dissolved air when cold than it can contain when hot. You may have noticed how bubbles of air form on the inside of a saucepan of water when it is heated. This is because as the water becomes hotter, it cannot contain as much dissolved air, so the air precipitates out of the water.

Also, water under pressure will contain more dissolved air than at atmospheric pressure. This is similar to how a bottle of soda water stores gas until the bottle is opened and the gas is released. Air dissolved in cold water entering the water heater remains dissolved when the water heats, because it is under pressure. As soon as the pressure is reduced, by opening a tap, air, in millions of tiny bubbles, is released with the water. Therefore, the water has a milky appearance because of the presence of all these bubbles. This milky appearance is harmless and quickly disappears as the air bubbles rise to the surface.

If soap is used while the water is still milky in appearance, the air bubbles become trapped in the lather. This may lead to complaints the water is hard. If soap is not used until the water clears of the air bubbles, then the “hard water” effect does not take place.

The Milky water appearance often occurs when pipe services are new and clean or in areas close to a reservoir or dam. This appearance tends to disappear in old pipe services where the oxygen in the dissolved air is consumed by pipe corrosion.

Noise is emanating from the hot water system or pipe work. What is causing this?

There can be a number of causes of the noise and these should be investigated.

  • A “thumping” noise may be experienced whenever a hot tap is turned off suddenly. This only occurs if a non-return valve is installed. The sudden stopping of the water flow at the hot tap causes a small “energy wave” to travel back down the water pipe. This “energy wave” generally diminishes but if a non-return valve is installed, the energy comes to an abrupt halt causing a “thump” to be heard. This is a form of water hammer.
  • A continuing light vibration or whining noise may be heard if there is a defective tap washer in the tap housing. Replace the tap washer.
  • A continual tapping or knocking noise may be heard when a tap is running. This is water hammer. It occurs when long runs of hot water pipe are not adequately fastened inside the wall, ceiling or floor cavities, and results from the pipe vibrating against a wall, ceiling or floor cavity as the water passes through the pipe causing it to resonate. This is overcome by securing the hot water pipe, to prevent movement.
  • If an electric water heater is installed and noise is evident only during the heat-up cycle, check for mineral build up on the heating element and check the quality of the water supply for sludge. Clean the heating element or drain sludge from the water heater if required.
  • If a noise is evident only when water is flowing through the water heater, and if it’s not water hammer, then check for restrictions in the pipe work or for faulty valves. It may be necessary to install a pressure limiting valve if the water pressure is excessive (above 700kPa).

A Rheem storage water heater is essentially a vessel filled with water with no moving parts. Therefore, it is unlikely any noise in the hot or cold water system can be attributed to the function and performance of the water heater.

The valve on the side of the water heater (temperature and pressure relief valve) is dripping water. Is the valve damaged or is there another problem?

A Temperature and Pressure Relief Valve is supplied with each Rheem water heater. Its function is to provide both thermal and pressure relief when required. This will be in the form of discharging water from the valve.

There are four types of flow which could be observed from the T&PR Valve.


  • Normal Operation


When water is heated through a 50°C temperature rise, its volume will expand by approximately 2%. This expansion is discharged through the T&PR valve. It is normal operation for the T&PR valve to discharge or dribble water during the heating cycle. The T&PR valve will discharge water equivalent to 2% in volume of the total hot water used in one day. Therefore, a drain line must be connected to the T&PR valve to take discharge clear of the heater to a suitable termination point. This first type of discharge occurs only when a non-return or check valve is installed on the cold water supply line after the stop cock. It is normal and desirable that this valve allows a small quantity of water to escape during the heating cycle. However, if it discharges more than a bucket full of water in 24 hours, there may be another problem.

  • Continuous dribble


If there is dirt or grit present under the seat of the valve, it may prevent the valve seat from closing properly. Therefore, a continuous dribble may be experienced. This dirt or grit may be dislodged by operating the easing lever and allowing a full flow of water to wash over the valve seat.

  • Steady flows for long period (often at night)


It may be possible for the incoming cold water pressure to be in excess of the pressure rating of the T&PR Valve. This will result in a continuous flow of water from the T&PR Valve without cycling. Ask your installing plumber to fit a pressure limiting valve. NEVER replace the relief valve with one of a higher pressure rating.

  • Heavy flows of hot water until water heater is cold – then stops until water reheats


The T&PR Valve also operates as a temperature activated safety device. If by chance both the thermostat and over temperature energy cut-out fail on a water heater, then the electrical element or gas burner would operate continuously. Therefore, when the water temperature reaches 98-99°C, the T&PR Valve will open allowing a flow of water in quantity discharging the full capacity of the water heater then stopping, only to repeat this at intervals. This thermal relief is provided under high temperature conditions by expansion of the polythene rod inside the temperature probe, which causes the stainless steel push rod to lift the valve off its seat. The valve stays open until cooler water surrounds the temperature probe, allowing the polythene rod to contract.

If this thermal relief does occur, the water heater must be switched off at the switchboard if it is an electric water heater. If it is a gas water heater, the gas control must be turned off using the knob on top of the gas control thermostat. Phone your SA HOT WATERTM plumbing services agent to arrange for inspection. (08) 8444 7310

Note: It is very important the T&PR Valve is connected to a drain line to allow the discharge to terminate at a location where the water will not cause damage. The drain line should be as short as possible (maximum 9 metres), have no restrictions, maximum three bends and have a continuous fall to the outlet. The drain line should terminate in a drained position so any discharge can be readily seen.

Remember: If a Temperature and Pressure Relief Valve requires replacement, then the correct valve must be installed. Never install a T&PR Valve with a pressure rating higher than the rated working pressure of the water heater.