The hot water valve started dripping in the heatwave. That is usually the valve working

Every summer, after a stretch of genuinely hot days, people find water dripping from a pipe near the hot water unit and conclude the system is failing. It is one of the most common reasons we get called out here, and a good proportion of the time nothing is broken at all.

This district does heat differently from the rest of Sydney. Penrith reached 48.9 degrees on 4 January 2020, the highest temperature ever recorded in an Australian metropolitan area, and university research recorded around 52 degrees a few kilometres north-east of the CBD on the same day. Equipment designed for average conditions behaves differently under that, and the relief valve is where you notice it first.

The short answer

The temperature and pressure relief valve on a storage hot water system exists to release water when pressure inside the tank rises. Water expands as it heats, and after a run of extremely hot days there is more expansion to release. A short discharge during or just after a heatwave is often the valve doing exactly what it was fitted to do. A valve that discharges constantly, or that has never stopped, is a different matter and needs a licensed plumber.

1. Note whether it drips occasionally or runs continuously
2. Note whether it started during a heatwave or independently of one
3. Never cap, plug or restrain the discharge pipe, for any reason
4. Ring if it runs continuously, or if the water is discoloured or very hot
5. Have the valve checked periodically rather than only when it acts up

On this page

1. What the valve is for
2. Why heat here matters
3. When to leave it
4. When to ring
5. The related heat problems
6. Getting ahead of summer

1. What that valve is actually for

A storage hot water system holds a fixed volume of water and heats it. Water expands when it is heated, and a sealed vessel with expanding contents inside it develops pressure. That is not a defect in the design, it is physics, and every storage system is built with a way to release it.

The temperature and pressure relief valve is that way. It opens when pressure or temperature inside the tank reaches the point it is set for, releases a small amount of water down a discharge pipe, and closes again. It is a safety device, and it is the last line of defence on a vessel full of very hot water.

So water coming out of that pipe is, in itself, the system behaving correctly. What matters is how much, how often, and whether it stops.

2. Why a heatwave here changes the picture

More heat in means more expansion, and more expansion means more to release. After a run of days in the forties the system is working harder and the valve has more work to do, so a discharge people have never noticed before becomes noticeable.

The other thing extreme heat does here is to the space the unit sits in. Roof spaces and unshaded exteriors get far hotter than the outside air, and equipment installed up there is living at temperatures the general design assumptions do not describe. That affects the upper element on an electric storage system and it shortens anode life, which is a slower and less visible problem than a dripping valve but a more expensive one.

Research from Western Sydney University found communities out here experience extreme heat far more often than had previously been understood. That is the useful framing: this is not one freak day, it is a recurring operating condition, and equipment ages accordingly.

3. When it is fine to leave alone

• A small discharge during or shortly after a genuinely hot stretch, which then stops.
• A drip that appears when the system is heating and stops when it has finished.
• Occasional discharge that has happened for years in summer and has never progressed.

In those cases the valve is doing its job. It is still worth mentioning to a plumber the next time one is on site, because a valve that operates regularly is a valve that is wearing, and they do not last forever.

4. When it is worth ringing about

1. It runs continuously. A valve that will not close is either faulty or is telling you the system pressure is too high, and both need attention.
2. It started and has not stopped. The change from occasional to constant is the meaningful signal.
3. The water is very hot or steaming. That is a temperature discharge rather than a pressure one and it should be looked at promptly.
4. The water is rusty or discoloured. That is about the tank rather than the valve.
5. There is no discharge pipe, or it goes somewhere unsafe. Worth having corrected regardless of whether anything is currently dripping.
6. The valve is very old and has never been touched. A relief valve is a consumable component, not a permanent fitting.

5. The other things heat does that nobody connects

A dripping relief valve is the visible one. Three others turn up here every summer and rarely get attributed to the weather.

• Elements and anodes ageing faster. A tank in a hot roof space or on an unshaded wall is being worked harder than the general service-life figures assume.
• Long plastic runs moving. PEX and poly expand and contract with temperature, and on a long run in a hot space that cycling is real movement, repeated every day of a heatwave.
• Pressure dropping across a whole street. When everyone is running evaporative cooling and irrigation at once, the pressure at your tap is not only about your property. A pressure complaint that appears only on the hottest afternoons is worth describing that way.
• Air conditioner condensate lines. A split system produces water, that water has to go somewhere, and a blocked or badly run condensate line is a summer problem people attribute to a roof leak.

None of those are emergencies. They are worth knowing because they explain a cluster of "sudden" problems that always seem to arrive in the same fortnight.

6. Getting ahead of it

1. Have the relief valve and the hot water system checked before summer rather than during it.
2. Find out where your unit actually sits and how exposed it is. Position matters more here than the general advice suggests.
3. Ask about the anode when anybody is servicing the system, because heat shortens its life and replacing one is far cheaper than replacing a tank.
4. Check the split system condensate line drains freely before the first hot week.
5. Note whether your pressure complaint is time-of-day and weather dependent, because that genuinely changes the diagnosis.

The theme is the same as everything else here: equipment in this district is not operating in average conditions, so the average maintenance advice is a floor rather than a plan.

The short version

• The relief valve exists to release pressure as water expands, so some discharge is correct.
• Penrith reached 48.9 degrees in January 2020, the highest ever in an Australian metro area.
• More heat means more expansion, so heatwave discharge is often the valve working.
• Never cap, plug or restrain the discharge pipe, for any reason at all.
• Continuous discharge, very hot water or discoloured water are all worth a call.
• Heat also ages elements and anodes faster, and moves long plastic runs every day of a heatwave.