Hot water system leaking? Where the water is coming from and what it means

Is a leaking hot water system an emergency? It depends on where the water is and how much

A leaking hot water system is an emergency when water is spraying, running steadily, coming from the tank body or getting into the house, or when you can smell gas. A slow drip from the small pipe that runs down from the relief valve is usually the system doing its job. Rheem says it is normal for the relief valve or expansion control valve to let water out during heating, roughly 1/50 of the hot water used, because water expands when it is heated (Rheem troubleshooting).

Use a simple rule of thumb. A few drips while the unit is heating: watch it. A steady trickle that never stops, or more than a bucket full in 24 hours: Rheem and Rinnai say that may point to another problem, so book a plumber today. Water pooling under the tank or water inside the house: switch off and isolate now. Gas smell: follow the gas steps below before anything else.

If you are in a rental, the Residential Tenancies Authority lists a breakdown of an appliance for hot water as an emergency repair (RTA emergency repairs). Tell your property manager straight away.

First 10 minutes: make the area safe before you look closely

The first job with a leaking hot water system is to stop the heating, stop the water and keep people clear. None of these steps touch a valve a plumber should handle.

1. Electric or heat pump unit: switch it off at the switchboard.
2. Gas unit: turn the unit off at its own control. If you smell gas, stop and go to the gas steps below.
3. Close the cold water isolation valve on the pipe going into the unit. This stops more water feeding the leak.
4. Keep children and pets clear. Water from the relief valve and the tank can be hot enough to scald.
5. Move boxes and rugs out of the way of the water.
6. Take photos of where the water is coming from. They help the plumber bring the right parts.

If you smell gas, the Queensland Government says not to light anything or use light switches, to turn off the gas appliance and open doors and windows, and to call 000 if you can still smell gas after turning off the supply (Queensland gas safety). Your gas supplier's emergency number is on your gas bill.

Find where the water is coming from: a leak location table

Where the water comes from tells you most of what you need to know about a leaking hot water system. Follow the wet patch back to its highest point and match it to a row below.

If you cannot tell where the water starts, leave it switched off and isolated and let a plumber trace it. Our guide on finding a hidden water leak covers leaks that are not near the unit.

Relief valve dripping or running: a few drips are normal, constant flow is not

The temperature and pressure relief valve sits near the top of a storage hot water system, with a copper drain line running down to the ground. The relief valve lets out a little water while the tank heats, because heated water expands. Rinnai's owner manual says it is normal for the valve to release a small quantity of water during heating, but that continuous leakage from the valve and its drain line may indicate a problem (Rinnai owner manual).

Watch the drain line over a full day. Drips that start when the unit heats and stop afterwards are expected. A line that runs all day and night, or fills more than a bucket in 24 hours, needs a plumber. Dux lists excessive water supply pressure, a faulty relief valve or a faulty thermostat as possible causes of continuous leakage, and says to turn the heater off and call for service (Dux owner manual).

Never block, cap or tie off a relief valve drain line to stop the water. The relief valve is a safety device, and Rinnai warns never to block its outlet or drain line for any reason.

Water on the cold side usually means the expansion control valve or high mains pressure

Water dripping from a valve on the cold water pipe going into the unit usually comes from an expansion control valve. Rinnai's manual says an expansion control valve is fitted on the cold water line in areas with scaling water, and that this valve normally lets a small quantity of water out during heating. That protects the relief valve from scale. Not every hot water system has an expansion control valve, so yours may only have a relief valve.

A relief valve that discharges at high flows, especially overnight, can point to water pressure above what the heater is designed for. Rinnai says high flows at night may be caused by water pressure exceeding the heater's design pressure, and to ask the installer to fit a pressure limiting valve. Fitting or replacing a pressure limiting valve is work for a licensed plumber.

Low pressure is a different problem with different causes. If your taps are weak rather than your valves running, see why water pressure drops.

Leaking from the top usually means fittings, connections or valves

Water on top of a hot water system usually comes from a fitting, a pipe joint or a valve body rather than the tank. Look for drips at the hot outlet, the cold inlet, the relief valve thread, a tempering valve on the hot line, or any union nut.

Water from the top can run down the outside of the case and puddle at the base, which makes a small fitting leak look like a failed tank. Dry the case with a towel, wait a few minutes and see where the first bead of water appears. A photo of that spot is useful for your plumber.

Do not tighten, undo or replace fittings or valves yourself. Work on hot water fittings is regulated plumbing work for a licensed plumber. If a fitting is spraying, close the cold isolation valve, switch the unit off and call.

Leaking from the bottom or tank base can mean the tank has failed

Water coming from the bottom of a storage hot water system is the leak to take most seriously, because it can mean the inner tank has started to leak. Water seeping from under the outer case when there are no drips from the fittings or valves above can point to the tank.

Before you assume the tank has gone, check the drain lines. A relief valve or expansion control valve drain that ends near the base of the unit can look like a tank leak. Heat pumps also have their own condensate drain line, which can end in the same place as the relief valve drain. If all the drain lines end somewhere else and water still appears under the tank, switch the unit off, close the cold isolation valve and call a licensed plumber.

Do not try to drain the tank yourself. Rinnai's manual says draining or filling the system must be carried out by an authorised person, and a hot tank holds scalding water.

Heat pump, solar and gas systems each have their own leak points

Each type of hot water system leaks in its own way. Heat pumps cool the air around them, and Rinnai's manual says the condensate that forms needs to be plumbed to a suitable drain through its own condensate line, separate from the relief valve drain. Clear water from that condensate line is usually condensate, not a leak from the tank.

Solar hot water systems have pipes running to the collectors on the roof as well as a tank on the ground or roof. Water dripping from a ceiling, eave or roof line near those pipes needs a plumber to inspect, and you should not climb onto the roof to look.

Gas storage and continuous flow units leak water like any other system, but anything to do with the burner, gas line or gas connections is gas work. The Queensland Government says only a licensed gasfitter can carry out gas work. Switch the gas unit off at its control, isolate the water and leave the rest to a licensed plumber and gasfitter. For more on how gas and electric systems compare, see continuous flow versus storage.

What a homeowner can and cannot do under Queensland plumbing law

Queensland law keeps most hot water work for licensed trades. The QBCC says most plumbing work in Queensland is regulated and needs a QBCC licensed plumber or drainer (QBCC unregulated plumbing guide). The Queensland Government lists installing or replacing hot water heaters, including electric, solar and heat pump units, as notifiable work done by a licensed plumber (Queensland plumbing laws).

The work an owner may do is a short list, such as changing a shower head or a tap washer. Hot water valves, fittings and tanks are not on it. You can look, listen, take photos, read the water meter, switch the unit off and close the isolation valve. The QBCC homeowner guide also says owners and tenants can operate the relief valve lever themselves, with care to avoid scalding and to close the valve properly afterwards (QBCC Plumbing Matters guide).

Repair or replace depends on where the leak is, the age of the unit and the tank's condition

Whether a leaking hot water system is repaired or replaced comes down to what is leaking. A relief valve, expansion control valve or fitting is a part a plumber can replace. Rinnai says relief and expansion control valves should be replaced by an authorised person at intervals not exceeding 5 years, sooner where the water is scaling. Water coming through the tank wall is not fixed by a new valve, so the conversation turns to a new unit.

• Where the water is coming from: a valve or fitting, or the tank itself
• The age of the unit and its service history
• The condition of the tank, case and connections
• Whether parts are still available for that model
• The system type, and whether it still suits your household

If replacement is on the table, our guide to choosing a hot water system walks through the options. Jim Monger Plumbing and Gas repairs and replaces gas, electric and heat pump units, and fits isolation and pressure-limiting valves (QBCC licence 15504687, gas licence L90908). See our hot water repairs and replacements page, or hot water service in Victoria Point, or call 0418 871 269.