Common Problems with Instant Gas Hot Water Systems

We are so used to lots of hot water that when there’s none left, it can be incredibly frustrating.

One minute you are basking in the warmth of your hot water and the next minute ice cold water wakes you up!

Not fun at all. If your instant gas hot water system is giving you nightmares, we can help. Our gas plumbers have created a guide to help you understand the reasons for your hot water troubles.

Need professional help? Contact our expert Sydney gas fitters on 02 9191 7374 for gas hot water repairs.

Note: DIY plumbing and gas fitting can void the warranty on your continuous flow hot water system. It can also void your insurance in the case of something going wrong. So when there's a plumbing issue, call the licenced professionals.

How does a continuous flow hot water system work?

When the hot water tap is turned on, it triggers the flow sensor and the burner starts heating the water. Cold water flows through a series of coiled pipes, called the heat exchanger, and is heated. When the tap is turned off the hot water system shuts down. Only a certain amount of water is heated and the hot water is not stored making these systems very energy efficient. In a storage style system, the water is heated by a heating element. As the water is not stored in a continuous flow system, there is no heat loss from the unit.

#1 - Water heater shuts down during use

Hard water can leave mineral deposits on the heat exchanger and if you haven’t had your system flushed recently it can cause your instant hot water system to shut down and also cause water leaks. It is important to get the unit flushed out regularly to remove the mineral buildup.

However, there can be other reasons that can cause your water heater to turn off unexpectedly including:

• Dirty burner
• Faulty ignitor
• Improper installation
• Loose electrical connections
• Dirty cold water filter
• Faulty flow sensor
• System overload

#2 - No hot water

Tankless systems do not store hot water and may take time to heat up the water.

If you are using multiple hot water outlets at the same time, it could overload the system. In that case, you may not be getting any hot water. Limiting the simultaneous use of hot water applications would resolve the problem.

Running multiple appliances can also cause your system to stop working totally. In that case, resetting the system might work.

'No hot water' could be due to other issues like:

• Mineral buildup in the heat exchanger due to hard water - routine flushing of a water heater is required to remove the mineral buildup and improve the functioning and efficiency of the water heater
• Blocked gas vent
• Faulty ignitor
• Temperature is set too low
• No gas supply - Check your mains gas supply or gas bottle to see if there’s enough gas to power your water heater. If there’s a problem with your gas supply, contact your gas provider or gas fitter.

To resolve the problem:

• Limit the use of hot water to one appliance/tap at a time
• If your system is not able to meet your hot water demands, you could consider upgrading your unit to a storage hot water system, a larger instant unit or more than one instant units.
• Check the temperature on the thermostat.
• Make sure your water heater is connected to an electrical supply (if model requires)

#3 - Hot water, then cold, and then hot

This intermittent hot water supply is also called a "cold water sandwich.”

The heat exchanger or the component that heats the water takes time to work its magic so you may get some cold water at the start and then hot water. It can also happen when the shower is used back-to-back.

You may get bursts of hot, cold and hot water again some water may be stored in your tap or shower and that’s why you might get hot water and then a burst of cold water. As the water gets heated you should start getting a steady supply of hot water. The hot and cold water bursts can be inconvenient when you are taking a shower, one way to avoid this is by limiting the back-to-back use of shower.

How to prevent a cold water sandwich?

• If you have a water-saving or flow restrictor shower head, the flow sensor might not be able to register the hot water tap. Fortunately it can be easily fixed by removing the flow restrictor device.
• This problem is not so common in gas hot water heaters but if you are facing this problem it could be due to insufficient gas supply.
• A faulty diaphragm can cause the intermittent supply of hot water as well and replacing it will fix the problem.
• You can consider installing a storage tank heater. The storage tank will have some hot water at all times and will eliminate the cold water sandwich effect.
• If you have an older hot water system, this problem could be more pronounced. You could upgrade your HWS to a storage HWS or a more efficient instantaneous gas HWS.

#4 - Lukewarm water

Another common problem is lukewarm water coming from the taps instead of hot water. This issue could be due to a faulty thermostat or diaphragm. Replacing the thermostat/diaphragm would fix this problem.

#5 - Low water pressure

Continuous flow HWS supply hot water at a lower pressure than storage HWS. If you are experiencing a lower hot water pressure than usual, it could be due to:

• Blocked strainer valve on the cold inlet.
• A broken or faulty thermostat.

#6 - Pilot light won’t turn on

If the pilot light isn’t turning on your system, check the:

• Gas supply from the mains - Check if the gas valves are fully open.
• Check if the other gas appliances are turning on - if the other gas appliances are working but not your water heater, there could be an issue with the ignition switch.
• Gas tank - Make sure your gas tank has enough supply.

In some models of instantaneous HWS, the pilot light starts working on running the tap. Try running your hot water tap for a few minutes and see if the pilot light turns on.

Troubleshooting Guide for Hot Water Brands

Need help with your Rinnai, Rheem or Vulcan hot water system? Check out these resources before you call our plumbers.

• Rheem Hot Water System Troubleshooting
• Rinnai Hot Water System Troubleshooting
• Vulcan Hot Water System Troubleshooting

If you are experiencing any of the above issues, talk to our expert gas fitters. We are available 24/7 and can help resolve your problem ASAP. Give us a call on 02 9191 7374 or complete our online enquiry form and we will be happy to help.