Instant Gas Hot Water System Problems

How to troubleshoot your continuous gas hot water system

Our troubleshooting guide offers step-by-step solutions to the most common problems with instant gas hot water systems. So whether you’re having trouble getting your system up and running or it’s not providing enough heat, we can help.

How does an instant gas hot water system work?

Before you get started with troubleshooting, it helps to understand how your instantaneous gas hot water system works. Rather than explain it in this article, you can read one of our other articles about how the system works here: How does an instant gas hot water system work?

Now you have a good idea on how the system works, it makes it easier to understand what may be happening when troubleshooting your instant hot water.

Common Problems with Instant Gas Hot Water Systems

Water coming through tap Cold

This would suggest that the heat exchanger isn't working, so we need to work out what is causing this. Below are some common problems to investigate:

No electricity to the unit so the ECU isn't working. Make sure the power point is turned on and it is working (eg. are other appliances off as well). Make sure the controller is turned on (if applicable)
No gas to the unit. Check gas is turned on and it is working on other gas appliances.
The flow rate is too low for the flow sensor to trigger. Have you recently installed "low flow fixtures"? Try running several taps to see if an increase flow fixes it.
The flow rate is too high triggering a safety shut down. If the ECU needs to turn the heat exchanger up too much it can trigger a safety shutdown to prevent overheating. Test with just one tap.
The pilot light isn't on. Watch the pilot light when a hot water tap is turned on to see if it triggers; make sure there is enough of a flow to trigger the flow sensor. If not, and you've checked the above, there could be an issue with the pilot light or ECU. Call a Gas Fitter/Plumber.
Blocked gas vent in heat exchanger. This stops the exchanger from firing. Call a Gas Fitter/Plumber.
Blocked exhaust. Your hot water system needs to bring in oxygen to burn and remove the burnt gases. If the exhaust is blocked your ECU will shut down the system for safety. Call a Gas Fitter/Plumber.

Water isn't very hot

This would suggest that the heater is working, but the heat exchanger isn't working at the level it should be. Check:

Thermostat turned too low. Sometimes the thermostat may be set at a level which seems fine but as the weather gets colder it is no longer high enough.
Your hot water usage is too high. If you need a higher hot water flow rate than what the hot water system is designed for you'll have this problem. Restrict hot water usage or consider an upgrade.
Thermostat not working. Call a Gas Fitter/Plumber.
The diaphragm is broken. This regulates how much gas goes into the heat exchanger. Call a Gas Fitter/Plumber.
You have mineral build-up in your heat exchanger. If you have hard water, which is water high in minerals, these minerals can come out of solution from the heat and form solid deposits. Your system should be flushed regularly. Call a Gas Fitter/Plumber.
Blocked gas vent in heat exchanger. This stops the exchanger from firing fully. Call a Gas Fitter/Plumber.
The ECU isn't working properly. Try turning your hot water system on and off to see if it resets the ECU. If still not working, call a Gas Fitter/Plumber.

Hot water pressure is low

The first thing you need to check is if the cold water also has low pressure. If this is the case then the problem most likely isn't your hot water system. Remember that continuous gas hot water systems ca restrict the flow slightly to ensure the water is at the required temperature. Otherwise check:

The strainer valve is blocked. The strainer valve removes solids near the inlet valve. This can be cleaned or replaced if faulty.
You have mineral build-up in your system. Flush your system as per the manual and see if that fixes the problem.
Faulty thermostat or ECU restricting water flow. Call a Gas Fitter/Plumber.

Water is Hot then Cold and then Hot when first turned on

This is called a cold sandwich. It is simply caused by someone using the hot water just before you and this hot water being left in the pipes. When you turn on the hot water it pulls cold water into the hot water system where it triggers the heat exchanger. During this process it is normal for the cold water to make it through the exchanger before it is fully fired and heating the water. Once the second lot of hot water comes through it should stay hot. No real solution to this, other than not use hot water straight after somebody or run it until the second lot of hot water comes through.

Water heater shuts down during use

Faulty Thermostat, ECU or Flow Sensor. Call a Gas Fitter/Plumber.
Dirty burners in heat exchanger. Your ECU can shut down your system if it detects an incorrectly firing heat exchanger. Call a Gas Fitter/Plumber.
Improper installation. Who knows where to start with this. Better to call a Gas Fitter/Plumber.

Water intermittently goes cold when in use

Insufficient gas flow. Low gas flow will affect the heat exchanger efficiency. If using bottled gas is the bottle big enough or is it nearly empty.
Faulty diaphragm affecting gas flow. Call a Gas Fitter/Plumber.
Water flow is too low. This can affect the flow sensor and trigger the ECU to turn the heat exchanger on and off.

Hot water heater isn't working and showing an error

In this case you need to refer to your manual to work out what the error message means. If this doesn't work, call a Gas Fitter/Plumber.

So there you have it. Whilst this isn't a complete list of possible problems, it covers the majority of them. Whilst some of these you can easily do yourself, many of them require the assistance of a qualified Plumber/Gas Fitter.