What is a flue on a hot water system, and why does it matter?
The flue is the pipe or vent that takes combustion gases away from a gas hot water system. You can look at it, but you cannot fix it.
A flue is the duct that carries exhaust gases from a gas appliance to the outside air. On a gas hot water system it is the pipe or vent you can see on top of a storage tank. Or the vent on the casing of a continuous flow unit.
Electric and heat pump systems burn no fuel, so they have no flue. If your system has one, it burns gas.
What does a flue actually do?
Burning gas makes water vapour, carbon dioxide and, if combustion is poor, carbon monoxide. The flue gives those gases a safe path out of the building. Without it they would build up where you live.
That is why the flue is a safety part, not a trim detail. It is also why the rules on where it may sit are strict.
What types of flue are there?
| Type | Where you will see it | What to know |
|---|---|---|
| Natural draught | Older indoor storage tanks, with a pipe through the roof | Relies on hot gas rising, so it needs a clear, correctly sloped run. |
| Fan assisted | Many continuous flow units | A fan pushes the gases out, so a failed fan stops the unit. |
| Balanced or direct vent | Sealed units on an outside wall | Draws air in and sends gases out through one fitting. |
Your data plate and installation manual say which type you have. Photograph the plate and keep the picture.
What can you check by looking?
You can look. You cannot repair. From the ground, check that nothing is stacked against the flue terminal and that no nest or leaves are packed around it. Check too that there are no brown or black stains around it or on the wall above.
Do not climb onto a roof to inspect it. Do not remove a cap or cover to look inside.
Which signs mean call a gasfitter now?
- Soot or scorch marks on or around the appliance or flue.
- A yellow or orange flame where a blue one used to be.
- A pilot or burner that keeps going out.
- Condensation or rust streaks at the flue joints.
- Headaches or drowsiness when the system runs.
Why do clearances around a flue matter?
The terminal has to sit a set distance from windows, doors, vents and the edge of eaves. Gases that leave the flue must not drift back inside. That is why a gas unit cannot go just anywhere on a wall.
If you are building a deck, fitting a fence or planting near an existing unit, check the clearances first. A neat fence that sits a few centimetres from the terminal can turn a compliant install into a hazard. Ask your gasfitter for the exact distances that apply to your model and state.
Does an old flue need replacing?
Not by age alone. A flue that is intact, clear and correctly sloped can serve for the life of the appliance. What matters is its condition and whether it matches the unit now connected to it.
When you replace a gas system, ask the installer to confirm in writing that the existing flue suits the new model. A different unit can need a different flue, and reusing the old one to save time is a common shortcut. Do not accept it without that confirmation.
This far you can go yourself
You can identify the type of flue, keep the area around it clear and photograph the data plate. That is the end of it.
Anything beyond that, including replacing, extending, re-sealing or moving a flue, is licensed gasfitting work in every Australian state and territory. Your state or territory plumbing and gas regulator lists licensed tradespeople and explains the rules for your area. Do not touch a flue, a burner or a gas connection yourself.
If you are weighing up a replacement, our guide to gas hot water explains how gas systems compare. And our storage versus continuous flow comparison covers the siting differences.
Frequently asked questions
What is a flue on a hot water system?
It is the duct that carries exhaust gases from a gas hot water system to the outside air. Electric and heat pump systems have no flue because they burn no fuel.
Does my electric hot water system have a flue?
No. Electric storage and heat pump systems burn nothing, so there is nothing to vent. A flue means the system burns gas.
Can I fix or move a flue myself?
No. Any work on a flue or gas appliance is licensed gasfitting work in every Australian state and territory. You can keep the area clear and look for warning signs from the ground.
What are the warning signs of a faulty flue?
Soot or scorch marks, a yellow or orange flame, a burner that keeps going out, rust or condensation at the joints, and headaches when the system runs. Stop using the appliance and call a licensed gasfitter.
Sources
What this page explains is based on these official sources. If you check one and it does not match what you read here, send it to us and the article gets corrected.
- Australian Government, energy.gov.au. Hot water systems — types, installation and energy performance
- Australian Building Codes Board. National Construction Code — Volume Three (Plumbing Code of Australia)