Instant Gas Hot Water Not Working: Troubleshooting Guide

Instant gas hot water not working the way it should? Cold showers and icy dishes aren't the end of the world, but they throw out everything from cooking to laundry, so it's worth sorting quickly.

Instant gas systems, also called tankless or continuous flow units, are popular for their compact look and the promise of hot water that doesn't run out. They're usually very reliable, but they do hit the odd snag. Before you boil the kettle or call anyone, work through these ten checks. They'll help you pinpoint the problem, or tell you when it's time to get a plumber in.

Your instant gas hot water checklist

These units heat water on demand with a gas burner, which is what sets them apart from storage tanks. Whether yours is a small wall-mounted model or a big one feeding a full household, these checks apply across the board.

1. Confirm the gas supply

Is the gas still on? Check the gas meter valve is open and that other gas appliances, like your stove, are still working. A closed valve or a supply interruption, which happens during maintenance or a billing issue, will stop it heating. On LPG, check the bottle isn't empty. A disrupted gas supply is one of the most common causes, and sometimes the quickest to fix.

2. Check the power supply

Plenty of instant systems need electricity for the ignition and controls, even though the heat comes from gas. Head to the meter box and look for a tripped breaker or blown fuse. Reset it once. If it trips again, you've likely got an electrical fault that needs a professional.

3. Inspect the pilot light or ignition

Older units have a pilot light; newer ones use electronic ignition. If yours has a pilot light and it's out, relighting it to the manual's instructions might fix it. On electronic ignition, a failure to spark can mean a faulty igniter, which is a likely cause if the unit keeps cutting out after it fires. Relighting and testing ignition on a gas unit is fiddly and carries real risk, so if you're not confident, leave it to a licensed gasfitter.

4. Check water flow and pressure

Instant systems need enough water flow to fire the burner, usually 2 to 3 litres per minute. If the water's going cold, low flow could be why. Check whether taps or showerheads are clogged with debris or limescale, which is common in hard-water areas. Open a few hot taps to test it. If pressure's low across the whole house, a blocked inlet filter or a pipe issue may need a professional clean.

5. Watch for hot-then-cold

If the water runs hot then cold, the system's struggling to hold a steady temperature. That often comes down to gas pressure or a faulty temperature sensor. Check the temperature setting if you can get to it, usually 50°C to meet Australian standards. If it keeps cycling, the heat exchanger might be clogged or the gas valve faulty, and that needs a proper diagnosis.

6. Look for error codes

Most modern units have a digital display or indicator lights. If yours keeps shutting off, look for an error code or a flashing pattern. Codes can point to ignition failure, overheating or a sensor fault. Check the manual, or search your model number, to decode them. This step takes the guesswork out of the whole job.

7. Inspect for leaks or corrosion

A hot water leak can make it look like the unit's failed completely. Check around it for puddles, damp patches or corrosion, especially on the heat exchanger and the connecting pipes. A faulty pressure relief valve can cause pressure drops that mimic a breakdown. If you spot a leak, turn off the water and gas and call a plumber before it does any damage.

8. Check the flue and ventilation

Gas units need proper ventilation to burn safely, and a blocked flue will shut the unit down as a safety measure. Have a look at the flue (the pipe exiting the unit) for debris, bird nests or corrosion. In dustier or leafier suburbs this is common. Don't ever try to clear a flue yourself, though. That's a job for a professional.

9. Think about age and servicing

Most instant gas systems last 10 to 15 years with regular servicing. If yours is getting on, parts like the heat exchanger or gas valve may be wearing out. A unit that's never been serviced, with a dirty burner or a scaled-up heat exchanger, loses efficiency and reliability. If it's old and neglected, a replacement can work out cheaper than chasing repairs.

10. Know when to call a professional

Worked through the lot and still no hot water? Time to get the experts in. Faulty igniters, gas valve problems and heat exchanger blockages need a licensed plumber or gasfitter to keep it safe and compliant. Knowing when to stop the DIY is part of getting it fixed properly.

How we do it at Fix-It Right

When an instant gas system lets you down, we'll get to the bottom of it, whether that's a quick repair or a full replacement. We keep it simple: there's $0 call-out, we give you three written quotes so you can pick what suits your budget, and you'll always know the price before we start. If the number's a stretch, Fix-It Now Pay Later gives you 12 months with no interest and no repayments. Every job is backed by our 10-year workmanship guarantee, because the whole idea is to fix it properly the first time. Plumbing fixed for good, not fixed for now.

We look after hot water systems across Melbourne, Geelong, Ballarat and Canberra. Book online or give us a call and we'll get your hot water running again.