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5 Common Instant Gas Hot Water System Problems (And How to Fix Them)

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Instant gas hot water systems are the compact, energy-efficient and cost-effective additions to any Melbourne home. But when your unit starts delivering freezing cold water, it helps to know how to diagnose the problem and what fixes are available.
Instant gas hot water systems are the compact, energy-efficient and cost-effective additions to any Melbourne home. But when your unit starts delivering freezing cold water, it helps to know how to diagnose the problem and what fixes are available.

Read on for the 5 most common instant gas system problems and how to fix them.

One minute you’re enjoying a steaming hot shower and the next…an icy blast of water to the face.

Losing water is one of life’s greatest frustrations (more so on those cold Melbourne mornings), and although an instant gas hot water system offers cost-effective and performance advantages over tank systems, there are still potential problems for you to run into.

Our expert plumbers have rounded up the most common instant gas hot water system problems to help you spot trouble before it gets out of hand, help you understand a few simple (and free) DIY fixes, and have your water flowing again in no time.

How instant gas hot water systems work

An instant gas system, also known as continuous flow, doesn’t have a storage tank full of water waiting to be used.

While gas and electric systems heat water in your tank before being siphoned into your pipe and delivered to your shower or sink, an instant gas hot water system warms water as it passes through a heat exchanger to deliver heated water at the source.

This offers a virtually limitless supply of water (which means no unwanted icy showers on those chilly Victorian mornings) at a flow rate between 10 and 32 litres per minute.

Instant gas heater helps to save money compared to traditional tank-based systems (which can be a significant saving as hot water uses 25% of your Melbourne home’s power), and since you don’t need to find space for a bulky water tank, you’ll have more options to use your space.

But with innovative water heating technology comes the risk of problems.
Just as you’d need to keep an eye on your car’s engine or your computer’s performance, there are problems that can arise with your instant gas hot water system – and we’ll show you how to spot them (and how to fix them).
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Step-by-step instant gas hot water system process

Step One: You turn on your hot tap and cold water flows into your instantaneous system.

Step Two: The water flow is detected via a sensor that ignites an internal gas burner to heat water in a heat exchange.

Step Three: The instant water follows a serpentine pattern through the heat exchange to absorb as much heat as possible before exiting via your faucet.

Step Four: An electronic control unit regulates the gas burner to maintain a set temperature.

Step Five: You enjoy sizzling hot showers, heated water in the sink, or in your appliances.

Common instant gas hot water system problems

#1 - Mineral buildup

All systems are exposed to water each day and while this sounds like a unit is doing its job, that water can fall into one of two categories:

Soft water refers to water that is free of harsh minerals that can damage your plumbing and gas system. While hard water refers to water that is high in mineral content and can damage your piping (and your skin). These minerals are commonly calcium and magnesium and don’t just leave your skin and hair feeling dry, but can ruin the performance of an instantaneous gas system too.

Over time, these minerals build up inside your tankless unit and impair optimal function. Whether it’s blocking valves, leaving components damaged, or preventing heated water from reaching you, mineral build up needs to be regularly checked and cleaned.

HOW TO FIX THIS PROBLEM: You can flush your tankless unit periodically to avoid excess buildup of minerals. This can be done with white vinegar or other approved cleaners, but be careful not to use harsh cleaning agents as these can damage your system.

It’s recommended to schedule a professional clean once every 12 months, so reach out and speak to our experienced Melbourne plumbers to arrange your affordable service.

#2 - Cold water at the start of your showers

Not every instant gas hot water system needs to be fixed by a professional plumber.

If you’re experiencing an initial burst of heated water, then cold, then a return to hot, this is known as a “cold water sandwich” (and it’s not a meal we recommend).

This problem is to do with the way your instantaneous system works. Remember, water passes through a heat exchange before exiting your taps. When one of your family members finishes their shower, for example, a small amount of heated water remains in your pipes.

When it’s your turn to shower, you get this leftover burst of hot water, but the water further back in the pipes has cooled down and comes out as an icy blast. The further your water needs to travel in your pipes, the more likely you’ll have a section of cold water.

Thankfully, this cold water is only temporary. Once your igniter fires up your system, the subsequent water will be heated and come out nice and hot, so you’re only dealing with a small amount of water that’s cooled in your pipes before the temperature rises again.

HOW TO FIX THIS PROBLEM: There’s no professional fix needed. Once you’re aware of a “cold water sandwich” you can leave your water running for a few seconds to ensure the excess cold water is drained from the pipes, then jump in the shower when the hot water comes back.

#3 - Cold water during your showers

Are you finding yourself blasted by icy water during your daily shower?

An older tankless gas system may have problems with its minimum flow rate. This is the amount of cold water that the system needs flowing through each minute (measured in litres) to produce hot water.

It’s easy to assume any amount of water should be sufficient, but remember that your instantaneous system has a heat exchange set at a high temperature. If you’re not asking for enough water to flow through that heat exchange, you risk overheating your system.
To avoid damaging the heat exchange (and burning you with scalding water), older tankless systems may shut off to protect you, which is why you’re being hit in the face by freezing water.

HOW TO FIX THIS PROBLEM: If you’re constantly ending up in a cold shower, contact your local plumber and request a service. They’ll be able to tell you if your system can be upgraded or if you’re better off installing a new tankless unit.

#4 - Ignition failure

Your instant gas system needs to ignite an internal burner to heat your water, but if your heater fails to ignite then your water will come out cold.

The most common cause of this problem is your gas supply, so start by making sure your gas valve is fully open and there is a connected supply of gas to ignite your internal burner.
If gas supply isn’t an issue (which you can test by running other gas appliances in your home), you may have a problem with your ignition pack, and that’s going to be tough to solve on your own.

HOW TO FIX THIS PROBLEM: If you’re concerned about ignition failure, reach out and schedule a maintenance service today.

#5 - Blocked exhaust

Your instantaneous system is pretty clever, so if there’s an internal problem, you’ll typically be able to see a warning error on your external unit.
While the errors vary, the most common for Melbourne homeowners is a blocked exhaust. This means your unit is struggling to vent, or bring in fresh air from outside. Some tankless units require air for combustion, and draw that air from outside. Your unit will also need to vent via an exhaust (as it is powered by gas, after all), both of which may become blocked or damaged.
HOW TO FIX THIS PROBLEM: Check for obvious signs of blockages by your water heater, like debris or other obstructive material. Make sure any openings are also free of debris and unobstructed. As malfunctions can occur inside your unit, if you can’t spot an obvious blockage, it’s best to call in the pros.

Is having no hot water an emergency?

It’s not just burst pipes or blocked toilets that require your local Melbourne plumber on speed dial, a lack of hot water is enough to ruin anyone’s day.

If your tankless system is giving you trouble, don’t hesitate to contact the Reed Plumbing and Drainage Solutions team for free advice over the phone and an obligation-free quote.

Plumbing problems typically start out of sight, and can become expensive headaches to fix if left untreated. So if your showers are getting colder, or your hot water seems to be hit and miss, it’s an emergency worth following up.
Wondering how to find the best emergency plumber in Melbourne? Click here for 8 signs you’ve found a winner

Looking for fast and affordable plumbing help?

There’s never a good time for your hot water to turn off.
Thankfully, the team at Reed Plumbing and Drainage Solutions offer 24/7 emergency plumbing help to get your instantaneous system up and running quickly. As a family-run business we know how important it is to keep your household in order, and how much chaos cold showers can cause.
When you’re looking for Melbourne’s most competitive prices and a team of insured, experienced and trained plumbers, we’re here to help.
Call 9331 6633 today or click here to request a fast and FREE quote now

Hire a Good Plumber in Melbourne

The factors that make an excellent plumber include a considerable portfolio of services, five stars Google ratings, as well as being easy to communicate with.

At Reed Plumbing and Drainage Solutions, we are fully licensed and certified, have more than 15 years of experience.

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If you think we can be of service, please give us a call on 9331 6633, book online through the website (book online here), email us ( or reach out on our social media social media, etc.

We are experts in all areas of plumbing, so give us a buzz and we’ll able to discuss the options with you to find the best solution in the quickest time.

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