No Hot Water on a Gas System: Storage, Continuous Flow, and Which One You Have

“No hot water” means two completely different things depending on which kind of gas system you have, and the fault is usually specific to the type. So the first useful step is working out which one is on your wall.

Which do you have?

A storage unit. A tank, often quite old, sitting outside or in a cupboard. Plenty of these around Auckland are the older natural gas storage heaters, Stella and similar. The important thing about them is that many have no electrical connection at all. They run on a standing pilot flame, and the pilot is what lights the main burner when the tank needs heating.

A continuous flow unit, sometimes called a califont. A flat rectangular box on an exterior wall, no tank. It heats water only while you are using it. These do need power, and modern ones have a display that shows an error code when something is wrong.

If it is a box on the wall with a power cable, it is continuous flow. If it is a tank with no power going to it, it is storage, and the pilot is where to start.

Storage units: the pilot is the usual culprit

On an older storage heater with no electrics, the failure mode is almost always the same. The pilot has gone out, or the part that keeps it lit has failed.

It goes out for ordinary reasons: wind, a gas interruption, bottles running empty, or simply age. Once it is out, nothing heats, and the tank of hot water you had gets used up and never gets replaced. That is why these fail the way people describe them, which is usually “we had hot water this morning and now we don’t.”

Relighting is often possible, and there are instructions on the unit. What matters is what happens next. If it lights and stays lit, fine. If it will not light, or it lights and goes out again within a day, stop relighting it and get it looked at, because the usual cause is a failed thermocouple or a gas supply problem rather than bad luck, and repeated relighting attempts on a unit that keeps dropping out is not a good habit.

Age is the other factor. These units are frequently thirty years old or more, and once parts are the question, replacement usually wins.

Continuous flow: read the code before anyone drives anywhere

Modern units tell you what is wrong, and that number saves a visit.

At a rental in Karaka a tenant had no hot water over a weekend. We attended Monday morning, checked the existing Rheem 27 continuous flow unit and confirmed that both gas supply and power to the unit were operational. Despite that, the unit kept displaying the same error code, which pointed at an internal fault rather than anything feeding it.

That is the useful sequence: eliminate the supply first, because gas and power are cheap to check and are the cause often enough to be worth ruling out. Only once both are confirmed good does the fault belong to the unit.

In that case it was not economically repairable, so we quoted a replacement the same morning, the client approved it, and we returned that afternoon to remove the old unit and install a new Rinnai Infinity continuous flow.

A new continuous flow gas water heater installed on an exterior wall

If you ring us about a continuous flow unit, read us the code. It is on the display and it is the single most useful thing you can tell us.

Water leaking at the unit is often not the gas unit

Worth knowing before you assume the worst, because it changes who needs to come and how urgent it is.

At a property in Henderson a leak was reported at the gas continuous flow unit. We checked the water meter first and found it was not running, which ruled out a continuous leak. Then we looked at the unit and found water staining beneath the cold water supply line feeding it, leaking only while water was being drawn off. That is an intermittent fault under pressure, not a fault in the heater.

We isolated the water, removed the failed 20mm stainless steel connection pipe and replaced it. The gas unit itself was fine.

The water connections and pipework below a continuous flow gas water heater

A gas water heater has water plumbing, gas plumbing and, on continuous flow, electrics all arriving in the same small space. Water appearing there is at least as likely to be the water side as the appliance.

When both the cooktop and the hot water are gas

If your gas appliances are of a similar age, they tend to reach the end together, and it is worth looking at them as one decision rather than two callouts.

We attended a property on Melrose Road where both the gas hot water and the gas cooktop were reported as not working. The outdoor continuous flow unit was faulty and beyond economical repair given its age, and the cooktop’s ignition was faulty and operating in an unsafe, unstable condition. Both were replaced in the same visit, with the new cooktop chosen to suit the existing benchtop cut-out so the bench did not have to be touched, then gas leak tested and commissioned together.

Two appliances, one visit, one certificate process.

Two dwellings on one gas connection

Common in converted houses and minor dwellings, and it causes arguments about bills.

On a property in Sunnybrae we separated the gas and the water lines between the upstairs and downstairs units, fitted a gas sub-meter and a water sub-meter so each dwelling is measured on its own, pressure tested everything, and installed a continuous flow gas heater for the downstairs unit.

If you have two households sharing one meter, that is fixable, and it is much easier to do while other work is happening than as its own project.

No hot water right now? Ring us and tell us two things: whether there is a power cable going to the unit, and if there is a display, what it says.

Common questions

Can I relight the pilot myself?

On most older storage units yes, and the instructions are usually on a label on the unit. Follow them exactly, and if it does not light after a couple of attempts, stop and ring us rather than persisting. If you can smell gas at any point, stop, leave it, ventilate the area and get it checked. A pilot that lights and then goes out again within a day is telling you something has failed, not that it needed another go.

Why does my continuous flow unit go cold for a moment?

A brief cold patch when you turn the tap off and straight back on is normal for continuous flow, because the unit has to sense flow and fire again. What is not normal is water that never gets properly hot, cycles hot and cold repeatedly during one shower, or runs hot then cools off and stays cool. Those point at the unit, the gas supply or the water flow rate rather than normal behaviour.

Is a gas unit cheaper to run than electric?

It depends on your rates, your usage and whether you are on reticulated gas or bottles, and bottled LPG changes the arithmetic considerably. The part people forget is the fixed daily charge for having a gas connection at all, which you pay whether you use gas or not. If gas hot water is your only remaining gas appliance, include that charge in the comparison before deciding.

How long do these last?

Continuous flow units commonly run fifteen to twenty years, and older storage heaters often go a great deal longer than anyone expects, which is precisely why parts become the problem. Once a unit is old enough that a repair means hunting for components, price the replacement alongside the repair before committing to either.

Do I need a certificate when a unit is replaced?

Replacing a gas appliance is gasfitting work, so yes, you should end up with certification for it. Keep it with the house paperwork. It matters at sale and it matters if an insurer ever asks whether an appliance was legally installed. More on that in gas work at home.