Water Under the Hot Water Cylinder: Four Things It Could Be

“My hot water cylinder is leaking” is one call where the answer is almost entirely about where the water is coming from. Not how much, not how old the cylinder is. Where.

There are only three places to look, and they lead to very different outcomes:

At a pipe connection — usually a tighten or a fitting. Cheap.
At or around a valve — a replacement part. Straightforward.
From the bottom or the top of the cylinder itself — the tank has failed. That is a replacement, and there is no version of this where it gets repaired.

So before anything else: get a torch, find where the wet trail starts, and take a photo of the whole cylinder plus a close-up of where the water appears. Send it to us. That one photo usually tells us which of the three you are in, and whether you are looking at a service call or a new cylinder.

First: is the power off?

Before anything else, if there is water under an electric cylinder, turn that circuit off at the switchboard.

We attended a property in Totara Park where the call was a blocked shower, a dishwasher not draining, and no hot water with the electricity tripping on one side of the house. The 135 litre mains pressure cylinder had cracked and was leaking, and the escaping water had reached the wiring, which is what was tripping the power. The electrical fault was a symptom of the plumbing fault.

Water and a cylinder’s element circuit belong apart. Switch it off, then work out the rest.

Wiring beside a hot water cylinder, wet from a leaking tank

Looking at the three places

Water runs down the outside of a cylinder and pools at the base regardless of where it started, so look above the puddle, not at it. A dry paper towel held against each fitting in turn finds it faster than staring into a dark cupboard.

1. A pipe connection. The cheapest ending. At a property in Ōtara the report was water leaking from outdoor cylinder pipework. The cause was loose crox nuts at the connections. We retightened them, tested, and that was the job. If the wet trail starts at a fitting rather than at the tank, this is where you are.

Brass fittings on the pipework at a cylinder connection

2. A valve. Cylinders have several, and they are consumables. In Hillcrest a cylinder was reinstated and the existing TPR valve was then found to be leaking. The valve had failed, and it was replaced. Valves are a repair, not a replacement, and they are the most common cause of water at a cylinder.

3. The tank itself. Water coming from the bottom of the jacket, or from the top of the cylinder, with no fitting involved. Rust staining running down the casing is the same signal. This is the Totara Park outcome: the cylinder was cracked and required full replacement.

A leaking tank cannot be repaired, and this is not us upselling. The inner cylinder is a welded, enamel-lined pressure vessel inside an insulated jacket. There is nothing to access, nothing to reseal, and no patch that holds mains pressure. Once it has gone through, it is done. If your photo shows water from the base or the top of the tank itself, you are replacing it, and the useful conversation is about what goes back in.

4. The tray or the drain from it. This one surprises people, and it is worth checking before you agree to anything expensive.

One thing that turns a small job into a big one

Valves and fittings are normally a quick swap. What changes that is finding the previous installation was done badly.

We see cylinders where the pipework has been improvised: wrong fittings, valves in the wrong order or missing entirely, dissimilar metals joined straight together, a drain line going somewhere it should not. In that situation there is no point renewing one valve, because the next weak point is a month away and it is sitting right beside it. The honest answer is to strip the pipework back and redo the valve set properly.

It costs more than a valve and far less than living with it. We will tell you which of the two you are looking at, and show you why.

The cylinder is not always the cylinder

At a unit in Flat Bush a leak was reported at the hot water cylinder. We inspected it and confirmed the cylinder itself was not leaking at all.

What had happened was that when the cylinder was installed, its drain line, the tundish, had been plumbed into the kitchen waste line. Because the tundish sits lower than the kitchen waste, any blockage or backflow in that kitchen line backed up and overflowed out through the cylinder tray. The water was genuinely appearing at the cylinder. Nothing about the cylinder was wrong.

Re-routing that drain to its own independent waste point meant opening wall and floor sections, so it was a proper job rather than a quick fix. But the important part is what it saved: nobody replaced a working cylinder.

The tundish and drain connection at the base of a hot water cylinder

Is a bit of water from the relief pipe normal?

Some discharge is normal. Water expands as it heats, and the expansion has to go somewhere, so an occasional drip from the relief line is the system doing its job.

Continuous discharge is not normal. A relief valve running steadily means either the valve has failed or the incoming pressure is too high for it, and both are fixable. It is also a leak you pay for twice, in water and in the power used to heat it. We have written separately about water running out of the pipe on your roof.

Repair or replace?

The deciding factor is where the leak is, not the age.

Valves, fittings and connections: repair. These are serviceable parts and replacing them is routine, whatever age the cylinder is.

The tank: replace. There is no repair for a failed cylinder, and patching one is not a thing.

Age matters for the decision around a repair, though, and we will say so plainly: a cylinder that is already old is usually better replaced than repaired. Putting a significant repair into a tank that is near the end of its life is money you spend twice, because the tank fails a year later and the repair goes in the skip with it. A cylinder only a few years old with a failed valve is a different matter entirely and is simply a repair.

Separately, if your problem is no hot water rather than water on the floor, that is a different diagnosis and often not the cylinder at all. The cylinder Q&A has the quick test: hot tap running cold points at the element or the power supply, while a hot tap giving no water at all points at the cylinder and needs a plumber.

If it is a replacement, read this before you buy

A new cylinder is rarely a straight swap into the old space. Modern cylinders carry more insulation, so the same number of litres takes up more room than the tank you are replacing — the old cupboard or the old alcove is often simply too small now. That is also the moment to ask whether the cylinder should stay where it is at all, because moving it outside frees up the space it occupies and can make the pipe runs better.

Size, location, low pressure against mains pressure, and where the quote can grow are all covered in Auckland hot water cylinder installation.

If you have water under yours and are not sure which of the three you are looking at, send us a photo of the whole cylinder and a close-up of where the wet trail starts.

Common questions

How urgent is this?

More urgent than the amount of water suggests, for two reasons. A cylinder holds well over a hundred litres, and if the tank has genuinely failed, the failure tends to get worse rather than stabilise. And the electrical risk is real, as the Totara Park job showed. Turn the power to it off, shut the cold feed valve into the cylinder if it is accessible, and get it looked at rather than watching it for a week.

Can I stop the water without losing all my hot water?

Usually yes. There is an isolating valve on the cold feed into the cylinder, and closing it stops more water entering the tank while leaving the rest of the house on. You will still have whatever hot water is in the cylinder until it runs out, and the cold taps keep working normally. Turn the power off as well so the element is not heating a tank that is emptying.

There is rust around the base but no water. Is it failing?

Rust staining with no active leak usually means there has been a small discharge or weep at some point, often from a valve rather than the tank. It is worth having someone look, because it is a much better problem to find early. Take a photo of it and mark the date, then look again in a month and compare.

How long should a cylinder last?

Electric cylinders commonly run fifteen to twenty years, and plenty go longer. What shortens them is water chemistry, unrestricted mains pressure, and a failed valve that nobody dealt with. If yours is in that age band and still fine, the useful thing to do is check the valves and the pressure rather than replace it pre-emptively.

If it has to be replaced, does it have to be the same type?

Not necessarily, and it is worth asking the question while the decision is open. Low pressure and mains pressure systems behave differently at the tapware, and a heat pump cylinder is a different proposition again on running cost. Swapping type can mean other changes in the house, so it is a conversation to have at quoting time rather than on the day.