No Hot Water at One Tap Only (When the Rest of the House Is Fine)

The good news is that single outlet hot water failures are the friendliest ones, as the diagnosis is already half done before you begin. If hot water is flowing in the house, your system is doing its job, it’s a fault that must occur along the short path between the water system and that one tap. It’s a short list of suspects, most of which could be a lot cheaper, and one of them is on the short list of things you can legally do yourself in NSW. Here’s the localisation guide.

The Rule That Does Half The Work

The first thing to remember is that one outlet affected equates to one delivery fault, and any other outlet affected too equates to one system fault. As soon as you switch on hot water at any other tap, you’ve eliminated the tank, the burner, the elements, the tariff and all of the high-priced suspects in the lineup. The only thing that’s left is the path, the branch pipe to that outlet, any valve on it, and the fixture itself, and that last one is a lot. Modern taps are more complex than they appear, and they leak much more frequently than pipes do.

Suspect One: The Mixer Cartridge (The Usual Answer)

A cartridge is a ceramic-disc package housed inside each of the mixer taps, which converts the movements of the mixing lever into mixed water. Cartridges wear and fail in predictable ways: one that spits cold water for the entire arc (the “hot” side is blocked or failed); one that stiffens or gets notchy before it breaks; one that eventually has a smaller and smaller portion of the arc that is “hot”; or one that drips when off. If your outlet is a mixer and its neighbours are hot, then the odds-on favourite is the cartridge, and the repair is a cartridge swap, not a new tap. A failed cartridge can also be hot and cold within itself, and inadvertently cross hot and cold at other outlets as well (the crossed-connection saga from our companion article), meaning a failing mixer sometimes tells more than a leaking one does.

Suspect Two: The Aerator (The Two-Minute Check)

The tiny little piece of mesh at the bottom of the tap, the aerator, is supposed to catch particles, and is a likely cause of any one tap losing water flow when it also loses hot or cold water. The critical part: sediment is not uniformly distributed on the hot lines (tanks release mineral particles, and anything recently done on the system flushes a surge of debris through the pipes), and a hot line that is blocked at the aerator while cold water still flows through the rest of the system is the standard show, particularly in the weeks following a hot water repair or replacement. It’s really up to you to take the check: unscrew the aerator by hand or with gentle grips, rinse the grit out and see if the tank sediment can be spotted as rusty crumbs, and refit. When the flow returns, you’ll have spent nothing, and again learned something about the inside of your tank that you might want to mention the next time you get service.

Suspect Three: The Isolation Valve (Check What’s Under The Sink)

Most modern supply lines have tiny, half-turn valves right under the basin, behind the toilet and in the vanity cupboard, which are responsible for a small proportion of “mystery” single outlet failures. The scenarios are endless: a valve that was slightly closed while cleaning supplies were shoved under the sink; a valve that was closed for a repair months ago and never opened; a new dishwasher install that knocked an adjacent valve. Check the problem fixture, identify the valve on the hot line (usually left supply, usually red tagged) and make sure that it is open. As you’re down there, take a look for flexi hoses kinking, a sharp bend in a flexi hose chokes one outlet with no warning.

Suspect Four: The Outlet’s Own Tempering Valve

While most homes have one tempering valve at the system, there are many that have more, such as renovated homes, extensions and homes with far-flung ensuites and outlets. If one of those satellite valves fails, only its part of the house goes lukewarm or cold, which is exactly the one-bathroom mystery this article is about, left running cold while the rest of the house goes hot. The fingerprint: typically a full zone (all the taps in one bathroom), and the water is probably not stone cold but more like lukewarm. If your problem area was subjected to plumbing reno, this suspect steps up the list, and it’s a simple valve replacement for a plumber who knows to check for it.

Suspect Five: The Airlock (The One That Follows Plumbing Work)

If the failure with one outlet occurred shortly after a plumbing repair or installation of a new hot water system, or when the water was off for street works, then suspect air is trapped. An airlock is a bubble stuck at a high point in a pipe run and acts just like a pipe blockage, the hot tap spits, gurgles, dribbles or stops, and everything else flows as normal. The classic airlock sufferers are gravity fed hot water systems (older house with a water tank in the roof), as they are not under the mains pressure that normally bulldozes air through. Mains fed systems predominantly self-clear, with the patience to run them for a few minutes. The folk remedy, giving the bubble a lift by opening the affected tap a bit and a hot tap a bit to push it through, works in the right hands, but is exactly the kind of cross-connection you don’t want done wrong; on a mains-pressure system, open the tap that is spitting a bit, and a hot tap a bit, give it a burst, and if you still have spitting after a day of this, it’s a quick visit by a professional.

The Honourable Mentions: Distance And Dying Branches

There are two final possibilities that warrant a line each. Distance: the further from your hot water heater, the longer it takes for the hot water to arrive, every metre of pipe contains a bubble of cool water to be pushed through, and so a far hot water tap slow to run hot is not faulty, it’s geography (which can be corrected with pipe insulation or, properly, a recirculation system, but not by fixing the tap). The dead branch: in older homes, where there is still a galvanised branch, one outlet can become choked by corrosion internally, and can go “cold” years before the others. No, it’s not a “fixture fix,” it’s the first chapter in the “repiping” conversation, one tap at a time.

The Ten-Minute Method, In Order

Work through it before you make any call and you’ll likely have your answer: (1) Check hot water at another outlet, delivery fault. (2) Unscrew and rinse the aerator, the free fix. (3) Under the fixture, check the hot isolation valve and flexi, the awkward fix. (4) Mark the pattern: single mixer (cartridge); whole zone lukewarm (satellite tempering valve); spit after fresh work (airlock); weak flow in older house (branch). (5) Make the call with the pattern in hand, and enjoy the rare pleasure of a plumbing call where everyone knows the suspect.

Questions We’re Often Asked

My shower is cold but the basin beside it is hot. How is that possible?

Different fixture, different internals, and it localises your fault beautifully. The problem is probably not the pipes, but the shower’s own mixer cartridge (or the integrated tempering system), as the basin demonstrates that hot water does reach that corner of the house. Cartridges are the first to go in a shower mixer, since they are given the heaviest work of any tap at home.

Can I replace a mixer cartridge myself?

This is at the edge. The legal DIY zone in NSW is washer-level maintenance, and a like-for-like change of cartridge in an existing mixer is likely to be considered washer-level maintenance (unless pipework is accessed and altered). The true disclosures: cartridges are all brand dependent, seats have a tendency to seize, and on some shower models a poorly executed refit will leak into a wall cavity. An easy-access basin mixer, easy to manage; anything in-wall or unfamiliar, hand it over.

Everything was fine until my new dishwasher was installed. Coincidence?

Almost never. Installations are disruptive to the neighbourhood under the sink; isolation valves are closed and then only half opened, flexi hoses are kinked in new positions, debris is shaken out into aerators. Follow the installer and the souvenir will be found in the valves, hoses or aerator.

Does one cold tap mean anything bad for the rest of the system?

Almost never, delivery faults are local in nature. The two exceptions that warrant action: an aerator that keeps refilling with rusty water (a flag on system health, since you’re losing sediment from your tank), and a bad mixer cartridge that can cross-connect and negatively affect the quality of water in the hot water system. Both are good arguments for correcting the minor problem quickly rather than dealing with it later.

The Bottom Line

This is a simpler “no hot water,” with one cold tap in a hot house; suspects are small, local, and cheap in most cases, mostly a cartridge or an aerator or a valve or a satellite tempering valve or an airlock or one tired old branch. Take the 10-minute method as it goes and claim the free fixes, then pass on the pattern to the others. Not all hot water problems are worthy of a whole detective story, this one’s a short story with a satisfying ending.

Written by the Premium Sydney Plumbers team, Sydney’s licensed plumbers repairing all types of hot water systems throughout Sydney, with same-day service where possible. If you are having trouble with hot water at any of your taps, you can call for help at any time of the day on 1300 026 452.