An instantaneous hot water system should be pretty straightforward in the way it operates. Turn on a hot tap, the unit fires up and heats the water. Turn the tap off, and the system shuts down.
So when an instantaneous hot water system keeps running when it shouldn’t, it’s a fault worth taking seriously.
We recently attended a home where the existing instantaneous hot water unit didn’t want to shut down properly. We tested the system, replicated the issue and confirmed that it wasn’t just a one-off glitch.
With the fault confirmed, the decision was made to replace the existing system. We returned the following day and installed a new Rinnai instantaneous hot water unit.
How Should an Instantaneous Hot Water System Work?
Instantaneous, or continuous-flow, hot water systems are different from traditional storage systems because they don’t keep a large tank of water heated throughout the day.
Instead, the unit detects when water begins flowing through the system. It then activates and heats the water as it passes through.
Once the hot tap is turned off and the water flow stops, the system should recognise that there is no longer any demand and shut itself down.
If the unit continues operating, behaves unpredictably or fails to shut down correctly, something within the system isn’t working as intended.
Confirming the Hot Water Fault
One of the important parts of diagnosing an intermittent hot water fault is determining whether it can be reproduced.
A system might behave normally when a plumber first arrives, particularly if the fault only occurs occasionally. Simply resetting the unit and seeing it operate once doesn’t necessarily mean the underlying problem has disappeared.
In this case, we were able to get the unit to repeat the shutdown problem during testing. That confirmed there was a genuine fault with the existing instantaneous hot water system.
Once we knew the fault was repeatable, we could make a proper decision about the next step rather than leaving the household with an unreliable system.
Replacing the Faulty System With a New Rinnai
After discussing the situation, arrangements were made to replace the existing unit.
We returned the following day and installed a new Rinnai instantaneous hot water system, giving the household a reliable replacement rather than continuing to rely on a unit that was no longer shutting down correctly.
Rinnai is a well-established manufacturer of continuous-flow hot water systems, with units designed to provide hot water on demand without the need to maintain a heated storage tank.
Using Google Translate to Keep Everyone Informed
There was another interesting part to this job that had nothing to do with the hot water system itself.
The family member who was available at the home spoke very limited English, while we didn’t speak their language. That meant the usual conversation about what we had found and what needed to happen next wasn’t going to work.
So we used Google Translate.
Using the app, we were able to explain the problem with the existing hot water system, communicate what we were doing and make sure the person at the property understood what was happening throughout the job.
It was certainly a different way of communicating on site, but it worked remarkably well.
Good plumbing work isn’t only about diagnosing and fixing the equipment. It’s also about making sure the people in the home understand what’s going on, even when that means finding a different way to have the conversation.
Signs Your Instantaneous Hot Water System Needs Attention
A hot water system doesn’t necessarily have to stop producing hot water completely before it needs to be checked.
- A unit that doesn’t shut down normally
- Water temperature that suddenly becomes inconsistent
- Repeated fault or error codes
- Unusual ignition behaviour
- Unexpected noises from the system
- The unit repeatedly starting or stopping
- A noticeable change in how the system normally operates
If something has changed, particularly with a gas hot water appliance, it’s worth having the system properly inspected rather than assuming the problem will disappear on its own.
Repair or Replace?
Not every instantaneous hot water fault means the whole system needs to be replaced.
The right option depends on the age and condition of the unit, the nature of the fault, the cost and availability of replacement parts, and whether repairing the existing system represents good value compared with installing a new one.
That’s why diagnosis comes first.
In this case, once the shutdown fault had been confirmed, replacing the existing system with a new Rinnai was the appropriate solution.
Having Problems With Your Instantaneous Hot Water System?
If your continuous-flow or instantaneous hot water system is behaving differently, failing to shut down, producing inconsistent temperatures or showing fault codes, RJT Plumbing & Gas can inspect the system and determine what’s causing the problem.
From diagnosing faults through to hot water system replacement and installation, we can help you work out the most practical option for your home.
Frequently Asked Questions
Why won't my instantaneous hot water system shut down?
An instantaneous hot water system should normally shut down when water flow stops. If it continues operating, there may be a fault within the unit or its controls. Because the exact cause can vary between systems, the unit should be inspected by a qualified technician.
Should an instantaneous hot water system keep running after the tap is turned off?
No. A continuous-flow hot water system is designed to activate when it detects water flow and shut down when the demand for hot water stops. If it continues operating after the tap has been turned off, the system should be checked.
Can an intermittent hot water fault be difficult to diagnose?
Yes. Some faults only occur occasionally, which can make them difficult to identify during an initial inspection. Reproducing the problem during testing can help confirm that there is an underlying fault rather than a one-off issue.
Is it better to repair or replace a faulty instantaneous hot water system?
It depends on the age and condition of the system, the nature of the fault, the availability and cost of parts, and the cost of replacement. A plumber can diagnose the problem and help determine which option makes the most sense.
What are some signs an instantaneous hot water system needs to be checked?
Warning signs can include failure to shut down normally, inconsistent water temperatures, repeated error codes, unusual ignition behaviour, unexpected noises, or changes in the way the system starts and stops.
Can an old instantaneous hot water system be replaced with a new Rinnai?
In many situations, an ageing or faulty instantaneous hot water system can be replaced with a modern Rinnai continuous-flow unit. The appropriate replacement will depend on the property's hot water requirements and the existing installation.
