One of the main reasons homeowners choose a continuous flow hot water system is right there in the name: continuous hot water.
Unlike a traditional storage system, there is no tank of heated water waiting to be emptied. Instead, the system heats water as it passes through the unit.
That means a correctly sized and properly functioning continuous flow system should be able to keep producing hot water for as long as the required water, gas and electricity supplies are available.
So when the shower suddenly goes cold halfway through, it can seem like something is seriously wrong.
The important thing to understand is that a continuous flow system usually has not run out of hot water.
More often, something has changed that prevents the system from continuing to heat the water correctly.
Continuous Flow Systems Need Enough Water Moving Through Them
A continuous flow hot water system needs to detect water moving through the unit before it starts heating.
If the water flow falls below the minimum level required by the system, the burner may shut down.
This can sometimes happen when:
- A hot tap is only partially opened.
- A water-saving fixture significantly restricts flow.
- Another tap or appliance changes the available water flow.
- There is a restriction somewhere in the plumbing.
- The inlet filter or strainer has become partially blocked.
This can produce the frustrating pattern of hot water, followed by cold water, followed by hot water again once the unit detects enough flow to restart.
The system has not emptied. It has stopped heating and then fired again.
A Blocked Inlet Filter Can Reduce Flow
Continuous flow systems commonly include an inlet strainer or filter designed to prevent debris entering the water heater.
Over time, that filter can become restricted.
As the restriction increases, the amount of water flowing through the unit can decrease.
In some cases, that reduction in flow can begin to affect the way the hot water system operates.
This is one reason a continuous flow unit that worked perfectly for years can gradually start behaving differently.
The Gas Supply Matters Too
A gas continuous flow system needs to generate a significant amount of heat while hot water is being used.
That means the gas supply to the unit needs to be correctly sized.
This becomes particularly important when replacing an older gas storage hot water system with continuous flow.
The fact that there was already a gas hot water system installed does not automatically mean the existing gas pipework is suitable for the new continuous flow unit.
If the system cannot receive sufficient gas when operating under load, hot water performance can suffer.
This is why changing from storage to continuous flow should involve looking at the whole installation, not simply replacing the appliance on the wall.
The System May Be Too Small for the Household
Continuous flow systems are generally rated according to how much hot water they can produce under particular conditions.
A unit that performs perfectly with one shower running may be placed under considerably more demand when two showers, a kitchen tap and another fixture are operating at the same time.
Incoming water temperature also affects performance.
The colder the incoming water is, the more energy the system needs to bring that water up to the required temperature.
As the required temperature rise increases, the amount of hot water the system can deliver may decrease.
This is why sizing a continuous flow system should not simply come down to choosing the highest litres-per-minute number you can find.
The number of people in the home, bathrooms, simultaneous hot water demand, incoming water conditions and the existing plumbing and gas installation all matter.
Mixer Taps Can Sometimes Contribute to the Problem
There is another quirk that can affect some continuous flow installations.
Homeowners sometimes set the hot water system to a high temperature, then add a large amount of cold water at the shower mixer to bring the final temperature back down.
Doing this reduces the amount of hot water actually passing through the heater.
If the hot-side flow becomes too low, the unit may approach its minimum operating flow and shut down.
A properly configured system should allow the hot water unit, temperature controls and fixtures to work together rather than producing excessively hot water only to dilute it heavily at the tap.
Does a Continuous Flow Hot Water System Need Maintenance?
Yes.
Continuous flow systems do not store a large volume of heated water inside a cylinder, but that does not make them maintenance-free.
Depending on the manufacturer, model, installation and water conditions, servicing may include checking:
- The water inlet strainer or filter.
- Burner operation.
- Ignition components.
- Gas pressure and gas supply.
- Water and gas connections.
- Flue or ventilation condition where applicable.
- Scale or restrictions within the water heater.
- Fault codes or abnormal operating behaviour.
Different manufacturers and models can have different maintenance requirements, so the recommended servicing schedule for the specific unit should always be followed.
In other words, tankless does not mean service-free.
Watch for Changes in How the System Operates
You do not necessarily need to wait until the hot water stops working completely before investigating a continuous flow system.
Changes in behaviour can be useful warning signs.
It may be worth having the system checked if you notice:
- The shower periodically going cold.
- Hot water temperatures fluctuating more than they used to.
- Reduced hot water flow.
- The system taking several attempts to ignite.
- Problems appearing when multiple fixtures are used.
- Recurring fault codes.
The cause is not always the hot water unit itself.
It could be a blocked filter, a restriction in the plumbing, insufficient gas supply, incorrect system sizing or another issue affecting the installation.
Find the Cause Before Replacing the Unit
If your continuous flow hot water system has started going cold, replacing the unit immediately is not necessarily the answer.
The better first step is to determine why the system is shutting down or failing to maintain temperature.
At RJT Plumbing & Gas, we can assess the hot water unit along with the plumbing and gas installation supporting it.
That means looking beyond the appliance itself to determine whether the issue relates to maintenance, water flow, gas supply, system sizing or another part of the installation.
If you are experiencing unreliable hot water in North Lakes or surrounding areas, contact RJT Plumbing & Gas to arrange an assessment.
Need Help With Your Continuous Flow Hot Water System?
Whether your system is intermittently going cold, struggling when multiple taps are running or simply not performing the way it used to, finding the cause early can help prevent ongoing frustration and unnecessary replacement costs.
Contact RJT Plumbing & Gas for continuous flow hot water servicing, repairs and installation advice across North Lakes and surrounding suburbs.
