A tankless water heater has no storage tank. It fires a burner or heating element only when you open a hot tap, and it heats the water as it passes through. That is why the supply does not run out in a long shower, and why the unit takes up so little room on a wall.
The same design means the unit works harder at each use. Scale, gas supply, venting and flow all matter more than on a tank, and small problems show up as error codes, lukewarm water or short bursts of cold. Getting those details right at installation and keeping up with maintenance is what makes a tankless heater last.
Why tankless units need descaling
Water carries dissolved minerals, and heat makes them settle out as scale on the inside of the heat exchanger. In a tank the scale falls to the bottom, but in a tankless unit it narrows the small passages the water flows through. The result is less flow, higher temperatures inside the exchanger, noisy operation and eventually an error code or shutdown.
A flush removes that scale. We connect pumps and hoses to the unit's service valves, circulate a descaling solution through the heat exchanger, rinse it, and check the inlet filter screen. Most manufacturers recommend doing this regularly, and the right interval depends on your water and use.
Hardness, chloramine and what Millbrae water means for your unit
Millbrae's water comes from the San Francisco Regional Water System. The city's 2025 water quality report lists hardness between 8.1 and 112 ppm, with an average of 62 ppm, which is on the soft side by common standards. For a tankless unit that is good news, since scale forms more slowly than in hard water areas, but it does not mean the heat exchanger never needs a flush.
The same report notes that the water is disinfected with chloramine. If you are thinking about adding a softener or filter to protect a tankless unit, the choice should fit that water, and our water softener and filtration page explains what we can set up. We would not recommend a softener just to recommend one, and with water this soft many homes do not need it.
Signs your tankless heater needs attention
Water runs lukewarm
If the unit never reaches the set temperature at a normal flow, scale in the heat exchanger, a partly blocked inlet screen or a low gas supply may be to blame.
Error code on the display
Codes point to specific faults, such as ignition, flame sensing, venting, water flow or temperature. We read the code, check the cause behind it and do not just clear it and walk away.
Short bursts of cold water
When hot water cuts out and returns, the flow may be too low to trigger the unit, or the unit may be cycling on and off because of scale or a sensor.
Noise from the unit
Banging, rumbling or whistling often follow scale, restricted flow or a venting issue. It is worth a look before it becomes a failure.
Leaks at or under the unit
Water at the connections, the condensate drain or the bottom of the unit needs attention soon. A small leak can come from a loose fitting, but it can also come from inside the heat exchanger.
How we service and install tankless units
Read the unit and the history
We start with the model, the age, any error codes and what you have noticed. That tells us where to look first.
Check the supplies
We check the gas line size and pressure, the water flow, the inlet filter screen, the vent and the electrical connection. A tankless unit is only as good as what feeds it.
Flush or repair
For a flush we circulate descaling solution, rinse and check the result. For a repair we replace the failed part, then test the unit through several cycles.
Install or swap with care
For a new unit, we size the gas line, plan the vent, fit isolation valves for future flushes and set the temperature. If you are replacing a tank, we explain what changes in the wall and the venting.
Millbrae homes and tankless conversions
A large share of Millbrae's housing dates from the postwar boom that took the city from 2,783 dwelling units in 1950 to 4,803 in 1960. In older homes the gas line serving the old tank is often sized for a smaller appliance, and a tankless unit can demand a lot more gas at once. Checking the line size is the first step before we quote a conversion.
Bay Area rules on new water heaters are changing, including rules that reach tankless units, so ask us what applies to your installation before you buy. Plumbing permits go through the Millbrae Building Division, and permits may apply to a new water heater, so we confirm what your job needs.
California also requires water heaters to be braced or strapped against earthquake movement. A wall-hung tankless unit is lighter than a full tank, but we follow the same safety approach and mount it to the structure properly.
Tank or tankless: how they compare
| Factor | Tank heater | Tankless heater |
|---|---|---|
| Hot water supply | Limited by tank size | Continuous at the unit's rated flow |
| Space | Floor space needed | Wall mounted, smaller footprint |
| Gas line | Standard size often enough | Larger line often needed |
| Maintenance | Flush sediment occasionally | Descale heat exchanger regularly |
| Upfront work | Simpler | Venting, gas and sometimes electrical changes |
What people try and what works
DIY or what people try
- Ignore an error code and reset the unit repeatedly
- Run a descaling solution through without isolation valves or proper pumps
- Add a tankless unit to a gas line that was sized for a tank
- Turn the temperature up to fix lukewarm water
- Skip the inlet screen when cleaning
Professional approach
- Read the code and test the part it points to
- Flush with proper pumps and rinse thoroughly
- Check gas line size and pressure before installing
- Test flow and temperature rise over several cycles
- Clean the inlet screen and confirm clear flow
What affects the cost of tankless service
We give you a clear price on the phone before work starts.
Type of service
A flush, a repair and a new installation are very different jobs.
Unit and brand
Parts and installation requirements vary by model.
Gas and venting
A larger gas line or a new vent adds time and material.
Access
A wall-mounted unit in a clear location is easier than one in a tight space.
Condition
A unit that has never been flushed may need more work than one on a regular schedule.
Call the 24/7 line for a clear price before work starts. If your unit is past repair, we explain why and discuss the options.
Not sure if you need a flush or a repair?
Call and describe what the unit is doing and any code on the display. We will tell you what we think is going on and what we would check first.
Related work
If you are thinking of installing a new unit, our water heater installation page explains the planning side. If your current unit has failed, see water heater repair for how we diagnose problems.
For work in nearby towns, see our Burlingame plumbing page, and the service area page shows our coverage. The call line is open 24/7 if you want to book a service.
Most manufacturers recommend regular descaling, and the right interval depends on your water hardness and use. Millbrae water is on the soft side, so scale builds slowly, but the unit still needs checking. Ask us about a schedule for your model.
Codes point to faults such as ignition, flame sensing, venting, water flow or temperature. We read the code, test the cause and repair it. Clearing the code without fixing the cause usually brings it back.
Yes. We check the gas line size, vent route and electrical supply, then install the unit with isolation valves so future flushes are simple. We tell you if a conversion needs more work than you expected.
Not always. Tankless units can draw much more gas at once than a tank, so the line size and pressure must be checked. We measure before we quote so there are no surprises on install day.
Not necessarily. Millbrae water averages 62 ppm hardness, which is soft by common standards. If you want to protect the unit further, we can talk through options on our water softener page.
We work on most common brands and models, and we confirm the make and model before the visit. If a part is not obtainable or the unit is past repair, we tell you plainly.
Yes. Call the line and we explain the work and give a clear price before we begin. If we find something that changes the job, we stop and discuss it with you first.
Sources and standards: The California Plumbing Code (based on the Uniform Plumbing Code). The City of Millbrae 2025 Water Quality Report for hardness and chloramine. The California Contractors State License Board (CSLB) C-36 plumbing classification.