A tankless water heater heats water only when a tap opens. Cold water passes through a heat exchanger, a burner or element fires, and hot water leaves the other side. No stored tank means no standby heat loss and no tank to rust through. It also means the unit has to heat every drop in real time, which is why sizing and installation decide whether you love it or regret it.
Is tankless right for your home?
Tankless tends to suit households that run out of hot water with a tank, homes short on space, owners planning to stay long enough to recover the higher installed cost, and properties where a leaking tank would do serious damage. It suits less well where the gas supply would need extensive upgrading, where the water is very hard and nobody wants to schedule descaling, or where a straightforward garage tank swap would do the job for much less.
Sizing for a California winter
A tankless unit's capacity is a trade-off between flow and temperature rise. The colder the incoming water, the more heat each gallon needs, and the less flow the unit can deliver at a comfortable temperature. Manufacturers publish flow ratings at particular temperature rises, and it's easy to read the most flattering one.
That matters here. Inlet water in the Bay Area, the Central Valley and the foothills gets genuinely cold in winter. Even in the southern metros, winter water is noticeably colder than summer. A proper sizing starts with your household's realistic peak demand, typically how many showers and appliances run at once in the morning, and the coldest inlet temperature you'll see, then picks a unit that meets it. If that unit is too large for your gas supply, two smaller units or a different approach may make more sense.
Gas supply and venting
Gas tankless heaters need a lot of fuel when they fire. The installer checks that the meter and gas line can deliver it alongside your furnace, range and dryer. Undersized supply is one of the most common reasons tankless units underperform or throw errors.
Venting is specific to the unit. Non-condensing models usually need stainless or special vent materials; condensing models, which extract more heat, typically vent with plastic pipe and need a condensate drain. Many are direct-vent, drawing combustion air from outside. The old tank's vent is usually retired. We don't publish venting rules because they depend on the manufacturer and jurisdiction; the installer designs to both.
An installer can assess supply and venting before you commit.
Planning for hard water from day one
Scale doesn't disappear with tankless; it just collects in the heat exchanger instead of a tank floor. In hard-water parts of the inland south and the Central Valley, that means regular descaling. A good installation includes isolation valves so descaling can be done quickly, an accessible inlet filter, and a conversation about whether softening or scale-control treatment makes sense. Hardness varies by district; ask your water provider for figures on your supply.
Indoor, outdoor and in between
Outdoor-rated units mounted on an exterior wall are common in mild coastal and southern areas and free up indoor space. Garage mounting is popular everywhere. In the foothills and the north, where freezes are real, indoor installation or robust freeze protection is important. Wherever it goes, it needs access for service, a route for venting and condensate, and fastening to the structure that holds it securely. Its gas and water connections should tolerate some movement; the installer confirms what applies at your address.
Not a DIY install
Leave these to the installer
- Gas line sizing, connections and any changes to the gas supply
- Venting, condensate routing and wall penetrations
- Electrical connections for the unit's controls
- Relief valve installation and discharge routing
If you smell gas at any point, leave the building and call your gas utility and 911 from outside.
What installation day looks like
The old tank is drained, disconnected and removed. Gas line work comes next if needed. The tankless unit is mounted and fastened, water lines are connected with isolation valves, the relief valve is fitted, and venting and any condensate drain are installed. The unit is powered for its controls, gas is connected, and the installer fires it, checks combustion and flow, sets the temperature, and confirms there are no leaks. Before leaving, they should show you the isolation valves and explain the descaling schedule.
What drives the cost
| Factor | Effect |
|---|---|
| Unit capacity and type | Larger and condensing units cost more. |
| Gas supply upgrade | Often the biggest variable in a tank-to-tankless conversion. |
| Venting | New wall or roof penetrations and vent materials. |
| Location | Outdoor mounting can simplify venting; indoor may need more work. |
| Extras | Isolation valves, recirculation, water treatment, electrical outlet. |
How long it takes
A replacement of an existing tankless unit is usually a single day. A first-time conversion from a tank, with gas and venting changes, commonly takes longer, and any permit that applies may affect scheduling. The installer will set out a realistic plan with you.
Sized for January, not the brochure.
Tankless installation questions
For many households, yes: continuous hot water, reclaimed floor space and no stored tank to leak. But tankless costs more to install than a tank, needs regular descaling in hard water, and must be sized for your winter inlet temperature. If you're replacing a working tank in a straightforward garage, a new tank is often cheaper. The installer can help you weigh it for your home.
By the flow of hot water you need at once and how much the unit must raise the temperature of incoming water. Add up the fixtures likely to run together at peak, then look at the coldest realistic inlet temperature where you live. A unit that handles two showers comfortably in summer may struggle on a January morning in the valley or the Bay Area.
Often. Gas tankless units draw far more fuel at full fire than a tank heater does, and many existing lines were sized for a tank plus the other appliances. The installer checks your supply against everything connected to it. If an upgrade is needed, it should be in the quote from the start.
Usually not. Most gas tankless units need their own specific venting, commonly sealed, direct-vent systems that run through a wall or roof, and condensing models often vent with plastic pipe. The old tank's vent may be capped or removed. The installer designs venting to the manufacturer's requirements for your unit.
Isolation valves on the water lines let a technician connect a pump and flush descaling solution through the heat exchanger without disconnecting the plumbing. In hard-water California, that makes regular descaling quick and affordable. Installing them at the start is far easier than adding them later.
No. Water still has to travel from the heater to the fixture, so the wait is similar to a tank, and sometimes slightly longer as the unit fires. Recirculation options can reduce the wait; the installer can explain what's possible with the unit and your plumbing layout.
Many models are designed for outdoor mounting, which is common in mild parts of Southern California and saves venting inside. In places with real freezes, freeze protection matters, and an indoor model may be the better choice. The installer will recommend what suits your climate.
Tankless units are mounted to the wall rather than standing free, so the question looks different from a tank. They still need to be securely fastened to the structure, and their gas and water connections should tolerate movement. The installer confirms what applies at your address.
Whole-house electric tankless units need very large electrical capacity, which many homes don't have. Smaller electric units can suit a single sink or a remote bathroom. For whole-house electric water heating, a heat pump unit is often a more practical option. The installer can check your panel.