What on-demand actually means
On-demand and tankless are the same thing. When you open a hot tap, flow sensors start a burner or electric elements, and a heat exchanger raises the incoming water to the set temperature as it passes. When you close the tap, the unit shuts down.
The practical result is a continuous supply with a ceiling. A tank has a finite volume and unlimited flow until it empties. A tankless unit has unlimited volume and a limited flow rate. If three showers run at once and the unit is rated for two, the third one cools off.
Who it suits and who it does not
A good fit
Households that run out of hot water with a tank, homeowners short on floor space, and people who plan to stay long enough for the longer lifespan and lower standby loss to matter usually do well with on-demand.
A weaker fit
A small household with a working tank in an easy location may not recover the extra install cost. If the gas line is undersized and the vent route is hard, the project grows quickly. A plumber should tell you that before quoting.
| Question | On-demand | Tank |
|---|---|---|
| Runs out during long use | No, but flow is limited | Yes, when the tank empties |
| Floor space | Small, wall-mounted | Larger footprint |
| Install complexity | Higher: gas size, venting, drain | Lower in a like-for-like swap |
| Standby heat loss | None | Some, all day |
| Maintenance | Periodic flushing | Periodic tank flush and anode check |
How to size one
Tankless sizing looks at two numbers. The first is flow, in gallons per minute, which is how many fixtures you run at once. The second is temperature rise, which is the difference between your incoming water temperature and the temperature you want.
Add up the flow of the fixtures that realistically overlap, such as a shower and a dishwasher in the morning. Then check the unit's rated flow at your temperature rise. A plumber can do this in a short site visit, and it matters more than the brand.
A mistake in either direction costs you. Too small and the temperature drops when two fixtures run. Too large and you pay for capacity you never use and may need a bigger gas line.
Weighing on-demand against a tank? Call for a site check.
Call (650) 860-1670What the house has to provide
- A gas line sized for the burner. Tankless burners draw far more gas than a tank, so a line that serves a tank may be too small.
- A vent path. High-efficiency units use plastic venting through a wall or roof, and the route has to be practical.
- A drain for condensate in condensing models.
- Electric power for the controls and fan.
- Water shutoffs and isolation valves for flushing later.
If any of those is a problem, the cost of the project shifts. See tankless water heater service for installs and flushing, and water heater installation if you are comparing a new tank.
Questions to ask before you book
- Has the gas line been checked for the unit's demand, and what changes if it is too small?
- Where will the vent run, and what wall or roof penetration does it need?
- What is the maximum flow at my temperature rise?
- Does the quote include the permit, isolation valves and haul-away of the old tank?
- What will flushing cost and how often is it recommended?
A plumber who answers those precisely is easier to trust than one who quotes a price from the doorway.
Running costs and upkeep over the years
An on-demand heater removes standby loss, the heat a tank gives up to the room while it sits full of hot water. Whether that saves money depends on your usage and on what you paid to install it, and we do not promise a payback period. A household that uses little hot water gains little from the change.
Upkeep is the trade. A tank wants an occasional flush and an anode rod check. A tankless unit wants a descaling flush on a schedule set by the manufacturer and your water, plus a clean inlet screen. Skip it and scale narrows the heat exchanger, flow drops and error codes appear. Put the flush on a calendar when the unit is installed.
Mountain View water, permits and what to confirm
The City of Mountain View's Building Division at 500 Castro Street requires a permit for every water heater installation and replacement, to be obtained before work begins. Ask your installer whether the permit is in the quote.
The City's published water information says its supply is mostly from the San Francisco Public Utilities Commission, with Valley Water and local groundwater for the rest. The mix can change over the year, and the provider's annual report is the place to look up hardness for your address. We do not quote a number here.
Homes in Los Altos are often served by California Water Service's Los Altos Suburban district, which uses local groundwater and treated imported water, so the water your unit sees can differ across the city line.
If you are outside the city limits, the service area page lists the neighboring towns we dispatch to.
Whichever unit you choose, the installation is judged against the California Plumbing Code and the manufacturer's manual, and a plumber who ignores either leaves you with a failed inspection.
Questions about this topic
Is an on-demand water heater worth it in a Mountain View home?
It can be if you have the gas capacity and a workable vent route and you value endless hot water and space. If the install needs major gas or vent work, a new tank may be the more sensible choice.
Does an on-demand heater need a permit in Mountain View?
Yes. The City's Building Division requires a permit for every water heater installation and replacement.
Will I notice a delay for hot water?
You may wait a short time for hot water to travel the pipe, as with a tank. A recirculation line can shorten the wait.
Can an on-demand heater run two showers at once?
Only if the unit's flow rating covers both at your temperature rise. Size it for the fixtures that overlap.
Which Mountain View neighborhoods are easier to retrofit?
It depends on the house, not the neighborhood. Gas line size, vent route and wall access decide the cost. A site visit answers it.