Plenty of people start thinking about a tankless unit the morning the old storage tank gives out. That is the worst time to decide, because the choice is less about the heater itself and more about the house it is going into: the gas line, the venting path, the electrical panel and how many people want a shower at the same time on a weekday morning.
What on-demand heating actually does
A conventional heater keeps a tank of water warm all day whether anyone uses it or not. An on-demand unit sits cold until a tap opens, then fires a burner or heating element and warms water as it passes through a heat exchanger. The appeal is obvious. You stop paying to keep a tank hot overnight, and you never hit the end of a tank halfway through a long shower.
The trade-off is flow. A tankless heater can only warm so much water at once, and that limit drops as the incoming water gets colder. Water coming out of the ground here runs cold for much of the year, so a unit that sounds generous on paper can struggle when a dishwasher, a shower and a laundry machine all call for hot water together. Sizing comes down to how the household really uses water, not a number on the box.
The house matters more than the heater
Older Seattle homes make this especially clear. A lot of them were built long before anyone imagined a high-output gas appliance on the basement wall. The existing gas pipe may be too small to feed a tankless burner, which means running a larger line back toward the meter. Venting is the next question: many units need their own sealed vent run to an outside wall rather than sharing an old masonry chimney, and the route that vent takes through a finished basement or a tight crawlspace can drive most of the work.
Electric models dodge the gas and venting issues but bring their own. Whole-house electric units draw a great deal of current, and an older panel often has no room for them without an upgrade. In a house with knob-and-tube remnants or a panel that has already been patched over the years, that conversation can grow quickly. That is a common thread with plumbing in Madrona and other neighbourhoods full of early-twentieth-century houses, where a simple swap rarely stays simple once you look behind the walls.
Hard water, scale and upkeep
The water supply here is relatively soft, which is good news for heat exchangers. Scale still builds over time, and a unit that is never flushed will lose efficiency and start throwing error codes. Most manufacturers call for periodic descaling with a pump and a mild solution run through isolation valves. If those valves were not installed with the heater, adding them later is worth doing so the job takes an hour rather than a half day.
Filters on the cold inlet also need an occasional look, especially in houses with older galvanized supply lines that shed bits of rust. A clogged screen can make the heater seem weak when the real problem is upstream. For a closer look at routine care, this rundown of keeping an on-demand system running smoothly covers the basics.
Signs the unit needs attention
Water that runs hot, then cold, then hot again usually points to a flow issue, a dirty sensor or a minimum-flow threshold the fixture is not reaching. Low-flow showerheads and bathroom faucets sometimes sit right at the edge of what triggers ignition. A burner that cycles on and off repeatedly, error codes that keep returning after a reset, or a noticeable drop in temperature at full demand are all reasons to have the system checked before a cold shower turns into a no-shower morning.
Noise is another clue. A unit that starts making a kettle-like sound may have scale on the exchanger. A rattle in the vent can mean a loose joint, which is more serious, because the vent is what carries exhaust out of the house.
Deciding whether it makes sense
A tankless heater is a good fit for a household that wants endless hot water and has a house that can feed and vent the unit without major surgery. It is a harder sell when the gas line, venting and electrical all need upgrading at once, or when the family regularly runs several hot fixtures in parallel. Some households end up happier with a high-efficiency tank or a heat-pump water heater instead.
The useful step is a walk-through before the old tank fails: check the gas supply, map a vent route, look at the panel and talk honestly about how the house uses hot water. If you want to compare approaches, this piece on service and repair for on-demand heaters lays out what a typical visit involves. Making the call ahead of time means the decision is made on your schedule, not the broken heater's.