Quick answer
A tankless water heater takes longer to deliver hot water than a tank because it adds an ignition delay of several seconds to the time needed to push cold water out of the pipe. On-demand means endless, not instant. A wait that has grown over the months points to scale or a failing flow sensor; a wait that was always long is the pipe run, and recirculation fixes it.
If your tankless water heater takes so long to get hot that you have started timing it, split the wait into two parts. The first is the heater: it has to notice flow, run its fan, light the burner and bring the heat exchanger up to temperature. The second is the plumbing: every litre of cooled water sitting between the heater and the tap has to leave before hot water arrives. A tank had the same pipe delay, but it started with hot water already made.
On-demand is not instant: where the seconds go
A gas tankless fires only after its flow sensor sees water moving, typically somewhere around two litres a minute. Then comes a short fan pre-purge, the spark, flame proving and a ramp up to full fire. Count on several seconds before the water leaving the unit is at setpoint, and that slug of unheated water travels down the pipe ahead of the hot.
The pipe does the rest. A 3/4-inch copper trunk holds roughly 0.3 litres per metre and a 1/2-inch branch about half that, so a 15-metre run to an upstairs ensuite holds four to five litres of cold water. A low-flow faucet passing five or six litres a minute needs most of a minute to clear it, while a tub spout clears it in a fraction of the time.
A trickle may never trigger the burner at all. Open the hot side a crack to rinse a razor and the flow can sit below the activation point, so the water stays cold however long you wait.
Why two-storey homes wait the longest for hot water
Distance explains most complaints. In the 1970s–90s subdivisions and 2000s builds across Milton, Oakville and Brampton, the mechanical room sits in one corner of the basement and the ensuite sits diagonally opposite, a floor and a half up. A tankless is usually hung close to an exterior wall to keep its vent run short, which can put it farther from the bathrooms than the old tank was.
Bare pipe makes it worse in winter. The water in the line cools between uses, so every draw starts from cold again, and the copper itself soaks up heat for the first few seconds. Insulating the accessible hot trunk in the basement is cheap and shortens the second and third draws of the morning, though not the first.
Recirculation: dedicated return, crossover valve or a built-in pump
Recirculation is the fix for a long pipe run: a pump keeps hot water moving through the trunk so it is already near the tap when you open it. There are three ways to set it up.
- Dedicated return line: a third pipe runs from the farthest fixture back to the heater. It gives the fastest hot water and leaves the cold line alone, but the pipe has to exist already or be fished through finished walls.
- Crossover valve: a thermostatic valve under the farthest sink lets cooled water in the hot line slip into the cold line and back to the heater. No new pipe is needed, but the cold tap at that sink runs lukewarm for a few seconds, and a valve that sticks open bleeds cold into the hot side.
- Built-in pump: models such as the Navien NPE-A2 and the Rinnai RUR series carry their own recirculation pump and work with either a return line or a crossover valve. A unit without one needs an external pump and a control.
A recirculation loop loses heat, so it costs some gas and electricity. Run it on a timer, a push button or the unit's own scheduling instead of around the clock. Our recirculation pump installation page lays out the options; the price depends on the pump, the valve and how the house is piped, so it comes as a written quote.
When the delay has grown: scale and the flow sensor
A wait that is clearly longer than it was a year ago is a heater problem, because the pipes have not changed. Three parts are usually behind it.
- Scale on the heat exchanger. Halton and Hamilton water is hard, and mineral crust on the exchanger walls slows heat transfer, so the unit takes longer to reach setpoint. A descale flush runs $150–$300; how to tell if a tankless needs descaling lists the other signs.
- A sticking flow sensor. The sensor is a small turbine on the cold inlet. When grit or scale drags on it, it under-reads, so the burner starts late, or not at all at low-flow fixtures. Replacing it falls in the $150–$800 repair range.
- A clogged inlet filter. The screen on the cold inlet catches debris, and as it plugs, flow drops toward the activation threshold.
Check this yourself first
- Time the nearest hot tap and the farthest one. If the near tap is hot in a few seconds and the far one takes a minute, the heater is fine and the pipe is the delay.
- Open the hot tap fully instead of part way, so the flow clears the activation point.
- If the house has recirculation, confirm it is switched on and that its schedule matches the hours you use hot water.
- Read the display for a fault code or a maintenance reminder.
- Find out when the unit was last descaled. On hard water, once a year is normal.
Call for service if
- The wait has grown noticeably, or the water goes hot then cold mid-shower.
- The burner will not fire at a low-flow tap that used to work.
- A code is showing on the controller.
- The unit is more than a couple of years old and has never been flushed.
ZK Mechanical services Navien, Rinnai and Bosch tankless units across Burlington, Oakville, Milton and Hamilton. A diagnostic visit is $90–$150 and is folded into the repair if you go ahead. See tankless water heater repair for what the visit covers, browse more water heater answers, or book a visit.