Quick answer
A tankless water heater that is not getting hot enough is usually being asked for more than it can deliver. In an Ontario winter, inlet water near 4 °C cuts the flow a unit can heat to setpoint by roughly a quarter to a third compared with summer. If the water is lukewarm with only one tap open, suspect the setpoint, a plumbing crossover, scale or low gas pressure.
When a tankless water heater is not getting hot enough, the first question is whether it happens with one tap running or only with several. A tankless has a fixed burner size, so the bigger the temperature rise and the more water you push through it, the sooner it runs out of capacity. Lukewarm water under heavy use is a sizing and season problem. Lukewarm water at a single faucet is a fault.
Why winter inlet water costs a quarter to a third of your flow
Cold inlet water is the most common reason a tankless falls short between December and March. Municipal water drawn from Lake Ontario reaches the house at around 4 °C in late winter and in the mid-teens or warmer by late summer. Heating to a 49 °C setpoint means a 45 °C rise in February against roughly 33 °C in August.
The burner's output is fixed, so the flow has to give. A 199,000 BTU condensing unit that can heat about 24 litres a minute through a 33 °C rise manages about 18 through a 45 °C rise. Same heater, same setpoint, a quarter less hot water.
What you feel depends on the model. Many current Navien and Rinnai units have a water-adjustment valve that throttles the flow to hold the setpoint, so the symptom is weaker pressure at the hot taps. Units without one let the temperature sag instead.
Too many fixtures at once, or a setpoint left at 49 °C
Two showers plus a dishwasher in January can exceed what one unit delivers, where the same load is fine in July. If the water is hot with one fixture open and fades when a second opens, the heater is working and is at its limit. Stagger the loads, or have the sizing reviewed: what size tankless water heater you need explains the flow math.
Setpoint is the quieter cause. Most units ship at 49 °C, the scald-safe default, and the water loses a few degrees along the pipe before it reaches an upstairs shower. Raising the setpoint is a button press on most controllers, but hotter water scalds faster, so read what temperature a water heater should be set to before going past the default. A higher setpoint also asks for a bigger rise, so it does nothing for a unit already at capacity.
Plumbing crossover and mixing valves that bleed cold into the hot line
If the water is lukewarm with a single tap open while the display shows the unit firing at setpoint, cold water is mixing in after the heater. The usual sources are a worn cartridge in a single-handle shower valve or faucet, a recirculation crossover valve stuck open, or a failed thermostatic mixing valve.
The test takes a minute. Close the cold-water isolation valve feeding the heater, then open a hot tap. The flow should die to nothing after a few seconds; if water keeps coming, cold is crossing into the hot side somewhere in the house. Closing the stops under each fixture one at a time finds the culprit.
Scale, a dirty inlet filter or low gas pressure limiting the burner
Anything that stops the burner reaching full fire caps the outlet temperature. Four things do it.
- Scale: mineral crust on the heat exchanger blocks heat transfer, and the control backs the burner off to protect the exchanger. Hard water in Halton and Hamilton makes an annual descale ($150–$300) routine maintenance.
- Inlet filter: a plugged screen on the cold inlet starves the unit of flow, and the burner modulates down or drops out.
- Gas pressure: a tankless can draw about five times the gas a tank did. A line sized for the old tank, or a furnace and a tankless firing together on a cold morning, drops the pressure at the unit and limits the firing rate. Upsizing the gas run adds $400–$1,200 when it is needed; whether a tankless needs a bigger gas line covers how it is checked.
- Intake and exhaust: a partly blocked termination can hold the burner back or trip a fault. Look outside for snow, leaves and nests.
Gas pressure has to be measured with a manometer while every gas appliance in the house is firing. In Ontario that is work for a TSSA-licensed gas technician, not a homeowner test.
Check this yourself first
- Read the setpoint on the controller and confirm nobody lowered it.
- Run one hot tap alone, then add a second, and note the point where the temperature drops.
- Do the crossover test: cold supply to the heater closed, one hot tap open, and watch whether the flow stops.
- Check the vent terminations outside for anything blocking them.
- Compare with last winter. A unit that coped a year ago and cannot now has lost output, most likely to scale.
Call for service if
- The water is lukewarm at a single tap with the setpoint correct.
- Output has dropped compared with the same month last year.
- The burner cuts out mid-use or a fault code appears.
- The crossover test fails and you cannot find the fixture responsible.
ZK Mechanical's TSSA-licensed technicians measure inlet and outlet temperature, flow, gas pressure under load and scale in the exchanger before recommending anything. The diagnostic fee is $90–$150, credited toward the fix, and most water heater repairs land between $150 and $800. Details are on our tankless water heater repair page, or contact us to book.