How a tankless heater works
A tankless (on-demand) water heater has no storage tank. When you open a hot tap, a flow sensor detects the water moving and the unit fires a gas burner or electric elements to heat it as it passes through a heat exchanger. When you close the tap, it stops.
That design has clear advantages: hot water doesn’t run out as long as the unit can keep up, there is no tank to rust through, and the unit hangs on a wall instead of taking floor space. It also has trade-offs that matter in Pennsylvania houses, mainly around winter capacity, mineral scale and the gas and venting a gas unit needs.
Winter is the real test
A tankless heater is rated by how many gallons per minute it can raise by a given number of degrees. The colder the incoming water, the more work it has to do.
In much of south-central Pennsylvania, more than a hundred nights a year are at or below freezing, and the water coming into the house in midwinter is much colder than in August. A unit that keeps up with two showers in September may only manage one in January.
When sizing a new unit, or troubleshooting one that runs lukewarm in winter, a plumber looks at:
- Peak demand: how many showers, the tub, the dishwasher and the washer might run at once.
- Winter inlet temperature, which sets how much the unit has to raise the water.
- The unit’s flow rating at that temperature rise, from the manufacturer’s chart rather than the headline number.
If a unit is undersized, the options are to stagger hot water use, add a second unit, or replace it with a larger one.
Common problems and what they mean
Error codes
Most tankless units show a code when they shut down. The manual lists what each means. Common categories:
- Ignition or flame failure: gas supply, a closed gas valve, a dirty flame sensor or a venting problem.
- Venting or air intake: a blocked vent termination, often from snow, leaves or a nest.
- Overheating: commonly scale in the heat exchanger.
- Flow or sensor faults: a clogged inlet filter or a failing flow sensor.
Note the code and how often it appears before calling. It speeds up the diagnosis.
The “cold water sandwich”
A short burst of cold water between two runs of hot is a known trait of many tankless units: the water left in the pipes cools, and the unit takes a moment to fire again. Some units have a small buffer tank or recirculation feature that reduces it. If the effect is new or getting worse, check the inlet filter and the flow.
Low flow doesn’t trigger the burner
Tankless units need a minimum flow to fire. A clogged aerator, a nearly closed mixing valve, or a low-flow fixture can fall below it, so the unit never turns on. Cleaning aerators and shower heads often helps; see faucet and fixture repair.
Snow and vent terminations
Gas tankless units vent through the wall or roof. After a heavy snowfall, check that the vent and air intake outside aren’t buried. A blocked vent is a safety issue as well as the cause of a shutdown. If you smell gas, leave the house and call your gas utility from outside.
Descaling: the maintenance that matters
Pennsylvania water varies from soft to quite hard depending on the source, and minerals in the water deposit as scale on the hot surfaces of a heat exchanger. Scale reduces efficiency, causes overheating shutdowns and shortens the unit’s life.
Most manufacturers call for periodic flushing, often yearly, more often with hard water. The job uses the service valves fitted to the unit: a small pump circulates a descaling solution through the heat exchanger, then the unit is rinsed and the inlet filter cleaned. Your water utility’s annual water quality report lists the hardness of your water, which helps set the schedule.
Ask whether a scale-reducing filter or a water softener makes sense for your house, especially if your water is hard.
Should you switch from a tank?
A tankless heater suits some houses better than others.
It tends to make sense when:
- Space is tight and a wall-hung unit frees up room.
- The household uses hot water in long stretches, such as back-to-back showers.
- The existing gas line and venting can be adapted without major work.
- You plan to stay in the house long enough to benefit from a longer-lasting unit.
It may not when:
- The gas line is too small for a high-output burner and would need to be upsized.
- Venting would require a long or difficult run.
- Peak winter demand would need a very large unit or two units.
- An electric tankless unit would need more electrical capacity than the house has.
A plumber can assess the gas line, venting route and electrical service before you decide. If a tank remains the better fit, see water heater repair for tank options, including heat-pump water heaters.
Installation and permits
Installing or converting to a tankless heater usually requires a permit, and gas work and venting are safety-critical. Expect the installer to:
- Size the gas line for the unit’s full output and check pressure at full fire.
- Install venting to the manufacturer’s specifications, with proper clearances.
- Fit isolation (service) valves for future descaling.
- Install a pressure relief valve and, where required, provide a condensate drain for high-efficiency units.
Some municipalities, including the City of Harrisburg, also have their own plumbing licensing requirements. Ask who pulls the permit.
Protecting the unit in winter
A tankless heater mounted in a garage, basement corner or on an outside wall can freeze if the power or gas is lost during a cold snap, because the built-in freeze protection on most units depends on electricity. If the unit will be without power in freezing weather, drain it following the manual, and insulate exposed supply lines nearby. Our page on burst and frozen pipe repair covers what to do if a line splits, and leak detection helps if water appears with no obvious source.
If the house still has old galvanized pipe, rust and scale from it can clog the unit’s inlet filter repeatedly; replacing that pipe is covered under repiping.
Repair or replace?
Many tankless problems are repairable: sensors, igniters, fans and flow components can be replaced. Replacement becomes more likely when the heat exchanger itself leaks, when repairs keep repeating on an older unit, or when parts are no longer available. Check the warranty first; heat exchangers are often covered longer than other parts.
What affects the cost
- The fault: a cleaned filter or descale costs far less than a new heat exchanger.
- Parts availability for the brand and model.
- Gas line and venting changes when installing or upsizing.
- Electrical upgrades for electric units.
- Water treatment, if a softener or scale filter is added.
Ask for the visit charge and repair price before work starts. Our how calls work page lists questions to ask, and emergency plumbing covers what to do if the unit is leaking.
Tankless water heater questions
How often should a tankless heater be flushed?
Follow the manufacturer’s schedule, often once a year and more often with hard water.
Why does my tankless heater run out of heat in winter?
Colder incoming water means the unit must raise it further. An undersized unit can’t keep up at full demand.
Why does it give a burst of cold water?
Water left in the pipes cools while the unit restarts. It is common with many models.
Can I replace my tank with a tankless unit?
Often, but the gas line, venting and electrical supply have to be checked first.
My unit shows an error after a snowstorm. What should I check?
Make sure the vent and air intake outside aren’t blocked by snow. If you smell gas, leave the house and call the gas utility.