A tankless water heater heats water only when you open a hot tap. A flow sensor fires a gas burner, and water passes through a heat exchanger on its way to your faucet. Output is limited by flow rate and how much the water must be warmed, so sizing, gas supply and periodic descaling all matter.
How does a tankless water heater make hot water on demand?
When you open a hot tap, a flow sensor detects water moving, the burner lights, and water heats as it passes through a heat exchanger. When you close the tap, it shuts off.
There is no tank of stored hot water to keep warm, so a tankless unit does not burn fuel to reheat water while you sleep or work. Its controls adjust the burner up and down to hold the outlet temperature you set, based on how much water is flowing and how cold it is coming in.
Because it heats water as you use it, a properly sized tankless unit does not run out the way a tank does. The tradeoff is that it can only heat so many gallons per minute at once.
What limits how much hot water a tankless unit delivers?
Two numbers: flow rate, measured in gallons per minute, and temperature rise, which is how much the unit must warm the incoming water. Colder incoming water means fewer gallons per minute.
A unit sized for one shower and a sink may struggle with two showers and a dishwasher at the same time. Incoming water is colder in winter, so the unit has to work harder then. Sizing starts with how many fixtures your household runs at once.
Tankless units also need a minimum flow to fire. A trickle from a faucet may not be enough to turn the burner on, which can feel like the hot water never arrives.
What does a tankless installation involve?
Gas tankless units often need a larger gas supply than a tank, dedicated venting, an electrical outlet for the controls and, for condensing models, a drain for condensate.
- Gas supply: the line must be sized for the unit's full burner demand, which is often more than the old tank needed.
- Venting: tankless units use specific vent materials and routes set by the manufacturer.
- Power: the controls and fan need electricity.
- Condensate: high-efficiency condensing models produce acidic water that needs a drain path.
- Location: wall-mounted units free floor space but need proper clearances.
What upkeep does a tankless water heater need?
Scale is the main enemy. Minerals in the water build up inside the heat exchanger over time, so manufacturers call for periodic descaling and cleaning the inlet filter screen.
Descaling circulates a cleaning solution through the heat exchanger to dissolve mineral buildup. How often it is needed depends on your water and how much you use. Your owner's manual lists the manufacturer's schedule. Cleaning the small screen at the cold water inlet and checking the vent for blockages are also part of routine care.
What goes wrong with tankless water heaters?
Most complaints trace back to sizing, gas supply, scale or venting. The unit's error codes help a technician narrow down which one.
- Cold water sandwich: a brief burst of cold water between uses as the burner relights.
- Lukewarm water when several fixtures run: the unit is undersized or the incoming water is very cold.
- Error codes and shutdowns under heavy use: often an undersized gas line or ignition issue.
- Reduced output or overheating codes: scale buildup in the heat exchanger.
- Unit will not fire at low flow: flow is below the minimum, or the inlet screen is clogged.
- Venting or combustion air problems that trigger safety shutdowns.
How does CASC install tankless water heaters?
CASC is a Rinnai PRO and installs Rinnai tankless water heaters. We also install gas lines, so the gas supply can be planned and sized as part of the same project.
Estimates for water heater installs are free, and we pull required permits. Repair visits on existing units start with a $99 service call on weekdays or $149 on weekends and holidays. Financing options are available.
What should you read next?
If you are deciding between a tank and a tankless unit, start with our side-by-side comparison, then see what happens at a free estimate.
Frequently asked questions
Will a tankless water heater ever run out of hot water?
Not the way a tank does. It heats water as it flows, so it keeps going. But it can only heat so many gallons per minute, so running too many fixtures at once can leave the water lukewarm.
Why do I need a bigger gas line for a tankless unit?
A tankless burner fires hard for short periods to heat water instantly. Its peak gas demand is often higher than a tank water heater's, so the gas line has to be sized to supply it.
What is descaling?
Descaling is flushing a cleaning solution through the heat exchanger to dissolve mineral buildup. Scale reduces output and can cause error codes, so manufacturers list it as routine care.
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Related guides
- Tank vs tankless water heater A tank water heater keeps a set amount of water hot and ready, while a tankless unit heats water only when you open a tap. Tankless gives you a steady supply and saves space; a tank is simpler to install and handles several fixtures at once. Your household's hot water use and your gas and venting setup decide the fit.
- Rinnai tankless water heaters Rinnai makes gas tankless water heaters that heat water as it flows, so you don't run out the way a tank can. CASC is a Rinnai PRO and installs Rinnai tankless units across Central Arkansas, sizing the unit to your fixtures, checking gas and venting, and registering your equipment.
- How a tank water heater works and how to make it last A tank water heater stores and heats water in an insulated tank using a gas burner or electric elements. Cold water enters at the bottom through a dip tube, and hot water leaves from the top. Flushing sediment, checking the anode rod and testing the relief valve help it last longer.
- Permits for water heaters, gas lines and sewer lines Replacing a water heater, running or changing a gas line, and repairing or replacing a sewer line commonly need a plumbing or fuel gas permit and an inspection in Central Arkansas cities. A licensed plumber usually pulls the permit. Requirements vary by city, so confirm with your local building department before work starts.
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