Start here

Quick, low-risk steps

  1. Record your cold-water temperature during winter and summer.
  2. List all fixtures that might run hot water at the same time.
  3. Check your gas line size or electrical panel capacity.
  4. Ask a pro to calculate the required flow rate and temperature rise.
  5. Get quotes that include venting, condensate, and maintenance access.

How Flow Rate and Temperature Rise Work Together

A tankless unit heats water as it passes through. The temperature rise is the difference between your incoming cold water and your desired hot water. In Denver, groundwater can be quite cold, especially in winter. A unit must be powerful enough to raise that cold water to your target temperature at the flow rate you need.

If you try to run more hot water than the unit can heat at once, the water temperature will drop. A professional can measure your flow rate by turning on the fixtures you typically use together and timing how much water comes out. They can then match a unit to your peak demand.

  • Flow rate is measured in gallons per minute (GPM).
  • Temperature rise is the difference between incoming and outgoing water.
  • Colder groundwater requires a larger temperature rise.
  • A unit's maximum flow drops as the required temperature rise increases.
  • Ask a pro to calculate your home's peak GPM and required rise.

Venting and Combustion Air for Gas Tankless Units

Gas tankless water heaters need proper venting to remove exhaust gases. They often use sealed combustion, pulling air from outside and venting directly through a wall. This can affect where the unit can be placed. The venting must meet local code and manufacturer instructions.

A professional installer will check that the vent termination is away from windows, doors, and air intakes. They will also ensure the gas line is large enough to supply the unit's high BTU input. Upgrading a gas line can add to the installation cost.

  • Sealed combustion units use outside air and direct venting.
  • Vent terminations must be clear of openings and obstructions.
  • Gas line sizing must match the unit's BTU rating.
  • Condensing units produce acidic condensate that needs a drain.
  • Always get a permit and inspection for gas and venting work.

Maintenance Needs for Tankless Water Heaters

Tankless water heaters need regular maintenance to work efficiently. Hard water can cause mineral buildup inside the heat exchanger. This reduces flow and can damage the unit over time. Flushing the unit with a descaling solution is a common maintenance task.

A professional can set up a maintenance schedule based on your water hardness and the manufacturer's recommendations. They will also check the burner, venting, and safety controls. Keeping up with maintenance can help the unit last longer and use less energy.

  • Flush the heat exchanger to remove scale buildup.
  • Clean the inlet water filter regularly.
  • Check the venting for blockages or corrosion.
  • Inspect the burner and ignition system.
  • Follow the manufacturer's maintenance schedule.

Energy Use and Operating Costs

Tankless water heaters can be more efficient than storage tank models because they avoid standby heat loss. However, the actual savings depend on your hot water usage patterns and local energy rates. A unit that is too small or poorly maintained may use more energy than expected.

Look for the Energy Factor (EF) or Uniform Energy Factor (UEF) to compare efficiency between models. A higher number means better efficiency. But remember that site conditions, like cold incoming water, affect real-world performance. A professional can help you estimate operating costs based on your specific situation.

  • No standby heat loss with tankless units.
  • Efficiency is rated by EF or UEF.
  • Actual savings depend on usage and energy rates.
  • Cold groundwater can increase energy consumption.
  • Proper sizing and maintenance are key to efficiency.

Take this with you

Evidence and scope checklist

  • What is my cold-water temperature in winter and summer?
  • What is my peak hot water flow rate (GPM)?
  • Is my gas line or electrical panel adequate?
  • Where will the unit be located and vented?
  • Do I need a condensing or non-condensing model?
  • What maintenance does the manufacturer require?
  • Will I need a water softener or scale inhibitor?
  • What permits and inspections are required locally?
  • What is the warranty and what does it cover?
  • Get quotes that include all installation and disposal costs.
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Common questions

What homeowners ask next

Can a tankless water heater run out of hot water?

A properly sized tankless unit should not run out of hot water because it heats water as it flows. But if you exceed its flow capacity, the temperature will drop. A professional can help you size the unit for your peak demand.

Do tankless water heaters need a larger gas line?

Many gas tankless units require a larger gas line than a storage tank heater because they have a high BTU input. A professional must check your existing gas piping and meter capacity.

How often should I flush my tankless water heater?

Flushing frequency depends on water hardness and manufacturer guidelines. In areas with hard water, annual flushing is common. A professional can test your water and recommend a schedule.

Evidence

Sources used for this guide

Research checked July 31, 2026

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