USA Expert Plumbing

Tankless water heater freeze protection: how it works and how to drain one

By USA Expert Plumbing Editorial Team. Updated . 2 min read.

Short answer

Most tankless water heaters have built-in freeze protection, usually electric heaters inside the unit, that only works while the heater has power. During a winter power outage, or if the heater will be unused in freezing weather, drain it: turn off power and gas, close the water isolation valves, and open the drain plugs as the manual describes.

In this guide
  1. How freeze protection works
  2. Which heaters are most at risk
  3. When to drain the heater
  4. How to drain a tankless water heater
  5. Restarting after draining
  6. Other ways to protect it

A frozen heat exchanger is one of the most expensive tankless repairs, and manufacturers often exclude freeze damage from warranty coverage. The good news is that it is almost always preventable.

How freeze protection works

Tankless water heaters hold only a small amount of water, but it sits in thin passages that crack easily if it freezes. Most units protect themselves with small electric heaters that switch on when the internal temperature drops. Some models also use recirculation or brief burner operation; older Rinnai non-SENSEI models, for example, can show code 02 if the burner does not run during freeze protection.

The catch is simple: freeze protection needs electricity. A power outage during a cold night leaves the unit unprotected.

Which heaters are most at risk

  • Outdoor-mounted units, common in warmer states that still get occasional hard freezes
  • Indoor units in unheated garages, attics or crawl spaces
  • Units whose outside air sensor is covered or damaged; on Rinnai outdoor models that can show as code 41
  • Vacation homes where the heater sits unused

When to drain the heater

  • A winter power outage is expected or has started
  • The house will be left unheated in freezing weather
  • The heater is being taken out of service for the season

How to drain a tankless water heater

Steps vary by brand and model, so check your owner's manual. The general sequence is:

  1. Turn off the heater at its controller, then disconnect its electrical power.
  2. Close the gas supply valve to the heater.
  3. Close the cold water inlet and hot water outlet isolation valves under the unit.
  4. Place a bucket under the unit and open a hot water tap in the house to relieve pressure.
  5. Open the drain plugs or drain valves on the unit as the manual shows, and let the water run out.
  6. Leave the drains open until you are ready to restart the heater.

Water inside the heater can be hot if it ran recently. Let it cool before draining.

Restarting after draining

  1. Close the drain plugs and clean the inlet filter while you have access.
  2. Open the water isolation valves and run a hot tap until air stops sputtering.
  3. Restore gas, then power, and turn the heater on.

Other ways to protect it

  • Insulate exposed water pipes near the heater, but keep insulation off any vents and outside air sensors.
  • Consider a battery or generator backup for the heater's power in areas with frequent winter outages; ask the manufacturer what is approved.
  • Some manufacturers offer freeze protection kits for outdoor units.

Freeze protection and yearly descaling together keep a tankless heater going for many years. See our guide to descale a tankless water heater for the other half of the job.

Common questions

Can a tankless water heater freeze with the power on?

It is much less likely, since built-in freeze protection runs on electricity. Extreme cold, strong wind on outdoor units or a failed sensor can still cause problems.

Does a tankless warranty cover freeze damage?

Freeze damage is commonly excluded. Check your warranty terms and protect the unit before a hard freeze.

Do I need to drain my tankless water heater every winter?

Not if it stays powered and in use in a heated space. Drain it for power outages, vacations in freezing weather or seasonal shutdowns.

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