Inspecting Gas Tankless Water Heaters

By Published On: October 1st, 2013Categories: Home Inspection Career Guide0 Comments

Tankless water heaters are starting to become more and more popular with home owners around the country because of their desire to save money on their energy bills.  Tankless water heaters heat the water directly without the use of a storage tank so you do not have the standby energy losses you would have with a storage tank water heater. This article describes how a tankless water heater works, as well as some details you should be looking for as you inspect the water heater.  Keep in mind, however, that depending on the climate in which you live, you may see a tankless water heater installed either inside or outside of the home.


  1. The hot water tap is turned on
  2. Water enters the heater
  3. The water flow sensor detects the entry of water into the unit, switching on the computer.
  4. The computer ignites the burner
  5. Water circulates through the heat exchanger
  6. The heat exchanger heats the water
  7. When the tap is shut off, the unit shuts down.

Things to look for when inspecting a tankless water heater

-Vent pipe is Category III (Stainless Steel) unless particular models are rated by the manufacturer to have PVC vent pipe

-It will be vented either vertically or horizontally (if the manual is readily available, take a look at it. Installers are required to either leave the entire manual mounted to the water heater or give the manual to the consumer)

-PRV is typically required where a TPRV is typically not required unless required by applicable codes.

-Check clearances for vent pipe. Some general clearances are as follows (again, they may very per code and manufacturer)

  • Should not be terminated near a dryer vent
  • Vent termination should not be under a soffit vent such that exhaust can enter the soffit vent
  • Vent termination  where exhaust and rising moisture will not collect under eaves
  • Vent termination should not be too close under the soffit  where it could present recirculation of exhaust gases back into the combustion air intake part of the termination
  • Should be a 24” clearance in front of the unit for servicing
  • Clearances will vary per manufacturers requirements to the top, bottom, front, sides of the unit
  • Clearances from the vent should be followed per the manufacturers requirements
  • The heater MUST NOT be common vented with any other gas appliance or vent stack.
  • B-Vent should not be used as the actual exhaust vent for the appliance
  • The condensate drain pipe should be corrosion resistant
  • These systems are like mini boilers so you should not see any water dripping from the unit
  • The current NEC code states that all unfinished basements and exterior receptacles have to be GFCI protected. However, Rinnai’s Installation Manual states, “a standard 3 prong 120 VAC, 60 Hz properly grounded outlet” so there is no mention of the receptacle being GFCI protected.   Some manufacturers may recommend a hard-wire with an on/off switch.
  • The gas line must be sized to the maximum BTU rating of the unit to deliver the maximum hot water. The larger BTU input of a tankless water heater may necessitate the installation of a larger meter.  A minimum of ¾” inside diameter piping for gas is required.
  • Horizontal vent runs should be supported every 4’ and all vertical vent runs every 6’ or in accordance with manufactures specifications. (indoor units)

If you are in the field and have access to the internet it is very easy for you to go to the manufacturer’s website and look up the installation guidelines to confirm if the unit was installed per the manufacturer’s installation guidelines.

Technical information sourced from, and Complete Code Check

About the Author: Kristin Warner

Kristin is the Marketing Director at AHIT. She has authored content for numerous real estate brands, and managed corporate communications for a public real estate company. She is passionate about the home inspection and real estate industries, and loves digging into research to provide insights that empower home inspectors and real estate agents in their businesses.