Tankless water heater and pressure relief valve

The home I’m purchasing has a Takagi tankless water heater but does not have a temperature/pressure relief valve or a discharge line. We asked for this to be added and the sellers told us that per the codes for tank-less hot water heaters (California) a pressure relief valve nor a discharge line are needed. Is this true? If so, is it still a good idea to have one installed or does that fact that the heater is tankless make these items not necessary?

Thanks for everyone’s input.



  1. The tankless water heater should have been installed only by a qualified, Tagaki approved installer so it should be proper.
  2. Tagaki and others, i.e. Bosch, Noritz, etc require a pressure only relief valve on the hot water output. Pressure only valves such as Cash Acme FWL-2, Zurn P3000BR, or Watts 174A would be required. Temperature sensing is done internally via an integral hi-limit switch.
  3. Manufacturer’s requirements supersede all others…it seems you do need a Pressure Relief Valve but it may depend upon your particular model. Contact your local Tagaki authorized installer for more information.

Here is an excerpt from a Tagaki installation manual:

Interesting since the regular tank models have expansion tanks on the cold side.

Gotta think about that one.

Expansion tanks and pressure relief valves serve two different purposes.

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Yes and no
They both deal with the problem of water expansion.
Though I understand what you mean.(just had to get off post.)

I meant to state TPR and expansion tank.

close enough

A relief valve is absolutely necessary with any type of water heating device. A combination temperature/pressure relief valve is only required with storage-tank type heaters.

The relief valve doesn’t necessarily need to be near the heater, although most competent installers will generally place it relatively close to the heater.

Makes sense

Good post Jeff. Tankless water heaters are subject to the same type of failures that tank type water heaters occasionally suffer from.

I’m starting to see these more frequently, usually coupled up with a holding tank and a high velocity heating system. They always have relief valves.

John, What is a High Velocity Heating System?
Is that another name for Instant Heater.

Jack did you make that diagram.?

No it came from one of takagi installation manuals.


This does not need a TPR valve right? or do all tankless need them…?

Up here a high velocity heating system is forced air and heat exchangers, smaller ducts carry the same amount of air at higher pressure. They are making a come back, they were popular in commercial high rises once upon a time, but dropped because they were less efficient and more troublesome. Not sure why they are coming back now, except maybe everyone forgot?

Agreed and even Mike Holmes knows that, as they did one show on this superb High Velocity System. Worth watching for sure.

Well I installed quite a few 100% Electric Tankless Water Heaters and never once put one on it. All were inspected and installed in accordance with the manufacturers instructions. The electric models move water based on turning on the faucet, the elements heat the water as they pass thought the chambers and no pressure was every in the unit…nothing to relieve so to speak.

So I would say not ALL need such a value…but I can only speak for the models (SETS) units.

They only relieve pressure when they take a leak. Right Paul. LOL