What PSI rating does a tankless water heater relief valve need?

150 PSI is the standard rating across residential tankless installs — every OEM isolation valve kit we've reviewed (Webstone, Watts, Eastman) ships with a 150 PSI relief valve built in, and the standalone Watts LF3L used as a direct replacement is also rated 150 PSI at up to 210°F. Confirm against your unit's manual before buying a replacement; a handful of commercial or high-BTU models spec differently.

Where does the discharge from a tankless relief valve need to go?

Per the International Plumbing Code (IPC §504.6), the discharge pipe from a temperature-and-pressure relief valve must terminate within 6 inches of the floor, run by gravity (no valves or restrictions in the discharge line), and not connect directly to a drain. Some local jurisdictions go further and require it to terminate at a floor drain or outdoors rather than just hovering above the floor — check your local plumbing inspector's amendment before finishing the install.

I already installed an isolation valve kit — do I still need a separate relief valve?

No, and buying one would be redundant. Every kit in our isolation valve kit roundup (Webstone 44443WPR, Watts LFTWHG2-FT-HC-RV, Eastman 60204) ships with a 150 PSI relief valve built into the cold-side valve body — it's not an add-on you buy separately once you have one of those kits installed. The scenario where you need a standalone valve is different: no isolation kit installed at all, or an OEM relief valve that's failed and needs a like-for-like replacement.

Can I use a generic pressure relief valve instead of the manufacturer's OEM part?

For most gas tankless units, yes, as long as the PSI rating, temperature rating, and connection size match what the install manual specifies — a Watts LF3L rated 150 PSI/210°F is a functional equivalent to most OEM valves. The exception is warranty-sensitive situations: Rheem's tankless warranty documentation is specific about parts and installation matching its own specs, so if the unit is still under warranty, the OEM part (like the AP12993C) is the lower-risk buy even at a higher price, since a generic substitution is one more thing a denied claim could point to.

Do You Need a Pressure Relief Valve for a Tankless Water Heater? (2026)

Q: Does a tankless water heater need a pressure relief valve?

Usually yes, but not for the same reason a tank heater does. A storage tank holds a standing volume of water that expands as it heats, so UL 174 makes a T&P valve mandatory. A tankless unit heats water on demand and holds only a few ounces at a time, so UL 499 doesn't impose the same blanket requirement — but the manufacturer's own installation manual almost always calls for one anyway, and once that manual says so, following it is what code enforcement and your warranty both check. Treat 'check your model's install manual' as the actual rule, not a generic yes-or-no.

Quick Answer: Tankless water heaters don’t automatically require a pressure relief valve the way tank-style heaters do under UL 174 — a tankless unit holds only a few ounces of water at any moment, so there’s no standing volume to build dangerous pressure as it expands. In practice, though, nearly every manufacturer’s install manual calls for one anyway, rated 150 PSI in virtually all residential cases, discharging within 6 inches of the floor per IPC §504.6. If you’ve already installed one of the isolation valve kits we recommend, you already have a 150 PSI relief valve built into the cold-side body — you don’t need to buy a second one.

This is one of the more genuinely confusing corners of tankless plumbing, because the honest answer is “it depends on your model’s manual,” not a flat yes or no you can Google once and forget. Here’s what’s actually going on, and which valve to buy in the cases where you do need a standalone one.

Why the tank-heater rule doesn’t just carry over

A tank-style water heater’s temperature-and-pressure relief valve exists because a 40-80 gallon tank of water expands as it heats, and if the tank is on a “closed system” (a check valve, pressure-reducing valve, or municipal backflow preventer somewhere on the cold supply), that expansion has nowhere to go — pressure climbs until something fails, and UL 174 makes the T&P valve mandatory to prevent that. A tankless unit heats water as it flows through, holding only a small volume in the heat exchanger at any instant, so the same expansion-buildup scenario doesn’t really apply the same way, and UL 499 (the electric tankless safety standard) doesn’t impose the same blanket requirement UL 174 does for tank units.

That said, “not required by the safety-listing standard” isn’t the same as “not required, period.” The International Plumbing Code and most manufacturer installation manuals still call for a relief valve on the cold inlet as a practical safeguard against a stuck valve, a malfunctioning flow sensor, or a closed-system pressure spike — and once the manufacturer’s manual specifies one, following it is what actually gets checked at inspection and at a warranty claim. Our tankless water heater installation guide and pressure-relief-inclusive isolation valve kits both build around that assumption for exactly this reason.

The part most buyers get wrong: you may already own one

If you’ve installed — or had a plumber install — one of the isolation valve kits we recommend, the relief valve question is already answered. The Webstone 44443WPR, Watts LFTWHG2-FT-HC-RV, and Eastman 60204 all ship with a 150 PSI, 200,000 BTU relief valve built into the cold-side valve body, the same safety function a T&P valve performs on a tank heater, just sized for tankless BTU input. It is not a separate line item you shop for once that kit is in place. The confusion happens because a lot of generic “does my water heater need a relief valve” advice online is written for tank-style units and doesn’t mention that a tankless isolation kit already bundles the equivalent part.

Scenario	Do you need a standalone relief valve?
Isolation valve kit already installed (Webstone/Watts/Eastman)	No — already built into the kit's cold-side body
No isolation kit, direct-plumbed install	Yes, if the manufacturer's manual calls for one (nearly all do)
Existing OEM relief valve has failed or is leaking	Yes — replace like-for-like
Unit still under manufacturer warranty	OEM part recommended over generic substitute

The two valves worth buying when you actually need one

Rheem AP12993C Tankless Water Heater Pressure Relief Valve

OEM Rheem/Ruud part · 150 PSI · 3/4" connection · ~$25-$45 depending on retailer

The direct OEM replacement for Rheem's Performance Platinum and RTGH-series tankless units — the safer choice if the unit is still under warranty, since Rheem's warranty documentation is specific about parts matching its own specs.
Priced close to, or sometimes below, a generic universal valve, so there's little cost reason to substitute on a Rheem unit specifically.

Check price on Amazon →

Ordering a replacement valve and want it fast? Try Amazon Prime free for 30 days and get it in two days instead of waiting on a plumbing-supply backorder.

Watts LF3L Lead-Free Pressure Relief Valve

Universal fit · 150 PSI · rated to 210°F · 3/4" NPT threaded · lead-free copper alloy body with test lever

Brand-agnostic — works as a functional substitute across Rinnai, Navien, Noritz, Takagi, and most other gas tankless brands as long as the connection size and PSI rating match your unit's manual.
The same 150 PSI rating used across every isolation valve kit's built-in relief valve, so it's a like-for-like match if you're replacing a failed part rather than buying a full kit over again.
Has a manual test lever, which the cheapest generic no-name valves sometimes omit — worth checking for on any substitute before buying.

Check price on Amazon →

How to choose

Already have an isolation valve kit installed? You’re covered — don’t buy a separate relief valve. See our isolation valve kit roundup if you’re not sure whether yours has one.
Rheem unit still under warranty? Use the OEM AP12993C rather than a generic substitute.
Any other brand, or an out-of-warranty unit, needing a replacement or first-time standalone valve? The Watts LF3L at 150 PSI/210°F covers the standard residential spec.
Not sure if you need one at all? Check your model’s installation manual for the relief valve requirement and PSI rating before assuming either way — this is the one detail that’s genuinely manufacturer-specific rather than governed by a single blanket code.

The bottom line

The honest answer to “does a tankless water heater need a pressure relief valve” is: probably, per your manufacturer’s manual, but not automatically under the same blanket rule that governs tank heaters — and if you’ve already installed one of the isolation valve kits we recommend, you already own one and don’t need to buy another. When you do need a standalone valve, match the OEM part to your brand if the unit is under warranty (the Rheem AP12993C for Rheem units), or use a universal 150 PSI valve like the Watts LF3L for everything else. Whatever you install, route the discharge within 6 inches of the floor per IPC §504.6 — that’s the one requirement that isn’t manufacturer-dependent.

Shop tankless water heater pressure relief valves on Amazon →