Quick Answer: A check valve on a tankless water heater’s cold inlet stops water from flowing backward — either from thermal expansion pushing hot water into the cold line, or from a street-side pressure drop siphoning water back out. For most residential installs, a single spring-loaded valve like the Eastman/EZ-FLO 20403LF (about $25–$35) or the budget Water Source CV75NL (about $13 at Home Depot) covers it. If your water utility specifically requires backflow protection as a condition of service, the Watts LF7R dual check valve (roughly $35–$125 depending on retailer) is the stricter option. Whichever you install, adding a check valve closes your plumbing system and almost always requires a thermal expansion tank too.
Our tankless pressure relief valve guide and expansion tank picks both mention check valves in passing as the thing that triggers a closed system, but neither one is actually about the check valve itself — what it does, when you need one, and which part to buy. This guide fills that gap.
What a check valve actually does here
A check valve is a one-way valve: water flows through it in one direction and the valve mechanically blocks flow the other way. Installed on the cold inlet ahead of a tankless heater, it solves two related but distinct problems. First, as water heats inside the unit it expands slightly, and without a check valve that expansion can push backward into the cold supply line instead of forward through the house — with a check valve in place, the expanding water has nowhere to go but into an expansion tank (more on that below). Second, according to InterNACHI’s inspection guidance, a sudden drop in street-side water pressure — a water main break is the classic example — can theoretically siphon water backward out of a home’s plumbing, and while a tankless unit doesn’t hold a standing tank of water to lose, that reverse flow can still stress the unit’s flow sensor and internal check components. A check valve stops both scenarios cold.
Some water utilities require a backflow preventer at the meter or at the water heater connection as a condition of service, independent of either reason above — that’s a cross-connection-control requirement, not a thermal-expansion one, so check with your utility before assuming your situation doesn’t need one.
The tradeoff nobody mentions: you’re creating a closed system
This is the part most installers gloss over. The moment you add a check valve — or a pressure-reducing valve, or the utility adds a backflow preventer at the meter — your home’s plumbing stops being an “open” system where expanding hot water can push back out toward the street. It becomes a closed system, and expanding water needs an expansion tank to absorb that pressure instead. The IPC (International Plumbing Code) and UPC (Uniform Plumbing Code) both require expansion control under these conditions, and the requirement applies identically to tankless and tank-style water heaters. If you’re installing a check valve as new work, budget for a thermal expansion tank in the same trip — retrofitting one after the fact because a pressure gauge starts creeping up is a more expensive, more annoying second job.
Compare the options
| Product | Type | Size | Best for | Price |
|---|---|---|---|---|
| Eastman/EZ-FLO 20403LF | Single spring check | 3/4" | Standard thermal-expansion protection | ~$25-$35 |
| Water Source CV75NL | Single spring check | 3/4" | Budget install | ~$13 |
| Watts LF7R | Dual check valve | 3/4" | Utility-mandated backflow protection | ~$35-$125 |
| Navien 30008360A | OEM internal subassembly | Navien NPE-fit | Replacing a failed internal check valve | ~$8-$48 |
1. Eastman/EZ-FLO 20403LF — Best Standard Pick
Eastman/EZ-FLO 20403LF Brass In-Line Check Valve
- A dedicated in-line check valve rather than a component bundled into a bigger kit — install it alone on the cold inlet ahead of your isolation valves, without paying for parts you don't need.
- Stainless steel spring resists the corrosion that can eventually stick a mild-steel spring open or closed, which is the failure mode that quietly defeats a check valve's whole purpose.
- Also sold in a 1/2" version (Eastman 20401LF) if your line runs smaller than the common 3/4" flagship size.
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The pick for most residential installs that just need standard thermal-expansion protection, no utility-mandated dual check required.
2. Water Source CV75NL — Best Budget Pick
Water Source CV75NL Brass Check Valve
- Same core function as the Eastman above — a single spring-loaded check — at roughly a third of the price, for anyone whose only requirement is the basic thermal-expansion fix.
- A straightforward, no-frills part; it doesn't carry the same corrosion-resistant stainless spring branding as the Eastman, so it's the budget pick rather than the default recommendation.
- Widely stocked at Home Depot, which makes it an easy add to a same-trip hardware run alongside other install parts.
The pick if you just need the code box checked at minimum cost and aren’t dealing with a utility-mandated backflow requirement.
3. Watts LF7R — Best for Utility-Mandated Backflow Protection
Watts LF7R Dual Check Valve Backflow Preventer
- Two independent check valves in one body instead of one — the redundancy is why water utilities commonly accept dual checks as secondary backflow protection at the meter or water heater connection.
- Priced well above the single-check options here; it solves a different, stricter requirement, not just the ordinary thermal-expansion case, so don't default to it unless your utility or inspector specifically calls for dual-check protection.
- Available in both union and straight FNPT connection variants — check which one matches your existing pipe before ordering.
The pick if your water utility or local code specifically requires backflow protection, not just the ordinary thermal-expansion check.
4. Navien 30008360A — Best OEM Internal Replacement
Navien 30008360A Check Valve Subassembly
- Not a supply-line accessory — this is the internal check valve inside a Navien NPE unit's recirculation buffer-tank plumbing, which controls flow between the buffer tank and the main heat exchanger.
- A worn or sticking version of this part is a secondary contributor some technicians check for "cold water sandwich" complaints on Navien recirculation units, after a flow sensor test comes back clean — see our [flow sensor guide](/guides/tankless-water-heater-flow-sensor/) for the more common cause first.
- OEM specialty parts houses (SupplyStop, RefCo, TanklessParts) price this significantly below big-box general retailers for the identical Navien part number — worth a price check across more than one source before buying.
The pick only if you have a Navien NPE-series recirculation unit and a technician has diagnosed the internal check valve as the cause of a flow or cold-water-sandwich issue — this isn’t a general-purpose supply-line part.
How to choose
- Standard residential install, no utility requirement: the Eastman/EZ-FLO 20403LF covers ordinary thermal-expansion protection.
- Lowest cost, same basic function: the Water Source CV75NL.
- Utility or inspector requires backflow protection: the Watts LF7R dual check valve.
- Navien NPE recirculation unit with a diagnosed internal flow issue: the Navien 30008360A OEM subassembly — an internal service part, not a supply-line add-on.
- About to install any of the above: budget for a thermal expansion tank in the same job — a check valve closes the system and code requires expansion control once that happens.
The bottom line
A check valve is a $13-$125 part depending on which of the four above fits your situation, and it solves a real problem — hot water migrating backward into the cold line, or a pressure-drop siphon stressing the unit — that most tankless buying guides skip entirely. For most residential installs, the Eastman/EZ-FLO 20403LF or budget Water Source CV75NL is all you need; save the Watts LF7R dual check for a utility-mandated backflow requirement, and the Navien 30008360A for a diagnosed internal fault on a Navien NPE recirculation unit specifically. Whichever you install, don’t skip the expansion tank that a closed system now requires.