Quick Answer: A tankless water heater only needs a thermal expansion tank if the cold supply line has a check valve, pressure-reducing valve (PRV), or municipal backflow preventer somewhere on it — a condition plumbing code calls a “closed system.” Both the IPC (§607.3) and UPC (§608.3) require expansion control in that scenario, tankless or not. For a standalone point-source unit, a 2-gallon tank like the Amtrol ST-5 or Zilmet ZEP1 (the latter built specifically for tankless and point-of-use heaters) is enough; hybrid units with an internal buffer tank, like Navien’s ComfortFlow-equipped NPE-A2 series, call for the larger 4.4-gallon Amtrol ST-12 instead.
Search “tankless water heater expansion tank” and you’ll find two conflicting answers: some sources say tankless units never need one because there’s no storage tank to build pressure, others treat it as mandatory. Both are half-right. The deciding factor isn’t the heater type at all — it’s whether anything on the cold supply line turns your plumbing into a sealed loop.
The one question that actually decides this
Is there a check valve, pressure-reducing valve, or backflow preventer anywhere on your cold water supply line — including inside the water meter itself?
If yes, your plumbing is a closed system, and code requires expansion control regardless of whether the water heater at the end of that line is tankless or tank-style. Heated water expands roughly 2% in volume as it warms; in an open system that extra volume simply pushes back toward the street main, but in a closed system it has nowhere to go except to spike line pressure — sometimes well above the pipe and fixture ratings.
This catches a lot of tankless owners off guard because the “no tank, no expansion” assumption is common — and wrong. It was true decades ago when most residential water meters had no built-in backflow preventer. Today, a rising share of municipal water meters ship with one integrated, which silently converts an otherwise-open system into a closed one with no visible check valve for a homeowner to spot. If you’re not sure, ask your water utility or have a plumber check your meter before assuming you’re exempt.
Compare the tanks
| Model | Size | Best for | Connection | Price |
|---|---|---|---|---|
| Amtrol ST-5 (Therm-X-Trol) | 2 gal | Standard closed-system tankless | 3/4" NPT | ~$65 |
| Zilmet ZEP1 | 2 gal, stainless | Purpose-built for tankless/POU | 1/2" NPT | ~$35 |
| Amtrol ST-12 | 4.4 gal | Hybrid/buffer-tank tankless (Navien ComfortFlow) | 3/4" NPT | ~$143 |
| Watts PLT-5 | 2.1 gal | Field-adjustable precharge, budget-flexible | 3/4" male | ~$30–$50 |
1. Amtrol ST-5 (Therm-X-Trol) — Best Standard Pick
Amtrol ST-5 Therm-X-Trol Expansion Tank
- The most widely stocked residential expansion tank at Home Depot and plumbing supply houses, so replacement units and fittings are never hard to find.
- 2-gallon capacity covers typical single-family static pressure in the 40–80 psi range for a standalone (non-buffer) tankless unit.
- Ships pre-charged at 38 psi — field-adjust it to your home's actual static pressure before final install, not after.
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This is the default answer for most closed-system tankless installs: a plain, well-stocked 2-gallon tank with no tankless-specific branding, because it doesn’t need any — the physics of absorbing a thermal expansion spike doesn’t care what’s heating the water downstream. If your plumber or inspector says “you need an expansion tank” without specifying a brand, this is almost always what they mean.
2. Zilmet ZEP1 — Best Tankless-Specific Pick
Zilmet ZEP1 Stainless Expansion Tank
- One of the few expansion tanks marketed by name for tankless and point-of-use water heaters rather than generic residential plumbing.
- Stainless construction and a compact footprint suit the tighter mechanical closets tankless installs are often squeezed into, next to a wall-mounted unit with no floor space for a bulkier tank.
- Lower price than the Amtrol ST-5 for the same 2-gallon capacity, without giving up NSF-rated potable-water construction.
If you’d rather buy a tank that’s explicitly labeled for this exact use case instead of a generic residential model, this is it. Functionally it does the same job as the Amtrol ST-5 at the same 2-gallon size — the difference is smaller physical dimensions and a lower price, which matters more in a cramped tankless mechanical closet than it does in a basement with room to spare.
3. Amtrol ST-12 — Best for Hybrid/Buffer-Tank Systems
Amtrol ST-12 Therm-X-Trol Expansion Tank
- Sized up for tankless units that ship with an internal buffer tank, like Navien's ComfortFlow-equipped NPE-A2 series, where the plumbing behaves more like a small storage heater than a pure point-source unit.
- Same Therm-X-Trol diaphragm design as the ST-5, just scaled up — a 2-gallon tank undersized for a buffer-tank system fails to fully absorb the pressure spike.
- Costs roughly double the ST-5, which is the tradeoff for correctly sizing a hybrid install instead of installing a standard tank and hoping it's enough.
Don’t default to the standard 2-gallon tank if your unit has a buffer tank built in — check our Navien lineup guide if you’re not sure whether your specific model ships with one. Undersizing here is the most common sizing mistake we see, because most generic “which expansion tank do I need” advice online is written for pure point-source tankless units and doesn’t account for the buffer-tank exception.
The precharge mistake that causes most failures
Amtrol’s own installation manual for the ST-series calls out incorrect precharge as the single most common installation error on this product category — installers leaving the factory-set 38–40 psi in place instead of field-adjusting it to the home’s actual static water pressure. A mismatched precharge doesn’t just reduce the tank’s effective capacity; it shortens its working life, since the diaphragm cycles through a wider pressure swing on every heating cycle than it was designed for.
The fix takes five minutes: shut off the water supply, open a hose bib to relieve system pressure, then use a standard tire-style gauge on the tank’s Schrader valve to check the air charge. Adjust with a bicycle pump (add air) or by briefly depressing the valve (release air) until the precharge matches your home’s static pressure — most residential systems run 40–80 psi, and your municipal water utility can tell you the number for your street if you don’t already know it.
How to choose
- Confirm you actually need one first: check whether your cold supply has a check valve, PRV, or backflow preventer — including inside the meter — before buying anything. No closed-system condition means no code requirement.
- Standalone tankless, no buffer tank: the Amtrol ST-5 or Zilmet ZEP1 at 2 gallons covers typical residential static pressure.
- Hybrid/buffer-tank tankless (Navien ComfortFlow and similar): size up to the Amtrol ST-12 at 4.4 gallons.
- Tight mechanical closet: the compact stainless Zilmet ZEP1 fits where a bulkier generic tank won’t.
- After installation, every year: press the Schrader valve — air means the bladder is fine, water means it’s failed and needs replacing.
The bottom line
Most tankless water heaters don’t need an expansion tank purely because they’re tankless — the real trigger is whether a check valve, PRV, or backflow preventer anywhere on the cold supply makes the system closed, a rule set by the IPC and UPC alike and enforced regardless of heater type. When it applies, the Amtrol ST-5 or Zilmet ZEP1 covers a standard standalone install at 2 gallons, and the Amtrol ST-12 is the right size step-up for buffer-tank hybrid units like Navien’s ComfortFlow series. Whichever tank you buy, field-adjust the precharge to your home’s actual static pressure before you walk away from the install — it’s the single most common mistake that shortens the tank’s life.