Quick Answer: Most people searching for a “tankless water heater buffer tank” don’t need what that term actually refers to in the plumbing trade — a 25-300+ gallon ASME-rated commercial component built for low-mass boilers, sold by names like Amtrol, Caleffi, and Wessels with 2-4 inch connections and no consumer price tag. For a real residential short-cycling problem, the two things that actually work are a tankless unit with a small buffer already built in (Navien’s NPE-A2 series carries roughly 0.5-1 gallon via its ComfortFlow system) or a small point-of-use mini-tank like the Bosch Tronic 3000T ES4 (~$230-$295) plumbed in as a flow-through buffer. Watch out for Amtrol’s own ST-1, sold “for Tankless Water Heaters” — it’s an expansion tank, not a buffer tank, a completely different job.
Search “buffer tank” and the results split into two worlds that don’t talk to each other. One is the plumbing-supply world, where Amtrol, Caleffi, and Wessels sell ASME-stamped steel cylinders sized in the hundreds of gallons for commercial boiler rooms. The other is the residential world, where someone’s tankless water heater keeps clicking on and off every few seconds and a forum thread told them a “buffer tank” would fix it. Those two things are not the same purchase, and conflating them is how a homeowner ends up quoting a 4-inch-connection commercial tank for a problem a $250 appliance solves.
The two products hiding behind one search term
A buffer tank, in the plumbing-trade sense, adds thermal mass to a closed heating loop so a small, fast-cycling heat source doesn’t short-cycle against a light load. Caleffi’s ThermaCon line runs 25 to 120 gallons; Wessels’ Hot Water Buffer Tank (HBT) series and Taco’s buffer tanks are built the same way, rated to 150 PSI, sold with 2-4 inch pipe connections. These exist because commercial condensing boilers and multi-unit systems really do need hundreds of gallons of buffered thermal mass — but that’s a different scale of problem than one house’s domestic hot water line.
An expansion tank, by contrast, absorbs pressure surge from thermal expansion in a closed plumbing system — a completely different physical problem, required by code the moment a check valve, pressure-reducing valve, or municipal backflow preventer closes off your supply line. Our expansion tank guide covers that rule in full, comparing the Amtrol ST-5 and ST-12.
Here’s where it gets genuinely confusing: Amtrol’s own ST-1 Therm-X-Trol is marketed explicitly “for Tankless Water Heaters,” and it is an expansion tank, not a buffer tank. If a short-cycling problem sent you shopping and a retailer’s autocomplete served up a product with “tankless water heater” in its own name, it’s easy to buy the wrong part. The ST-1 solves pressure surge. It does nothing for short-cycling.
What actually solves residential short-cycling
| Situation | What actually fixes it | Approx. cost |
|---|---|---|
| Well pump cycling the tankless on/off at low flow | A tankless with built-in buffer (Navien NPE-A2 ComfortFlow) or a flow-through mini-tank | $1,499+ (unit) or ~$230-$295 (mini-tank) |
| Solar-thermal preheat confusing the burner | Small insulated buffer tank + circulator between collector and heater | See our solar guide |
| Multiple condensing units on one commercial system | True ASME buffer tank (Caleffi, Wessels, Amtrol, 25+ gal) | Commercial quote only — not a consumer purchase |
| Cold-water-sandwich delay at distant fixtures | Built-in recirculation buffer (Navien ComfortFlow) or a recirculation loop | Varies by setup |
Option 1: Buy a unit with the buffer already built in
Navien’s NPE-A2 series (NPE-180A2, NPE-210A2, NPE-240A2) ships with ComfortFlow — a small integrated buffer, roughly 0.5-1 gallon per forum reporting on Navien’s own engineering, paired with a recirculation pump. It’s enough thermal mass to bridge a brief low-flow moment without a separate accessory purchase. See our full Navien tankless water heater roundup for current pricing and the NPE-240A2 vs NPE-210A2 sizing breakdown.
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Option 2: Plumb in a small mini-tank as a flow-through buffer
The DIY approach documented on forums like GreenBuildingAdvisor is simpler than it sounds: a small, well-insulated electric mini-tank plumbed as flow-through downstream of the tankless unit, with the tank’s own thermostat and the water heater’s output determining a minimum run length that stops the short-cycling. This is a different installation than a point-of-use appliance feeding one sink — same hardware, different job.
Bosch Tronic 3000T ES4 (4-Gallon Mini-Tank)
- Already reviewed in depth as a point-of-use fixture heater in our point-of-use water heater guide — the same unit works here because the job is identical at the plumbing level: add stored volume between the heat source and the draw.
- 4 gallons is enough stored mass to bridge the few seconds a well pump or solar-preheat line needs without the bulk or expense of a commercial buffer tank.
- Plug-in 120-volt install keeps this a same-day job rather than a re-plumb, unlike a true ASME buffer tank's 2-4 inch hard-piped connections.
If your draw is heavier than one mini-tank comfortably buffers, the 7-gallon Bosch ES8 (hardwired rather than plug-in) is the next step up — see the full spec comparison in our point-of-use guide.
Option 3: A real commercial buffer tank — only if you’re outside the residential case
If you’re feeding multiple condensing units off one system, or pairing a modulating commercial boiler with a hydronic loop, that’s the actual use case Caleffi’s ThermaCon, Wessels’ HBT, and Amtrol’s ASME buffer tanks are engineered for — 25 to 300+ gallons, quoted through a mechanical contractor or plumbing-supply distributor rather than bought off a retail shelf. If that’s not your project, you almost certainly don’t need to go here.
The bottom line
“Buffer tank” is trade jargon for a 25-300+ gallon commercial component most residential tankless owners will never need to buy. If a well pump, solar-thermal preheat line, or distant-fixture delay is short-cycling your tankless water heater, the real fix is either a unit with a small buffer already built in — Navien’s NPE-A2 ComfortFlow line — or a small flow-through mini-tank like the Bosch Tronic 3000T ES4 plumbed in ahead of the draw. And if a retailer serves up Amtrol’s ST-1 “for Tankless Water Heaters,” check twice: that one’s an expansion tank, solving pressure surge, not short-cycling. Our recirculation-loop guide and solar guide cover the two other setups that get lumped into the same search.