A.O. Smith Signature 6.6 GPM
Amazon customer rating as of
Tankless Geek editorial score: 4.3/5
The A.O. Smith Signature 6.6 GPM is the retail-channel condensing tankless — the Signature Premier indoor natural-gas unit sold through Lowe's — rated at 160,000 BTU/h, up to 6.6 GPM, roughly 0.94 UEF, and ENERGY STAR certified. It's aimed squarely at the mainstream homeowner replacing a tank: condensing efficiency, lighter PVC venting, and a long 15-year heat-exchanger warranty, at a price below the pro-channel ProLine models.
Six-and-a-half GPM is a solid two-bathroom capacity in warm and mild climates, tapering to a comfortable single-shower-plus in the cold. Owners consistently praise quiet operation and steady temperatures. The recurring caution — common to A.O. Smith's tankless line — is service: a minority report early failures and slow or hard-to-reach warranty support in some regions. For a typical home wanting condensing efficiency from a familiar brand at a retail price, it's a sensible pick; just line up a qualified installer and register promptly.
Key Features
Best Use Cases
Our Verdict
A well-priced condensing tankless for mainstream two-bath homes, with real efficiency and a long heat-exchanger warranty from a familiar brand. Quiet and steady in daily use. The asterisk is warranty service, which owners rate unevenly — register on time and know your local authorized-service situation before buying.
Technical Specifications
| Input | 160,000 BTU/h (natural gas) |
|---|---|
| Max flow | Up to 6.6 GPM (at 35 °F rise) |
| Efficiency | ≈0.94 UEF, condensing · ENERGY STAR |
| Venting | PVC (condensing) |
| Warranty | 15-yr heat exchanger · 5-yr parts |
| Install | Indoor, wall-mount |
Specifications verified against the linked source documents. See Sources below.
How much hot water you'll actually get
Usable flow drops as incoming water gets colder. Figures are calculated from rated output (160,000 BTU × 0.94 UEF) and labeled as such — not bench-tested by us.
| Temperature rise | Typical scenario | Usable flow |
|---|---|---|
| 35 °F | Warm climate (75 °F in → 110 °F) — capped at max | 6.6 GPM |
| 45 °F | Mild climate (65 °F in → 110 °F) — capped at max | 6.6 GPM |
| 60 °F | Cool climate (50 °F in → 110 °F) | 5 GPM |
| 70 °F | Cold climate (40 °F in → 110 °F) | 4.3 GPM |
Usable flow at a 110 °F target: BTU × UEF ÷ (ΔF × 500), capped at the 6.6 GPM maximum. Warm/mild output is flow-capped; cold-inlet flow falls as shown.
Pros
Cons
Buy it if…
- Two-bathroom homes wanting condensing efficiency
- Homeowners replacing a tank who want a familiar retail brand
- Warm/mild climates where 6.6 GPM covers simultaneous fixtures
- Buyers who value ENERGY STAR and a 15-yr exchanger warranty
Skip it if…
- Large homes needing 3+ bathrooms of simultaneous flow (see the ATI-540H)
- Cold-climate households wanting full 6.6 GPM in deep winter
- Buyers worried about warranty-service availability in their area
Installation Requirements
Professional vs DIY
Gas sizing plus sealed venting and condensate handling put this beyond typical DIY — use a licensed installer to keep the 15-year heat-exchanger warranty intact.
Gas and electrical work should be done by a licensed professional, and most installs require a permit and must meet local building codes — DIY work can void the manufacturer warranty. Always confirm the unit's own manual for exact clearances, venting, and sizing.
What owners report
Themes summarized from retailer customer feedback (4.2/5 Amazon customer rating, 34 ratings, as of 2026-08-23). These are owner opinions, not our own bench testing.
The most consistent praise: near-silent operation and stable water temperature during use.
Owners upgrading from a tank cite lower gas bills and endless hot water for a two-bath home.
A minority report failures within months and slow warranty support — some note authorized plumbers hard to find locally.
As with the brand generally, timely registration and professional installation matter for honoring the 15-year exchanger warranty.
A.O. Smith Signature 6.6 GPM FAQ
Is this a condensing unit?+
Yes — the Signature Premier indoor 6.6 GPM is condensing at roughly 0.94 UEF and ENERGY STAR certified, using lighter PVC venting and a condensate drain.
How many fixtures can it run?+
About two bathrooms' worth (6.6 GPM) in warm and mild climates. In deep cold it settles to a strong single-shower-plus-sink as deliverable flow drops.
How does it compare to the A.O. Smith ATI-540H?+
The ATI-540H is a bigger, pro-channel 199,000 BTU / 10 GPM heater for heavy use and recirculation. The Signature 6.6 GPM is the retail two-bath unit at a lower price.
What's the warranty?+
15 years on the heat exchanger and 5 years on parts for residential use. Register promptly and use a qualified installer to keep coverage valid.
Product sources
Specifications verified 2026-08-23. Manufacturer documents are treated as the primary source for model specifications. Retailer pages are used only for customer ratings and owner-feedback themes.
- A.O. Smith Signature Premier 6.6 GPM indoor — Amazon (accessed 2026-08-23)
- A.O. Smith Signature Premier 6.6 GPM indoor NG (GT15-240-NI) — Lowe's (accessed 2026-08-23)
- A.O. Smith warranty overview — ConsumerAffairs (accessed 2026-08-23)
Price range from leading retailers
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