AO Smith is the largest US residential water heater manufacturer by volume — but their tankless lineup is sold under the Takagi subsidiary rather than the AO Smith brand. So for tankless specifically:
For tankless
Compare Rinnai to Takagi (AO Smith's tankless). See our Rinnai vs Takagi for the head-to-head. Summary: Takagi undercuts on price, Rinnai wins on ecosystem and recirculation.
For tank water heaters
If you're cross-shopping Rinnai tankless against an AO Smith tank water heater, you're really asking: do I want a tank or tankless?
- Tank pros: Lower upfront cost ($600-1,200 vs $1,800-2,500 for tankless install). Simpler. No recirc needed. Familiar service technicians
- Tankless pros: Endless hot water. 30-40% lower gas bills. 30% less space. Longer lifespan (15-20 vs 10-12 years)
- Payback: Tankless typically pays back in 6-10 years on gas savings alone in a typical 2,500 sq ft US home
Hybrid heat pump option
AO Smith's Voltex hybrid (and Rheem's ProTerra hybrid) compete with tankless on operating cost. If your gas is expensive or unavailable, hybrid heat pump can beat tankless on lifetime cost. See AO Smith's Voltex line for that conversation.
Brand reputation
- AO Smith: the broad-line US specialist. Strong on tanks, strong on hybrids, weaker (via Takagi) on tankless
- Rinnai: the focused tankless specialist. Doesn't sell tanks at all
Bottom line
If you've decided on tankless: buy Rinnai (or Takagi as a value alternative). If you're not sure tank vs tankless: do that decision first; then come back to brand. For tank, AO Smith is excellent. For tankless, Rinnai is excellent.