A.O. Smith Adapt ATHR-199M vs Rheem IKONIC S11i

Last updated: October 31, 2026

The Rheem IKONIC S11i is the better tankless water heater overall, rated 4.1 to 3.8. The Rheem IKONIC S11i leads on value. The Rheem IKONIC S11i costs $498.65 less ($1,847.43 vs $2,346.08).

A.O. Smith Adapt ATHR-199M vs Rheem IKONIC S11i

Prices were last checked October 31, 2026 and are subject to change; the price and availability shown by the retailer at the time of purchase apply.

Which should you buy?

Scores side by side

Each factor is scored 0-100 against the same fixed reference points for every gas tankless water heater; a clear win is highlighted. How we rank explains each one.

Factor weightRheem IKONIC S11iA.O. Smith Adapt ATHR-199M
Overall rating4.1/53.8/5
Value 35%24 $9.24 per 1,000 BTU/h ($1,847.43, 199,900 BTU/h)2 $11.79 per 1,000 BTU/h ($2,346.08, 199,000 BTU/h)
Heating power 25%100 199,900 BTU/h100 199,000 BTU/h
Efficiency 15%100 0.96 UEF94 0.95 UEF
Warranty 25%100 15-year heat exchanger warranty100 15-year heat exchanger warranty

Specifications

Rheem IKONIC S11iA.O. Smith Adapt ATHR-199M
Heating
FuelGasGas
Gas typeNatural gas or propaneNatural gas or propane
Gas input11,000 to 199,900 BTU/h9,000 to 199,000 BTU/h
Max flow11.2 GPM10.5 GPM
UEF0.960.95
CondensingYesYes
ENERGY STARYesYes
Installation
VentingBothBoth
Vent type2 in or 3 in PVC/CPVC/PP2 in or 3 in PVC/CPVC/PP/ABS
Weight77.6 lb104 lb
Features
Built-in recirculationNoYes
Wi-FiYesNo
Heat exchanger warranty15 years15 years
Parts warranty5 years5 years

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