A.O. Smith Adapt ATHR-199M
#13 in GasFull review
A premium whole home heater for buyers who want built in recirculation and top efficiency and can pay for it.
Amazon $2,346.08$2,346.08
The A.O. Smith Adapt ATHR-199M is the better tankless water heater overall, rated 3.8 to 3.4. The A.O. Smith Adapt ATHR-199M leads on warranty. The Mizudo GEN 3 199K wins on value. The Mizudo GEN 3 199K costs $1,147.08 less ($1,199 vs $2,346.08).
Mizudo GEN 3 199K
#16 in GasFull review
A connected indoor heater with full whole home power, though major brands give a longer warranty for similar money.
Amazon $1,199 Amazon $1,199$1,199
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?
- Choose the A.O. Smith Adapt ATHR-199M for warranty (15-year heat exchanger warranty vs 5-year heat exchanger warranty).
- Choose the Mizudo GEN 3 199K for value ($6.03 per 1,000 BTU/h ($1,199, 199,000 BTU/h) vs $11.79 per 1,000 BTU/h ($2,346.08, 199,000 BTU/h)).
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 weight | A.O. Smith Adapt ATHR-199M | Mizudo GEN 3 199K |
|---|---|---|
| Overall rating | 3.8/5 | 3.4/5 |
| Value 35% | 2 $11.79 per 1,000 BTU/h ($2,346.08, 199,000 BTU/h) | 63 $6.03 per 1,000 BTU/h ($1,199, 199,000 BTU/h) |
| Heating power 25% | 100 199,000 BTU/h | 100 199,000 BTU/h |
| Efficiency 15% | 94 0.95 UEF | 50 UEF not published (neutral score used) |
| Warranty 25% | 100 15-year heat exchanger warranty | 0 5-year heat exchanger warranty |
Specifications
| A.O. Smith Adapt ATHR-199M | Mizudo GEN 3 199K | |
|---|---|---|
| Heating | ||
| Fuel | Gas | Gas |
| Gas type | Natural gas or propane | Natural gas or propane |
| Gas input | 9,000 to 199,000 BTU/h | 199,000 BTU/h |
| Max flow | 10.5 GPM | 10.3 GPM |
| UEF | 0.95 | - |
| Condensing | Yes | No |
| ENERGY STAR | Yes | - |
| Installation | ||
| Venting | Both | Indoor |
| Vent type | 2 in or 3 in PVC/CPVC/PP/ABS | 3 in x 5 in concentric |
| Weight | 104 lb | - |
| Features | ||
| Built-in recirculation | Yes | No |
| Wi-Fi | No | Yes |
| Heat exchanger warranty | 15 years | 5 years |
| Parts warranty | 5 years | 2 years |