A. O. Smith TK-310U-I 2** vs Envo WH-AZ52-199-NGO: which uses less energy?
An asterisk in a model number stands for any character: one filing covers several variants.
At the high draw pattern the A. O. Smith TK-310U-I 2** and the Envo WH-AZ52-199-NGO use the same energy, 209 therms a year each, on the same federal test. Both are filed as a gas tankless unit, so the comparison holds; what separates them is below. Enter your ZIP to price either one on your own rate.
Side by side
Compared within one class only: models filed as a gas tankless unit at the high draw pattern.
| Figure | A. O. Smith TK-310U-I 2** | Envo WH-AZ52-199-NGO |
|---|---|---|
| UEF (Uniform Energy Factor) | 0.81 | 0.81 |
| Energy use per year (therms) | 209 | 209 |
| Max flow (GPM) | 4.7 | 4.8 |
| Installed cost range NLR REMDB | $1,698–$2,632 | $1,698–$2,632 |
| CPSC recall naming it CPSC | none on file | none on file |
DOE CCD · 2026-09-19 NLR REMDB · 2026-04-01 CPSC · 2026-09-24
What it costs to run
Reliability
Not published: Reliability, failure rates and each model’s actual warranty term.
The registries these figures come from certify energy performance under a federal test; none of them records how long a unit lasts or how often it fails, so this page does not rank the two on reliability.
What is here instead:
- The filed efficiency, capacity and tank size of both models, side by side above.
- No CPSC recall on file names either model (list checked 2026-09-24).
- The actual warranty term is printed in each model’s documentation, not in any registry.
Which should you pick?
- Pick the Envo WH-AZ52-199-NGO if you run more fixtures at once: it lists 4.8 GPM against 4.7.
- Neither if you need more than 4.8 GPM at the same time — neither unit lists more. Check how to size a tankless unit by GPM.
Each model on its own
- A. O. Smith TK-310U-I 2** 209 therms a year
- Envo WH-AZ52-199-NGO 209 therms a year