A. O. Smith Signature 100 Review
The default 50-gallon gas replacement, and the one where the burner size on the box changes both the hot water and the warranty.
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Buy the 50,000 BTU version if you want hot water, and the 40,000 BTU version if you want warranty. The Signature 100 50-gallon gas tank is rated at 91 gallons in the first hour on the 50,000 BTU burner against 77 gallons on the 40,000 BTU one — but the longer cover, 9-year tank and parts with 2-year labor, sits on the smaller burner. The 50,000 BTU model carries 6-year tank and parts with 1-year labor. Both are 22 in dia x about 61 in H and both use an ultra-low NOx burner.
This is the tank most gas replacements end up being, and for good reasons: it is stocked at Lowe's, it fits the alcove the old one came out of, and the first-hour rating is the strongest of any 50-gallon unit we cover. It is also the unit where the decision is not which brand but which version of the same brand, and the box does not make that obvious.
The specification that matters
| 50,000 BTU model | 40,000 BTU model | |
|---|---|---|
| Capacity | 50 gallons | 50 gallons |
| First-hour rating | 91 gallons | 77 gallons |
| Burner | 50,000 BTU/h ultra-low NOx | 40,000 BTU/h |
| Dimensions | 61 in H x 22 in dia | 60-3/4 in H x 22 in dia |
| Warranty | 6-year tank and parts, 1-year labor | 9-year tank and parts, 2-year labor |
| Venting | Atmospheric — chimney or B-vent | Atmospheric — chimney or B-vent |
Fourteen gallons of extra first-hour delivery is close to one additional shower. Three extra years of tank cover and one extra year of labor is real money if the tank fails early. There is no version of this heater that gives you both, and which way you go should follow how many people use the house rather than which number looks better on the shelf.
Quick picks
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| # | Product | Best for | Price |
|---|---|---|---|
| 1 | A. O. Smith Signature 100, 50 gal gasThe straightforward 50-gallon gas replacement, and the reason to pay attention to the burner size: the 50,000 BTU version is rated at a 91-gallon first-hour rating against 77 gallons for the 40,000 BTU one. | Most gas tank replacements | Check price on Amazon No live price feed on this site, so we show none. #ad |
| 2 |  Rheem Performance 40 gal gasThe honest small-house gas tank: 36,000 BTU/h, around a 65-gallon first-hour delivery, and a recovery rate of about 36 gallons per hour — which is why a 40-gallon gas tank serves a family that would need a 50-gallon electric. | Smaller gas households | Check price on Amazon No live price feed on this site, so we show none. #ad |
| 3 |  Bradford White RG2PDV50S6NThe direct-vent tank to specify when combustion air is the problem: 83-gallon first-hour rating from 40,000 BTU/h, and it draws its air from outside instead of from the room. | Sealed closets and tight mechanical rooms | Check price on Amazon No live price feed on this site, so we show none. #ad |
The Signature 100 and its two alternatives
Pick 1 — Most gas tank replacements
A. O. Smith Signature 100, 50 gal gas
The straightforward 50-gallon gas replacement, and the reason to pay attention to the burner size: the 50,000 BTU version is rated at a 91-gallon first-hour rating against 77 gallons for the 40,000 BTU one.
| Capacity | 50 gallons |
|---|---|
| First-hour rating | 91 gallons (50,000 BTU model); 77 gallons (40,000 BTU model) |
| Burner | 50,000 BTU/h ultra-low NOx, or 40,000 BTU/h |
| Dimensions | 61 in H x 22 in dia (50,000 BTU); 60-3/4 in H x 22 in dia (40,000 BTU) |
| Warranty | 6-year tank and parts with 1-year labor (50,000 BTU); 9-year tank and parts with 2-year labor (40,000 BTU) |
What is good
- A 91-gallon first-hour rating from a 50-gallon tank covers back-to-back showers that empty an electric tank of the same size
- Widely stocked at Lowe's, so same-day replacement is realistic when the old one fails
- Ultra-low NOx burner meets the stricter air-district rules in California and Texas
- 22-inch diameter fits most existing alcoves without re-plumbing
What is not
- The longer 9-year warranty is on the smaller burner, so warranty and recovery pull in opposite directions
- Atmospheric venting needs a working chimney or B-vent
- Standing pilot and flue losses mean it idles energy away all day
Skip it if: Skip it if your water heater sits in a sealed closet or a space with combustion air problems. An atmospheric gas tank needs real combustion air and a real flue — the power-vent or direct-vent models exist for exactly that case.
No live price feed on this site, so we show none. #ad
Specifications above from Lowe's listing, Signature 100 50-gal 50,000 BTU; Lowe's listing, Signature 100 50-gal 40,000 BTU 9-year. We have not tested this unit ourselves.
Pick 2 — Smaller gas households
Rheem Performance 40 gal gas
The honest small-house gas tank: 36,000 BTU/h, around a 65-gallon first-hour delivery, and a recovery rate of about 36 gallons per hour — which is why a 40-gallon gas tank serves a family that would need a 50-gallon electric.
| Capacity | 40 gallons |
|---|---|
| Input | 36,000 BTU/h |
| First-hour delivery | 65 gallons (Rheem PROG40-36N data); 68 gallons on the Performance retail model |
| Recovery | 36.4 gallons per hour |
| Energy factor | 0.62 (0.58 UEF on the retail Performance model) |
What is good
- Gas recovery means a 40-gallon tank behaves like a much larger electric one
- Cheapest route to a working hot water supply for one or two bathrooms
- Short and tall variants both exist, so it fits awkward spaces
- Parts on the shelf at every supply house in the country
What is not
- Published figures vary between the professional and retail versions of the same nominal unit — read the EnergyGuide label on the box you are buying
- 0.58 to 0.62 energy factor is the low end of current gas tanks
- 40 gallons is genuinely tight above three people
Skip it if: Skip it for a household of four or more with two bathrooms. The first-hour rating is adequate for one bathroom running at a time, and the complaint that follows a too-small tank is always the same: the last shower is cold.
No live price feed on this site, so we show none. #ad
Specifications above from Young Supply listing, Rheem PROG40-36N; Rheem Performance 40 gal retail listing. We have not tested this unit ourselves.
Pick 3 — Sealed closets and tight mechanical rooms
Bradford White RG2PDV50S6N
The direct-vent tank to specify when combustion air is the problem: 83-gallon first-hour rating from 40,000 BTU/h, and it draws its air from outside instead of from the room.
| Capacity | 50 gallons nominal, 48 gallons rated storage |
|---|---|
| First-hour rating | 83 gallons |
| Input | 40,000 BTU/h |
| Efficiency | 0.71 UEF, 78% recovery efficiency |
| Venting | Direct vent — combustion air from outdoors |
| Tank lining | Vitraglas enamel |
| Safety system | Defender FVIR with ScreenLok flame arrestor |
| Warranty | 6-year limited |
What is good
- 83 gallons in the first hour from a 40,000 BTU burner is the best ratio on this page
- Direct venting solves the sealed-closet and tight-house combustion air problem outright
- Hydrojet sediment reduction genuinely helps first-hour delivery on hard water
- Defender FVIR is standard, not an upcharge
What is not
- Sold through plumbing wholesalers rather than big-box retail, so weekend DIY sourcing is harder
- 6-year warranty where some competitors sell 9- and 12-year tanks
- 0.71 UEF is ordinary — this is a reliability and venting pick, not an efficiency one
Skip it if: Skip it if your old heater vents into a shared chimney with a furnace and you were not planning to touch the venting. Direct vent means cutting a new sidewall penetration, and that changes the job from a swap into an install.
No live price feed on this site, so we show none. #ad
Specifications above from PEX Universe listing, RG2PDV50S6N; Supply House listing, Bradford White RG2PV50T6N power vent. We have not tested this unit ourselves.
What it does well
- 91 gallons in the first hour from 50 gallons of storage. That is the burner, not the tank, and it is why a gas unit covers back-to-back showers that empty a 50-gallon electric tank.
- 22 inches in diameter. It fits most existing alcoves without re-plumbing, which keeps a replacement a replacement rather than a remodel.
- Ultra-low NOx on the 50,000 BTU burner, which meets the stricter air-district rules in California and Texas.
- Stocked at Lowe's. When the old unit failed this morning, availability beats specification.
What it does not do
- It needs a real flue and real combustion air. Atmospheric venting is a hard requirement, and in a sealed closet or a tight modern house it rules this unit out entirely.
- No leak detection. If the tank sits above a finished room, pair it with a pan and a sensor, or look at a unit with an integrated shutoff.
- Standing pilot and flue losses mean it idles energy away all day — the category sits around 0.58 to 0.62 energy factor.
- The warranty and recovery pull apart, which is the single thing to get right at the point of purchase.
How it compares
| A. O. Smith Signature 100, 50 gal | Rheem Performance 40 gal gas | Bradford White RG2PDV50S6N | |
|---|---|---|---|
| Capacity | 50 gallons | 40 gallons | 50 nominal, 48 rated |
| First-hour rating | 91 gallons | 65 gallons | 83 gallons |
| Input | 50,000 BTU/h | 36,000 BTU/h | 40,000 BTU/h |
| Venting | Atmospheric | Atmospheric | Direct vent — air from outdoors |
| Efficiency | Not published on the retail listing | 0.58 - 0.62 EF | 0.71 UEF, 78% recovery efficiency |
| Where to buy | Lowe's | Big-box retail | Plumbing wholesalers only |
| Warranty | 6 or 9 years by burner size | Not published on the retail listing | 6-year limited |
The Bradford White is the better engineered unit on the venting question and the worse one on availability — wholesale-only distribution rules it out as a Saturday purchase. The Rheem 40-gallon is the honest small-house alternative: less hot water, less money, and 36.4 gallons an hour of recovery. 40 vs 50 gallon water heater works through that choice properly.
Verdict
For a gas replacement in a house with a working flue and three or more people, the 50,000 BTU Signature 100 is the straightforward answer and the first-hour rating is the reason. Take the 40,000 BTU version instead if the household is small enough that 77 gallons in the first hour is plenty, because the extra three years of tank cover is worth more to you than hot water you will not use. If there is no flue, this is the wrong tank entirely and water heater venting is the page to read first.
What goes in with it
A water heater is a system, not a box. These are the parts that go in with it, and the ones people discover they need halfway through the job.
- Expansion tank
Required by code on a closed system, which is most homes with a pressure-reducing valve.
- Drain pan
Cheap insurance anywhere the tank sits above something you care about.
- Anode rod
The part that decides when the tank leaks. Replacing it is the highest-value hour of maintenance there is.