Tankless vs Tank Water Heaters
The tank wins most replacements. Here is exactly when that flips.
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The tank wins most replacements. It is cheaper, it is a like-for-like swap, and on gas its first-hour rating is excellent — 91 gallons from a 50-gallon unit. Tankless wins when your gas line already carries 199,900 BTU/h, you value the floor space, or you are already doing construction. The answer flips on the install, not the appliance.
Both of these are good technologies. The reason this comparison is contentious is that the marketing compares appliances while the money is spent on installations, and the installation difference is enormous.
Where the tank wins
- It is a swap, not a conversion. Same gas line, same flue, same circuit, same footprint. Often a half-day job.
- Lower purchase price, consistently, at every capacity.
- Excellent first-hour rating on gas. A 50,000 BTU, 50-gallon unit delivers 91 gallons in the first hour, which covers five sequential showers.
- No annual maintenance requirement. Flushing a tank is good practice; descaling a tankless unit is a manufacturer requirement with warranty implications.
- It keeps working in a power cut if it is an atmospheric gas unit with a standing pilot. No tankless unit does.
- Simplicity. Fewer components, fewer error codes, and any plumber can service one.
Where tankless wins
- It cannot run out. Five showers in a row, or ten. This is the complaint tankless actually fixes, and nothing else fixes it as completely.
- No standby loss. A tank reheats 50 gallons all day whether you are home or not.
- Floor space. A wall-hung unit is 18 by 10 by 27 inches against a 22-inch-diameter, 61-inch-tall cylinder.
- Longer service life — up to 20 years against 10 to 15 for a tank, across the sources we checked, provided the annual descale happens.
- Higher efficiency. 0.93 to 0.98 UEF against 0.6 to 0.7 on a gas tank.
- No catastrophic flood risk. There are no 50 gallons sitting in it waiting to let go.
| Gas tank (50 gal, 50,000 BTU) | Gas tankless (199,900 BTU) | |
|---|---|---|
| Hot water available | 91 gal in the first hour, then recovery | 5.5 GPM at a 70°F rise, indefinitely |
| Efficiency | About 0.6 to 0.7 UEF | Up to 0.93 to 0.98 UEF |
| Standby loss | Continuous | None |
| Footprint | 22 in dia x 61 in tall, on the floor | 18-1/2 x 9-3/4 x 27-1/2 in, on a wall |
| Service life | 10 to 15 years | Up to 20 years |
| Annual maintenance | Optional flush | Required descale |
| Gas supply | 40,000 to 50,000 BTU/h | Up to 199,900 BTU/h |
| Venting | Existing flue, usually | New PVC or stainless run |
| Condensate | None | Yes, on condensing units |
| Works in a power cut | Yes, if atmospheric with a standing pilot | No |
| Install complexity | Low | High |
Cost of ownership, as honestly as we can state it
We are not going to print an install cost figure. Published ranges for overlapping scopes of work in this category differ by thousands of dollars, and averaging them would invent a number nobody published. What we can do is name what actually drives the difference:
| Item | Tank | Tankless |
|---|---|---|
| Gas line | Usually reused as-is | Frequently needs upsizing — often the largest single cost |
| Venting | Existing flue | New run; PVC if condensing, stainless if not |
| Condensate | None | Drain or a condensate pump |
| Electrical | None for atmospheric | An outlet at the unit |
| Isolation valves | Standard shutoffs | Service valves, needed for the annual flush |
| Labor hours | Half a day | A day or more |
The best of each
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 — Most whole-house gas jobs
Rheem Prestige RTGH-95DVLN
The condensing gas tankless most houses should start with: 5.5 GPM at a 70°F rise covers two showers in a cold-inlet climate, and the 12-year residential heat exchanger warranty is the longest on this list.
| Fuel | Natural gas (LP variant sold separately) |
|---|---|
| Gas input | Up to 199,900 BTU/h |
| Flow at a 70°F rise | 5.5 GPM |
| Maximum flow | 9.5 GPM at a 35°F rise |
| Efficiency | Up to 0.93 UEF, stainless condensing heat exchanger |
| Minimum activation flow | 0.4 GPM |
| Venting | Up to 150 ft of 3-inch PVC; 60 ft of 2-inch PVC |
| Dimensions | 18-1/2 in W x 9-3/4 in D x 27-1/2 in H |
| Warranty | 12-year heat exchanger (residential), 5-year parts, 1-year labor |
What is good
- The 70°F-rise figure is published, so you can size it honestly instead of guessing from the headline number
- Condensing design vents in PVC, which is far cheaper to run than stainless category III pipe
- 0.4 GPM activation means a trickle at a bathroom faucet still fires the burner
- Long PVC vent runs let you put it where the gas line already is
What is not
- Needs a gas line sized for nearly 200,000 BTU/h — many existing 1/2-inch runs are not
- Condensate has to go somewhere, which means a drain or a condensate pump
- Annual descaling is not optional on hard water
Skip it if: Skip it if your gas meter and line cannot carry 199,900 BTU/h. Upsizing the gas line is often the single largest cost in a tankless conversion, and no amount of unit quality fixes a starved burner.
No live price feed on this site, so we show none. #ad
Specifications above from Rheem RTGH series specification sheet (PDF); Pace Supply listing, RTGH-95DVLN, 199 kBtu/hr. We have not tested this unit ourselves.
Pick a tank if...
- Your current heater has failed and you need hot water today.
- Your gas line feeds 40,000 BTU/h and nobody has established it can carry 200,000.
- You are not planning to be in the house for another decade.
- The heater is in a basement or garage where floor space is worthless.
- You want an appliance that keeps working when the power is out.
- You will not reliably do an annual descale. Be honest about this one.
Pick tankless if...
- The complaint is running out of hot water, and a bigger tank will not fit.
- Your gas supply is already adequate — a house with a large furnace off a generous trunk often is.
- You are remodeling and the walls are already open.
- The heater's floor space is genuinely valuable, as in a small condo or a converted closet.
- You travel often and standby loss on an empty house annoys you.
- You want the longer service life and will do the maintenance to get it.
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.
- Flush kit
The annual descale is not optional on a tankless unit, and you need a pump, hoses and a bucket.
- Descaler solution
What actually dissolves the scale once the kit is hooked up.
- Recirculation pump
Fixes the cold-water delay that makes new tankless owners call a plumber.