The decision matrix at a glance
| Factor | Tankless wins if | Tank wins if |
|---|---|---|
| Stay in house | 12+ years | <5 years |
| Gas line | Already 3/4" dedicated | 1/2" or shared branch |
| Household demand | 3+ bath, simultaneous showers | 1-2 bath, low simultaneous |
| Budget for install | $3,500-$7,000+ | $1,500-$2,200 |
| Mechanical room space | Tight (wall-mount saves floor) | Plenty of room for a 50-gal tank |
| Local utility rebate | Available (PG&E, ConEd, Mass Save) | None or minimal |
Annual operating cost (the number that drives 20-year math)
| Unit type | UEF | Annual energy use | Annual cost |
|---|---|---|---|
| Heat pump water heater | 3.88 | ~1,150 kWh | $184 |
| Gas tankless (condensing) | 0.95-0.98 | ~205 therms | $267 |
| Gas tank (standard) | 0.65 | ~270 therms | $351 |
| Electric tank (standard) | 0.92 | ~4,800 kWh | $768 |
Costs at $0.16/kWh electric, $1.30/therm gas. Heat pump beats every other option on operating cost. The reason tankless wins so often in comparison articles is they're comparing tankless to electric tank — which is the wrong baseline for most households. Compare to gas tank, and the annual savings is $84 — recoups a $3,300 install-cost premium in 39 years, longer than the unit lasts.
Install reality — the part that decides everything
Tank install: Swap-out is straightforward. Same gas line, same vent, same TPRV discharge. $1,500-$2,200 installed. Half-day job.
Tankless install:
- Gas line upsize. 199k-BTU calls for ~200 cu ft/hr at peak. 1/2" line of any length cannot deliver. Minimum 3/4" dedicated, often 1" if run exceeds 50 ft. Cost: $400-$1,500 accessible, $2,500+ walls open.
- Venting. Non-condensing: Category III stainless ($25-$45/ft). Condensing: PVC or polypropylene ($8-$12/ft).
- Condensate drain. Condensing units produce 0.8-1.3 gal/hr acidic condensate. Plumbing code requires neutralizer cartridge ($60-$120) and gravity drain or pump.
- 240V circuit (electric tankless only). 36 kW unit needs three 50A double-pole breakers on a 300A panel. Most 100A/200A homes need a panel upgrade ($2,500-$4,500) before electric whole-house tankless is legal.
Total installed cost (gas condensing tankless): $3,500-$7,000. Total installed cost (gas tank): $1,500-$2,200. The install delta is the number that decides whether the payback math works.
The "we'll be here 30 years" payback
Take a $5,000 condensing tankless install vs a $1,800 gas tank install. Install delta: $3,200. Annual operating savings vs standard gas tank: ~$84. Payback on operating cost alone: 38 years. Tankless lasts ~20 years. The math doesn't pencil on operating savings.
Where it pencils: If you'd otherwise pay for the second tank replacement at year 10. Tank dies → another $1,800 install. Tankless still running. So real comparison is:
| Year | Tank scenario | Tankless scenario |
|---|---|---|
| Year 0 | $1,800 install | $5,000 install |
| Years 1-10 | $3,510 in operating cost | $2,670 in operating cost |
| Year 10 | +$1,800 replacement | (still running) |
| Years 11-20 | $3,510 more operating | $2,670 more operating |
| 20-year total | $10,620 | $10,340 |
Tankless wins by ~$280 over 20 years if you stay in the house. Not a dramatic margin. The real win comes from utility rebates — PG&E, ConEd, Mass Save offer $300-$1,000 on condensing gas tankless. Check DSIRE for your zip.
Why heat pump water heaters often beat both
Annual operating cost of $184 vs $267 tankless vs $351 gas tank. That's $167/yr savings vs gas tank, $83/yr vs tankless. Over 20 years, that's $3,340 vs gas tank, $1,660 vs tankless — without counting state rebates. The 120V plug-in models from Rheem (ProTerra Plug-In) eliminated the install-cost barrier that used to kill the math on gas-to-electric swaps. If you have a basement with 700+ cu ft of free air and ambient stays above 40°F, this is now the smart-money pick — not tankless.
FAQ
Is tankless better than a tank water heater?
Better at energy efficiency + longevity. Worse at upfront cost + install complexity. Wins if you stay 12+ years AND have 3/4" gas line.
How long does tankless last vs tank?
Tankless: 20 yrs (with annual descaling). Tank: 8-12 yrs.
What's the payback period?
9-14 yrs on gas savings alone. Faster if you also amortize the second tank replacement.
Do I need to upsize my gas line?
Almost always. 199k-BTU needs 3/4" minimum. Upsize cost: $400-$2,500.
Should I consider heat pump water heater instead?
Yes — operating cost $184/yr beats both. 120V plug-in models from Rheem eliminate the panel-upgrade barrier.
The bottom line
Picture three doors. Behind door 1 is a gas tank — cheap install, 10-year life, $351/yr operating. Behind door 2 is a condensing gas tankless — $5,000 install, 20-year life, $267/yr operating. Behind door 3 is a heat pump water heater — $2,800-$5,000 install, 12-15-year life, $184/yr operating. Most homeowners assume door 2 is the right answer because that's what the SERP sells. The math says door 3 beats both — if your basement can host it. If it can't, door 1 is the right answer for <12-year stays and door 2 wins for long-term owners on gas with a 3/4" line already in place.
Editorial standards: Cited authorities include DOE Energy Saver tankless water heater guidance, ENERGY STAR Most Efficient list, DOE UEF test procedure (10 CFR Part 430), DSIRE database of state incentives. Reviewed by Al, Building Doctor — IUOE Local 39 Stationary Engineer.