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Gas tankless vs electric tankless water heaters: which to spec
Both heat on demand with no stored mass, but fuel supply, venting, and the cold-inlet rise decide which one actually performs.
Short answer
Pick gas tankless for a whole-home or heavy steady draw, especially in a cold climate where the winter inlet forces a large temperature rise, provided the building has the gas line, meter, and venting to feed it. Pick electric tankless where there is no gas, where you cannot run venting and combustion air, or for a point-of-use load with a mild inlet. The single biggest deciding factor is the utility the building can actually feed: a gas tankless fires 150,000 to 199,000 BTU/hr and needs a 3/4 to 1 in gas line plus a vent, while an electric tankless is vent-free but its output is capped by the amperage you can run to it.
Gas tankless vs Electric tankless: side by side
| Factor | Gas tankless | Electric tankless |
|---|---|---|
| Efficiency (UEF) | ~0.80 to 0.96; condensing models reach the top of the range | High at the unit; nearly all input goes to the water, no standby loss |
| Upfront and install cost | High; add an upsized gas line and venting to the unit price | Unit often cheaper, but heavy 240V circuits can drive the install cost up |
| Utility the building must feed | 3/4 to 1 in gas line and a meter sized for 150k to 199k BTU/hr | Large dedicated circuits; a whole-home unit can swing the panel or service size |
| Venting and combustion air | Required: atmospheric, power, or condensing PVC, plus combustion air per NFPA 54; CO is a life-safety item | None: vent-free, no flue, no combustion air, no CO risk |
| Cold-inlet performance | High gas input holds output across a big winter temperature rise | Output drops hard on a cold inlet; best where the rise stays small |
| Performance limit | Flow ceiling in GPM at the rise; exceed rated GPM and every fixture goes lukewarm | Same flow ceiling, but the ceiling is set by available circuit amperage |
| Maintenance | Descale on hard water; plus vent checks and a condensate neutralizer on condensing units | Descale on hard water; no vent, combustion, or condensate to service |
| Lifespan | Commonly ~15 to 20 years with descaling | Long service life; replaceable elements, no combustion side to foul |
| Best use | Whole-home or heavy steady draw, cold climate, gas building | No gas, mild inlet, or point-of-use and lighter loads |
Which should you pick?
Choose Gas tankless when
- Whole-home or heavy, steady draw where the winter cold-inlet rise is large
- Gas service is present with a 3/4 to 1 in line and a meter that can feed 150k to 199k BTU/hr
- The electrical service is too tight to add the heavy circuits a whole-home electric unit needs
- You want condensing efficiency near 0.96 UEF and can route plastic venting plus a condensate path
Choose Electric tankless when
- No gas at the building, or no practical way to run venting and combustion air
- A point-of-use or lighter load where the required amperage stays manageable
- A mild inlet climate where the temperature rise stays small year round
- You want a vent-free install with no flue, no combustion air, and no CO risk
Bottom line
It depends on three things: the fuel the building can feed, the coldest inlet temperature you have to overcome, and the size of the load. Gas tankless is the workhorse for whole-home and steady heavy draws, and it holds output across a large winter rise because gas delivers high heat input cheaply, but it demands an upsized gas line, a meter that can feed it, and venting with combustion air that carry real life-safety weight. Electric tankless is simpler and vent-free with almost all input going to the water, but its output is capped by the circuit you can run, so a whole-home unit in a cold climate can push the electrical service hard while the same unit is a clean fit at a remote point-of-use fixture or in a mild inlet climate. Price the fuel over the life of the unit, not on install day, and confirm the utility is actually at the heater before you commit the type.
FAQ
Is gas or electric tankless cheaper to run?
It turns on local rates. Where gas is cheap it usually costs less per unit of heat, and a condensing gas unit runs up to about 0.96 UEF. An electric tankless converts nearly all its input to hot water, but electricity at a high rate makes it expensive to run. Neither one moves heat the way a heat pump water heater does, so both cost more to operate than a heat pump. Price the fuel over the unit's life.
Can an electric tankless heat a whole house?
It can, but its output is capped by the amperage you can feed it, and the rating collapses as the inlet gets colder and the temperature rise climbs. A whole-home electric unit in a cold climate can demand a heavy service and may need a panel or service upgrade. It fits best with a mild inlet or as a point-of-use unit for a single fixture; for a large cold-climate load, gas tankless usually holds output better.
Does a gas tankless need venting when an electric one doesn't?
Yes. A gas tankless burns fuel, so it needs venting and combustion air, and that is life-safety: a starved or backdrafting unit spills carbon monoxide into the room. Depending on the model it vents atmospheric, power vent, or condensing PVC, with the combustion-air rules of NFPA 54 in play. An electric tankless has no flue and no combustion air requirement, which is a real install savings where running a vent is hard.