The best tankless water heater is the one whose flow rating survives your coldest month, on a fuel your house already has. That sentence rules out most of what a search for the phrase returns: ranked model lists, each entry a winner for a household nobody describes. It also rules out any honest durability ranking, because no public failure database exists for water heaters, a gap I spelled out in the piece on water heater brands. I would not trust a top ten that pretends otherwise.
What can be ranked is fit. Four facts about your house pick the machine before a brand enters the conversation: the fuel at the meter, what the building can supply, the flow you need at your own groundwater temperature, and how hard your water is. This page walks them in that order and prints the arithmetic so you can redo it. If you are still weighing whether tankless earns its place, how long a tankless unit lasts is the fair place to start.
The short answer
- Gas at the meter, four or more people, cold-winter groundwater: a condensing gas tankless sized for about 5 GPM at a 70 °F rise. It runs about $220 a year at the site’s prices, against $315 for a gas tank.
- Gas at the meter, two people, short vent run: a non-condensing gas tankless. The efficiency premium of a condensing unit rarely comes back inside the life of a small household’s appliance.
- All-electric house in a cold climate: probably not a tankless at all. A heat pump tank runs about $194 a year, and an electric tankless about $694.
- All-electric house in the warm south, one or two people, panel with room: a whole-house electric tankless is workable, because a 45 °F rise needs far fewer kilowatts than a 70 °F one.
- One remote sink or guest bath: an electric point-of-use unit. This is the job it was built for.
- Hard water and nobody who will ever descale: buy the tank. A tankless heat exchanger punishes neglect faster.
How to pick the best tankless water heater: four questions, in order
Start with the fuel, because it is the cheapest constraint to check and the most expensive to change. A house with a gas meter and a route for exhaust can take a gas unit. A house without gas is choosing among electric options, and the cold-climate answer there is often not tankless. What the building can supply is a question for a licensed plumber on the gas side and a licensed electrician on the electrical side. I will not answer it on a web page, and the answer can delete an option from your shortlist in a single visit.
Third comes flow, and this is where most lists quietly fail. A unit does not have one flow rating. It has a flow that depends on how much the water must be heated. Delivered heat in BTU an hour is gallons per minute times 500 times the temperature rise, where 500 is 8.34 pounds per gallon times 60 minutes. Two showers running together, call it 5 GPM, in a northern winter with a 70 °F rise: 5 × 500 × 70 = 175,000 BTU an hour. A condensing burner at UEF 0.95 has to be fed 175,000 ÷ 0.95, about 184,000 BTU an hour, which is why whole-house gas units cluster near 199,000.
Now the electric version of the same shower pair. Kilowatts equal GPM times rise times 0.1465, so 5 × 70 × 0.1465 = 51 kW. The largest whole-house electric units on the market top out around 36 kW, and 36 divided by 70 × 0.1465 gives 3.5 GPM. In a house with a 45 °F rise, the same 18 kW unit that struggles up north covers 2.7 GPM. Same appliance, different house. The worksheet for your own numbers is the flow times rise sizing guide.
The fourth question is your water. Scale builds on the heat exchanger or the elements, output falls, and the lifespan on the label quietly becomes a lifespan that depends on maintenance. I will say how often it matters and leave the how-to to the manufacturer’s manual. If nobody in the house will act on a descaling schedule, the honest answer is that a simpler machine suits you better.
Side by side
| criteria | GAS, CONDENSINGtwo heat exchangers | GAS, NON-CONDENSINGone heat exchanger | ELECTRIC, WHOLE HOUSEresistance, 18 to 36 kW |
|---|---|---|---|
| Flow at a 70 °F rise | 4.1 to 5.4 GPMfrom a 199,000 BTU burner at UEF 0.95 at the topcovers a house | About 4.7 GPM199,000 BTU at UEF 0.82, worked out | 1.8 to 3.5 GPMone shower, nothing else |
| Typical UEF | 0.90 to 0.96latent heat recovered | 0.80 to 0.85heat leaves in the flue | 0.98 to 0.99no flue at allhighest on paper |
| Running cost per year | $220UEF 0.93 at $1.50 a thermcheapest | $249UEF 0.82 at $1.50 a therm | $6944,082 kWh at $0.17 |
| Ten-year energy | $2,200site model | $2,490site model | $6,940site model |
| Equipment price | $1,400 to $2,000whole-house capacities | $1,000 to $1,500simpler machinecheaper to buy | $500 to $900the box is cheaplowest price |
| What the house needs first | Gas and a draincondensate is acidic | Gas and a flue routestainless vent material | Electrical capacitythe constraint most older houses fail |
| Expected lifespan | 15 to 20 yrsscale is the clock | 15 to 20 yrsscale is the clock | 15 to 20 yrsscale on the elements |
Gas at the meter and a busy household? Condensing gas, and the running-cost gap is not close. Gas at the meter and two people? Non-condensing. All-electric and warm? Whole-house electric can work. All-electric and cold? Look at a heat pump tank first.
The ten-year bill, done out loud
The site’s model household draws 64 gallons a day at a 70 °F rise. That puts about 136.4 therms, or 4,000 kWh, of heat into the water each year. Divide by efficiency to see what the meter records. Condensing gas at UEF 0.93: 136.4 ÷ 0.93 is about 147 therms, and at $1.50 a therm that is $220 a year. Non-condensing at 0.82: 136.4 ÷ 0.82 is about 166 therms, or $249. Electric tankless at 0.98: 4,000 ÷ 0.98 is 4,082 kWh, and at $0.17 that is $694 a year, or $6,940 over ten years.
Here is the number worth carrying to your own bill. For an electric tankless to match the condensing gas figure, electricity would have to sell for roughly 5.4 cents a kilowatt-hour: $220 divided by 4,082 kWh. Ordinary residential rates sit at several times that. The gap between the two is $474 a year, $4,740 over the decade, and no difference in the price of the box comes near closing it. If your prices differ, redo the two divisions above with your own rates before you believe mine.
An honest footnote: a heat pump tank at UEF 3.5 needs about 1,143 kWh a year, $194, and beats every tankless in this table on running cost while using the fuel that just lost. In an all-electric house the question on this page is often the wrong one. The condensing decision itself is worked through in condensing against non-condensing, and the fuel decision in electric against gas tankless.
How to read any best-of list, including this one
Ask three things of every list. First, at what temperature rise is the flow figure quoted? A big number at a mild rise says little about your January. Second, which fuel is assumed, and does it match your meter? A winner in the wrong fuel is not a winner for you. Third, who is claiming reliability, and on what evidence? There is no public model-level failure record, so a confident durability verdict is one installer’s memory or a marketing page. The Department of Energy’s building science pages give the typical tankless flow as 2 to 5 gallons per minute. That range is the honest ceiling for a house, and it is why the 5 GPM household above is at the top of it, not the middle.
Where I would not buy one, and what nobody can tell you
I would not pay for a tankless in a low-use house, a rental, or a place where the water is hard and maintenance is a fantasy. The savings are proportional to the gallons, and a small household leaves the efficiency edge with nothing to work on. On the site’s model, a light household at 40 gallons a day saves only about 22 dollars a year moving from non-condensing to condensing, so the extra cost of the better unit never comes back.
Now the concession, which cuts against my own verdict. Every figure here assumes a unit that keeps its rated efficiency. Scale erodes that, and neither of us knows your hardness or your habits. And there is a real thing I cannot tell you: which brand’s condensing unit will outlast another’s in your house. Nobody can, because the data does not exist. What I can say is that both designs can be serviced at the component level, and that sizing the flow honestly matters more than the badge on the front.
Sources
- US Department of Energy, Building Science Education Solution Center, page on gas-fired tankless water heaters: the typical flow range of 2 to 5 gallons per minute.
- The modeling assumptions published on the method page: delivered-heat anchor, prices, UEF bands and equipment ranges.
Work the questions in order: fuel, what the building can supply, flow at your own rise, then your water. A model name is the last thing to pick, not the first. Every price, efficiency figure and running-cost model here is built the same way, and the workings are on the method page.