Tankless Water Heater Cost: What Drives Your Quote

Updated September 2026 · Home Project Resources

Most of a tankless water heater cost estimate is not the heater. It is the gas line, the venting, the electrical, the water treatment and the wall you have to open to run any of it. Two houses can buy the identical unit and get quotes that look like they came from different planets, because one house already has a 3/4 inch gas line and an exterior wall to vent through, and the other needs a meter upgrade, forty feet of new pipe, and a vent path that crosses a finished ceiling.

That means your job before you collect bids is to find out which house you live in. Six questions settle most of it, and you can answer them yourself in an afternoon with a tape measure and a flashlight.

Updated September 2026

A note on numbers. There are no dollar figures here. Gas work, electrical work and drywall repair are priced completely differently in different markets, and the install scope swings far more than the equipment does anyway. What follows is the scope, the sizing math, the line items that belong in a written quote, and the things that legitimately push a number up after demo starts.

Budget the wall repair, because almost nobody does. Gas line runs, vent penetrations and relocating the unit all mean opening drywall, and patching is a separate trade. Measure the openings and get your sheet, mud and tape quantities from our free Drywall Calculator, then ask each bidder in writing whether patch, tape, texture and paint are in their scope or yours.

What drives a tankless water heater cost estimate

Work down this list in order. The first three account for most of the spread between quotes.

Notice that only one row on that list is about the appliance. If a bidder gives you a single number with no breakdown, you cannot tell which rows they priced and which ones they plan to discover later.

Size it before you shop, not after

Tankless units are rated by temperature rise at a given flow, and that is the only spec that matters. The formula is simple:

BTU per hour needed = gallons per minute x 500 x temperature rise in degrees F

Temperature rise is your target output temperature minus your incoming water temperature. Incoming water runs roughly 40 to 55 degrees F across the northern half of the country in winter, and 65 to 77 degrees F in the deep south. Target output for household use is usually 120 degrees F at the heater, with 105 to 110 at the shower after mixing.

Typical fixture flows, which you should confirm by reading the face of your own showerheads and aerators:

Add up the fixtures that genuinely run at the same time in your house on a bad morning. Two showers plus a kitchen faucet is a realistic worst case for a family of four. A single bathroom household almost never needs more than one shower plus one faucet.

One caution about reading spec sheets. Manufacturers advertise the input rating in BTU, but the number you need is the published gallons per minute at your temperature rise. A 199,000 BTU residential gas unit typically publishes somewhere around 5 gallons per minute at a 60 degree rise, not the number the raw math suggests, because efficiency and internal limits derate it. Always read the flow chart in the manual for the rise you actually have.

Worked example: sizing a 2.5 bath house in a cold climate

Here is the whole calculation with real numbers.

Step 1. Measure incoming water. Run the cold tap for two minutes and put a thermometer in the stream. Say it reads 50 degrees F in February.

Step 2. Pick the output. Target 120 degrees F at the heater. Rise = 120 minus 50 = 70 degrees F.

Step 3. Add up simultaneous flow. Two 2.0 gpm showers plus a 1.5 gpm kitchen faucet = 5.5 gpm.

Step 4. Do the math. 5.5 x 500 x 70 = 192,500 BTU per hour of delivered heat. Divide by a 0.95 thermal efficiency and you need about 202,000 BTU per hour of input, which is past the top of the common 199,000 BTU residential range.

Step 5. Decide what to change. Three honest options. Drop to 1.75 gpm showerheads and accept a 4.0 gpm peak, which brings the requirement down to about 140,000 BTU per hour of delivered heat. Install two units in parallel, which doubles the gas demand and the vent work. Or stay with a tank, which handles short simultaneous peaks by having hot water already sitting there.

Step 6. Check the same house in July. Incoming water is 68 degrees F, so the rise is only 52 and the same unit delivers far more flow. That is why winter is the only season worth sizing for.

That example is the reason so many tankless installs disappoint. Nobody sized for the coldest month, and the unit is fine in August and thin in January. Our full comparison of tankless vs tank water heaters walks through the households where each one wins.

The gas problem, explained plainly

A tank water heater usually burns 30,000 to 50,000 BTU per hour. A whole home tankless unit burns up to 199,000. That is roughly four times the demand through the same pipe, and gas pipe capacity depends on diameter, total run length and how many other appliances are on the line.

What that means in practice:

• A 1/2 inch line that fed the old tank almost never feeds a whole home tankless unit.
• 3/4 inch is the common minimum, and long runs often need 1 inch.
• Your gas meter has a rated capacity in cubic feet per hour. Add up the furnace, range, dryer, fireplace and the new heater. If the total is over the meter rating, the utility has to swap the meter, and that is a separate appointment on a separate schedule.
• Flexible CSST is fast to install but has its own sizing tables and bonding requirements, so it is not automatically a shortcut.

Ask every bidder two direct questions: what pipe size are you running and how did you size it, and have you confirmed meter capacity with the utility or am I finding that out on install day. The answers separate the professionals immediately.

The electric version has a bigger problem

Electric tankless skips the venting entirely, which sounds great until you look at the amperage. Take the same 5.5 gpm at a 70 degree rise from the worked example. That is 192,500 BTU per hour, and one kilowatt equals 3,412 BTU per hour, so you need about 56 kW. At 240 volts that is roughly 235 amps of dedicated load.

Even a modest whole home electric unit at 27 kW draws about 113 amps and typically lands as three 40 amp double pole breakers. Most homes with a 100 or 150 amp service simply cannot do it without a service upgrade, and a service upgrade brings in the utility, the meter base, the panel and often a permit and an inspection.

Electric tankless makes real sense in one place: point of use. A small unit under a distant bathroom sink or feeding a single shower in a warm climate is a genuinely good application, and it runs on a circuit the panel can actually spare.

Every line item that belongs in a written quote

Print this and check it against each bid.

1. The unit, listed by model number, with its published gpm at your winter temperature rise.
2. Gas line: size, material, total run length, and whether the meter is being verified or upgraded.
3. Venting: material, diameter, total length, number of elbows, termination location and clearance from windows and soffits.
4. Combustion air, and confirmation that the unit is direct vent or that the space has adequate makeup air.
5. Condensate: drain line, trap, condensate pump if needed, and a neutralizer for condensing units.
6. Water connections: new shutoffs, a service valve kit with isolation valves and hose bibs, and a sediment screen.
7. Expansion tank or thermal expansion control, where the home has a closed system with a check valve or pressure reducing valve.
8. Electrical: the circuit for the controls and ignition on a gas unit, or the full breaker and wire schedule on an electric one.
9. Mounting: backing board, anchors, and any framing added inside the wall.
10. Removal and disposal of the old tank, including draining it and getting it out of the space.
11. Drywall, insulation and paint repair, or a clear statement that they are excluded.
12. Permit and inspection fees, plus who pulls the permit.
13. Startup: gas pressure verified under load, temperature rise verified at a fixture, and the flow chart demonstrated to you.
14. Warranty: heat exchanger, parts and labor, each with its own term, plus what voids it. Skipped descaling voids more warranties than anything else.
15. Recirculation, if you want fast hot water at a far fixture, including whether it is a dedicated return line or a crossover valve at the far fixture.

Line 15 catches a lot of people. Tankless units have a small delay before the burner fires and hot water arrives, and if your old tank sat closer to the bathrooms than the new location does, the wait gets noticeably longer. Recirculation solves it, but it is a scope item, not a setting.

What makes the number go up after work starts

These are the legitimate surprises. A good contractor tells you they are possible before the first swing of the hammer.

• The wall the unit mounts to turns out to have a stack, ducting or a structural post exactly where the vent needs to go.
• The gas run has to leave the joist bay it was supposed to follow and take the long way around a beam.
• The utility says the meter needs upgrading, which stalls the job on their schedule.
• The old tank was leaking slowly, and there is rot or mold under it that has to be dealt with before anything gets mounted.
• The water main pressure reads high, which means a pressure reducing valve and then an expansion tank to go with it.
• Local code requires a specific vent termination clearance that pushes the unit to a different wall.

And here are the ones to push back on: a permit "we usually skip," a vent run reused from the old tank, and any quote where the gas line size is not written down.

Hard water is the maintenance line nobody quotes

Tankless heat exchangers have narrow passages and very high surface temperatures, which is the perfect recipe for scale. In hard water, an annual descaling flush with a pump and a food safe acid solution is not optional, and most manufacturers make it a warranty condition.

Two things make it painless. First, insist on an isolation valve kit at install, so a future flush is a 45 minute job instead of a plumbing project. Second, find out what your hardness actually is. Run through the signs of hard water and test with a strip. If you are treating the water anyway, the heat exchanger is one of the biggest beneficiaries.

For comparison on the tank side, the same principle applies with a different procedure. See how to flush a water heater for that one.

Comparing bids apples to apples

Fill this in for each bidder before you look at any bottom line.

If one bid is dramatically lower, run down the column and find the empty cells. That is where the difference lives, every single time. Our guide on how to get contractor bids covers how to send the same scope to all three so the numbers mean something.

Frequently asked questions

Is a tankless water heater cost worth it for a small household?

For a one or two person household in a warm climate with a single bathroom, the sizing math works easily and the install is usually simple, which is the best case. In a cold climate house with simultaneous showers, you are buying a bigger unit, more gas capacity and more vent work for the same daily hot water. Size for your coldest month before deciding.

Do I need a bigger gas line for a tankless water heater?

Usually yes. A whole home tankless unit can demand roughly four times the gas of the tank it replaced, so the 1/2 inch line that fed the old heater rarely qualifies. The installer should size the pipe from the total connected load, the run length and the meter capacity, and write that size into the quote.

Can I install a tankless water heater in a garage or attic?

Often yes, with conditions. Gas units in garages usually have ignition source elevation requirements, and any unit in an unconditioned space needs freeze protection, which means keeping it powered and sometimes adding heat or insulation. An attic install also needs a drain pan and a leak path that does not run through a ceiling.

How long does a tankless install take?

A straight swap in the same location with an existing 3/4 inch gas line and an easy exterior vent is often a one day job. Add a gas line run, a relocation, a meter upgrade or an electrical service change and it becomes a multi day job with a scheduling dependency on the utility or the inspector.

Does a tankless water heater need a permit?

In most jurisdictions yes, because it involves gas piping, venting and sometimes electrical work. Permits also protect you: the inspection is a free second opinion on the vent, the gas sizing and the combustion air. Check the rules for your area first with our guide on whether you need a permit.

What maintenance does a tankless unit actually need?

Annual descaling with a circulation pump through the isolation valves, a rinse of the inlet screen, an inspection of the vent termination for blockage, and a check of the condensate neutralizer media on condensing units. Skipping the descaling in hard water is the fastest way to lose both efficiency and warranty coverage.

Should I just replace my tank with another tank?

That is a perfectly good answer for a lot of houses, especially when the existing gas line, vent and location all still work. Our guides on water heater installation and water heater replacement cover that path, including the sizing and first hour rating math that makes a tank feel bigger or smaller than its gallon count.