Tankless water heaters: are they actually worth it?

Your tank water heater is getting old, you've started shopping around, and every contractor and YouTube video is telling you the same thing: just go tankless. Endless hot water, lasts forever, takes up no space. What's not to love?

Quite a bit, depending on your house. Tankless is a great fit for some homes and a genuinely bad financial call for others, and the contractors quoting you aren't always being straight about which one you have. Here's the honest version.

What tankless is

A traditional water heater keeps 40 or 50 gallons of water hot in a tank, around the clock, whether you're using it or not. A tankless unit heats water on demand — you turn on a hot tap, water flows through the unit, a burner or heating element fires up, and hot water comes out the other side. When you shut the tap off, the unit shuts off.

That's the core idea, and it's where most of the pros and cons come from.

The pros

The benefits are actually quite real, and the ones contractors lead with are mostly accurate.

• Endless hot water. As long as the unit can keep up with your flow rate, you'll never run out mid-shower because someone else used it first.
• Smaller footprint. A tankless unit mounts on the wall and is roughly the size of a carry-on suitcase, which frees up floor space in a closet, basement, or garage.
• Longer lifespan. A typical tank heater lasts 10 to 12 years. A well-maintained tankless unit can run 20 years or more, which matters if you're staying in the house.
• Lower standby losses. A tank is constantly losing heat to the surrounding air and reheating itself to make up for it. Tankless skips that entire cycle, which is where the energy savings come from — usually somewhere in the 10 to 30 percent range on water heating, depending on how much hot water you use.

But be sure to also carefully consider the cons listed below to make sure you're thinking it all the way through!

The cons

This is the part a contractor selling you a tankless unit may not volunteer.

• The install is the expensive part. The unit itself isn't dramatically more than a quality tank. The install is what hurts. A like-for-like tank replacement might run you $1,500 to $2,500 all in. A tankless install commonly runs $3,500 to $6,000, and can go higher.
• Gas-line upgrades. A gas tankless unit fires at a much higher BTU rate than a tank — often 150,000 to 200,000 BTU versus 40,000 for a tank. Your existing 1/2-inch gas line may not be sized to feed it, and running a new 3/4-inch line from the meter is a substantial chunk of the install cost. This is the line item that most often gets buried in the quote.
• Electrical panel upgrades. An electric whole-house tankless unit can pull 100 to 150 amps. If you have a 100- or 150-amp panel, you may be looking at a panel upgrade before this is even possible. That alone can be $2,000 to $4,000.
• Cold-water sandwich. When you turn the tap off and back on quickly, you get a brief slug of cold water between two hot ones, because the unit takes a second to fire. It's a minor thing, but it's a real thing.
• Hard water eats them. Tankless units are much more sensitive to scale buildup than tanks. If your water is hard and you skip maintenance, the heat exchanger scales up, efficiency drops, and eventually the unit fails early. A water softener helps a lot here.

Maintenance

A tank water heater is mostly install-it-and-forget-it. You might drain a few gallons off the bottom once a year if you're conscientious, but plenty of people never touch theirs and get a full lifespan out of it anyway.

Tankless is not like that. To get the 20-year lifespan, you need to do this:

• Annual descaling flush. Once a year, you hook a small pump up to the service valves on the unit, circulate white vinegar or a commercial descaler through the heat exchanger for 45 minutes to an hour, then flush it with fresh water. A descaling pump kit runs around $150 and lasts forever, or you can have a plumber do it for $150 to $300 a visit.
• Inlet filter cleaning. There's a small screen on the cold-water inlet that catches sediment. Pull it, rinse it, put it back. Five minutes, but it has to happen.
• Venting matters. Gas tankless units need their own dedicated venting — usually stainless steel or special PVC depending on the model — and the vent has to stay clear. This is the contractor's problem at install but yours forever after.

Skip the annual flush in a hard-water area and you can kill a tankless unit in five or six years, which wipes out the entire economic argument for buying it in the first place.

How to decide

Four questions worth working through.

1. How big is your household, and how does it use hot water? A family of five with three teenagers showering back-to-back is the textbook tankless win — the endless-hot-water benefit pays off every single day. A couple with no kids gets very little from that benefit and is mostly paying extra for the privilege.
2. How hard is your water? If you're on city water that's reasonably soft, or you already have a water softener, you're fine. If you're on a well with hard water and no softener, tankless is a much riskier bet unless you're committed to the annual flush or willing to add a softener to the project.
3. What fuel are you on, and what does the infrastructure look like? Gas tankless with an existing properly sized gas line is the cleanest case. Electric tankless with an old panel is the worst case — you may spend more on the panel than on the heater. Get the quote to itemize the gas-line work and any electrical work as separate line items before you agree to anything.
4. How long are you staying in the house? The payback on tankless versus a high-efficiency tank is usually somewhere in the 10 to 15 year range once you account for the install premium. If you're moving in five years, you'll never see the savings, and you don't recover the install premium on resale.

If you've got a big family, reasonable water, an existing gas line that's sized right, and you plan to be in the house for 15 or 20 years — go tankless. It's a good call.

If you're a smaller household, on hard water, facing a gas-line or panel upgrade, or unsure how long you'll be there — a high-efficiency tank is very likely the smarter buy, and a good one will still get you better efficiency than the unit you're replacing.

One last thing. When you get the tankless quote, ask the contractor to break out the unit, the install labor, the gas-line work, the venting, and any electrical work as separate line items. If they push back on itemizing it, that tells you something about the quote... and the contractor.