10 Things to Know Before Buying a Tankless Water Heater

Have you ever stepped into the shower expecting soothing heat… and instead got blasted with ice-cold water that hits you like a slap in the face? That kind of shock snaps you awake fast, and it’s usually the moment you realize your water heater isn’t keeping up anymore.

That’s when most homeowners start looking at tankless water heater systems. But here’s the part people never think about: switching to tankless without understanding a few key details can still leave you standing in that same cold shower, just with a more expensive unit on the wall.

Before you spend the money, here are 10 things you need to know so your next water heater actually delivers the endless hot water you’re expecting.

1. Know How a Tankless Water Heater Actually Works

A tankless water heater heats water the moment you turn on a faucet. Cold water flows through internal heating coils, and within seconds, you get hot water, no storage tank, and no reheating all day long.

Because it works on demand, the size and setup matter. If the unit is too small or installed incorrectly by a plumber, the system may struggle to meet your home’s hot-water needs. That’s why proper tankless water heater installation is essential for steady, reliable performance.

2. Size Matters More Than You Think

A tankless system heats water on demand. That means the unit’s size must match the amount of hot water your home uses at once. Two showers running? A shower plus a dishwasher? A laundry load at the same time?

If the system is undersized, the water may turn lukewarm, or the heater may fail to keep up. Oversizing can raise installation costs without improving performance.

That’s how crucial a professional plumber is during tankless water heater installation. They calculate your home’s flow rate (GPM) and the required temperature rise to ensure you get steady, comfortable hot water in every room. 

Choosing the right size upfront saves you from cold surprises, wasted money, and performance issues later.

3. Your Gas or Electrical System May Need Upgrades

A tankless unit needs more power at once than a traditional tank. That means your current gas line or electrical panel might not be strong enough to support it.

For gas systems, the line may need a larger diameter to deliver the higher BTU demand. For electric units, the panel may need new high-amp breakers before the system can run safely. This is why a professional plumber checks your home’s energy supply before moving forward with any tankless water heater installation.

It’s part of making sure the heater runs smoothly, avoids shutdowns, and doesn’t lead to future tankless water heater repair calls.

4. Venting Requirements Are Stricter

Tankless units, especially gas models, need a safe path for exhaust. Because they burn hotter and work faster, they require a dedicated vent system to safely exhaust the exhaust outside. You may need new vent pipes, proper clearance, and the right materials, not the same duct your old heater used

Others need special materials rated for high heat. A plumber will evaluate venting before installation because an improper setup can trigger shutdowns, reduce performance, or pose safety risks.

Good venting keeps the system efficient, protects your home, and prevents surprise issues that lead to tankless water heater maintenance down the road.

5. Hard Water Can Damage a Tankless Heater

If your home has hard water, a tankless unit will feel the impact fast. Minerals like calcium and magnesium don’t just stick in your pipes; they collect inside the heat exchanger, the most important (and most expensive) part of the system.

Without protection, that mineral buildup forces the unit to work harder, reduces heating efficiency, and can lead to sudden shutdowns. Many homeowners think a tankless system “went bad,” when the real issue is untreated hard water slowly clogging the system.

This is why plumbers often recommend a softener, pre-filter, or regular plumbing maintenance to keep things running smoothly. A small step now prevents major repairs later, and protects the investment you’re making in your tankless upgrade.

6. You Still Need Yearly Maintenance

Tankless heaters don’t store water, but that doesn’t mean they run forever without care. Over time, minerals, debris, and everyday use leave wear inside the system that affects performance.

If it’s never cleaned, the unit can lose efficiency, heat inconsistently, or trigger error codes that stop the system entirely. 

That’s why a quick tankless water heater maintenance is necessary. It’s a simple routine that prevents bigger problems and keeps your tankless heater lasting as long as promised

7. Installation Costs More, But for a Reason

Many homeowners are surprised when the quote for a tankless system is a bit higher than for a standard tank, but there’s a reason. The installation is more complex, which is why the upfront price is higher. You’re paying for proper sizing, correct setup, and long-term reliability, which avoids tankless water heater repair down the road. The upfront investment protects you from larger expenses later.

8. Tankless Doesn’t Mean “Instant Hot Water”

Many homeowners expect a tankless system to deliver hot water the second they turn the tap, but that’s not how any plumbing system works. A tankless heater warms water immediately inside the unit, but the hot water still has to travel through your pipes to reach the faucet.

That means you may wait a few seconds, just like with a traditional heater. The difference is what happens after that wait: the hot water never runs out. Once the flow reaches you, it stays consistent, whether you’re showering, washing dishes, or running multiple fixtures at once.

So tankless gives you endless hot water, not instant hot water, and knowing that upfront helps set the right expectations before you buy

9. Tankless Heaters Last Longer and Save More Over Time

One of the biggest advantages of going tankless is longevity. While a traditional water heater usually lasts 8–10 years, a well-installed tankless system can often run for 15–20 years with proper care. That extra lifespan alone offsets a good portion of the upfront cost.

But the savings don’t stop there. Because tankless systems only heat water when you need it, they use less energy, resulting in lower utility bills. And when something does need attention, most units allow quicker diagnostics and targeted tankless water heater repair rather than replacing a whole tank.

10. Not Every Home Is Immediately “Tankless Ready”

Before switching to a tankless system, some houses need plumbing adjustments, new venting paths, or upgraded energy lines before a tankless water heater installation can even begin.

Older homes may not have enough water pressure. Some houses need stronger electrical circuits or larger gas lines. Even the wall location matters, because tankless systems require safe venting and enough room to breathe.

This doesn’t mean you can’t upgrade to a tankless system; your home might need a few adjustments before an installation works properly. A plumber can check your system and tell you what your home needs before tankless water heater installation.

Upgrade To a Tankless Water Heater with the Help of a Reliable Plumber

Switching to a tankless system is a smart move, but only when your home is assessed correctly, sized, and prepared. A tankless heater offers unlimited hot water, lower energy bills, and a modern setup, but installation must be done correctly.

That’s why having a dependable plumber matters. A trained plumber checks your home’s gas lines, electrical load, plumbing layout, venting, and water quality before installation, so you avoid surprises, extra costs, or disappointing performance later.

If you’re ready to upgrade, Cornwell Plumbing can guide you through every step. Their team explains what your home needs, handles the technical work safely, and ensures your new tankless system runs exactly as it should. This type of system is an investment, so let the right plumber handle your tankless water heater installation.