Tankless Water Heaters: 3 Installation Factors Homeowners Often Overlook

Tankless water heaters are known for their energy efficiency, compact design, and endless hot water. But installing one is not as simple as removing the old tank and plugging in the new unit.

There are technical details that can affect performance, safety, and the overall success of your installation. Here are three important factors that many homeowners overlook; each of which can impact the cost, comfort, and reliability of your system.

Your gas or electric supply may not support a tankless unit.

Before a tankless water heater can operate, it needs the right fuel source in the right quantity. Traditional tank systems use lower gas flow rates or electrical loads. In contrast, tankless models often require a significantly higher input to heat water on demand.

In gas-powered systems, most homes need a gas line upgrade to meet the BTU demands. Many tankless models require over 150,000 BTUs, which exceeds what older residential gas lines were designed to handle. For electric models, installation often requires a dedicated 240-volt circuit and a panel with enough capacity to handle the load. This isn’t always available in older electrical systems.

Without the proper utility setup, the unit will not function efficiently—or may not function at all. These upgrades can also affect installation costs and timelines, so it’s important to evaluate your current setup before choosing a unit.

Incorrect sizing can lead to cold showers under load.

Tankless water heaters must be sized according to household demand. This includes the number of people, the number of fixtures, and how many appliances use hot water at the same time. Many homeowners assume a single model will work in any home, but that’s not the case.

Every unit has a maximum flow rate it can support. If too many faucets or appliances draw hot water at once, and the unit is too small, the temperature will drop or hot water will run out. Sizing a system involves calculating peak demand in gallons per minute (GPM) and matching it to a unit that can handle that load without lag or interruptions.

Choosing the right size ensures consistent hot water delivery, even during busy times like mornings or evenings when demand is highest. A professional can assess your home’s needs and recommend a system that matches your usage patterns.

Ignoring water quality can void your warranty.

Water quality directly affects how well a tankless water heater performs and how long it lasts. In areas with hard water, mineral buildup can quickly accumulate inside the unit, reducing efficiency and increasing the risk of damage.

Manufacturers typically require the installation of a filtration system or descaling valve in hard water zones. This helps protect internal components from limescale, which can lead to overheating, reduced flow, and system failure. In many cases, failure to address water hardness can void the manufacturer’s warranty.

Including a water treatment solution as part of the installation process reduces long-term repair costs and extends the life of the system. It also helps maintain energy efficiency by keeping the heat exchanger clean and clear.

Conclusion

Installing a tankless water heater offers long-term benefits, but only if the system is matched to the home and installed with attention to key technical details.

Proper fuel supply, accurate system sizing, and water quality management are critical to performance, reliability, and warranty protection. Before moving forward with an installation, it’s important to have your home assessed by a licensed professional who can plan the job correctly from the start.

If you're considering a tankless water heater, make sure you work with a plumbing team that understands these details and can help you avoid costly mistakes.