A tankless water heater heats water as it flows through, so it draws everything it needs at the moment of use instead of drawing from a stored reserve. The house has to supply that demand. Before you commit, check four things and ask the installer about each one in writing.
Gas supply
Ask the installer to size the gas line for the full rated input of the unit you are considering, measured from the meter to the unit. Find out whether the same line also serves a furnace, a range, or a dryer, because those appliances share the pipe and draw from it together. Ask who confirms the line size and whether the answer will be written down.
Venting
Most units need a dedicated vent path or a specified venting system. Note where the vent would exit the house, how far it would run, and how many bends it would need. Ask whether the existing chimney or wall path is suitable, and what would change if it is not. Some condensing models also produce condensate that needs a drain. Ask where that drain would go.
Electrical
Many tankless models need a dedicated circuit and a specific power supply. Ask what electrical requirement the chosen model carries and whether your panel has room for a circuit of that size. If the panel is already full, the electrical panel page on this site explains typical service life and when to plan a panel change, which affects the timing of this work.
Water flow and incoming temperature
Ask the installer what flow rate the unit delivers at the temperature rise you need in winter, when incoming water is at its coldest. Ask whether the cold supply pipe is large enough to carry that flow. Ask whether your water calls for scale protection or a softener, and whether the manufacturer requires one.
What to get in writing
Ask for the gas line size, the vent route, the circuit requirement, the expected flow at winter temperature, and the maintenance schedule the manufacturer specifies. Keep those answers with your house file. If an answer is missing, ask again before the crew arrives.