Tankless Gas Heater Sizing Guide
A tankless gas water heater must match real peak demand. The unit does not store hot water. It heats moving water as fixtures run. Correct sizing starts with gallons per minute. It then checks the required temperature rise. A cold climate needs a larger rise. A warm climate may need less burner power.
Why Flow Matters
Every fixture adds flow. A shower may use more water than a sink. Two showers can double the demand. Laundry and dishwashing can add short peaks. The calculator totals selected fixtures first. It then applies a simultaneity factor. This factor reflects how much demand may happen together. A higher factor gives a stricter design.
Why Temperature Rise Matters
Temperature rise is the outlet target minus inlet water temperature. The heater must raise every gallon by that amount. A small rise allows more flow. A large rise lowers available flow. This is why the same model can supply different homes differently. Groundwater temperature is very important.
Efficiency and Gas Input
The heat delivered to water is output. The gas burned by the unit is input. Efficiency connects these two values. A condensing model wastes less heat. A noncondensing model may need more input. The calculator also allows altitude or derate adjustment. This helps avoid undersizing where gas performance drops.
Using the Result
The result gives adjusted flow, temperature rise, output load, and minimum gas input. It also estimates usable flow at the chosen rise. Add a safety margin when comfort matters. Use higher margins for large families. Use lower margins for simple cabins. Always compare results with the heater rating chart. Manufacturer charts may show capacity at several rises. Choose a model that meets or exceeds the calculated demand. Local plumbing codes and gas line capacity still matter. A qualified installer should confirm venting, pressure, and pipe sizing before purchase.
Practical Safety Notes
Do not size by bathroom count alone. Fixtures vary by aerator, valve, and user habits. Check the coldest inlet temperature expected during winter. Include future fixtures when remodeling is planned. Keep maintenance in mind too. Scale buildup lowers heat transfer. Annual flushing can protect performance and help the heater keep its rated flow longer with less strain.