If you are replacing your storage type water heater with a tankless water heater (also called a demand water heater), you will need to make sure that you choose a water heater large enough to meet your demands. At the same time, you don’t want to choose one that is unnecessarily large which would waste money and energy.
Storage type water heaters are sized based on their capacity and first hour rating which is the amount of hot water they can deliver in an hour. Demand water heaters don’t have a tank and are sized based on their flow rate and rise in water temperature.
To choose an appropriately sized demand water heater, you will have to determine the maximum flow rate for all of the appliances and fixtures that you will be using at the same time and how much of an increase in the water temperature you will need.
For example, if you will never have more than one shower going and can remember not to run the dishwasher or clothes washer while you are in the shower, you can choose a smaller unit. On the other hand, if you have a larger household where two people might be taking showers at the same time, or if you run the dishwater, clothes washer, bathtub or shower at the same time, you will need a larger capacity water heater.
Your plumber can help you determine the flow rate of the various fixtures and appliances in your home. With that information, you can take into consideration your lifestyle and hot water usage patterns to determine the proper size tankless hot water heater. The maximum flow rate for a typical tankless water heater is around five gallons per minute. If your needs are greater than that, consider installing two tankless water heaters to handle the load.
