About once a week we get an email asking about modulating tankless water heater.
Modulating is basically matching the burner output to the demand. A good analogy is a car. Your car has a wide range of speeds. You control the speed by the gas pedal. You want to go faster you step on the accelerator resulting in burning more gas. A modulating tankless works in the same since, but instead of “speed” its hot water. When you need more hot water your tankless burns more gas.
For example, if you have a tankless water heater rated at max flow rate of 8.0 GPM with 152,000 BTU. This unit will use 152,000 BTUs to heat up 8 gallons of water. So if you are washing your hands in the bathroom, you will most likely need around 0.5-1.5 GPM. A modulating tankless water heater will only use enough BTUs (roughly 25,000 BTUs) to heat up the 0.5-1.5 GPM. Now if you are taking a shower (2 GPM), running the dishwasher (1 GPM) and doing a load of laundry (2 GPM) for a total of 5 GPM, the water heater will use roughly 95,000 BTUs.
One thing to consider, which many manufactures don’t openly share is, the energy factor is based on max BTU operation. The energy factor drops as the BTU’s used is drops. For a more in depth understanding of this we recommend reading this article.
This modulating helps reduce the amount of gas you will burn, thus reducing the total cost of operating the water heater. Almost all the tankless water heaters we have reviewed are modulating. Our opinion is that this is an absolute must have.