The output of a tankless direct vent water heater is rated by a temperature rise for a given flow rate. Most water heaters sold today can typically provide between two to five gallons of hot water per minute (GPM) at a 77 degree fahrenheit temperature rise.
To understand how they arrive at this, let’s look at these numbers. The temperature rise is defined as the difference in temperature between the supply water entering the hot water heater, and the temperature of the hot water that leaves the water heater. Sounds simple enough, but there is a caveat. Since the incoming temperature of supply water can vary widely, especially depending on the source – water well versus city water supply, it’s best to be conservative and stick with the 77 degree temperature rise rating when reading the manufacturers product literature.
Often manufacturers will list a flowrate that appears to be quite high, but they do this by decreasing the temperature differential. For instance, a water heater that’s alleged to have a 9 gallon per minute output is doing so by only providing a 35 degree rise in temperature.
If your supply water comes from a well, it may be quite cold. As an example, if your well water is 45 degrees fahrenheit, a conservative estimate as some wells provide water that’s considerably cooler than that, a water heater that supplies 9 gallons per minute is delivering water that’s only heated to 80 degrees fahrenheit. So you can see, that’s not very practical when you’re trying to shower with it, or do laundry and dishes. Because of all this, you should keep this in mind when you see very high advertised flow rates, and expect a very small temperature rise.
So what temperature should you set your water heater? Most guidelines suggest that a temperature of 120 degrees fahrenheit is the perfect temperature. This is because it’s low enough to save you money and prevent scalding, which is especially important if you’ve got young children.
And it’s high enough to keep the water safe from some bacteria that could grow in the water. Legionella, the bacteria that causes Legionnaires’ disease, could possibly survive at water temperatures below 120 degrees fahrenheit. By keeping the water temperature at or above 120, you’ll significantly reduce the chances of this happening.
So with a 77 degree temperature rise, and a water temperature setpoint of 120 degrees, your water supply into the water heater can be as cool as 43 degrees and you can be assured that it will deliver it’s rated flowrate @77 degrees.