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There's a lot of misinformation and confusion surrounding the performance of tankless water heaters. Claims like "endless hot water" or exaggerated performance are too common in discussions about tankless water heaters. The reality is that a British Thermal Unit (BTU) can only generate a certain amount of heat.
So, what exactly is a BTU? Simply put, it's the amount of energy needed to raise one pound of water by one degree Fahrenheit in one hour. Now, let's explore how this relates to the performance of tankless water heaters.
Most tankless water heaters in the market are rated at 199,999 BTUs, compared to the typical 40,000 to 50,000 BTUs of a 40- or 50-gallon tank-type water heater. Naturally, tankless water heaters can heat more water per hour than tank-type heaters.
Tankless water heaters can indeed provide a seemingly endless supply of hot water under specific conditions. These conditions include the incoming water temperature, temperature rise, and flow rate. Just like running your hand quickly over a campfire won't transfer much heat, if the incoming water temperature is too low and the flow rate too fast, the heat transfer from the flame to the water won't be optimal.
However, with an optimal flow rate—around 5 gallons per minute in the Chicagoland area—the heat transfer is at its peak. For instance, considering an incoming water temperature of 47 degrees in the Chicagoland area, running a tankless water heater at 5 gallons per minute should result in a temperature rise of about 80 degrees, leading to an outlet temperature of around 127 degrees. If the flow rate exceeds 5 gallons per minute without adjustments, the temperature rise would be less than 80 degrees due to insufficient time for heat transfer into the water.
How does this apply to a household?
Here are the flow rates for most household plumbing fixtures:
- Standard shower head: 2.5 gallons per minute (GPM)
- High-efficiency showerhead: 1.5 to 2.0 GPM
- Kitchen sink faucet: 2.2 to 2.5 GPM
- Bathroom sink faucet: 1.0 to 1.5 GPM
- Water-saving faucet: 0.5 to 1.0 GPM
These flow rates can be used to determine the appropriate application of a tankless water heater to ensure optimal performance.
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