Tankless hot water heaters are known for providing a never-ending supply of hot water, but they will only turn on if the flow rate exceeds their activation rate. Whenever the flow rate remains below their activation rate, they will remain off -- and your water will remain cold. Therefore, if you hope to enjoy long, hot showers after installing a tankless water heater, you should first ask a plumber to make sure that your showerhead's flow rate is at least as fast as the activation rate of the tankless water heater you're considering having installed.
Tankless Water Heater Activation Rates
Unlike traditional water heaters that supply hot water regardless of how low you turn the faucet on, all tankless water heaters have an activation rate. For single-family homes, M. Scott Gregg recommends models that have an activation rate of 0.5 gpm, meaning that they turn on whenever half of a gallon of water flows through them in one minute. Some models used in homes, though, have activation rates of 0.75 gpm, as Gregg notes.
Low-Flow Showerheads Have Low Flow Rates
Low-flow showerheads use less water by lowering their flow rates. According to A Great Shower's chart, efficient showerheads can have flow rates between 1.25 gpm and 2.0 gpm.
Although these rates are well above the activation rate of tankless water heaters, their actual flow rate will probably be lower than the claimed flow rate. In his article, Gregg explains that flow rate decreases as water travels through pipes in a home. For example, if you were to install a low-flow showerhead that had a flow rate of 1.25 gpm right next to a tankless water heater, it's flow rate would be close to 1.25 gpm.
If this same showerhead were installed three stories up and on the other side of the house, though, the flow rate might be much lower than 1.25 gpm. In fact, it may be below 0.75 gpm and not activate your tankless water heater, depending on your heater's activation rate.
Have a Plumber Check Your Showerhead's Flow Rates
To avoid a situation where a showerhead doesn't use enough water to turn on your tankless water heater, ask your plumber to check the flow rates of all your showerheads. If your plumber expresses concern that the rates are too low for your new tankless water heater, there are a few possible solutions.
The cheapest option is to run two faucets at once. While your showerhead might not have a high enough flow rate, turning the sink on when you take a shower will increase how much hot water is needed. Their combined flow rate will exceed your tankless water heater's activation rate, even if they're located away from it.
Alternatively, your plumber could install new showerheads. Since your plumber will already be coming to install your water heater, you may be able to save on labor costs by having them replace any low-flow showerheads at the same time.
Finally, your plumber might be able to adjust the activation rate on your new tankless water heater so that your current showerhead's flow rate will activate it. This is the best solution, as it doesn't waste water or require new showerheads. Your plumber should only lower the activation rate as long as doing so won't void your tankless water heater's warranty.
A tankless water heater will let you shower as long as you'd like, provided that your showerhead's flow rate is above its activation rate. Before you have a tankless water heater installed, talk with your plumber to make sure you'll have hot water. If your showerhead's flow rate is below the activation rate, your plumber will be able to help you find a solution. Check out websites like http://lowryservices.com for more information.