Get more from your tankless water heater with these practical tips on temperature settings, energy savings, recirculation, and seasonal care.
Unlike traditional tank heaters that store and constantly reheat a reservoir of water, tankless (or on-demand) water heaters heat water only when you need it. Cold water flows through a heat exchanger, and gas or electricity brings it up to your desired temperature in seconds. This means you never run out of hot water — but it also means the system works differently than a tank heater.
With a tankless heater, your setpoint temperature is the actual temperature the water will be heated to as it passes through the unit. There's no tank of pre-heated water to draw from, so the relationship between your setpoint and the water at your faucet is more direct. Understanding this helps you make smarter choices about scheduling and temperature settings.
The U.S. Department of Energy recommends 120°F for most households. At this setting, you get comfortable showers and baths while minimizing scalding risk and energy consumption. Water at 120°F is hot enough for comfortable bathing and effective hand washing, but significantly reduces the risk of burns — especially important in homes with children or elderly family members.
If you need to sanitize dishes without a dishwasher or require extra-hot water for cleaning, temporarily raising the temperature to 130°F or 140°F is reasonable. Just remember to lower it back to 120°F when you're done. At 140°F, water can cause a serious scald burn in about 5 seconds, so exercise caution and never leave the heater at this setting if small children or vulnerable individuals are in the home.
If you're trying to minimize energy costs, 110°F works well for hand washing and light use. This is cooler than most people prefer for showers, but if you mix hot and cold at the faucet (as most people do), you may find that lowering the heater temperature and opening the hot tap further achieves the same shower temperature with less energy consumed overall.
If you're leaving home for more than a day or two, put your heater in vacation mode rather than just lowering the temperature. In vacation mode, the heater enters a standby state and won't fire at all — saving the most energy possible. With Onsu Home, you can toggle vacation mode remotely, so you can turn it off before you arrive home and have hot water ready when you walk through the door.
If your tankless heater has a recirculation system, it can circulate hot water through your pipes so it's ready at the faucet the moment you turn the tap — no waiting for cold water to clear the pipes. Here's how to get the most out of it:
The most energy-efficient approach is to run recirculation only when you need it. Set up schedules for 15-20 minutes before your typical hot-water usage times:
Continuous recirculation wastes energy by repeatedly heating water that's sitting in your pipes. Even with an insulated recirculation loop, heat loss is constant. Running recirculation only during the times you actually need hot water can reduce energy consumption significantly — often saving 15-25% compared to a heater running its recirculation pump without a timer.
The time it takes for hot water to reach your most distant faucet depends on the length of your plumbing runs. A typical home needs 1-3 minutes of recirculation to prime the pipes with hot water. If your master bath is far from the heater (say, on the opposite end of the house or a different floor), you might need a slightly earlier schedule start time to account for the longer travel distance.
Your tankless heater works harder in winter than summer. Here's why and what to do about it.
In colder months, the incoming water temperature drops — sometimes dramatically. In northern climates, incoming water can be as cold as 40°F (4°C) in January, compared to 75°F (24°C) in summer. This means your heater has to raise the water temperature by 80°F to reach a 120°F setpoint, versus only 45°F in summer. That's nearly double the energy per gallon.
You probably don't need to change your setpoint, but be aware that:
Warmer incoming water means less energy needed per gallon. This is a great time to lower your setpoint by 5-10°F and see if you notice a difference. Many people find that 115°F is perfectly comfortable for summer showers and saves energy. With Onsu Home, it's easy to experiment — just use the temperature dial and adjust up or down until you find your comfort zone.
In spring and fall, incoming water temperature can vary noticeably week to week. This is when scheduled recirculation is most valuable — it ensures consistent hot water at the tap regardless of fluctuating ground water temperatures.
First, check your setpoint in Onsu Home — it may have been lowered by someone else or by a schedule you forgot about. If the setpoint is correct, the issue may be high flow rate overwhelming the heater's capacity. Try reducing flow at the faucet. In cold weather, this is especially common. If the problem persists, check your gas supply and consider descaling the heat exchanger (recommended annually).
Check the Activity tab in Onsu Home to see if the recirculation command was sent and acknowledged. If the command shows as "sent" but you still don't get hot water quickly, the issue might be plumbing-related — check that your recirculation line isn't blocked and that the pump is operational. Also verify that your plumbing has a dedicated recirculation return line or a crossover valve at the furthest fixture.
This means Onsu Home can't reach your heater through Rinnai's cloud. Common causes: your heater's Wi-Fi module lost connection (check your home network), Rinnai's servers are temporarily down (usually resolves within hours), or your internet connection dropped. Try power-cycling your heater's Wi-Fi module by turning the unit off and on, then check the Status tab again after a few minutes.
If your gas bill seems higher than expected, check for continuous recirculation (running the pump 24/7 wastes significant energy), a setpoint higher than needed (every 10°F above 120°F increases energy use noticeably), or multiple simultaneous draws (multiple showers running at the same time forces the heater to maximum output). Use Onsu Home's activity log to see exactly when your heater has been firing and identify patterns.
Tankless water heaters are low-maintenance, but a few annual tasks keep them running efficiently:
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