In short, the answer is yes. There are a number of reasons why your tankless water heater may be experiencing temperature fluctuations. The good news is that there are steps you can take to minimize this problem.

Low Water Pressure

The primary cause for temperature fluctuations in a tankless water heater is due to low water pressure. If the water flow in your tankless water heater is too low and dips below the flow activation rate, the main gas burner will turn itself off. If your model features a standing pilot, the pilot light will not turn off ““ only the gas burner. The owner’s manual for your tankless model will tell you what the minimum required flow rate is.

Other causes of low water pressure that can cause tankless water heater temperature fluctuations include:

  • Improperly designed heating systems
  • Additional fixtures in the home being opened and closed
  • Consistent flushing of the toilet(s) in your home
# Product Image Product
1 EcoSmart ECO 11 Electric Tankless Water Heater
2 Ecosmart ECO 18 Electric Tankless Water Heater
3 Rinnai RU199iN Sensei Super High-Efficiency Tankless Water Heater

Hot Water Flow Restrictions

With tankless water heaters, temperature fluctuations are commonly caused by restrictions on the flow of hot water. If you believe that this might be the source of your problem, carefully inspect all of your shower heads, faucet aerators, and plumbing fixtures. There is a possibility that they might have become clogged because of mineral deposits.

Each one should be cleaned and thoroughly flushed. If any of the items are damaged, have them replaced. Most experts recommend cleaning the fixtures in a solution of vinegar, which will break down the accumulated deposits, and to then rinse them well with water.

Newer tankless water heater models are equipped with flow control knobs. This knob allows you to control the unit’s activation rate. If you place it on the lowest setting, the water temperature will become much hotter.

Inlet Filter Screens

If your tankless water heater is outfitted with an inlet filter screen, this screen can become dirty and clogged, which prevents water from flowing through it. Water flow and pressure drops, and this causes temperature fluctuations in the water flow. Most filters can be located at the bottom of the water heater, and it is simple to remove and clean them. If you cannot adequately clean it, replace it.

Crossover Plumbing

Temperature fluctuations from your tankless heater can also be caused by problems in the plumbing. One common type of plumbing is referred to as crossover plumbing. It is caused by failed washers, faulty mixing valves, and/or incorrect plumbing. Check all of the showerheads and faucets in your home. Close the main valve, open your hot water tap, and leave it for five minutes. If the water continues to run after the five minutes, then you more than likely have a problem with crossover plumbing.

Mixing Valves

Check to see if your tankless water heater has mixing valves installed. These are a common source of temperature fluctuations in water heaters because they cool the temperature of hot water by mixing it with colder water. Look at the set temperature on the valve and take note of what it is. It may need adjusting.


If the aforementioned problems and solutions do not fix the issue of temperature fluctuations, then it is time to call in professional plumbers. It’s a good idea to research some new product options before the plumber comes just in case you need to buy a new one. Here is our full buyer’s guide and tankless water heater reviews for reference.

Make sure to look for a company that is certified and experienced in dealing with tankless hot water heaters. There may be an issue with the water heater itself, and the unit may need to be repaired or replaced. Do not attempt to make any modifications or repairs to the water heater yourself, as doing so will often lead to the voiding of the manufacturer’s warranty.


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