Hot Water Temperature Fluctuates in Tankless Heaters

How to troubleshoot the problem when the temperature fluctuates on the tankless water heaters, when the hot water temperature is changing during my shower from hot to very cold?

In this article, I will try to cover why there is a problem with the tankless hot water temperature, why the temperature is suddenly changing (fluctuating), what are the causes, how to avoid them and how to solve the problem:

Low water pressure

One of the main reasons why the temperature fluctuates is that your tankless heater stopped working due to the low water pressure. The flow within your tankless unit is low, actually below the activation flow rate, resulting in shutting down the gas burner. If your heater has a standing pilot, the pilot light does not go out, only the main gas burner. Check for the manual to see the minimal required flow rate.

Check out the article why my pilot light goes out, to see why your tankless heater shuts down and causes a sudden drop in the outlet "hot" water temperature.

If your heating systems is not designed properly where your tankless heater capacity is smaller than demand, if additional fixtures in your home are opened or closed, there will be a change in the flow, and you will see the fluctuation/drop in the temperature. The end result is what you will feel during the shower, the cold water coming out of your shower-head.


Restriction in the hot water flow

Another reason why water temperature fluctuates is the restriction in the hot water flow; check the plumbing fixtures, faucet aerators and shower heads, they might be clogged due to the mineral deposits. Flush and clean every single one, or if damaged use the new units. To clean and flush the fixtures, the recommendation is to soak them in the vinegar to dissolve and break mineral deposits... flush with water after.

Some tankless heaters, like Bosch 125 series, for example, are equipped with the flow control knob setting which allows you to control the activation rate. If you set it to the lowest, the temperature of the water will be the hottest.

Inlet filter screen

If your tankless unit is equipped with the inlet filter screen, it might be dirty, so water cannot pass through and the flow is below desired. Filter can be found at the bottom of the unit, simply remove it and clean it. If it is necessary, replace it.

Plumbing crossover

Problem in the plumbing can also be the reason of an inadequate flow which is why tankless water temperature fluctuates, hot-cold. One of them is the plumbing crossover; caused by the faulty mixing valve, incorrect plumbing or if you have a single lever faucet, failed washer. Check all the faucets. Close the main valve, open the hot water tap, wait for 5 min, and if the water is still running you have a problem with plumbing crossover.

Water pressure

Water pressure has to be according to the manufacturer specs. Constant pressure of 30 psi has to be maintained, even if using other sources of water, like well, for example.


Mixing valves

Check also for mixing valves if they are installed, as they lower the temperature of hot water by mixing it with the cold... check the set temperature on the valve.

Before you do any changes on your plumbing system or trying to fix the temperature fluctuation problem on your heater, I suggest you to check the activation flow rate problem first.

Measure the fill time to see what is the proper activation rate. If you have the fill time longer than usual (when you are not experiencing changes in temperatures) the odds are activation rate is not met.


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