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Why Hot Water Temperature Fluctuates In Tankless Heaters? (How To Fix It)

Note: This post may contain affiliate links. This means that at no cost to you, we may receive a small commission for made purchases.

Have you ever noticed that water from your tankless water heater changes temperature from hot to cold, and back to hot again?

There are a number of different reasons why the hot water temperature in tankless heaters fluctuates.

In this article, we will take a close look at problems with consistent hot water temperatures in your tankless heater, and we find out how to fix it.

Why Hot Water In A Tankless Heater Fluctuates

The fall and rise in temperature in a tankless heater is commonly known as the “cold water sandwich”.

Just like in a sandwich, there appear to be different temperature layers.

First, your tankless heater delivers hot water for a short time, then it goes cold, and then the water turns hot again.

When you are under the shower, this can be unpleasant. 

While you may think that this is a technical fault, temperature fluctuations with tankless water heaters are very common.

Some tankless heating systems have a built-in feature that minimizes these fluctuations but most tankless heaters work with the temperature changes.

When the heater is turned on, it delivers warm water. When the heating product is turned off again, the water inside the pipes cools down.

This means that next time you turn the heater on, the cool water in the pipes gets pushed out first, creating a burst of cold water.

Causes Of Fluctuating Temperature In Tankless Heaters And How To Fix Them

If you do not have a tankless water heater system that adapts to the temperature fluctuations, then there are a number of ways you can fix this.

However, first you will need to find out exactly what causes the temperature changes.

Incorrect Temperature Setting

It’s important to check the instructions with your tankless water heater.

This will give you a range of insights on how to set up the device, and also how to troubleshoot.

Why Hot Water Temperature Fluctuates In Tankless Heaters (How To Fix It)

The manual typically also states the manufacturer’s recommended temperatures settings.

The majority of tankless water heaters require a hot water temperature of around 120 degrees Fahrenheit.

This should be set at the safe, default setting. However, the manufacturer also specifies a recommended minimum temperature setting of around 100 degrees Fahrenheit, and a maximum temperature of 140 degrees Fahrenheit (180 degrees Fahrenheit in commercial buildings). 

Low Water Pressure

This is one of the most common reasons for fluctuations in water temperature.

When the water pressure is too low, then your tankless water heater will stop working.

Your tankless water heater requires something called an activation flow rate. This is the minimum flow through the unit to activate the heater.

If the flow inside the unit is too low, then the gas burner shuts down and there is no hot water.

If your heater also has a pilot light, then you will notice that the pilot light doesn’t go out. Only the gas burner will shut down.

In order to avoid any low water pressure issues, check your water pressure and the required flow rate of your water heater. This is typically between 0.4 and 0.5.

You may still see some temperature fluctuations when taps and fixtures are opened and closed in your home.

This will cause the water pressure to vary, and may briefly change the water temperature.

Also the demand on your water system in your home may have an impact on the water pressure in the house leading to temperature fluctuations.

The overall water pressure in your home for a tankless heating system should be a consistent pressure of 30 psi.

You may be able to use other water sources, such as a well, but it is important that the water pressure is constant.

Restrictions In Hot Water Flow

Another common reason for water temperature fluctuations is a restricted hot water flow.

The best way to fix this is by checking any shower heads, faucet aerators and plumbing fixtures.

They may be blocked due to any deposits, hair or other debris.

If you notice any obstructions, just remove them and clean the fixture thoroughly with vinegar or a specifically designed cleaning solution.

Why Hot Water Temperature Fluctuates In Tankless Heaters (How To Fix It)

If you notice that any fixtures are damaged, then this may also cause the temperature of the water to fluctuate.

Try to replace the fixture and then check the water temperature again.

There are a couple of things you can do to ensure that the temperature doesn’t change to any clogs in the hot water flow.

Check what type of tankless heater you have. The older heater models have a flow control setting. This allows you to manage the flow activation rate.

If you choose to set it to the lowest setting, then the water temperature will be highest.

In comparison, new tankless heaters, come with a small buffer tank inside. These models don’t require a minimum water flow.

Some models even have an automatic bypass system with a specific water adjustment valve. 

This changes the system to keep the water temperature stable for the time when water is drawn from the system.

Dirty Inlet Filter Screen

Inlet filter screen catch any debris in the water that’s flowing through the system.

Check the inlet filter screen of your tankless heater to make sure that it isn’t clogged up.

Dirty inlet filter screens block the water flow and they need to be cleaned regularly, in order to ensure the smooth running of water through the unit.

Typically, the filter is at the bottom of your tankless heating system. It can be removed and cleaned very easily.

If you are not sure where you can find the filter on your unit, it’s best to check the manual or contact the manufacturer.

Faulty Plumbing

The plumbing in your home is a carefully designed system. It’s a network of pipes and fixtures that provides water to the rooms where you need it.

Any problems in your home’s plumbing system can lead to issues with the water flow.

This can make the temperature of your tankless water heater fluctuate.

These problems may be caused by any number of things, such as a faulty mixing valve, failed washers or faucets.

The best way to check for a plumbing issue is to close the main water valve. Then open a hot water tap and wait for a few minutes.

You will notice some water running still through the pipes and out the tap but after five minutes there shouldn’t be any water left inside the pipes.

If the water is still running after five minutes, then you likely have an issue with the plumbing in your home.

It’s best to contact a plumber and he will be able to advise on how to find the exact cause of the issue and how to fix it.

Mixing Valve Missing

A mixing valve does exactly what its name suggests. It mixes the hot water with the cold water, keeping a consistent temperature without fluctuations.

Check that there is a mixing valve installed with your system, and that it is set to the correct temperature settings.

Why Hot Water Temperature Fluctuates In Tankless Heaters (How To Fix It)

No Power Source

Most tankless heaters are run with electricity, so when your water temperature fluctuates, then there may be an issue with the power source.

Check that the power supply to your home is consistent. Try to turn on some other devices and see if they work as they should.

Sometimes power cuts happen at short notice, particularly in bad weather.

This can also lead to short bursts of electricity coming through, which can cause your heater to go on and off, leading to fluctuating temperatures.

Cold Weather

In extremely cold weather, your tankless water heater has to work hard to get decently warm water through your cold pipes.

When it’s very cold outside, pipes may also freeze. This can lead to temperature fluctuations in the water, or not hot water getting through your pipes at all.

Problems With Your Recirculation System

In order for water to circulate properly through your pipes, you need a recirculation system. This is operated by a circulation pump.

Make sure that the pump is working properly, and the water is flowing correctly.

There may also be air inside the pipes or the recirculation system. This can have an impact on the pump.

Some systems also have a too small pump installed. If your water demand has suddenly gone up, then a small pump may not be able to supply the water enough.

This can lead to a drop in water flow as well as water pressure. 


Although the hot water temperature fluctuations in a tankless heater is a common problem, there are a number of different ways to fix this.

We recommend that before you start making a any changes to your plumbing system or you try to change any temperature settings, you should check the activation flow rate.

This is one of the most common issues with tankless heating systems. If you are not sure how to fix any of the issues we mentioned above, then it is best to contact your trusted plumber.

author avatar
Charlie Hardcastle


On Key

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