Shower Water Temperature Problems

Having a shower at the correct temperature is one of the simple joys in my life. As an electrician and a part-time plumber, I am often called to repair clients’ shower water temperature problems.

Most people complain about the shower water temperature being too cold, but there is a growing list of issues that can now cause the water temperature to be too hot. So what is causing the hot water to go cold and then hot again?

In this post, I will be showing you how to fix the more common shower water temperature problems and solutions.

Shower water temperature problems

There are a number of reasons you are having shower water temperature problems these could be due because of

  • A small-sized hot water tank is not suitable for a large family.
  • A tripped internal water thermostat due to over-temperature. Reset it inside the tank control panel.
  • Tripped main circuit breakers. Reset it at the switchboard.
  • A faulty thermostatic valve or pressure-balancing valve. Clean or replace the valve.
  • A blocked shower filter cartridge or failed mixer tap. Clean filter.
  • Limescale in the tank or tankless water heater. Flush the heat exchanger with white vinegar.
  • Clogged pipes leading away from the water heater. Unblock.
  • The water tank element has failed. Replace the heating element.
  • Failure of the off-peak or booster contactor circuit.
  • Flame failure of the tankless hot water system. Investigate air, gas, and electrical systems.

If you have the main tank with hot water storage then this is less prone to water temperature fluctuations unless you are the last person to have a shower and all the hot water is gone. These tank hot water storage systems are not efficient and tend to fail after 10 years due to internal scale build-up or heating element failure.

Many people nowadays are going to gas tankless hot water which will give you endless hot water but these can be problematic if not sized correctly. It is important for these instantaneous hot water systems to be serviced every few years due to scale build-up.

The main issues of these tankless systems are fluctuating hot water temperature, flame failure, blocked inlet/outlet air vents, and internal scale build-up of the hot water heat exchanger.

Fluctuating hot water temperature

lady screaming in the shower Fluctuating hot water temperature

If your shower temperature fluctuates in your apartment work out if you have a gas instantaneous water unit or a tank storage unit. Hot and cold apartment showers are often caused by gas instantaneous hot water.

This can be solved with proper sizing of the hot water unit. If someone opens another hot water tap then the tankless hot water system has to run hotter and harder to heat up the extra hot water. An oversized heater will have no issues supplying the extra hot water.

Another reason for fluctuating hot water temperature is due to the low flow of hot water. Many low flow/water saver shower heads use so little water that the tankless hot water heaters tend to trip out. This causes fluctuating hot water temperatures. I just remove these restrictors which are located in the showerhead. No more problems.

Some LED light-up shower heads have a small filter that can clog up over time. So check this filter and clean it if you have started to have fluctuating hot water temperatures.

Many tankless hot water systems will have a minimum and maximum water flow, take note of these flow rates.

Flame failure

Shower water temperature problem Flame failure on a tankless hot water system

A tankless hot water heater that fails to start is a real problem. Often it will show an error code that will help you diagnose the problem. The most common causes I have come across for flame failure are dirty spark/detector contacts, blocked air or exhaust flue, gas solenoid issues, and failed internal thermistors.

Scale build-up of the hot water heat exchanger.

Scale build-up in a hot water tank cut open.
Scale build-up in a hot water tank cut open.

Over time scale build-up of the heat exchanger will cause issues with the heating of the hot water. An easy fix is to do a tankless hot water flush using white vinegar. I use the Kelaro tankless flushing kit to flush my client’s gas water heaters. This is usually performed every few years during the service. It will only take around 1/2 hour. If it is really scaled up it may take up to 1 hour.

How to flush a tankless hot water system.

If you have the flushing connections on your tankless gas hot water system then it is really easy to do. You will need the following items.

  • 3 Gallons of white vinegar or Flow-Aide Biodegradable Descaler
  • Large bucket
  • 1/8 HP Submersible Utility Pump
  • Water hose pipes and connections.

Start by turning off the water, gas, and electricity supply to the tankless hot water system.

How to flush a tankless hot water system.

Connect the two flushing hoses to the cold and hot water bypass lines of the gas hot water system and turn on the flush valves. Make sure that the house water hot and cold valves are turned off. This is so that only the gas hot water heat exchanger will be flushed.

How to flush a tankless hot water system with vinegar

Pour the three gallons of white vinegar into the bucket and turn on the sump pump. I find on the heavily scaled heat exchangers that you will need to continuously flush for around 1 hour. 1/2 hour will be fine for a preventative service on a tankless hot water system.

How to flush a tankless hot water system with a sump pump and bucket

Once the vinegar flush has been complete you can turn off the sump pump and empty the bucket. You can either pour the dirty solution down the drain or put it in a spray bottle to use as a natural weed killer.

You can fill the bucket up with fresh water and flush out the sump pump and heat exchanger a few times to get rid of the excess vinegar. This is optional as you can always open a hot water tap and flush it through later on.

Make sure all the appropriate taps are open and or closed. You can then turn on the gas and electricity supply and enjoy your new high-flow hot water system.

Instant gas hot water low pressure

Low-pressure water in your gas hot water could be a number of reasons. Often it is a simple fix like a blocked water inlet filter or scale build-up in the heat exchanger.

Instant gas hot water low pressure

Remove the water inlet filter first to check for dirt or scale build-up. If this is clear then do a water flush using white vinegar. I use a tankless hot water flushing kit from Amazon every few years on my instantaneous hot water systems.

The shower goes cold after a few minutes

If your hot water is supplied from a tankless hot water system such as a Rennai or Bosch then check for any error codes.

It could be because the tankless hot water is tripping out on over temperature. This could be from a number of reasons such as scale build-up, flame failure, fan failure, low inlet water flow, blocked exhaust, or inlet air.

I once has a client who had just installed a low-flow water shower head. The water flow rate was so low that the tankless hot water was tripping out on over temp and then starting up again after a few minutes. Removing the low-flow showerhead restrictor fixed the problem of the shower going cold after a few minutes.

Gas hot water goes hot then cold

Gas hot water goes hot then cold

This is a common problem that mainly happens to instantaneous tankless hot water systems. There are a few possible causes for this.

  • Gas Hot water unit rating is too small for the water flow rate.
  • Gas Hot water unit rating is too large for the water flow rate.
  • Scale build-up in tankless hot water unit.
  • Faulty hot water outlet sensor.
  • Faulty thermostatic valve.
  • Using a flow-restricting shower head.

Make sure that your gas hot water system is rated correctly for your house. If your instantaneous gas hot water system is too small and more than one tap is turned on the hot water goes hot and then cold.

The opposite is true if the gas tankless hot water system is too large and you are using a water saver shower head then the water flow rate may be too small to be heated up correctly. I have found that water saver shower heads are a real problem for gas hot water systems. The water is heated up to fast and at too high of a temperature. This causes the gas heater to start and stop then the hot water goes hot and cold.

If the hot water system has a faulty hot water thermistor then the outlet temperature will be erratic. It will need to be replaced.

Faulty thermostatic valves can also cause gas hot water to go hot then cold. It will need to be replaced.

Shower too hot or too cold

If your shower is too hot or too cold then this is usually a problem if you have a single mixing shower handle valve. These can be adjusted internally to be either hotter or colder.

Take an Allan Key and unscrew the locking screw to the tap handle. Remove the handle and look for the hotter/colder locking ring. Turn the locking ring to the desired temperature and then replace the tap handle and screw to lock it back into place. This is the proper way how to adjust the shower mixing valve.

The below video will explain it in more detail.

You could also check if the hot water temperature is set to hot enough at the controller. 50 deg Celsius is generally hot enough for most people.

Hot and cold water not mixing in the shower

Hot and cold water not mixing in the shower

Make sure to check the water pressure at both inlets to the hot and cold tap. A water filter can drop the pressure at either tap. The water pressure should be equal for both hot and cold water. You could also have a faulty thermostatic valve. These valves will not work properly if there is a lack of water pressure due to clogging, a damaged shutoff valve, scale build-up, or a damaged check valve.

thermostatic mixing valve (TMV) is an automatic valve that blends hot water with cold water to ensure constant outlet temperatures.

I have found that thermostatic valves often fail due to scale build-up. The best way to clean a thermostatic valve (TMV) is to remove the valve assembly and soak it in white vinegar for 2 hours. This will dissolve any scale that is preventing the TMV from operating correctly.

If you have a single tap mixer you could have a blocked internal filter or a failed internal o-ring gasket. Most are not user repairable so just replace the mixer tap as it will be cheaper and quicker.

If you have normal shower taps and water is not mixing then there could be an issue with the valve seal in the shower tap staying closed or not opening enough. You can replace these seals quite easily.

Gas electronic ignition water heater won’t light

If your electronic ignition water heater won’t light then check that you have power at the tankless water heater. You also could have an issue with the ignition spark and detector. This is easy to check especially if there is an error code.

Electronic ignition water heater won't light
Tankless How Water Ignightor and Detector.

Turn off the electricity, water, and gas supply to the hot water unit. Remove the High voltage ignitor and detector. Clean the terminals of both the detector and the ignitor. Replace and check for the correct operation. Listen if there is a spark.

Also, check that the air supply and exhaust vents are clear. You could also have a sensor failure but this would bring up an error code.

It is also a good idea to check another gas appliance in your house just in case the gas supply has been turned off to your street for maintenance.

What temperature should hot water be at the tap?

For a hot water main tank, the temperature must be set above 60°C. This is to prevent bacteria growth such as Legionella. Hot water systems installed after August 1998 must have a tempering valve installed also known as a thermostatic mixing valve. This ensures that the water that comes out of the tap is at a lower temperature for safety reasons to prevent scalding.

This temperature at the tap will be set between 45°C and 50°C depending on your plumber.

YPRM66XB bosch controller repair fix
Bosch tankless hot water controller

For tankless hot water systems, you can have a lower temperature like 40°C at the tap due to no bacteria that can grow in a tankless system.

My family likes the hot water temperature to be set at 105°F or 41°C during summer and 44°C during winter. This is due to the small amount of heat loss in winter of our long hot water pipe traveling through the walls and roof space of the house.

Tankless hot water systems are easy to adjust the temperature with the main controller. Just use the up and down buttons of the controller to set the temperature. If your controller fails it may need a clean on the push button contacts. Check out how to repair a hot water controller.

Make sure that your hot water pipe is well insulated to prevent heat gain/heat loss and freezing during the summer and winter months.


Hopefully, you are able to fix your shower water temperature problems. Having inconsistent water temperatures in the shower is not fun. Often it is an easy fix that any handyman or woman should be able to repair themselves.

You can quickly check if it is a gas, water, or electrical problem and then dive down further to diagnose the issue. Often rebooting a tankless hot water system will reset any non-serious faults. If they continue you will need to work out why. Often an error code will show up on modern systems.

Fluctuations with the hot water temperature can be dangerous so check the gas heater for scale build-up. Also, check the thermostatic valve if there is one installed for correct operation.

If you have a water saver shower head remove the restrictor. These cause more issues than they are worth. Often it is located close to the showerhead nozzle.

Let me know in the comments below if you have any other shower water temperature problems and I will attempt to point you in the right direction.

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