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Why Does My Shower Go Red Hot When I'm In It??

So we've all been there, we are in the shower, then all of a sudden the pressure drops and it goes red hot and you let out a little yelp and jump out the way of the water!!!


Well, let me tell you why this happens.......

This is all down to what boiler you have installed in your property, and will happen with the much loved combi boiler. This, for me, is the only draw back with having a combi boiler installed in your property.

Hot Water

The hot water on a combi boiler works simply by opening up a tap which causes the water to flow, inside of the boiler is a flow sensor which tells the PCB inside the boiler there is a demand for hot water and then fires the boiler up to heat the water. So cold water comes in to the boiler, gets heated through a plate heat exchanger and comes out hot. Its as simple as that!!

Combi boiler hot water diagram

As a combi boiler works purely from your cold water main it has no stored water, so every time you run two taps at the same time, whether its hot or cold, you are reducing the flow of cold water going in to the combi boiler. So, as the flow of cold water reduces going into the boiler to get heated, it is going through the heat exchanger slower causing it to get heated up more and that causes the water to become hotter.


The pressure will drop due to the cold water main being used by another tap. So, with your cold water main (which we measure in litres per minute) you have set a flow rate which is coming in, each property will be different, if you are only using one tap then you will get the full flow of water from the main BUT as soon as you open up another tap then the flow is shared between two taps so will decrease as there is a demand from two outlets instead of just one. Remember that washing machines & dishwashers also use cold water.

What Can I Do To Prevent This?

The only way to prevent this from happening is to ensure that no other outlets will be used whilst you are in the shower e.g don't put the washing machine or dish washer on and then go in the shower as this will happen.

If you are currently considering getting a new boiler installed then this is one thing to bear in mind. If you don't want this to happen then you will need a cylinder, whether its vented or un-vented, as these will give you plenty of stored water and you will be able to run multiple outlets at the same time without it effecting the temperature or the flow.

If you would like more information or looking for a new boiler installation then get in touch today on 01922 503579

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Matt Hughes

Matt Hughes is the Managing Director of Platinum Boiler Installations and his mission is to make boiler installations as simple and hassle-free for customers as possible. What Matt doesn't know about boilers simply isn't worth knowing! Connect with Matt Hughes on LinkedIn >>

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