Do mixer showers cause combi boiler overheating?

I have posted this before but the subject line might not have been clear
When I use my shower my combi boiler overheats. It doesn't overheat at any other time, and the overheating is independant of whether the central heating is on at the same time.
I have been told that showers using mixer taps can cause the boiler to overheat because the flow of hot water is reduced compared to when the tap is running. To make matters worse the pressure from the cold tap passing through the mixer causes the hot water flow to be reduced further.
I am sceptical of the above because sometimes I can stand in a hot shower for ages with no overheating, and other times the boiler will overheat and cut out after a few minutes. I suspect that I have a faulty overheat sensor, but without any in-depth knowledge of combi boilers I cannot confirm this.
If it is the reduced flow causing the overheat where can I buy a high flowrate shower head? The one I use at the moment has quite a high flowrate as it is!!
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This can be caused by the cold feed to the mixer fighting back against the hot supply and making the hot flow even slower. So it is a good idea to balance the hot and cold feeds to the mixer valve.
Because the hot supply is going through the boiler before it reaches the shower and the cold isn't, then it is true to say the pressure of the hot supply is already being slightly reduced. Therefore, if you turn the cold supply down until it flows from the shower head at the same rate as the hot, and not being allowed to overpower the hot supply, then the problem should clear.
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Cheers for the reply. I have been turning down the hot water output temperature oof my boiler due to this overheating to the point where I only need a trickle of cold water to get the right temperature. I still get the overheating. Is my boiler faulty??
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sounds like your thermosatate has gone. as the boiler should cycle the gas on and off to maintaine the correct temp. with other taps the flow is never slow enough for this fault to show.
anon wrote:

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What make and model boiler is this? In particular, does it have a modulating burner? Combis without modulating burners are often not very suitable for thermostatic mixers. They work better using a manual mixer so you can set full hot water flow and adjust temperature only using the cold flow.
As the flow temperature is directly related to flow rate, but with a long time lag, a thermostatic mixer may go into weird oscillations as it tries to stabilise.
If you already have a suitable valve, or your boiler is modulating, then you may be able to improve things by replacing your shower head and hose with something less resistant. Choose a model that claims to be suitable for power showers. They may have larger bore hoses and bigger holes and allow more water through.
Christian.
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It is a Vokera Excell 80E. Not sure about the spec of the burners.
I have a bog standard mixer, not a thermostatic one. I also put hot on full and add a little bit of cold through it to get the right temperature.
Today I had the PCB replaced due to another fault today so maybe that was the cause of the overheating problem (faulty PCB). I will have to see if it happens again.
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