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Any non return valves or filters on your valves. Sounds like interruption in cold supply that causes thermo valve to do what its meant to and cut off scalding water.
 
Ah that sounds interesting. I think I'll have to have another look at the installation instructions. It makes sense that that's what's happening so perhaps there's just an adjustment I can make. I noticed that when I put the water on the hottest setting it wasn't that hot. Maybe related to this problem. Thanks!
 
Is your shower suitable for your system? Some are made for Gravity hot and cold only.
What mske snd model is the mixer?
 
If all your other outlets are ok the problem is the shower , but does it not happen with the bath aswell ? I see you wrote bath / shower , plus what extra pipe did you need to instal , was it an electric shower previous then you put in a hot for the mixer?
 
Turn down temperature of hot water at combi until you get 43 C at sink with just hot tap.
If shower then functions OK, cold supply problem confirmed.
 
I have a new combi boiler and the mixer is this one. The issue must be with the mixer valve as it feeds the shower and bath which are the only places where I have a problem.

https://victoriaplum.com/product/matrix-oval-triple-valve-conc19

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Following your link you have both check valves and filters. Remove, flush pipework, clean filters and reassemble.
 
I was just watching a YouTube video out troubleshooting shower valves. Looks like that's what I need to do, thanks. Should this happen on a new fitting or did I miss a step out when installing?

Sounds like I might not have to take off my new tiling...yay!
 
You missed the bit about taking out the check valves and flushing the pipework leading to the valve.
 
That valve is just for a shower with two outlets , a fixed head and a sliding rail, did you say it feeds the bath or am I picking u up wrong , ???
 
Yes, it has a temperature control and two outlets. One for the shower head and one for the bath. I have two of these in my home...both working well. This is the first time I've installed myself though!
 
Yes, it has a temperature control and two outlets. One for the shower head and one for the bath. I have two of these in my home...both working well. This is the first time I've installed myself though!

If you read the specifications for the shower it's designed for two outlets to run simultainesly , either a shower fixed head , shower sliding rail or shower jets , it's not designed for a bath just a shower , the flow rates will probably take ages to fill,
 
So I have exactly the same shower valve mixer in my house in two bathrooms and have used for two years without a single problem. I get that I'm totally new to plumbing and will keep trying until I figure it out but even if the product is not designed for this purpose, there must be a way of making it work like the two in my house?

Anyway, I'll stop posting and try some of the good advice that has been posted previously.
 
If you read the specifications for the shower it's designed for two outlets to run simultainesly ,
I read that in thinking it gives you the option too run at the same time, pretty sure the top and bottom valves are just 1/4 turn stop valves? , agree like you say poor flow rate for bath filling,

Taryn is there any chance that you have piped up the hot to the cold on the valve?
 
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