hot water flow stops when cold is turned on

had a combi installed 6 months ago, now im finding that when in the shoer (also just running the hot tap) when i turn on the cold the hot flow drops off to a trickle.
Any ideas where to start looking?
Cheers
Chris
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What's the make and model Chris?
Has is just started doing this (over the last month or so now the ambient water temp is colder) or has it been like this over the summer? What sort of flow rate do you get through?
Mike
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"> What's the make and model Chris?

More noticably as ive just installed a shower in the last month or so, cant say i noticed b4
chris
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Chris,
Is it just the shower that show a problem?
A friend of mine has just bought the same model and gets the same problem (well sort of.....). This boiler will raise 9.8L per minute through 35 degrees C. This isn't huge, but should be Ok. If the water in the pipe is 5 degrees then you will be getting it at maximum flow rate at 40 degrees whcih is only just above body temperature. The water outside will be getting much colder now and the amount you get through at a decent shower temperature will be less as the boiler has to heat it through a higher temperature and therefore you get less water.
Check your flow rate as you suggest, use a jug and then do the maths.
I guess this doesn't really tell you much....
Mike
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happens with sink taps as well, will see what flow i get later on
cheers
chris
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