Hot water doesn't stay hot.

[happypig]

I have a pressurised system with a separate hot water cylinder and a problem has developed with my CH/HW in that if I go for a shower in the middle of the day, the water isn't very hot.
The heating and water comes on from around 0600-0900 and if I get in the shower anytime up until about 1100 , it's well hot so it seems to heat up ok (rads all heat up ok too).
It seems to me that the boiler is ok, as is the pump, but somehow the cylinder is losing heat.
It's not losing water as I have visually checked everything I can see and I figure if it was losing that much water, there would be a puddle somewhere.
I'm starting to think it's either the cylinder stat or the 3-way valve.
Any suggestions ?

 

[twgas]

If you get a full tank of hot water at the set temperature it's unlikely to be anything to do with the boilers and/or it's controls.
By pressurised I assume you mean central heating not an unvented cylinder.
I believe you may have a mixer tap that is letting cold into the hot, or a leak somewhere on the hot tap supplies.

 

[electronicsuk]

Could be a problem with the controls if, for some reason, the pump was running running continuously with zone valves open, dumping all the heat from the cylinder into the rads as soon as the boiler stops running.

If it were a mixer tap letting by, the OP would presumably have noticed the header tank overflowing, if we're assuming it's a vented system.

 

[happypig]

It's not a vented system, there is no header tank.

I hadn't considered it being a dodgy tap; I have mixers on the bath, shower, basin (all fairly new) and kitchen sink (probably 25 years old).

Would a dodgy tap let cold into the hot when turned off, or only when turned on ?