Cylinder cools when CH turned on

[ARRANPILOT]

Hi,

I wonder if anyone can help me? In a fit of enthusiasm I fitted a thermometer to my HW Cylinder, and this has been very revealing. One thing I do notice is that when the cylinder is hot for the morning wash and shower, if the CH is then turned on, the water temp in the cylinder drops by as much as 2 degrees. I have motorised valves controlling the circuits, and I thought this should prevent any cross flow - but maybe not. Any thought on where I should go to solve this? many thanks, Mike.

I forgot to say open, vented, fully pumped system.

 

[45yearsagasman]

It may be that the valve for the dhw is not 100% closed, and this would allow ch water into the cylinder coil thus having a cooling effect on the stored water until the system rads come up to temp. I'm assuming you have 2 x 2 port valves, but this could also happen with a mid position valve. I can also have the reverse effect in raising the cyl temp when ch is on.

 

[ARRANPILOT]

Thanks for the really quick reply - does this mean that the motorised valve is on its way out, and that I should change it then?

 

[45yearsagasman]

With such a minimal effect on your system, I would leave well alone.
Depending on make and age of the valve the head can be removed to check that the spindle is free to move. Any thing else requires removal of the valve body to clean or replace the brass body. Less commonly, the head may not be returning to the closed position, but this needs to be inspected on site.

 

[Agile]

In any case the stratification within the cylinder should enable the water above the heating coil to remain hot.

Check that the motor valve is really closing the port to the cylinder.

Tony