feed/expansion tank location?

[amstel]

Evening all,
been reading some posts tonight with great interest about warm water getting into the cold water storage cistern then overflowing,etc.
I understand the mains cold feed/mixer valve back feed scenario and also the pinhole in the coil as well, but one of the reply,s back to this particular op asked,....what is the difference in height between the f/e cistern and the cold water storage system?.....should the f/e cistern be higher, and by how much, or aslong as it is not lower than the cold storage tank.
can somebody spare the time to explain please, im very interested.
Thanks.

 

[kirkgas]

i think the reason for it stems from the chance of the coil in the cylinder pinholing, if this happens the heating circuit can fill from the domestic as the storage tank is higher the pressure is greater (gravity) and forces its way into the coil then back up into the (lower) F&E tank which then back fills and overflows, under usual conditions the water from the 2 systems doesnt mix so the position of the tanks should effect each other, so i can only see it being a prob under fault conditions, but dont worry someone will be along soon to advise me of the error of my ways and to keep you right

 

[seco services]

Agree with kirkgas.
you wouldn't want your water from your heating system and chemicals/inhibitor in your cws would you.

 

[spraggo]

As I see it, please correct me if I am wrong. As the coil inside the cylinder is under positive pressure from the pump if the coil develops a pin hole etc It would make no difference to height of the expansion tank in relation to the cold water tank as the heating circuit water would be forced into the domestic water by the force of the pump.

spraggo.