You should have a small tank as F&E for your CH and a large tank for domestic water storage?
Which tank is warm / hot? It makes a difference as the cause is not the same?
Guessing for a moment that its the large CWST from your description the commonest reasons are:
1. Boiler or immersion stat too high. Water is boiling in the cylinder and rising up the expansion into the CWST.
2. You have a mixer fitted which has cold mains and hot from the cylinder. The cold is at far greater pressure than the hot, overwhelms it and forces the hot water back into the tank where it has nowhere to go except up the expansion pipe into the CWST.
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