New shower installation - hot water entering cold tank

[why did I start]

With a friend, I installed an Insignia shower cabin over the weekend, I installed a 50 gallon cold water tank on reinforced timbers across the joists, the outlet of the tank is connected to a Trevi 45 Shower pump. The hot water tank was fitted with a Surrey Flange. the top take off, ( not the side exit) runs to the shower pump. The hot water immersion cylinder is around 18" higher than the cold tank.Monday night water was dripped out of the over flow of the cold tank, so I replaced the ball valve, Tuesday night the same issue and I am convinced water from the immersion cylinder is finding its way into the cold tank, as the cold outlet pipe is definately warmer than other cold water pipesWhat have I messed up here. Thank you in advance

 

[Chalked]

Your tank needs to be the same height as the tank (cistern to be correct) that feeds the hot water or the hot will creep up the cold feed and therefore fill the cold storage.
Could you take a cold feed to the shower direct from the hot water storage tank?

 

[why did I start]

Thank you for your reply, on looking again, we had an existing 20 gallon tank above the immersion heater, then added a larger 50 gallon tank (Across the rafters) to cope with the demands of the shower pump, this is below the immersion tank. Would I be right in thinking, we should of replaced the smaller water tank with the larger one, so the new larger tank sits above the immersion heater?

 

[Killy Bing]

also that top take off is going to get all the air in it, not good for the pump ! that should be your vent tbh

 

[why did I start]

Thanks for that, so to sort this mess I have created:1. Feed the pump from the side exit of the Surrey Flange2. I left a smaller water tank that fed the immersion tank in situ, hence siting the larger tank on the rafters. so remove this small tank and replace it with the larger tank to support the volume of water needed to run the pump. Of course reinforcing the support mounting to allow for the extra weight.

 

[Howsie]

Check valves?

 

[stevetheplumber]

yes thats right feed both from same tank and it has to be above the cylinder