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Hi there I'm a builder working in Portugal renovating old property we have got a solar water heater system from China itd a great system but did not realize I should have ordered an assistant tank with it (as that was in the extras option ) so rather than waiting another 6 weeks for delivery I'm wondering if there is any alternatives out there that would replace the assistant tank . the assistant tank is basically a cistern with ballcock it controls the water going into the big tank and screws on top with half inch thread. The system is non pressurized and water coming straight from the mains no storage tanks .swh is on roof terrace and is gravity fed ..also the mains pressure is quite good ....

Can anybody help this chap?

Sorry I know nothing about such things.

Not entirely sure as its not something i cover.
are you refering to something like this?

https://gondwanagreenheat.co.uk/thermal ... -cylinder/

A ball rooster is the whole valve assembly. Hot water should come from the top of the storage tank and cold water added at the bottom (just like a domestic water heater). "Assistant tank" is probably a poor Chinese translation for expansion tank. If you have much plastic pipe in your loop, then three liters may not be large enough. The only way for pressure to build in your expansion tank is for the vent line to be plugged.