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Solar heating DIY system using tubes from various sourses, good idea. problem with connecting exsisting water heating

I have found that the expansion of water in the heating solar heating system, creates a very high pressure, and cannot be connected to your existing water heater supply. Some kind of expansion tank is required, or a blow off value. The water supply should be 2 bar or 28 psi. Any direct water heating on this water line will increase the pressure, both in the house system and back up the cold water supply system. What's the cheap way of resolving this