New to the forum. I am installing a Biomax 60 and a 1200 gallon tank as a buffer. I have gained lots of knowledge by searching and lurking. I haven't been able to find a solution for expansion if I pressurize my 1200 gallon tank. I thought I might use a couple 80 gallon well pressure tanks but I don't think they can stand the temperature. To buy a large enough boiler expansion tank or combination of tanks would cost more than my budget allows. Can I leave the top of the tank empty and use that air space for expansion sort of like the older bladder-less air control expansion tanks? Total system volume would be 1100 gallons if I left 200 gallons out of the tank. If I could do this the obvious advantage would be not having to use a heat exchanger for the tank input and output. The top 1/3 of the buffer tank is above the rest of the system.