franco b wrote:I am under the impression that the tank only provides an air cushion and has nothing to do with the pressure in the system other than to absorb the increased pressure when the water is heated.
I think you are right, with the currently-used bladder tanks. My old-style tank sat near the ceiling, upstairs, several feet above the top of the highest cast iron radiator. A pipe entered the tank at the bottom and the tank itself was half full of water. A pipe also exited the tank at the top; that was simply an overflow pipe open to the air at its other end (in the cellar). The pressure came from the total height of water in the system, about 20 feet, so I estimated the pressure at the boiler to be about 9 psi. The pressure within the expansion tank itself, of course, would be very low. With the radiators hot, the water level in the expansion tank was higher than with the radiators cold. System pressure did not change when the water was heated, since the whole system was essentially open to the atmosphere -- unlike the modern closed systems.
franco b wrote:You could put any pressure you wanted, with a tank or without.
Without an adequate tank, when you heat the water, something will give either leak-wise or kaboom-wise.