I currently have 4 Zones in my house I want to add three more. Currently a series 100 Bell and Gossett pumps the water back to the Boiler. I am wanting to re-configure the piping in the boiler room. The new header will have three pumps pumping water from the header to the new individual zones and back to the return header.The four existing zones I want to continue using the four zone valves with one pump pumping water from the supply header to the zone or zones, depending on how many are calling for heat. I was told a lot of circulators are way over sized for what I am currently doing. Question is do you do a head calculation for all four zones, then size a circulator for the total? I read that you should do a head calculation for all zones, then take the zone with the largest head and size a circulator for that zone and it will be large enough for all four zones. Any thoughts or opinions on this. NJT you woke me up on pump curves sizing, Thanks

Conventional expansion tanks are large with an air-to-water interface, which requires continuous air separation from the circulating water and re-injection into the tank. Bladder-type tanks continuously allow air to diffuse (leak) across the bladder, depleting the air cushion. The bladder can eventually rupture. 50+ years ago I was involved in naval nuclear propulsion. The reactor plants had a closed circulating water system. They had an expansion tank, called a "pressurizer." It was a vertical tank with electric heaters in the bottom. A steam bubble was maintained in the pressurizer to keep a controlled pressure on the main system. Such a pressurizer could be designed for a residential hot-water heating system? Of course, the pressure would be limited to the saturation pressure of the water temp in the pressurizer, which would be a bit above the normal operating pressure of residential hot-water heating system. I'm putting on my flack vest.