converting a gravity system to a pumped system

Help with some of it.

The tappings in modern boilers (and the quantity of water they hold) are generally WAY too small for them to work under gravity. Dan's book "How Come" shows good ways to add the circulator and pipe for a modern boiler. There's a good discussion of sizing the circulator elsewhere on this site.

You [probably] won't need to add balancing valves in the mains--with such "wide open" spaces, you're pretty well assured of circulation through the entire system. As long as you get sufficient circulation in the highest, farthest rad (and it's not too fast in the lowest, closest) you should be reasonably balanced. If you have circulation problems FIRST look for flow restrictor plates in the upper rads (they're generally on the hand valve side "inside" the union). If found in a rad that's not getting enough heat, remove it (save it for possible use downstairs if you find a rad--usually a short one connected bottom-bottom--that doesn't seem to be heating properly.)

Old-timers sometimes used the small amount of air at the top of rads as the compression "tank." If still working under gravity you should fine an expansion tank in the attic with an overflow to the roof. Disconnect the line to the tank and add a new tank by the boiler. If the new boiler has a "built-in" air separator, they're generally effective for a gravity system.

Rarely is the layout of a gravity system such that you can divide into logical zones. Kind of a strange t-stat setup, but if it works and the customer is pleased with the arrangement... It's possible that the 2nd thermostat was added as the pipes corroded (roughened) a bit and gravity circulation slowed.

If you're not piping primary/secondary you generally have the boiler and circulator come on in unison with a call from the t-stat. It's may not be the best arrangement, but it's what I see most and it seems to work OK. You [generally] don't have to be concerned about condensation issues with simple iron boilers on gravity conversions. BUT the more efficiency that is wrung from a conventional (non-condensing) design, the greater the potential for problems. Flue dampers in particular seem to be notoriously short-lived on such systems.

How much does the diameter of the supply and return of a boiler set up for gravity operation matter as compared to the size of the pipes to the radiators? I'm wondering whether the heat and pressure of the water being forced up and out will over come the size of the opening.

The low pressure of the large mains is for circualtion to the rads.

ESBE mixing valve

I have seen several postings regarding the use of these mixing valves. Where can I find info on them?