Once filled, you should never have to add water to your system unless it leaks. Keep in mind that not all leaks are visible, especially around the boiler. The heat from the boiler can evaporate the water from a small leak before you see it and eventually, in some cases like the domestic coil, rust right through the boiler.

Keep your system pressure where it should be and think of your system loops as a circle and bleed accordingly. It's hard to tell how to bleed an unseen system.

Water makeup is a normal requirement of cast iron boilers. Boilers will loose up to a quart a month through the porous cast iron and burned off going up the flue. In addition depending on whether you have push nipples or gaskets, additional water may depart there.

Barring any leaks, auto vents will loose a bit of water since the seal is minimal and some water is expelled when venting air. So you see the water make up although minimal is perpetual.

Regarding the 1/2" piping, many systems back then with baseboard and still today were designed/sized to satisfy heat requirements of a space at 180 degrees with 1-3 GPM. 1/2" copper can flow 3 GPM easily with the right circ. It sounds like you have a B&G 100 so, having this flat curve medium volume pump would work well for you.

Point of interest, my dad's 1965 home which has 2,500 sq ft, 5 zones, 1/2" Edwards baseboard and is piped all 1/2" with exception to common 3/4" returns in basement, heated by a Weill McLain HE4 (104,000) and has a single Grundfos Brute UP15-58 with a tekmar 256 outdoor reset heats beautifully since installed in 1987.

""Water makeup is a normal requirement of cast iron boilers. So you see the water make up although minimal is perpetual. ""

Let's not forget about the high quality steel boilers out there that don't lose water and will probably be in service when the cast iron boilers are shot.

I didn't mean to pick on material types, all systems loose water, especially ones with B&G 100 circs with water seals.

Baymee, I realize you are/was connected with a steel boiler company. Of course that brings up other points and the effects of oxygenated water and steel boilers. As with all boilers proper pressure and air elimination is key to all boilers surviving.

Any thoughts on copper to iron connections?

Twenty years ago I installed a piece of copper in a radiator run and just this fall I went to move the radiator and the iron elbow connected to the copper run was so thin it crushed. Also, I recently replaced an iron riser up to a faucet from a copper main line. The copper fitting had to be replaced as well because it was nearly plugged with deposits.

Copper and steel/iron are used commonly in heating systems. The farther apart they are on the list below, the more chance there is for corrosion. Magnesium and High Chromium SS would be a poor choice. Di-electric unions are sometimes used, but I haven't seen alot of them.

Magnesium
Zinc
Aluminium
Cadmium
Mild steel
Wrought Iron
Stainless steel (Low Chromium)
Lead
Tin
Brass
Copper
Bronze
Copper-nickel alloy
Nickel-copper alloy
Stainless steel (High Chromium)

Here is a link that might be useful: Example of a di-electric union

"Di-electric unions are sometimes used, but I haven't seen alot of them."

They are actually required, though in a well constructed and operated closed loop hydronic system you can sometimes get away without them.

Galvanic corrosion requires ions in the water to operate.
In a closed system the corrosion tends to be self limiting as the ions are taken out of the water and trapped as boiler scale, rust, etc.

Every time new water is added to the system the corrosion starts again until the various ions and gasses are removed from the water. This is one of the reasons that well sealed systems with minimal losses last longer.

When we remove them on purpose the process is called gettering. They are absorbed by sacrificial materiel in the system so they no longer are available to support corrosion and reactions.

Cast iron and steel boilers in a residential application will have about the same life expectancy. There are many systems with manual make-up valves, out there that are water tight and do not have to add water from one year to the next. There are systems which add a lot of water every year. Both steel and cast iron boilers will serve your needs well as long as properly piped and sized.

Quoting: "Di-electric unions are sometimes used, but I haven't seen alot of them."

They are actually required, though in a well constructed and operated closed loop hydronic system you can sometimes get away without them. Quoting end.

This has popped up before about the requirement of di-electric unions. I've been querying the various heating contractors I come in daily contact with and none of them use them, or rarely and yesterday I had to order some stuff for my own commercial boiler installation and the supply house said they sometimes don't sell any di-electric stuff in a year's time. As far as PA Labor and Industry who inspects commercial boilers, they don't specify either.

So, I'm guessing it's a local thing.