Must be an older home that used cast iron or steel pipes leftover from the gravity heat days perhaps, converted steam or an old time plumber.

The boiler will have a pressure reducing valve to keep water in the boiler typically in the 12-15 lb range, cold. It will expand as heated thus; there most likely is an expansion tank too.

As far as antifreeze, most often use in radiant systems where the tube is buried and if frozen and breaks difficult to access.

Antifreeze can be used in your system, NEVER AUTO ANTIFREEZE, soNon-Toxic Hydronic antifreeze is used in about a 30% concentration offering freeze protection to about -10 and burst protection to about -50. Most antifreezes contain corrosion inhibiters and if maintained prolong seals, pumps, piping and boiler however antifreeze is also a insulator and affect heats transmission as much as 10%.

Good points about anti-freeze trade-offs, though it's really anti-corrosion I'm worried about.

Is COPPER proper for plumbing this application (mostly baseboard w/ a few remaining radiators)? Seems copper would solve the corrosion problem. Then again, I have no idea what the baseboard tubing is made of. I'm pretty sure the radiators are cast iron... hmmm...

Acutally oxygen and entrained air is the killer of systems via corrosion. Being that the systems consist of a ferric metals, just add water and air and you have rust.

Remove the trapped air and corrosion in system will cease.

Google sprirovents. They are about the best auto air vents when installed right.

Copper is better for hot water systems as they are less mass to heat up and sized right tend to circulate the water more efficient than steel or cast pipe. On newer systems PEX has taken over which works well with its inherant insulation properties, oxygen barrier and less fittings for smoother delivery.

Here is a link that might be useful: Spirovent

zl, thanks much for all the feedback. That's exactly the kind of real world nuances I need to assimilate. Those spirovents look like the ticket, especially as the house is UNoccupied most of the year, and gets only casual checkups from local relatives. I'll be installing one or more vents next week.

If existing runs were re-plumbed in copper, would di-electric fittings be required when joining to ferrous items?

PS: Though small-town, old-time plumbers may be the reason for ferrous piping, I'm also wondering if it's to avoid dissimilar metals (galvanic) issues w.r.t. the cast iron boiler and radiators? IIRC, the GIGANTIC old-school pressure-relief valve, pumps, and other fittings also appear (under UMPTEEN LAYERS of paint) to be cast iron. Original install was 1949, so it's... interesting... lol.

Antifreeze is rarely used in normal residential hydronic systems.
It reduces the heat carrying capacity of the system.

The mix of metals is not as much of a problem as it first appears.
While new water will have entrained air, and some dissolved solids.
The oxygen in the air and any ionic solvents can cause corrosion, but since the amount is limited it stops after the oxygen and dissolved solids react.
Adding 'new' water repeats the corrosion again until the reactions are completed.
The plumbing codes do require galvanic isolation in new systems.

You will see engineers write specs for the addition of di-electric unions on commercial systems yet; they are extremely rare in residential systems unless going in a fussy plumber's house.

You mention of adding vents, makes me add one more thing. Unless youÂre adding them to nuisance air trapping radiators, too many people make the mistake of adding too many auto vents and worse yet in the wrong places. Under certain circumstances they can actually draw air into system. One good system vent in the proper place is all thatÂs needed. This vent placement should be in a straight piece of pipe, in the lowest pressure point of the system, which tends to be on the suction side of the circulator.

One properly placed spirovent in time is capable of removing air from a 3rd floor radiator.

brick, thanks much... can the integrity of iron pipe in such systems be reliably checked by pinging it with a hammer???

... One good system vent in the proper place is all thatÂs needed. This vent placement should be in a straight piece of pipe, in the lowest pressure point of the system, which tends to be on the suction side of the circulator.

One properly placed spirovent in time is capable of removing air from a 3rd floor radiator.

The circ pumps are in the basement, adjacent to furnace. Are you saying the spiro should go there, near pumps, and as dissolved air is removed from water, the water will eventually re-absorb all bubbles everywhere in the loop?

Yes,

The water is scrubbed of it's air to a point lower than water pressure raised atmospheric pressure, thus absorbing air where ever it can

I have seen in occur in a clear pipe circulating piping demonstrator.

Depending on the age of the house and the heating system, it may have been installed BEFORE copper came into common use in the United States in the 1920s.

Also, it may have been a gravity flow hot water system, which requires larger-diameter pipes.

OK, finally on scene... takes a few days to get that thar "innanet" thang goin' in these small burgs... lol.

Hydronic system was originally installed 1949, with many mods since. Boiler is a PENNCO brand, from Warren, Pa.

Anyway, I see there's a huge 4' long, 1' dia. Thrush-brand expansion tank mounted horizontally in the ceiling near the boiler; how can a spirovent ever remove all the air in the loop if there's several sq. ft. of water being pressed on by a large air bubble @ 15psig?... is there a piston in there providing separation?... no duh, eh wot? ;')

The word 'hydronic' was not in my vocab, so now I at least of a starting point for self-education.