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Small steam leaks add up fast! I had a leak in between two sections of one of my radiators. It didn't leak enough to drip, it was at the top of the push nipple on the bottom of the radiator but it would bubble a bit.

For the longest time I dismissed it because I thought it was too minor to make a difference….all the while I was adding make up water weekly. I fixed that leak and guess what…I top the boiler off once a month maybe.

Set the thermostat up high to get the system full of steam and a little pressure. Use a small mirror and move it around all the joints, valves, vents, etc. You will see little wisps of fog on the glass of the mirror if steam is leaking. Sometimes leaks are so minor you can't see or hear them….but they are there.

Bingo!

The work fine on a conventional two pipe system. Just adjust the metering section to suit the system working pressure, that's all.

That's one of those questions… how big is a box? The metering orifice should generally be set so that the radiator is about 80 to 90 percent hot at the end of a long run. Then you can control the heat in the space by reducing the throttling valve (the accessible handle) to what you want.

Be aware that the above is pressure sensitive, and any adjustments should be made when the system is operating at its normal pressure — usually no more than a few ounces. Not that the valves can't be used on higher pressure systems — they can — but the orifice will be set smaller.

Venting and steam velocity may have an influence.

Um, wrong link ? Maybe this one.

1 —- no. @EdTheHeaterMan wasn't referring to a steam vent, but to the exhaust from the boiler burner up the chimney.

Let me re-word this "

1. If the boiler has a leak above the water line, then steam could be leaving out the vent along with the flue gas. You may never see water on the floor if that is the case.

1. If the boiler has a leak above the water line, then steam could be leaving out between the cast iron sections. That steam will then go up the chimney vent along with the byproducts of the combustion process. You may never see water on the floor if that is the case. (steam will go up the chimney as steam not water)

It's almost certainly leaking vents (radiator and/or main) and/or radiator valves. There is a much less likely chance that some radiator runout or section of main is leaking, but it's possible. It could be a wet return, especially if any of them are buried where you won't see the water.

Even a non-obvious leak could let out a gallon or so per day, and for those boilers, a gallon of condensate might be 1/2" to 1" of height on the gauge glass.

There's almost a zero chance that either boiler itself is leaking IMO

Edit: sorry everyone when I posted it wasn't showing me those earlier responses so I may have repeated what others said, I have to go back and look!

Edit 2: Yeah i was confused by that comment about the chimneys being plugged up and unused too!