HVAC is not my trade. I have often suspected that fresh water is one of the places that a boiler gets additional oxygen to create additional rust. If a boiler was sealed so that no air could get in, and no water was ever added, it would seem that the only rust would be from the dissolved oxygen in the initial water fill and after that oxygen is captured in rust, no additional rusting would occur. But boilers apparently are not sealed.

If there is no power, then how will the thermostat turn on the boiler when there is a call for heat?

"We routinely add and draw water out of the system to drain rust and yuck "

You should try to NEVER add or remove water form a sealed hot water system.

Evey time you add new water it brings another load of oxygen to add to corrosion, and nitrogen to form bubbles in the system.

The old water may be stinky and look bad, but it has been de-oxygeneted and cannot cause any more rusting, and dissolved nitrogen has come out of solution.

You bleed out the nitrogen, and add some new water.
After a few cycles you do not add enough new water to release significant nitrogen.

From that point on the system just runs and should be left alone.
You do not need to do anything when it is not being used except make sure it never freezes.

It probably also has some oil in it from any circulation pump you are using. The oil will not hurt anything, and is just from bearing seals leaking.

One of the ways to 'clean' gasses and fluids is called 'gettering.'
A chemical (the 'getter') is placed in the sealed system to react with and remove the offending item.

Boilers do it by default.
Any oxygen in the water forms rust, and the oxygen is no longer available for further corrosion.

The OP stated it is a steam system. The radiators should have air vents in order to allow the steam to enter. It is an open system. You can't avoid adding water to this type of system.

My steam system is two pipe so I have one air vent in the basement by the boiler to purge the air out of the system until it comes up to full pressure.

Yes I must add water periodically as the system is not closed, at which time I draw water out.

My thermostat runs on batteries and kicks the heat off regardless of the power being on/off.

Thanks for the replies, does anyone know the lifespan of such a beast as this boiler? What may I reasonably expect to pay to replace it were it to die? It seems simple so maybe it is indefinitely repairable?

I would expect a boiler to last 25 - 30 years.

Your thermostat may have a battery in it, but I doubt it powers the control to your boiler.

Try this experiment. Shut off the main breaker in the house, and then push the thermostat up and see if the boiler fires.

I know that my thermostat runs (maybe magic?) when the power is out b/c we have had multiple long power outages during which the boiler cycled appropriately. I was told it was a "millivolt system" whatever that may mean.

Well I learned something new today. There are furnaces and thermostats which operate on a millivolt system. The gas pilot heats up a thermopile in the furnace which creates a 0.7V DC voltage source. This powers the thermostat during a blackout. I assume the battery in the thermostat is powering the clock and any programmable functions.

Manufacturers have eliminated the standing pilot flame in order to achieve higher efficiency ratings. So if you ever had to replace the boiler, you would need to provide some other means of a electric supply backup. It is unfortunate since your present system is simple and reliable.

I grew up in a house with a one pipe steam system. Every spring I would flush the boiler until the water was clean. This kept the water in the sight glass crystal clear. Keep the water level in the proper range and the boiler should last for many years. It is difficult to say how much a replacement would be. I would expect a new boiler would be in the $4000 - $6000 range. I have no experience with boiler repair, so I can't advise you whether if it is even feasible.

Hi Joslin, We also have an old house with radiator heat. I think it is the best kind of heat. Very warm. Any way, we got a new boiler a few years ago - maybe 3 yrs. It cost $6,000 or $6,800 installed.(I can't remember exactly). It is a Dunkirk. I am sure we tried to do tons of research on boilers before we bought it, but I think that was very expensive. Our house is only 1800 sq ft. At the time all the advice I found on the internet said a boiler should cost between 2k and 4k if I remember correctly, but we could not find any at that price. (NY suburb - everything seems to be more expensive here!)

Our old boiler was 30 years old and still working, but we figured it was at the end of useful life and was somewhat inefficient, and had to be repaired the year before, so we thought it was time to replace it. We used to have a pilot light and never had to worry about the heat going off if there was a power outage. It was great. Now, if there is no electricity, there is no heat. There is also supposed to be a thing that controls the water level, but it does not work, so my husband has to check the water level every day or two. Every once in a while the heat goes off, since the water dissapears. We need to have them come back again for like the 10th time but keep putting it off. I sometimes wish we had never replaced it. Keep your old boiler for as long as you can!

Thanks so much for all of your replies. I think this year I will hedge my boiler bet and get my asbestos removed from my pipes in anticipation of needing to replace it in the forseeable future.