Understanding oxygen in my complete steam system

Fishin_Rod

I am a homeowner with a 106-year-old steam radiator system. The original boiler (coal-fired and later converted to gas) lasted about 90 years. My replacement boilers (or sections thereof) have not been having a long life.

During my fall start up, I have to circulate some heavy brown sludge out of the bottom of the boiler (which turns to dry rust particles when dried), but I also have to clean out some black sludge and even some iron chunks out of the return lines and check valves.

I believe I suffer some significant corrosion every year in my boiler, in my piping, and in my radiators. Further, I believe my corrosion rates are increasing, because I can't believe my total system would still be functional after 106 years if this much iron was removed every year!

I want some good advice on all kinds of maintenance, so I can do a much better job on my system. However, I do NOT have a good understanding of the oxygen distribution in my system. I think this is important because iron + hot water and steam + oxygen is pretty much the perfect recipe for corrosion!

I am going to state what I think I know, so I can be corrected in the comments to the actual circumstances within my system. (Can you please simplify the answers a little bit, to the "non-expert homeowner" level?)

Background:

After my boiler shuts off, my main air valve on the condensate return lines stays open and hissing for several minutes. Putting a cheap grocery bag over the outlet, shows that it is on a strong vacuum. I am therefore drawing lots of air (and oxygen) into my system.

I think that my steam supply lines and radiators should end the cycle full of steam (water vapor), and if my steam traps at the radiator outlets are all functioning, then my supply lines and radiators should have very little air in them under normal operating conditions. Correct?

However, my condensate return lines and the air space above the water interface in the boiler will all be filled with "fresh air" and the full atmospheric ambient level of free oxygen. Correct?

The water in the boiler should be at a very low oxygen level at the end of a heating cycle after the heat drove out all of the dissolved gasses in the boiler water. Correct? Does it quickly become oxygenated from the air above the interface, or does it mostly stay oxygen depleted - and then the next heating cycle starts?

I believe my fall start up procedures should be to perform some partial high rate water flushes at all of my outlets to clear the rust and debris that formed in the lines during the summer shut down period. Only after the lines and bottom of the boiler are mostly clear, would I then add my Rectorseal 8-way to the system and then try to get through the heating season with only some small water additions (manual re-fill supply water).

Questions:

Will all of that flushing add a damaging amount of oxygen to my system?

Are there any common failure points that add a damaging amount of oxygen to the system? (For example, failed air vents and other items that I will NOT notice compared to items that I do notice that cause an obvious water loss in my boiler sight glass.)

Finally, am I totally off base worrying about oxygen in my system - since the main vent lets in fresh air? Is the CO2 (and subsequent carbonic acid) created by heating dissolved carbonates in my water supply the main cause of corrosion in the system? If so, how should I think about managing THAT when I am considering better practices for my system maintenance?

Thanks for any help on this topic. I am just asking such a basic question, so I can better understand the maintenance and care discussions on the forum that I obviously need to read to extend the life of my old system.

Jamie Hall

A few comments, though I'm not sure they will be direct answers…

Dissolved oxygen in the boiler water will be very close to nil after a few runs up to steam. Furthermore, it will stay that way, as even if there were to be a significant amount of oxygen in the air over the water in the boiler when the boiler was off, there would be very little interchange of air as the surface area is small and turbulence is also nil. The primary source of oxygen in the boiler water is from makeup water — and that is detrimental, so makeup water is to be avoided if possible.

The water chemistry of the water in the wet returns is a somewhat different matter. While the oxygen concentration will be low, the carbonic acid concentration may be significant (though the actual concentration of the carbonate radical will very small) and this can and does lead to corrosion in the wet returns and, indeed, the wet returns are the first place to go looking for leaks in a steam system for this reason.

It is quite true, of course, that air — and oxygen — is drawn into the steam mains and radiation through the vents when the system stops steaming. This is not a large volume of air, however, in comparison with the volume of steam which comes into the system, and the air doesn't mix with the steam — hence all but a very small amount of the initial condensate will have almost no oxygen at all (although this is the source of the carbonic acid mentioned above, not the heating of dissolved carbonates, which occurs, in any event, only in the boiler from makeup water). Further, the metal surfaces will be mostly if not entirely dry, although the relative humidity inside the system will be quite high.

As to maintenance/operation… hmm. Well, first thing is to avoid makeup water to the extent possible. Some people add pH buffers, such as 8-Way, to their systems, some do not. For residential boilers the evidence for benefit is not conclusive. The second thing, however — and overlooked, in my view — is run as long cycles as possible compatible with comfort. In my view — and there are strong contrary opinions — the system, once asked to produce steam for instance by a thermostat should never go off steam until the thermostat is satisfied (if it is shut down briefly to let the radiation catch up to the boiler if the boiler is slightly to even significantly oversize, that's OK — so long as the system never actually goes off steam. Keep that catching up off time as short as the burner will permit). This is possible for any two pipe system. It is also possible for one pipe systems, but only if thermostatic radiator vents are NOT being used; if they are, the system must go off steam for the vents to have control. Again, an overlooked point.

Fishin_Rod

Mr. Hall,

Thanks for typing that long reply and the clarifications of my concerns.

Our old house is almost the same size as the one you take care of. I wish you could download your steam system knowledge onto a thumb drive and I could then upload it to my own head.😁

How long are the heat cycles that you consider "optimal" on your museum? I can somewhat manipulate the heat cycles by opening or closing some of the radiators in seldom used rooms. A few years back when gas prices were very high, I shut down a bunch of rooms to save money on our family budget. Our actual gas usage was about the same.

I was a little surprised, but did notice that with a significant number of radiators shut in; 1.) that made my boiler effectively "oversized", and 2.) that made my boiler cycle on and off on much shorter cycles as the room with the thermostat heated up much more quickly, and I think(?) cooled off more quickly.

Jamie Hall

Good observations there! Keep it up.

I happen to be very fortunate in that Cedric — that's the boiler — was chosen carefully and set up by an excellent steam man, @Charlie from wmass , so that it is only very slightly oversize for the radiation. As a result, it only cycles on pressure towards the end of very long runs — say 45 minutes to an hour. When it does start to cycle on pressure, it runs a cycle of one minute off (that's the stop.start delay on the burner) and about 8 to 10 minutes on. It never actually drops off steaming; the pressure never goes below about 2 ounces (the cutoff is 7 ounces; it's a Hoffman Equipped System).

Shutting off radiators will, inevitably, make the system oversized. Again, I would try to keep the "off" part of cycling on pressure as short as possible, but when the "on" and "off" parts start to get close, you may have to introduce a timer to hold off the system after it reaches pressure; there are several threads on that approach.

Going a little beyond steam, though: be wary of closing off rooms. Yes, it will save some fuel, though nowhere near as much as one might think. What it may do, though, is allow the relative humidity in those rooms to rise to the point where condensation becomes an issue. In most houses this may be seen by dripping on the windows, but even before then it may be a problem if there are things like fine furniture, books, musical instruments and the like in there — or even real plaster. One cracked piano soundboard, or a few square feet of failed plaster, is going to cost a lot more than some extra fuel…

EBEBRATT-Ed

@Fishin_Rod

How much MU water are you adding to the boiler and how often? Do you have any leaks in the system?

Fishin_Rod

When the boiler (or boiler repairs) are new, I add a 1/4" of water maybe every 3-4 weeks during heating season.

When our first "replacement" boiler rotted a large hole in one section, I only caught on because I lost so much water (as steam) that the boiler quit operating because the low-water controller properly shut off the burners. I was able to add water daily at that time to keep our house heated until we could repair the boiler after the end of the heating season.

I was very surprised that our gas bill was NOT outrageous during that period. Apparently, the percentage of live steam that I was losing up the exhaust stack was not that great compared to the amount of steam that went up the headers!

I fear I may have a leak this season, but I just turned on the system and have been fixing anomalies. I want to do a "correct" start-up procedure, but need to learn more about steam radiator systems to properly follow the discussions on the forum.

I do not believe we have ever had a "water loss" event from the piping behind the walls or due to a radiator rotting out. I am asking about oxygen in the system in an effort to reduce the likelihood of failures in difficult and/or expensive places to repair. Plus, I just want to do best practices on my boiler and the adjacent piping and controls.

TKPK

@Fishin_Rod ,

I recently found a setting in my thermostat that changed the behavior of my heating cycle.

it is fast or slow acting … it is similar I think to the anticipator in older thermostats … I changed it to slow and no I have one long continuous heating cycle at each cal for heat instead of 2 or 3 calls separated by a few minutes. Total run time is also reduced which is a big bonus to me.

Jamie Hall

Yup. Most modern thermostats come from the dealer set for hot air heat — which is the most wrong setting possible for steam. Check the instruction manual for how to set the thermostat for steam.