A question asked on The Wall: "Is it okay to use copper on steam systems?"
Brad White replies:
It is not so much temperature as cyclical heating and cooling. Hot water runs at reasonably constant temperatures minute to minute. Even with outdoor reset, the swings in temperature with all of that mass is gradual, both warming up and cooling down.
With steam, a 60-degree F. pipe will be 212-215 degrees moments later. Between cycles (and especially if uninsulated) it will be at ambient or nearly so in short order.
Take a copper soldered joint in your hands – say a coupling in a length of 3/4" copper. Gently flex it back and forth while idly watching TV, to replicate stress cycles (or to alleviate your own :)
Eventually you will get hairline cracks in either the solder or the copper itself.
Copper-bearing metals are "work-hardened"- take soft copper and work it by rolling, hammering or listening to Heavy Metal and it will become more rigid. Heat it past its first critical temperature and let it cool to room temperature (annealing) and it will become dead soft once again. Such critical temperatures are well above what a steam system generates so that annealing does not apply here.
Iron piping is quite adaptable to those stresses, moreover, the joints are.
Now, no one is saying that 100% of steam systems piped in copper will fail. But a high enough percentage will, enough compared to proper iron pipe-work, to say, "Don't do that".
Like any risky behavior, yes you can do it without harm quite often. Just that the odds are you do not want to make a habit out of it.
Gerry Gill replies:
It isn't so much about what will happen to the copper, the copper will probably be fine. Copper will, however, degrade the iron or steel faster. This discussion has come up before and gets intense. Personally, I've grown weary of the fight.
If you do your homework you will answer your own questions. Study about cathodes and anodes. If you do, you will learn you want a small cathode-to-anode ratio. The more cathode (copper), the shorter the life of the anode (steel/iron).
I work on a customer's Iron Fireman SelecTemp system that is piped entirely in copper. It has to be by design. The cast-iron boiler on this job looks like crap. The water always looks like chocolate milk. There is nowhere else in the system that the rust could have come from, other than the anode-to-cathode reaction of the dissimilar metals.