steam copper piping?

My plumber is replacing a leaking steam boiler with a new (gas fired) one. I have a two pipe steam vacuum heating system. My plumber is inserting a valve into the bottom of the condensate return line ( a near horizontal section) to enable it to be flushed from time to time. But he's also replacing that part of the line (a straight piece about 8 feet long) with copper repeat copper line that is threaded to allow connection to the existing piping which will remain. The existing piping is all iron/steel.



I asked him why he's using copper not iron or steel as surely it will expand at a different rate to the iron/steel? His response is that copper will last much longer for this application than steel/iron and as the condensate line doesn't get that hot there's not an issue regarding different metals. He acknowledges that you should never use copper for steam piping, especially close to the boiler. He also says the copper line has threaded connections.



So my question is: Is he correct that it's better to use copper pipe at this location or is he completely wrong? I'd appreciate earliest response given that the plumber is installing pipe as I write.



Thanks!



Michael