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Did you check the supply pressure?

Yes, that boiler has a draft hood built-in.

@KarlW I think with good system design, probably yes - I see no reason in theory for a steel multicolumn radiator to fail - but we have seen issues from other manufacturers of steel radiators on 2PS so I can't say for certain. We definitely won't be changing our guarantee terms to include steel for steam sytems any time soon I'm afraid.

Cast iron seems to have fared ok on steam for the past 100+ years so that's what we'll continue to recommend!

I think we'll just agree to disagree on that one. But then again, Chicago isn't Jersey.

I started to write a longish commentary on this — that is, some of the considerations for sub atmospheric steam systems — but rapidly realised that it was getting long — even for me!

While there are indeed some advantages to operating a steam system at such pressures, there are also some major problems associated with it. Most of them revolve around establishing the initial pressures required and then maintaining them while at the same time preventing loss of refrigerant (water) and preventing the entrance of non-condensibles (air). This is no mean feat even for conventional heat pump systems (heat pumps, air conditioners, refrigerators, what have you) which operate well above atmospheric pressure.

On balance, the complexities which are added would not outweigh the advantages to be gained.

However, that is not to say that a small efficiency gain could be made in one of two ways is systems were designed to operate part of the time at subatmospheric pressures. One would be to have the boiler operating at condensing temperatures for some or all of the time after the initial firing in a cycle. I haven't worked out whether the cycle would need to be long enough to entirely fill the system with steam, however. If that were true, there would be no real advantage. The other is that forcing the system to drop subatmospheric at the end of a cycle would enable transferring the residual heat in the boiler to the target space. This second can be done in any two pipe steam system which is piped using crossover traps and a single main vent by the simple expedient of making the main vent (or vents) Hoffman #76 vents — which are still made. Some folks have also suggested accomplishing the same objective by placing a low cracking pressure check valve before a regular vent (the cracking pressure, however, has to be very low). Either of these methods will work. There is a question of is it worth it, monetarily? Both #76 vents and low cracking pressure checks do not come cheap, so one needs to look at how much energy can be recovered this way. In the days of large high mass boilers, and particularly in the days of coal fires, which don't just turn off, the answer was —-a significant amount, with the result that such systems, while not common, weren't that unusual. Today, however, the amount of heat stored in a modern boiler is, in relation to the overall system, almost trivial.