Where's The Venting?

If orifices are sized right, no steam should make it passed the last section on the radiators.

Broomell, VECO and other similar systems vented into the chimney like that. The idea was that the chimney draft would help pull air from the system. See chapter 15 of Lost Art.

But AFAIK this is the first time we've seen a chimney-vented Hoffman. Which components on this system did Hoffman build?

OK. This is what Gordo and I call a "vernacular" system, in that it was probably the work of a local contractor who went around installing systems his own particular way.

The pattern seems to be that the contractor knew how to build a workable system, and ordered his materials from whatever the local suppliers stocked.

If the Hoffman valves are the #7 type, as used on systems with Differential Loops, it fits the pattern. These valves had orifice shutters built in, so it would have been a simple matter to set the shutters on installation to the point where they only allowed the steam to fill 80% of the radiators. We've seen such systems built with all different makes of valves, and standard union elbows on the radiator return connections.

But this is the first one I know of that was chimney-vented.

Of course the original systems had a continuous fire, so actual significant air removal was a rare event at startup. Ongoing the chimney draft would drop the pressure in the dry return and thereby reduce the pressure required at the header from what it would be with the dry return vented to the atmosphere. A small advantage, yes, but a continuous one and easily worth it.

With intermittent fire, if an open pipe in the chimney from the dry return is the only vent, I assume the system backfills from the chimney between cycles. Fire is off, and the collapsing steam wil surely create a lower pressure in the pipes than is in the chimney.

I have suggested for some time now that very little venting is required in two pipe systems if you simply don't let the air back in. If you don't let air in steam continues to move from the header to the rads the whole time the fire is off between cycles. On refire a very small amount of air needs to be pushed out each cycle at the end of the burn when the vacuum is gone. Makes a huge difference in the performance, really does.

Beautiful install as always @New England SteamWorks.

Now that's a good looking header