New Wallie here, posting for the first time, from the Great Wasteland o' Scorched Air no less (hydronics exist, but are not common in residential work, and steam? unless you're on district/campus heat, what's that?), with a bit of a steamy theorycrafting question as I've been mulling over loads of alternative ways to get heat around a building, if nobody minds.
One of the goals that I have for any non-scorched-air heating system (hydronic, steam, or the likes) is that it be able to service all types of heating demand in the building or space from a single plant, as there's no sense in having two appliances that take fuel and cold water in and spit heated water and flue gases out. It's more fuel piping, more combustion air intake work, more venting, more places for gas to leak, more potential for venting problems, and generally just not as good as having one combustion plant that does all the work. (If you need redundancy, it's better to have multiple boilers in one plant instead of multiple plants serving individual loads; that way, no single load can be rendered unserviced by a single failure.)
For hydronics, this is relatively easy using (regular or reverse) indirect tanks to feed DHW loads, or brazed-plate exchangers if you want a fully tankless approach for that matter. On first blush, the steam folks have taken that gambit and matched it, with a variety of heat exchanger setups (steam in shell, steam in tube, and even brazed plate) available for generating hot water from steam, in both instantaneous and storage setups. However, while I was reading the IOMs for several different makes and types of steam-fired hot water generators, I noticed something odd: every single unit I can find provides installation instructions that are exclusive to two-pipe steam systems, even for your most basic steam-in-tube storage configurations.
This, to me, makes not a whit of sense. How is a heat exchanger coil in a tank of water any different from a radiator in a room in terms of thermal massing and ability to condense steam? What stops you from connecting the branch line to the lower port on the steam side of the exchanger and a one-pipe TRV with an air vent and a suitable actuator/remote sensor (Danfoss makes a version of the RAVK that's set up for hot water cylinder service, with a maximum setpoint of 65degC) to the upper port on the steam side? Would this not work, for some reason or another? What am I missing here? Surely, I'm not the first bloke on the planet to think of this idea!