Heater
Water is piped from the cold water pump the to the heater in the smoke box top. Water does not mix with the oil-laden exhaust steam but the steam heats the water. (If the exhaust gases from the firebox are hotter than the exhaust steam, then the steam may actually cool the heater.) A vent to the track is provided for exhausted steam. Exhaust of the hot water pump is probably enough to do this job without using the main steam cylinder exhaust. Note that if the water is too hot, then it might flash to steam at the hot water pump inlet valves. You may want to insulate the heater body from the hot firebox exhaust gasses.
Availability
The body is no longer sold, although the heater tops are still available.
The body casting is not required to pre-heat the feed water since a simple pipe/tube run into the smoke box will do the job. The heater body is not outside the smokebox and is an unneccesary casting. The heater tops can be set on top of the heater well and look like the prototype. A drawing furnished with the tops shows how the well is made.
Over the past 20 years only 6 heater bodies were ordered, so I stopped having them cast.