Piping an indirect water heater into a steam boiler is not an uncommon application with gas or oil fired steam boilers. There are a few bits of information we need to talk about before we do the piping.
1. We can return the water from the indirect to a boiler tapping normally in the opposite end of the boiler from the supply. This will avoid short cycling of the water within the boiler.
2. The boiler should have at least one 1" or larger tapping higher in the boiler than the return tapping. Two tapings would be better although one can be as small as 1/2" to use for a control. A control on the piping works but a control in the boiler is much more accurate.
3. It is best to use a bronze circulator to pump this water. Although many have used the smaller wet rotor circulators it is best to use a three piece circulator. The circulator must pump away from the boiler. Think of the boiler as the expansion tank connection.
4. The piping from the supply tapping must elbow down toward the floor and mount the circulator close to the floor to keep the live steam away from the inlet of the circulator in case of surging water.
5. It is a good idea to install a strainer before the circulator as some dirt my get disturbed in the bottom of the steam boiler with the circulation of water.
6. The use of a check valve is recommended.
7. Use an aquastat to keep the boiler from creating steam on a demand for DHW only.
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Piping IWH to steam boiler See control wiring for IWH.
Piping IWH to a steam boiler with a baseboard loop below the boiler water level