Domestic coil on steam boiler

@EBEBRATT-Ed has a good point. If you use the boiler water in the indirect water heater coil HX, all you need is the pump for an open system. I have used a tankless coil on a steamer to add a small baseboard HW zone to an otherwise huge boiler for that small space. A church with over 10,000 sq ft of floor space. The restrooms, the secretaries office and the pastor's office were heated by steam when I bid the new boiler. I offered the option to heat the 4 small rooms with their own hot water zone, so now you don't need to send steam to the entire building to get heat on any given weekday when only those small rooms are occupied.

This involved making the tankless coil a part of a closed system with a circulator, expansion tank and relief valve as if that coil was a regular boiler. The steamer will heat the water around the coil without making steam with a limit control set at 170°F.  I felt that was a better system since the baseboard radiators were going to be above the boiler’s water line.  More dependable and less knowledge needed to keep the baseboard full of water, compared to an open system boiler water running through the baseboard loop.   

In your situation when the indirect water heater HX is lower than the boiler’s water line you can very easily use the boiler water to heat the tank without the extra closed system parts required with the tankless coil as the heat source.