steam to hot water brazed plate exchange

Boilergeek

I am tapping into an existing low pressure steam boiler to add heat for a new addition requiring 50,000 btu's. I am going to be using a 3642 Spacepak with a hot water coil. My question is how to best control the steam side of the heat exchanger. I was thinking of using a flow switch to enable a three way floating steam control valve. Any ideas for piping or control valve recomendations would be appreciated.