This document describes how to rewire a Vapor thermostat to allow it to be used to control modern, SSR-switched electric heaters. You'll still get that famous, "Always at the set point" Vapor control plus the traditional steam heat look and feel but have the convenience of safe, reliable electric heat.
To facilitate the conversion of the thermostat, one of the internal jumper wires must be reconnected so that a normally open contact is made available to run the SSR.
To make this alteration, remove the existing jumper wire between external connector pins G and H. Next, add a new piece of wire from the NO contact on the relay (opposite pin D) and connect it to pin G of the connector.
The drawing Electric_Vapor can be downloaded as a PDF. This drawing shows the schematic of the altered Vapor thermostat as well as its application in a heating control circuit.