I am trying to change an old Honeywell thermostat for a programmable Horstmann DRT2unit . The old one has 3 wires Red, Blue & Yellow which go into terminals 1,2 & 3 respectively on the 5 terminal block. The Horstmann has 3 terminals NO, COM & NC. Can it be connected to the old wiring and if so how?

Old Honeywell thermostat model number?

The only number is on inside of cover. EE2 4200 3879-001

That number does not compute. No info available. Sorry