Ok. All 126's with the OM 617 have a monovalve. Monovalves are easy to repair, here is a
link to a nice write up/.
BUT -
before you jump to conclusion that the monovalve is faulty, tell us a little more. 126's don't turn on the heater blower motor unless the temperature gauge reaches 40C. The idea is not to blast the passengers with cold air. So, your blower probably is fine as long as warm air comes out when the gauge hits 40.
If it takes 15 miles for your temperature gauge to heat up that much, you probably have a bad thermostat. Test it and swap it if bad. If your engine reaches operating temperature normally, the next place I'd look would be the monovalve.