I used something like this for controlling my heater water flow:
http://www.summitracing.com/parts/EAR-230502ERL/
Best Regards,
Steve Thomas
Years ago I built up a Chevett for my daughter. (because I got it for free) and it needed a heater valve.
Autozone got me one for a few bucks. Light simple, cable operated and it works.
Lynn E. Hanover