It doesn't work. From the information you gave that's the only answer possible.
If it's 200 degrees! Your heater wont go that hot. Trust me.
THERMOCOUPLE or defective gas valve
Have you turned the control from Pilot to On? Have you increased the temperature setting?
Your heater core is likely bad
check heater fan to see if it is working.
Low coolant? Thermostat not opening? Plugged or restricted heater core? Heater core airbound? Heater valve control cable detached or broken?
What temp. thermostat did you put in? Try a higher temp. one. or check to see if heater core is clogged
most likely your heater controls run via vacuum lines. cracked or broken lines could cause this.
it could be that you've put too much coolant to percentage of water in
verry common problem its the heater tap stuck open, you can do a quick fix and bypass the heater, but than you wont hav a heater when its cold of find the tap and close it manually, so i recommend that you replace the heater tap
It is a water heater, not a hot water heater. There would be little point in using a hot water heater. I assume that is your point.
you have a bad heater core