With the engine warmed up & the heater on, grab the heater hoses one at a time. The in going hose (usually smaller in dia.) & the out going should both be hot enough that you can not hold on to them for more then 3-5 seconds. If one is not as hot as the other then you have a restiction. It could be in the control valve if it has one. You may need a new therm. like Fleetside say's. A 195 would be ok but you may have to change it back in the summer.
