Heater not hot

Coolant level could be low enough that not much is reaching the heater core but then your car would run a little hot.Could also be your Stat is stuck open...

There is a Vac operated valve that opens and closes to regulate the amount of water that goes to your heater core. It's controlled by your hot.cold slider. That valve may be stuck closed or the vac hose (and come to think of it some are actual cables and not vac operated) or cable could be broken. If that all checks out then look at the short hose that rund through the firewall to the heater core. In it, there should be a restrictor. It keeps the core from springing a pressure leak. If the hose is worn that restrictor can sometimes end up sideways blocking off flow to the heater core. If all that looks good and your airconditioner is not continually running then I'd say that something is either jammed up in the core or there is so much scale buildup inside that it's not tranferring heat. Hope this helps.