1994 Honda Accord

I recently had quite a bit of work done to my honda, water pump, timing belt, oil gasket repair, spark plugs, etc. Since i've had all this fixed though my heater doesn't seem to heat well, even after 15-20 min of warming up, it's blowing cold air. What areas do I need to look at or could it possibly be as simple as a thermostat? Thanks for your help!