2004 Civic heating issues

Hello. I have newly purchased a 2004 Honda Civic EX 1.7l vtec (D17a1) with a manual transmission. The vehicle has 220,000 miles on the body, however the engine was replaced and its exact mileage is unknown, though the previous own told me 100,000 for whatever that is worth. I am trying to get some help with diagnosing a heating issue. The problem is that the car does not blow hot air at idle. From a cold start (0°F outside at the moment) the engine take about 20 minutes to reach operating temperature, however, once it does (dead center on the gauge) the engine temperature is stable and does not fluctuate. Now when I say that there is no heat at idle, it is not ice cold, but no where near what it should be. If I had to take a guess without actually getting a reading, I would say that it blows out at around 55°F. If I pick up the rpm’s to 1500 or higher it blows nice and hot, then goes cold when it drops back down to 900. Again, for what it’s worth, which is next to a grain of salt, the previous owner claimed to have replaced the timing belt and water pump when he put in the new engine. So I would like some advice on where to start diagnostically. Could this be a thermostat issue even though the engine maintains operating temperature? Could this be a sign that the “new” water pump is wearing out? Could this be as simple as changing the coolant? Yes, I have check the coolant level and it is properly filled. Also, the climate control knob is functioning properly. Thank you in advanced for your input.