Honda Civic VII 1.4 16v, 66kW, HB 5D, 2001. poor interior heating.

I was looking everywhere, but unfortunately did not find a solution.
Problem:
Honda Civic VII 1.4, heats poorly regardless of the position of the "heat / cold" knob. All control cables are in place, the bulkhead valve responds to the cable. Coolant level normal. It heats better in closed air circulation, but unfortunately everything evaporates even in the climate. It is known that in a closed circulation system, air is always warm, taken from the car, heated, so maybe for this, but what can be the cause of poor heating and lack of response to air temperature regulation? It is also strange that as the control knob turns the cold one, it is also unchanged. I suspect an airflow control system, but how do you check it?
Perhaps someone had such a case and dealt with it?

The control of the heater's flaps may be damaged, or maybe the classic heater to be replaced.

Can you elaborate on the topic? Any particulars? I will be very grateful...

First of all, are the engine temperature readings normal?

Of course, I take for granted what the speaker asks about, i.e. that the most obvious obviousness (read: thermostat) works properly, because that is where the discussion should start.
So it works? Does it catch operating temperature quickly?
It's gasoline, therefore, no problem, even in frosts like this morning, in my case -13 * C, it should catch 70 * C after the first three kilometers of driving.

Yes, sorry I didn't refer to the cooling system. Display and engine temperature and its heating OK.

I do not know if there is a thermostat regulating the water temperature for the heater (because you are squeaking something about one) or a classic flap closing the air supply from the outside:
- if it's the first - then you have to check with your hand on a hot engine whether the hose behind it is as warm as in front of it
- hereafter this hatch - can be jammed in the open and packs masses of cold air
- the worst option: the heating system is dirty - these are small sections, years of unchanged fluid, mixing anything, or applying some sealant in the cooling system and after heating ...
Sometimes rinsing helps, but in most cases it is necessary to replace the heater with a new one, which costs little in relation to the cost of the replacement itself because only in a few cars it can be replaced without disassembling the entire cockpit.

Piasek80, I am grateful to you for your willingness and valuable comments, but I would rather ask for advice on this particular vehicle, not a lecture on the theory of vehicle construction What you write are timeless truths ...
Maybe some help in checking the operation of the air stream blades in the heater unit (how to check if they are working properly), or other substantive advice that would solve the problem.
There is a lot of advice on all topics, but not solving the problem and not very substantive. Maybe we can create a helpful pill here?