2005 Renault Scenic heater

First of all, you have to understand that at higher fan speeds you're drawing in more cold air with more humidity but you have a limited amount of heat from the heater. You'll be blowing colder air onto the windows, and that humidity is going to condense there. If the car came with air conditioning, be sure that is turning on when you select the defrost mode. The AC is supposed to run to remove the humidity first before the air is warmed up and blown onto the cold glass.

Nope. You should not have to turn the AC on yourself. The compressor should run automatically when you select the defrost mode or any other mode that allows you to direct the air flow to a window. In really cold weather that may not occur because the condensed humidity will collect on the evaporator in the heater box and could freeze into a block of ice and block air flow. Normally humidity isn't a problem in low temperatures anyway.

I never heard of a heater matrix. Regardless, you haven't provided a clear description of what is happening other than to say it isn't. Is the air not blowing onto the windshield? Is the air blowing from where it should but it's not hot enough? Is there fog and condensation forming on the inside or are you trying to melt snow on the outside? What are the weather conditions?

Raining is when you get the worst condensation inside. First be sure the AC compressor is running. It should be warm enough for that to run. Next, if you think the air isn't warm enough, feel the heater hoses under the hood when the engine is warmed up. They should be too hot to hold onto for very long. If they are cool but the radiator hose is much hotter, a good suspect is a plugged heater core. Often those can be cleared by running water through from a garden hose. If the radiator hose is also cool, suspect the thermostat. If both heater hoses are hot, the best suspect is a problem with the temperature blend door in the heater box, or its able or actuator.

As for air volume, be sure the fan speeds up on every switch setting. Some cars use a relay for just the highest speed. When it fails, on setting "4" it will run at the same speed as on setting "3".

If you hear the fan speed up on each switch setting but the air volume doesn't increase, suspect blockage at the heater core. Usually the fan will make a lot of noise with little air flow to show for it. If that happens right away, leaves and mouse nests are good suspects. If air flow is normal at first but goes down after a few minutes, suspect that condensation being removed by the air conditioner is freezing into a solid block of ice. That can occur quickly, but it can take hours for it to melt. That would be related to how the air conditioning system is regulated. A sensing bulb filled with gas, or a temperature sensor, on the evaporator turns the AC compressor on and off to keep the evaporator temperature no lower than 40 degrees F. To prevent that freeze-up. Those sensors don't fail very often, but they can become dislodged so they don't measure the temperature accurately. To melt that ice faster, use the heater only, but not on defrost mode. Increasing the fan speed won't help with that. It's just the hot coolant that's circulating through the heater core that's going melt that ice.