My heater core became partly clogged during the first two years or so, and didn't heat very much any more. The passenger side was heated worse than the driver side. There are a few threads about clogging heater cores in this forum. In short, especially 2014 - 2015 cars can suffer clogged heater cores because of chemical reactions between OEM coolant and the heater core. This was cause for flushing out and replacing the OEM coolant with some better and more suitable product four years ago. The clogging neither worsened or improved after that, but stayed as it was at the time of flushing and changing the OEM coolant. After some clarifying research I finally got round to fix it.
This is how the partly clogged heater core was fixed:
For better access the air filter box was removed and put out of the way. The heater core was disconnected from the coolant circulation. To do that, both hoses were disconnected from the core, and a U-shaped bypass connected to the hoses instead. The bypass is about 19m or 3/4 inch in diameter. See:
Two short pieces of 3/4 inch garden hose were connected to the core, and the open ends left pointing upwards. A special cleaning fluid was filled into the core through the hoses, and it remained there for two days.
The recipe for the cleaning fluid:
Two full mugs of water plus two tablespoons of 9% ammonia water (spirit of ammonia), and one tablespoon of your favourite household cleaner.
After soaking for two days, the two short snips of hose were disconnected and discarded.
A working garden hose was connected to one side of the heater core, and the tap turned on full blast for one minute. Then the hose was connected to the other side of the heater core and flushed full blast in the other direction for a minute. The hoses were swapped and flushed in both directions a few times. The obstructing substance was sucessfully flushed out!
NB: There is not much point in doing this if the OEM coolant was not thoroughly flushed out and replaced. If not done yet, this may probably be your last chance.
A test showed the heater core works properly again, the temperature on both sides is practically the same. This procedure is easier and much cheaper than a replacement.