Honda CR-V 1998 model.

The heater works ok at 5-10 degrees celsius, the air gets sufficiently hot. Approaching zero and below zero it really struggles to get any temperature going, it never gets really hot. I'm not using the airconditioning system.

There was a hole in the radiator which was repaired with a radiator sealant at an auto repair shop. Since then the radiator have been replace with a new one. I've later read that these sealant products could cause serious problems, so I fear this could be the root of the problem? I've had help flushing the hole system and I doesn't appear to be clogged, which is good. The thermostat has also been replaced.

The auto repair shop couldn't help me without going through the whole system, that would cost too much.

I would greatly appreciate any tips for things that I can check, I have semi-qualified help, not a mechanic but more knowledgeable than me.