Heater does not work in the winter

The engine I had to be rebuilt due to a faulty part, oil pump, the engine had 176,000 miles on it, oil was changed on average at less than 2,000 miles. The radiator was replaced 10 years ago, since it "failed" during a timing belt replacement, I assume non - OEM since it was done at Pep Boys no change in engine temperature performance or heater performance ever noted prior to rebuild of engine. Prior to rebuild, as suggested all hoses were replaced and a new radiator (non - OEM) was installed. During warmer months the engine runs hotter, still within OEM specifications so to speak, just a noticeable increase, during the winter, at cold outside temperatures, basically the heater is useless. The hotter/warmer temperature I "wrote off" or explained away as the result of the engine being painted, which raises thermal conductivity, and emissivity, the engine block and head was not painted by Toyota. What would be the best reason(s), wider swings in engine temperature, and the engine not heating up during cold temperature - outside temperature day 40 degree F or below? The engine now has over 6,000 miles on it, so it is well broken in.

It does not seem reasonable to assume a circulation issue, if the engine runs "slightly" hotter after the rebuild during the warmer months (again it does not overheat), then the engine should run slightly hotter during the winter months, and the heater should be even more "effective/efficient", but basically the heater during cold weather does not function.

I am at a loss for an valid reason, there is no engine warning light. So there is no diagnostic code to be obtained.