[danans]

> The problem is (especially with older furnaces; modern ones have safety features to prevent this) that if you have too much back-pressure

An everyday way in which people induce this scenario is by shutting the heating vents in particular rooms because they get too hot. Even on the newer furnaces, this results in the automatic controls shutting off the burner when the pressure and temperature gets too high, and then the fan starts pushing around cold air.

A secondary negative effect of this is that it pressurizes the ducts causes them to leak more, resulting in reduced efficiency, and also quicker failure.

A well designed system which has been configured to deliver the correct amount of heat to each room doesn't experience the same issues. Unfortunately, most older homes and even newer production built homes have poorly designed HVAC systems.

Heat pumps don't experience as many of these issues because they just don't get air as hot, and instead rely on higher throughput of lower temperature air to heat spaces, but that makes them far more reliant on good duct system design.