I own a home w/ central heating & air (heat pump). The house has 3 bedrooms that are not currently in use. In cold weather I shut the doors to the 3 bedrooms and shut the registers in the bedrooms in an effort to reduce heat costs. The thermostat is not located in any of these rooms. I figure, this saves on heating 3 extra rooms.

My question is whether this is effective or not. I'm not sure if the air return from these "closed off" rooms (cold) is mixing with air return of other rooms (warm) and actually causing the heat pump to work harder.

Thanks

The less square foot you heat or cool the cheaper it should be BUT that is not always the case. I am sure you will save some $$$ but your system was designed to heat and cool the entire area. When you restrict air flow it does have consequences both on system operation and utility cost.

I am sure you are saving a bit of money but there is no way I can calculate the amount. When your heat pump is running your system is not capable of removing all the heat generated due to the restriction on the discharge but the air volume will increase in the other open ducts due to the increase of pressure and here again I cannot calculate the savings.

Unless you did a complete manual D then calculated the difference there is no way anyone can figure it except to guess.