"• A samller unit is capacitated to cool or heat of big house, it will run continuously to render the desired comfort. This will shoot up the power bills. • It will be able to provide the desired comfort making the house hotter in summer and cooler in winter • If it is unable to provide you the desired heating, it will depend upon supplemental form of heating or cooling. This will be very costly in terms of operation. • It can create unnecessary noise. This is will more acute if the ducts are also undersized, compared to what is required by your house. • As it will be running continuously, its lifespan will be shortened; you will generate frequent repair bills due to breakdowns and mail-functioning. • A good side of an undersized unit is that it will provide effective humidity control as it will run longer each cycle. "