[MtnHam]

Electric heat can be relatively efficient  But the most efficient electric heat is combination infrared (that heat only objects in the room)/convention (designed to incidentally heat the air) units.  Ceramic/mica based portable heaters are relatively safe, inexpensive, no fans, and quite efficient.

One kwh of electricity = 3412.14 BTU. This is an immutable fact of physics and can not be argued. All resistive electric heaters are 100% efficient, regardless of design, unless they convert electricity to some other form of energy, ie motion. Thus an electric heater with a fan does not convert 100% of it's consumption into heat. However, small fans found in portable heaters use a very small amount of energy.

The exception to this is a heat pump, which does not directly convert electricity to heat, but instead uses a motor/pump to move heat from another place, and has an efficiency as high as 350%!

The average cost of electricity in California is $0.24/kwh, ranging up to $0.55/kwh making it very expensive for heating compared to natural gas. If you live in a area where electricity is cheap, say $0.05-0.10/kwh, consider yourself fortunate, and moderate consumption to spot heat a small space may make sense.

An interesting article: http://www.midwestrec.com/myHome/electricVsGas.aspx

[Martyn]

The efficiency with which a fuel is converted to heat is only part of the overall efficiency calculation. Consider two natural gas heating systems, one which uses a gas furnace and forced-air system, and one which uses a gas boiler and hydronic in-floor system. Even assuming that the furnace and boiler are of equal efficiency, the hydronic system will be more efficient at space heating, i.e. the kWh of energy required to heat the same house to the same perceived level of comfort will be lower for the hydronic system.

Incidentally, even the fan heater is 100% efficient at converting energy into heat. Frictional losses in the bearings, viscous losses around the rotor, and acoustic noise from the fan blades all end up as heat in the end. Unfortunately, air is a rather poor heat-transfer medium, whether it's in a fan heater, a baseboard convection heater, or a forced-air furnace.

There are numerous factors at work, but I'm aware that this subject is quite removed from the OP's original question.

[PRELUDE]

Since this project going on, I have been talking with people, guys at work and neighbors and I just realized two things.
1. Which is kinds of scary, most of the people does not even have a idea what is cold air return and why it is there.
2. I see a lot of new houses that have only one central return.
So far I get 50 50 answers and a lot of them are agree if I cut under the door for 1" then I am OK which is already has been done before(not by me) but I do not even close the doors anyway.