The primary job of a portable heater is to counteract heat loss and reduce the effect of cold infiltration.
  A simple way to gauge cold infiltration is to feel your walls and floors and ceiling or better yet use an infrared thermometer to test wall,  floor and ceiling temperature then compare that to air temperature which will always be higher than wall floor and ceiling temperature...this is your heaters battle ground, this is what it must overcome and this doesn't even include what may be coming in through seals, doors and windows.
  Now measure the amount of floor, wall and ceiling space and include doors and windows, this is the area where the cold air is forcing its way through and the heater must keep that from happening.
  Insulation, Insulated exterior walls of wood or plastic, fabric interior wall layers, dual layer windows or better yet plastic windows, door and window seals, insulated doors, insulated under house and insulated carpeting on floors will all help reduce heat loss and cold infiltration as well as proper house ventilation, the house must breath well...but cold infiltration will always be at work as long as it is colder outside than it is inside your home and once the temperature drops below 42 degrees the task becomes increasingly difficult as the temperature outside continues to drop.

  Humidity is another HUGE factor as humidity can make it feel sharply colder in a house while the temperature says you should be warm and toasty, humidity is a very tough nut to crack and solutions vary widely, so don't blame your heater.

1.   Portable heaters, no matter what make, are designed for 150 to 250 square feet maximum, any more and heating efficiency drops the larger the space...a small heater can only put out so much heat.

2.   An area to be heated must be isolated, separate from any others, not open to other larger areas, portable heaters are designed to heat a room at a  time.

3.   Heating a large area will require multiple heaters as well as multiple electrical circuits.

4.   Heating an area with cathedral ceilings is like removing the roof, the heat will just go up and dissipate, a low speed fan can help.

5.   If you require two heaters in one area, be sure they are on separate circuits otherwise you're going to spend a lot of time tripping circuit breakers.

6.   Keep heaters at least three feet away from anything flammable... Items, fumes and or living beings.

7.   Insulated walls, doors, ceilings, carpeted floors and storm windows all will help keep power use down and heat in.

8.   NEVER EVER leave ANY portable heater un-attended and NEVER use one in a bathroom or shower area without a GFR circuit.

9.   NEVER USE a portable heater without a thermostatic controller.

10. SAFETY: Any heater MUST have a power overload protector and tip over switch.

11. Always feel the power cord when heater is at high setting to see if is getting hot, a proper heater cord will have HEATER CORD stamped on the power cord and feel warm but not hot.

12. Also check heater cord plug when running for several minutes and be sure the plug fits snuggly into the wall outlet, loose fitting plugs do cause shorts and outlet fires!

13. All heater makers should have posted on or with the heater a Support number, Manufacturers Name and Physical Address.