What is Reddit's opinion of EconoHome Wall Mount Space Heater Panel - with Thermostat - 400 Watt Convection Heater - Ideal for 120 Sq Ft Room - 120V Electric Heater - Overheat Protection?

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there are many possible reasons for this. the most likely are poor insulation in the walls/floor, and generally poorly balanced air flow, which is very common for ducted system. it's hard to know which it is without being able to look at it.

here are some possible solutions:

a resistive wall panel heater with thermostat (like this)

you can try closing other ducts in the same branch and see if more air comes through into that room

you can have someone check your ducts for leaks

you can see if you can insulate the room better and/or seal up the window better. plastic over the window may not look the best, but it will likely make a noticeable difference to room temp.

the rattling is probably unrelated.

I used a wall mounted heater that was made of ceramic. It's electric, but doesn't use much energy. You can paint them and wall paper them. The one I used was also safe to have around children and pets. It wad given to me by a friend, but I think this is very much similar on Amazon: https://www.amazon.com/dp/B07BN2TH5V/ref=cm_sw_r_cp_awdb_t1_c.GwCbJSMKS9D

• they make resistive panels that I think don't require much clearance.
  
• heated blanket while you're sleeping and a regular space heater when you're awake and can watch it
• maybe an electric radiator. should be safe against fires, if memory serves.