In the corner of Paulina's room stood a heater

[alpacinou]

I'm trying to describe this heater.



Is this okay?

In the corner of Paulina's room stood an old-fashioned electric heater with a red box and grill-like bars. Sam walked to it, plugging it in. He then flicked the switch and was surprised to hear a strange whirring sound as the elements slowly glowed to life. Several seconds later, they were fully on and emitted a warm, orange light. Sam moved his hands closer to the elements, rubbing them as the heat seeped into his hands and the room.

 

[emsr2d2]

It's a three-bar electric fire.

 

[alpacinou]

It's a three-bar electric fire.

In the corner of Paulina's room stood an old, three-bar electric fire with a red box. Sam walked to it, plugging it in. He then flicked the switch and was surprised to hear a strange whirring sound as the bars slowly glowed to life. Several seconds later, they were fully on and radiated a warm, orange light. He moved his hands closer to the bars, rubbing them as the heat seeped into his hands and the room.

 

[emsr2d2]

In the corner of Paulina's room stood an old, three-bar electric fire with a red box surround.

Sam walked to it, plugging then plugged it in. (Using "plugging it in" means he was plugging it in as (while) he walked towards it.)

He then flicked the switch and was surprised to hear a strange whirring sound as the bars slowly glowed came to life/began to glow. Several seconds later, they were fully on and radiated a warm, orange light. He moved his hands closer to the bars, rubbing them his hands as the heat seeped into his hands them and into the room.

See above. I'm not keen on "They were fully on". Do you mean they had reached their maximum heat-generating potential?

 

[alpacinou]

See above. I'm not keen on "They were fully on". Do you mean they had reached their maximum heat-generating potential?

Yes, that's what I mean. We used to have an electric fire like that. It took them some time to reach their full potential.
How can I express it better?

 

[emsr2d2]

It took them some time to reach their full potential.

That's why I asked. I've never known a fire like that heat up in just a few seconds, as stated in your post. However, if you want to use it (it's fiction, after all), I'd say "A few seconds later, the fire had completely warmed/heated up".

 

[Tarheel]

I would call it a space heater. (American English)