here前可以加介词吗

先说一下here与in here的细微区别（参见朗文词典）。

• You use here on its own, when you are talking about a place or organization:

The people here are poor.

Do you work here?

✗Don’t say: The people in here are poor. | Do you work in here?

• You use in here when talking about the inside of a building, room, cupboard etc:

It’s crowded in here.

There aren’t enough chairs in here.

也就是说，有了in之后，突出是在“里头”。

问题中的句子It’s like a furnace (＝ very hot) in here.也就明确了是说屋里头热，没有in则可能会产生这个地区很热的歧义。

至于如何分析in here，感觉争议可能会比较多，常见的是in介词+副词here（一小时前我还是这么想的）。有个问题是类似的结构并不仅限于in，可以是各式各样的方位词（参见朗文词典）。

示例：up/down/in/out here

 What was she doing up here in the woods?

 Would you close the window? It’s cold in here.

 Come on. I’m over here.

 Will you be back here tonight?

 There are no good pubs round here.

如果按照介词理解，我们就需要一并把up、out、back等也看作介词，感觉似乎有些别扭。个人倾向于把方位词看作副词，和副词here以及there一起构成短语副词，方位词进一步明确了here和there的范围/方向等。

个人理解，仅供参考。