Below, “room temperature” takes the article “a” in one case but not the other. “Temperature” seems like a countable noun, so why no article in the first case? What is the grammar that is working here?
Keep the office at room temperature.
Keep the office at a room temperature of 22C.
Both are grammatical. The first makes a generic reference, that is, room temperature is a noun phrase which, in the words of the ‘Longman Student Grammar of Spoken and Written English', ‘refers to the whole class, rather than just one or more instances of the class.’
In the second sentence the prepositional phrase of 22C postmodifies room temperature and thus makes it specific. That in turn requires room temperature to be preceded by the indefinite article.
The Corpus of Contemporary American English has 2785 records for room temperature, of which 6 are for a room temperature. The figures for the British National Corpus are 267 and 4.
According to Macmillan, room temperature is an uncountable noun in its own right:
room temperature, n.: the normal temperature inside a building that is neither too hot nor too cold
In the second sentence of your example, room is redundant; the sentence is better rendered as
Keep the office at a temperature of 22C.
In the first sentence, the term room temperature, according to wikipedia
... is a general term describing common indoor temperatures. It is usually 20 °C (68 °F or 293 K)
In the second sentence, there is no term, rather two words room and temperature, which when bought together point to the temperature of the room.
In the first sentence, 'room temperature' is assumed as a general term, that is why there is no article.
In the second sentence, we are specifying the 'room temperature' to a fixed value, so we need to put an article in there.
Whenever talking about the general terms, we don't need to put an article, but when talking about a specific value, we need to put an article.
Both are correct, 'room temperature' is a standard phrase taken to mean whatever the average temp. may be for the house/office, etc. The second one simply gives more detail as to what the specific temp. is, but 'room' is not needed here because 'office' is already mentioned.
