First, we start with the image of a light illuminating a person—"you"—who, we can assume, is the man's lover. Then we hear about a different woman, stretching out, and a woman reading in a corner.
We skip from those images to hear more about the title theme, central heating. Except now we're getting into the metaphorical level of the poem, and central heating is standing in for the system of the speaker's heart, which seems to be having a breakdown. The speaker doubts both what's making him work and where his emotions are going.
Then we switch to hearing about the speaker's doubts about the reality of the world. He says that his love, and maybe his life, is nothing, only a spark. He thinks that a lot of things in the world are artificial, even though the way he feels when he's touched by the "you" from the beginning of the poem seems quite real. He sees that he has opportunities, but due to his doubts, doesn't act on them.
Then the speaker switches to the memory of a particular night spent with his lover. Though the speaker claims both he and his lover were unhappy, a joyous world surrounds them. The poem ends with a nostalgic and loving image, leaving us with the bittersweet of our speaker's emotions.