This poem's title ties the whole poem together with a metaphor. The poem explores different ways to view the heart and body as powered by a metaphorical central heating system. The system is kicking into gear as we start with a light, and a woman stretching out like a flare. Then we move to find out that central heating in our speaker's body is not working as it should—he feels as if there has been a breakdown.
This breakdown causes him to doubt the reality of the world, but he comes back to the title idea when he talks about the force that provides central heating for the universe—the sun. He accesses the sun by thinking about his lover's heart, showing that possibly our speaker's love for this woman is his central heating, the force that's keeping him running, despite all of his doubts.
We don't have a simple extended metaphor in this poem. It's not as if each separate part of the heating system is analogized to a different aspect of the love affair. It's more interesting than that. There are gaps, questions, things that don't quite fit into the metaphor. But that's the surrealist side of our speaker, forcing our minds to leap across gaps in logic, even though we're not quite sure we can. Then, after we've succeeded in landing safely, we're forever changed on the other side of the crevice.
And if you wanted to take this title to Big Picture-ville, you could ask, in reading this poem, what kind of central heating do we have as humans? What is the power that keeps our hearts pumping blood, our stomachs digesting, our temperatures at 98.6 degrees Fahrenheit? Science, theology, and philosophy present many ways to answer this question, a question that is not answered definitively in this poem, but at which our speaker takes a good stab.