There is an extended metaphor running through this poem, linking a central heating system to the human body and heart. As we read, we explore the different things that may give us humans the energy to keep our bodies and minds warm and healthy: a woman's touch, a memory, love, the sun. Yet, in the face of this metaphor, the speaker doubts the reality of the very world he explores. The force behind his heating mystifies him as he ventures to find out what it is. (And do you think he's getting warmer? Sorry, we couldn't help ourselves there.)
Line 6: This line is part of the extended metaphor for central heating. The wordplay in "heart-system" shows that the heating system here is not the kind that heats up a building, but the kind that heats up a human being and all of his or her emotions.
Line 7: This line is another part of the metaphor for human emotions. We know that something like a heating system is being talked about here because we're dealing with an engine that's breaking down, but since we already know that there are problems in the heart system, we can guess that this "engine" is really standing in for something else, like the speaker's ability to love, perhaps.
Line 8: This line continues the metaphor and the wordplay. An electromagnet is a magnet whose field is activated by electricity, so we can think of the speaker as run by a magnet. If he is activated, he'll have a magnetic force, which can metaphorically repel and attract other people, objects, moods. But the metaphor leaves us guessing, asking the same question as the speaker. This is another literary device, called a rhetorical question, or, a question that isn't really meant to be answered.
Line 11: The spark mentioned here could also be part of the heating metaphor. We think of a fire in a hearth, keeping the whole room and all of the people in it cozy and warm. Yet this spark, this sustaining force, is allowed to die. The heat fizzles out, leaving the humans cold. Perhaps this line is a metaphor for life in general, comparing life to a spark that is born only to die. Cheery thought, eh?
Line 16: Here, we explore another form of heating—touch. This touch seems to be one of the most reality affirming things in the speaker's life, heating him up when his world seems cold and distant.
Line 24: This copper wire is probably conducting electricity to a light bulb, thus causing light and, it's a safe bet, heat. As light illuminates, it heats, perhaps adding another metaphorical layer to this poem—that learning, becoming more knowledgeable and seeing life more clearly, is one of the things that keeps us alive and real.
Line 25: This line connects us to the central heating system of our whole solar system: the sun. The comparison of the heating system to the sun means that, perhaps, as the sun gives heat and light to the earth as a whole, the woman in the poem gives heat and light to our speaker, thus completing the metaphor on more than one level. We've got simple central heating as it would be in a building, central heating as it applies to one human life, and central heating as it applies to the whole solar system. That's a triple-stacked metaphor, folks, and a lot of heat to boot.