"If you capped both ends of the circulation going under your porch, you may have interrupted the flow of hot water everywhere else. Hot water heating usually runs the water in a large circle - in newer homes like mine, anyway. Joining the cut ends may be a better idea."
That's a good point, but like the porch itself, the heater must have been added later on. The pipes came out of the main heating pipe inches apart, one from the top and over, the other from the side and over, creating a miniloop. If I was really ambitious, I could have replaced the junction with straight pipe. I think I'll wait til summer for that.