The parallel port was really neat. I once connected my furnace to a PC, to track when the thermostats were opening the hot-water valves. My house has hot-water

The parallel port was really neat. I once connected my furnace to a PC, to track when the thermostats were opening the hot-water valves. My house has hot-water heating (as opposed to electric or hot air). I would occasionally observe the furnace coming on for a few seconds, then shutting down. I suspected a malfunction. So I connected the 24 v. valves that control the flow of hot water to the radiators, to relays. Each relay opened/closed a pin on the parallel port. I wrote a program to record the status of the parallel port. I was able to determine that the furnace was coming on at the very end of a heat cycle, just when the room had reached temperature. The furnace would kick in just before the room stopped asking for heat. So it was OK after all.