I have water continuously running from an outlet pipe on the side of my house (not the AC condensation pipe). The water is warm and it appears that my hot water heater is emptying and filling continuously. No other appliances are running and there are no signs of leaks anywhere in or around the house. What could be causing this and what do I need to do?

The PRV(pressure relief valve) on you water heater is being activated. It could possibly be because the heater is overheating due to a faulty thermostat. It could also just be a bad PRV that is letting the heater pop-off because of a weak spring.

Tonight, I would recommend shutting the heater down and then calling out a licensed plumber in the morning due to the uncertainty.

Check to see if a fitting near the top of your hot water heater is piped outside. Several things can cause a pressure relief valve to open, and once they open, often a little dirt keeps them from sealing. Usually the easiest thing to do is replace it, a simple DIY task. That does not solve the problem. You need to track down why the valve opened. If you have scalding hot hot water, you need to replace the thermostat, but that is rare. Do you have city water and a back flow preventer? When cold water flows into the tank, it expands when heated. The excess will go somewhere. In the past, it just went back out the inlet. City water systems now require back flow preventers which keep the water from flowing back out the meter. The solution is a expansion tank teed into the inlet between it and any valves. Look your system over and post back.

There's not a doubt in my mind that the discharge is coming from the T & P (temperature and pressure) valve on your water heater. Since they're so inexpensive replace the T & P valve. If the problem persists then install a expansion tank, (see image) and I'll bet you problem disappears. Regards, Tom