I have an oil fired boiler (baseboard) that also heats my hot water. The hot water has a line from the cold supply to feed the system then it circulates through the boiler and into a tank for storage.
I've noticed in the summertime, I get a lot of hot water in the cold lines. When I run the cold tap and it comes out hot, it goes back to cold in 30 seconds or so.
What I'm suspecting is that the pressure on the supply line is dropping here and there throughout the day; lots of irrigation around here and backflow prevention on the supply line. When the supply pressure drops, the hot water backs into the cold system through the line that feeds new water into the hot system. When I open the cold valve, it flushes that hot water through the lines and pressure is restored.
This is just my theory right now, maybe there's something more sinister going on; but I've noticed as the summer wears down this problem goes away - in the winter this never happens.
The line that adds cold water into the hot water system is just a straight pipe off of the supply; could I remediate this problem with a check valve on that line?
I've been close to just adding a check valve blindly to see if it does the trick, but then stopped. I think my hesitation is that if this is actually a problem with the pressure in the hot system (not lack of pressure in the cold system) a check valve would bottle that pressure up and could cause bigger problems.
Anyone here dealt with a similar situation? Is my hypothesis even possible? Can summertime irrigation cause enough pressure drop for hot-water to back into the cold line?