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?



  • Is your domestic hot water supplied directly by the boiler, or by an indirect coil (heat exchanger) from the boiler loop? Nov 6, 2016 at 14:09

1 Answer 1


It is totally possible for thermal expansion to back some hot water into your regular cold water supply. A check valve will prevent this but you are correct in that this could cause an unwanted (potentially damaging) increase in pressure on the hot water side of the system.

You should look into installing a thermal expansion tank if this is enough of a problem for you as to consider installation of a check valve. It is a fairly simple and inexpensive solution; do an internet search and you will find a lot of information. Check with your local permitting authority to see if this is a DIY install, or call a licensed plumber.

  • Thanks for the response. I went to check after I read your feedback and there's an expansion tank on the baseboard lines but not on the hot water lines. I'll look into adding one to that system. Any thoughts on why this seems to only be a summertime problem?
    – Matty U
    Sep 6, 2016 at 2:09
  • 1
    I don't know where you are geographically but in my locale the heat of the day can cause our cold water lines to get pretty hot when they run through unconditioned spaces (attic). So hot it seems to be heated by the boiler, they cool quickly with flow. So thats kinda what you described. Sep 6, 2016 at 5:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.