I have a Grundfos circulation pump. It is installed on top of the hot water heater and feeds directly into the hot water pipe feeding the entire house. It also has a timer that I use. When hot water is circulating, all water pipes, hot and cold have hot water coming out of them until we run the cold water a long time, after which only cold comes out of the cold water pipe. What is causing this?
Somewhere at the end of the line there should be a valve that bridges the hot and cold water lines, which is usually under a sink. If that valve fails or is installed backwards then you will get excessive hot water in the cold line.
To test the valve:
- Find the sink with the valve under it.
- Close the cold water valve under the sink.
- Open the cold water faucet.
- Water should SLOWLY come out of the faucet. (If it comes out quickly, you have a bad valve.)
- When the water warms up, the flow should slow down and eventually stop completely. (If it does not stop then you have a bad valve.)
Circulation pumps are typically installed at the end of the line, I'm not sure how it would recirculate water if it's at the beginning.
Some pumps, in order to avoid needing a dedicated return path, will recirculate the water via the cold water supply, so getting some hot water from the cold supply would be normal in this case. If your pump does this, then it is to be expected, otherwise it sounds like the hot and cold are mixing at some point and you'd need to find out where in order to troubleshoot.
It's also possible that your pipes run near a furnace duct and this is heating up the cold supply while the water sits in the pipes over night.