Plumbing – Why is the hot water circulator causing there to be hot water in hot and cold pipes

Here's a diagram of a hot water recirculating system

As I understand it, the idea here is that the recirculating system runs hot water around the pipework to keep the temperature hot near the faucets/taps. This allows for instant hot water at faucets/taps far from the water heater.

Even when no taps are running the pump should be actively maintaining the circulation as needed. It needs to do this as any unlagged pipes will cool the water in them if no faucets/taps are drawing hot water.

When the water near the faucets/taps is sufficiently hot, the "Aquastat" will turn the pump off.

Your power feed to the recirculating system should be via a timer, or other device, that lets the pump run whenever the "Aquastat" feels necessary. This should be "on" during any times of the day when you are likely to use the faucets/taps.

Some ideas:

• your timer isn't on or you are not allowing time for the pump to raise pipe temp.
• your "aquastat" is faulty.
• your "aquastat" is not near the faucet/tap you tried.
• your configuration needs additional "aquastats" or the "aquastat" relocated.

You should be able to test this with a thermometer and a voltmeter if you can locate the "aquastat"

If you are (as is common) "returning" via a cold water supply line, rather than a dedicated recirculation line, I think you need a check valve where the hot recirculation joins the cold line, not just at the pump.

Otherwise the cold can backfeed from the cold water line to the hot loop when the hot water is run, without the check valve at the pump being involved. All the system designs which use the cold water line as a return typically have the pump located at the point of joining the hot to the cold, NOT at the water heater, so that the check valve in the pump works for this application.

If you have a dedicated return line, and it has a check valve in the pump, that should be sufficient, and you should complain to the supplier of the pump if the check valve is not working.