Water pump won't stop running

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Wondering if anyone has had this problem. I replaced the pressure sensor on my sureflo pump because it would not shut off. That worked but then the problem started again but only with hot water. When I shut cold water supply to Raritan hot water heater then the pump shuts off properly when using cold water. If I open the valve that supplies HW tank then it runs and does not shut off. Have looked for leaks but nothing. No drips from heater relief valve. Raritan tech rep suggested a long shot that water from heater could be entering coolant loop but no evidence of this. If I had a pressure gauge then I could see if pressure in HW heater holds. Maybe I will try that but first wanted to check if anyone has experienced this.

Thanks

Don