hot water from cold tap, and tankless heater

I recently started getting hot water coming from my cold water taps. It stays hot for about 30 seconds before turning cold. I ruled out the taps, and now I realize that the problem is happening at my furnace.
Nothing on my furnace has been changed recently. I have a tankless water heater in an oil-fired Weil-McLain furnace with a 40 gallon holding tank. Cold water gets fed into the loop that goes from the holding tank to the tankless coil. This cold water line is heating up at the furnace and is causing me the headache. I'm suspecting that the tankless coil in my furnace is crudded up, and causing backpressure when the circulator kicks in. This must be forcing hot water into the cold water side of things. Does this seem possible?
Is changing out the tankless coil difficult? I can deal with stuck or broken bolts on the face of it if that's the tough part. How much should a coil cost?
Thanks in advance.
Tom
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Do you have a check valve on this system?
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There is no check valve on the cold water coming into the system (although there are check valves to keep the cold from flowing into the hot storage tank). I thought of adding one, but I'm wondering why it worked for so long without one?
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It might be caused by an expansion tank losing its air charge, filling with water, and no longer working. This might cause the hot water to back up into the cold line as it is heated and expands. Don Young

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