Reason for leaky pressure relief valves on water heaters

Hi. I just moved into an older house that has two water heaters (one connected to upstairs plumbing and the other connected to downstairs plumbing). The water heaters appear to be less than 10 years old, one possibly 5 years old. After moving in, I noticed that both top-mounted water heater pressure relief valves had a slow drip that came from the outlet part of the valve (not from the threads below). The drip rate is significant enough to make a rag sopping wet within a day, and also to form a damp circle around the bottom of the water heaters on the concrete. I figured that the valves may both be bad or have accumulated some crud that slightly kept the valves open, so I purchased two completely new relief valves and installed them. I also turned the temperatures down to about 110-115 degrees.
The same problem continued! New valves and they both still had a slow drip from the valve outlet. Does anyone out there have any good ideas on what may be causing this unusual situation?
Thank you for any flashes of brilliance or helpful suggestions!
Scott
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"Scott"
If you have some sort of backflow preventer on your water service, then as the water heats and expands, it'll come through the relief valve. I'd expect that to be intermittent, though, and not a constant slow drip.
Maybe the pressure on your system is unusually high?
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