Keith -
You are right about the cause. The water heater is originally filled with cold water, and there is an air space at the top of its tank. As the water warms, the air is also warmed. The air expands, forcing water backward out of the tank's cold water inlet. The air space is usually small, so the mount of water forced backward is small and doesn't cause a problem. Sounds like you are having more of an issue. However, you should experience the problem only once, when the water heater is first filled and turned on. It shouldn't happen each time you turn on the cold water faucet, so maybe you can live with it?
As far as you know, is the water heater mounted in its intended orientation - meaning right side up, not rotated, and not tilted end-for-end? If it has been remounted in such a way that the air space is too big, the expanding air will force out a lot of water, of course.
I agree with you - a check valve at the inlet will prevent you from draining unless you also install a valve to bypass the check valve, or find a bypass-able check valve (do they even exist?). I ran into this problem when we had a cabin in the Colorado Rockies. There was a household electric water heater in the basement. Every year I would drain the plumbing system. And every year a pipe would freeze and break. After several years of troubleshooting, I finally discovered that the water heater (and presumably every household water heater), had an invisible check valve pre-installed in one of its fittings. I opened up the fitting, pried out the plastic valve, and never had a problem again.
Bill
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