[villages07]

To close this one out....the problem was indeed the expansion tank. The bladder inside had broken and the tank was full of water rather than air.

We called Robert Isherwood ... took a couple of days to get him out, but, it was not an emergency. Seemed competent and had it fixed quickly. He said the chlorine in our water eats through these bladders in about 5 years. Newer tanks are good for more like 7 years. Since we had already replaced the pressure relief valve, he didn't mess with that. Normally, at the 5+ year mark he will replace both items at the same time.

Anyway, something to watch out for if your house is starting to get some years on it...