I have an oil fired boiler system with two circuits, both pumped. One
drives the rads - which works fine, the other goes to the hot tank.
Since September I've had no hot water from the tank. I fitted a new
pump but no joy. What happens is this:

When the boiler has been running for a while the feed to the coil gets
hot as expected but no matter how long the boiler runs, all day even,
the return is always cold. If I switch off the pump I can hear the
water "falling" back down almost like the pump has been pushing
against a blockage.

So I drained the system and disconnected both ends of the coil and
there is standing water at both ends. I would have assumed that the
coil would have drained out via the bottom pipe when I emptied the
system, is this correct?

Does this all seem like the coil is blocked inside the tank. If so can
it be unblocked or is it time for a new tank.

House was built in 1995.