"Ernie Bilko" wrote in message
...
On Sat, 06 Dec 2008 18:34:56 +0000, Ernie Bilko
wrote:

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.




Pump check valves are open. No motorised valve or anything else
between the pump and the coil. Very simple system, two loops two
pumps. Has worked perfectly for 13 years until now. When drained the
water looked pretty gunky. I was wondering if it is sludge and as the
system has been unused over the summer, it may have congealed and
blocked it. Top of the feed pipe is the expansion path to the F&E
cylinder so any air would go straing out. Also when I refilled I
filled from the system drain point using a hose so the system filled
from bottom to top. That should have shifted any trapped air.

Setting the pump speed to max has no effect either.

What concerned me most is that when I disconnected both ends of the
coil there was still water at the top coil connection on the tank,
i.e. the feed. If the coil was clear wouldn't the coil have drained
down with some water maybe at the bottom right angle joint, i.e. the
return.

Replacing the tank could be expensive as I have a Willis type
immersion heater, which is fantastic and I want to keep even if I need
a new tank.


The best way to find out exactly is to undo the coil and fish a drain spring
through it. If it's clear of any blockage the spring will pop straight
through. If it's blocked the spring should clear it out for you.

I do know, from my recent fluid dynamics learning (thanks Rod), that the
coil in the tank works like the bend in a river, and causes a fast and slow
running current as the water turns around the bends. The slow moving part
on the inside of the bend can deposit silt against the inside edge which is
not removed because the water is so slow moving. The silt build up over
time creates a narrowing in the pipe which only reinforces the slowing of
the water current even more, so more silt is deposited until the water is so
restricted that it blocks itself off.

You say you drained the system and found it really dirty, so it might be the
gunk that has built up in this part of the system, due to the effect
described. Push a drain spring through the coil to find out for sure. The
springs are cheap from the DIY sheds, and a fully re-usable for other
things.

It the only other thing I can think of.