Long tale of woe...

My mother's 14-month old boiler sprung a leak yesterday. The water
that came out was completely black. After finding a plumber out of
the Yellow Pages (Tesco Insurance's 24-hour emergency hotline, my
arse!), he said the leak is coming from the cast-iron heat exchanger.
He expressed surprise that the heat exchanger should spring a leak in
such a short period of time, and that, if the system had been flushed
properly, the water should be so black.

The boiler, an Ideal Classic FF340, was fitted as a replacement to an
original system. This was supposedly power-flushed by the installers,
but according to yesterday's plumber, this should have taken about 4
hours, whereas my mother reckons it was a lot less than that (mind
you, she is in her eighties and does get confused, so she's not a
reliable witness).

Further information that may or may not be relevant:
i) All the radiators are fitted with TRVs, I think there is a by-pass,
but it doesn't have any kind of flow valve on it. I fitted a room
thermostat as there wasn't one before, and told my mother to keep the
TRV in the same room fully open (replacing the TRV with a lockshield
valve would have required a round tuit, which wasn't available at the
time). My sister visited last week and turned the valve down.
ii) Since the boiler was installed, it's only been used full for about
three-four months. My mother went into hospital in early January, and
was there for six months. While she was away, apart from a couple of
weekends I stayed over, the thermostat was lowered to keep it ticking
over on frost protection. Last weekend was the first time it's been
used apart from hot water for 9 months.

Questions:
1. Can an unflushed system cause a cast-iron heat exchanger to leak
after 14 months (unless there's a manufacturing fault)?
2. If there isn't a by-pass circuit, could all the valves being up to
temperature have caused enough pressure to rupture the heat exchanger?
3. Would a long layoff or low level of usage have exacerbated this
problem?
4. Who's at fault here? The boiler's manufacturer's, or the
installers (name-and-shame time: Merseyside Central Heating of
Crosby)?
--
Hugo Nebula
"If no-one on the internet wants a piece of this,
just how far from the pack have you strayed?"