On Tue, 06 Feb 2007 11:00:37 +0000, John wrote:

A friend has an old back boiler heating system (Baxi, I think) and an
upstairs radiator is leaking - bottom of radiator has rusted through.
Due to lack of funds and as the system is so inefficient that they
don't use it, they wanted to drain it down or isolate the radiator.

All of the radiators have only one regulator valve, the other side being
a plain 90 degree union. There do not appear to be any drain points on
the system unless on the boiler behind the gas fire. The system appears
to be fed from the same header tank as the DHW system and there is no
obvious way to isolate the cold water supply to the heating without
also isolating the hot water tank - the only visible wheel valve being
to isolate the hot water tank.

Sounds like a 'Primatic' system - you may find this on a label on the hot
water cylinder. If you've definitely only got a large cold-water tank and
no small feed-and-expansion tank then it is this type of system.
If this is the case I think you'll have to drain down the system (from the
drain-off cock on the connection at the bottom of the cylinder) and
isolate the CH pipework from the cylinder (capping off the cylinder
connections) before you can drain the CH whilst having the HW working
again. DO NOT put any additives into the system before doing this - they
will get into your hot water!

As meow2222 says you can use a self-piercing drain-off cock (or any other
self-piercing fitting such as a self-piercing washing-machine connector)
to drain off the radiator pipework at a low point.