Brian Reay wrote:
"Ed Sirett" wrote in message
news
On Sun, 24 Sep 2006 09:47:38 +0000, Brian Reay wrote:

I'm having a problem with our CH. Upstairs rads and hot water are fine,
downstairs rads are very slow to heat up (even with all other rads off).
No
air in system, as far as I can tell and system was nicely balanced last
winter and worked fine- ie the problem probably isn't balancing.

Pump seems to be running- there is a slight vibration which you normally
get
when it is BUT the only time I've seen this sort of thing before was when
the pump had failed (in our previous house).

So, I'm thinking the pump may be rotating but maybe the impellor is shot
in
some way (broken shaft maybe). Is this a "normal" failure mode?
Yes it's one of the less common failure modes.

I decided to replace the pump and, after removing the old one, discovered
impellor was corroded but appeared otherwise intact. New one is in place but
I'm not 100% convinced I've found the problem- at least not all of it. There
seems to be an airlock that won't "bleed", not sure if this is real or
imagined due to being a tad cream crackered!

That is very possible. I had that problem here in a new install.

I solved it by running ONLY the CH, and shutting down any rads that were
actually getting hot completely, and cranking the pump up to full speed.
Eventually the bubbles got blown to somewhere I could bleed them out.

I SHOULD have put bleeds in the long loft piping that fed that
section..its the highest point of the CH circuity.

If you have plenty pof mains pressure, another trick is to simply run
with the pressuring loop wide open and take the fittings off the
radiators..this sort of pressure washes the whole system and probably
your carpets, but does clear the bubbles.

I.e. what you want is max flow through the bubbly pipework




The system is quite old (1984 or so) and I suspect the pump is the original.
Previously the system has been very reliable in the 9 years we've live here-
new motor in divert valve some years back and a new thermostat, only
replaced as it interfered with the radio.

Brian