CH Pump failure mode?

[Brian Reay]

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?

TIA

Brian

[Andrew Gabriel]

A friend of mine found there were no impeller blades left on the spindle of a central heating pump. He never did find where they actually went.

Your symptoms could also be that the system has sludged up.

Another possibility could be TRVs have got stuck closed during the summer.

Is downstairs a separate zone, in which case maybe a zone valve isn't working properly.

[Derek ^]

They get worn off by abrasives (rust ?) in the radiator water.

A similar affliction can happen to vacuum cleaners. The vac makes all the right noises but won't pick anything up.

DG

[Ed Sirett]

Yes it's one of the less common failure modes.

The major ones IME are: Electrical earth fault. Jams and continues to do so every time it stops.

[Brian Reay]

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!

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

[The Natural Philosopher]

Can be. Also siezed but still humming anyway.

[The Natural Philosopher]

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

[Ed Sirett]

Another good place for air bleed points is _near_ the top of a vertical sectional that's flowing downward.