Checking a pump?

[Johna7770]

I have a problem where I can't get all of the radiators working all of the time. (one radiator in particular seems to give me most grief). If I close off all rads apart from the troublesome one - it seems to work ok. I've had a go at balancing the radiators, and have managed to get the temperature fairly consistent across all rads - the only problem now is that none of the radiators seem to heat up properly. Could this be a weak pump? Someone told that a way of checking the pump is to remove the centre cap, and with the pump running, apply some pressure to the spindle (e.g. with a screwdirver or something) and try to stop it rotating - I was told that if the pump can be stopped, then it needs replacing. Well, I tried this and was able stop the pump. It doesn't seem very scientific, but can anyone confirm if this is a valid test? Is my pump past it's best?
If I do need a new one, where is the best place to buy it? It is currently a Grundfos 15-50 - do I have to replace it with the same type/spec or is there something better?

 

[breezer]

have you tried adjusting its speed?

(little "dial" marked 1 2 3)

 

[Johna7770]

have you tried adjusting its speed?

(little "dial" marked 1 2 3)

Yes - it is currently set to 2, and tonight I tried it briefly on 3. It seemed a bit noisy when set to 3 (although, I'm not sure if this is normal) - so I have turned it back to '2'. I probably didn't leave it on 3 long enough to notice a difference in radiator temperature.

I've also been told that if there are any leaks in the system (and I do have to bleed the radiators occasionally - so perhaps there might be a small leak somewhere), then increasing the speed setting to 3 can worsen the problem. Is this correct?

 

[ChrisR]

Having to Bleed is more often a sign that corrosion is going on, so not enough inhibitor.

I've never tried stopping a pump that way and I'm surprised that you can.

There have been a few posts here recently on testng pumps.

If you get another Grundfos you will be able to change just the "head" - remove the 4 allen screws.