1 cold radiator

[Alfie25]

I've moved into a house and inherited a fully pumped system, with Grundfos 15/50 pump, tanks in loft, 9 rads in total (though I've got one switched off). I re-balanced the system as some rads were taking longer to heat up than others. I managed to get a more even balance, and at this point the pump was on 3 (its highest setting).

Shortly after I turned the pump speed down to 2 as it was much quieter than 3 and I noticed no difference in the time taken to heat the rads up. A day or 2 after I had 2 radiators replaced (singles with doubles), put the pump up to 3 to clear some airlocks and all was fine - all rads heating up fine. I then put the pump back to 2 but one of the old rads isn't heating up now. Thing is, I'm sure it was fine for a few days with the pump on 2 after the new rads were fitted but now this one rad only heats up with the pump on 3.

The lockshields are open, it doesn't have a TRV on. So... could the new radiators be making the system require the highest setting on the pump? Or do I need to rebalance the system now the newer radiators have been fitted? I really don't want to have the pump on 3 all the time but unless I do this then one won't heat up.

Confused I can't see how having the pump on setting 2 is plenty sufficient for all but one rad? Surely if that was the case then at least one of the others would be cold or lukewarm? Likewise it can't be air or sludge can it, as the rad works fine on setting 3. It just stays stone cold if the pump is on 2.

 

[Burnerman]

The water always takes the easiest way around the system, so rebalance again (usually from fully off the valves only need 3/4 to 1 turn).
Turning other rads off should have the same effect.
This is assuming that there isn't loads of grot in the system......if you aren't sure, disconnect the faulty rad, then turn each valve on in turn to see if there is water there.
John

 

[Alfie25]

The system has a magnaclean so I wouldn't expect too much sludge, although when the plumber opened the olive on the radiator in question some mucky black water came out (this was when I'd just got the 2 new rads fitted and we couldn't get any heat to this old one at all, even with the pump on 3, so we tried to pull the air through).

The plumber did no re-balancing so I'll give this a try, although the only change is the 2 new rads so I'll turn down the lockshields on these a bit. Even so... could rebalancing fix it? As explained, with the pump on 3 the radiator works fine, which makes me think the balancing is already ok, it's more a matter of the pump struggling on 2. Or, if there was loads of sludge, would the pump need to be on 3 to power the flow through it?

 

[Burnerman]

I would have thought that pump speed 2 would normally be adequate but I'm not qualified to justify that statement....others will know better!
I reckon balancing is your best bet.
John