Fitting higher head Grundfos pump to Worcester boiler.

[vee-tail]

My Worcester Danesmoor 15/19 boiler has been intermitantly locking out since extra rads were fitted on the third floor.
All the usual checks (including flushing) failed to fix the problem. It seems the high limit thermostat is actuated by poor circulation in the system. The old UPS 15-50x18 pump is probably unable to push enough water through the 8 rad system on 3 floors, so I am thinking of fitting the new UPS 25-120 high head pump which is designed to cope with micro bore and high head systems. But the new pump is expensive and I would like to get the experts opinions before shelling out lots more money.

 

[SimonG]

Locking out of the boiler and circulation are not connected, unless you are talking about the over heat lockout

 

[vee-tail]

are talking about the over heat lockout

That's the one. The boiler runs fine for a while, cycling on the normal thermostats for up to an hour, but only heating one or two rads. Then the overheat sensor locks it out. Takes some time before it all cools down enough to reset the lockout and start up again. I figure increased circulation via a stronger pump will fix the problem.

 

[SimonG]

and the system is full?

To me it sounds as though you have an airlock. First try is to get rid of this before you start changing anything. Where the radiators a diy install?

 

[vee-tail]

Thanks SimonG. Have bled all the rads and found one on the top floor that had a bit of air. First I opened all the lockshields on all the rads, and ran the boiler. The top floor rads became very hot, the middle floor rads cold. As before the boiler locked out after approx one hour. Next day I closed the top floor lockshields and ran the boiler. As expected the top floor rads remained cold, and the middle floor rads warmed up but not as hot as the top floor had been. Lockout occurred again after approx the same time.
Maybe I have an airlock somewhere on the middle floor, but all the rads there have been well bled.
The middle floor is a DIY job using 15mm down pipes from the flow & return ring on the top floor above.
I am tempted to connect a mains water supply to the lowest drain point adjacent to the boiler, and perhaps force any air out of the system, but am aware that the pressure is quite enough to burst a steel rad.
It still seems possible that the existing pump (which was changed recently) may not be strong enough for the job.

 

[Howsie]

Needs balancing properly before you start throwing money at it.

 

[doitmyself]

How to balance a CH system

 

[vee-tail]

Hmnn... After a lot of hassle adjusting lockshields it is possible to get all the rads warm. But the boiler locks out sooner. So the original lockout fault remains, possibly made worse by the extra restrictions of nearly closed lockshield valves and four new rads tee d into the 15 mm feed & return loop.
Judging by experts comments here, the DIY additional rads need to be re-plummed into the main 22mm feed & return. But that would not fix the lockout problem which surely must be due to an inadequate pump? If I can find a cheap high head pump on e-bay it might be worth trying ... it could fix both problems. Ah well spring is nearly here!