Post leak repair, boiler runs for 15 seconds and cuts out before accelerating

[Tommo1806]

Background.
System had a leak at the first joint below the Grundfos pump, which was fixed by a plumber friend of a friend. System ran fine when tested immediately after the repair. Plumber left. That evening when the HW and heating should have kicked in, we noticed that the boiler wasn't running. The only thing we noticed was that the pump was bloody hot. Tried to reset the boiler, but this didn't work, so we deselected HW and heating on the control panel.

Called plumber and he suggested it was just air in the system and once everything had cooled, select heating only and reset the boiler but leave the boiler temperature set to low. This worked. After about 10 minutes in this configuration boiler temp was turned up to normal and HW was selected on. Everything was normal apart from 2 rads in the house not getting hot. After a couple of days, and work permitting, I decided to tackle the rads that weren't getting hot.

Bled them, but there wasn't any air in them. Turned the taps off and on, but still no flow through. Turned taps off, disconnected pipes at rad entry points, drained them down, no sludge came out. Opened taps to see if we had a flow to the rads and we did. Reconnected pipes, filled and bled rads. Turned system on and HW and heating worked normally, all rads were hot. System worked for 1 day.

Next morning, boiler had not fired up again. Tried the plumbers trick of resetting the boiler on a low temp setting and selecting heating only but it's still not working.

It goes through the start up sequence, lights for about 15 seconds, but cuts out at the point were it would normally 'accelerate' to full burn. Boiler is only 1 year old.

Any advice please?

 

[terryplumb]

Sounds like a lack of circulation. When turning on heating and boiler fires up,is pump spinning ?

 

[Tommo1806]

Thanks Terry.
Yes, the pump is running. After the boiler trips out, and if we leave heating switched on, the pump gets bloody hot!

 

[terryplumb]

Have you bled the pump ?
Pump gets hot when it spins with no water in it

 

[Tommo1806]

Yes. Thought this would solve it, but it hasn't.

 

[Tommo1806]

Is it best to bleed it with the pump running or not?

 

[terryplumb]

With pump not running ,and opening its bleed screw ,does water flow out constantly ?
Have a big towel handy and make sure no water gets on electrics.
I assume your system is fed from a loft tank ,is it full of water ?
Do you have a strong magnet ?

 

[Tommo1806]

It dribbles out of the bleed screw. No tank in the loft, it is a sealed system.
No strong magnet, i'm afraid.

 

[terryplumb]

What make and model is your boiler ?
What is the pressure in the sealed heating system When all cold ?

 

[Tommo1806]

Worcester Greenstar Ri
1 Bar pressure when cold.

 

[Tommo1806]

Could there be air in the system above the pump? And if there is, how do you bleed that out?

 

[Madrab]

Air locks are rare in a sealed system, purely down to the fact that the pressure it's operating at is usually plenty to push out any airlocks. Usually happens when other rads have been added or there's some strange additional pipework somewhere that wasn't original.

Are you still having issues with 2 cold rads? If so bleed them quite significantly, in other word draw a lot of water out of them, few cups full whilst topping up, there may be a block somewhere that's not allowing the airlock to move and it needs enough water above it removed to allow the water below to push the bubble up and into a rad somewhere. Are these 2 rads cold on an added section/extension?

 

[Tommo1806]

The 2 rads are not on any section/extension.
I'll try drawing the water off now.

 

[oilhead]

When bleeding again, try doing so through both the flow and return independently.

 

[MeldrewsMate]

The hot water circuit will also use the circulating pump, and its hydraulic resistance is low compared to the heating circuit. Sometimes the hot water cylinder return pipe will have a restricting valve installed.
Firstly note how many turns are needed to close this valve (if fitted), then open it fully.
Note the setting on the automatic bypass valve (should have been fitted!), then set it to MINIMUM.
Switch on Hot Water ONLY and wait until the hot water circuit runs quietly, indicating that air pockets have been dispersed. Bleed the high points of air only!
Set the ABV to MAXIMUM.
Close the restricting valve to its previous setting - usually 1.5 to 2 turns from closed with a 22mm gate valve, dependant on the type of HW cylinder you have.
Turn off the HW, turn on the CH.
Turn OFF all rads which get hot. Do this until the air is pushed into the non-working rads, then bleed the air from them.
Re-pressurise the system.
Open the closed-off rad valves.
Set the ABV to its original setting.

Note: Balancing of the cylinder restricting valve should only be done with a cold HW cylinder. Starting at 1.5 turns from closed, adjust its setting until there is a drop of about 15C between incoming and outgoing pipework, ie in at 65, out at 50C.

MM