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Andrew Smith
Andrew Smith, Engineer
Category: UK Plumbing
Satisfied Customers: 3629
Experience:  Domestic Gas, Commercial Gas, LPG, Unvented Hot Water, Part P Electrical, Commercial Catering
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My greenstar 30cdi boiler cuts out on hot water demand with

Customer Question

My greenstar 30cdi boiler cuts out on hot water demand with a u3 to u9 code
JA: How long has this been going on? And how severe is the problem?
Customer: Last 24 hours.
JA: Do you plan on doing the work yourself?
Customer: I left the central heating on over night but when I returned home the next morning the house was cold and the hot water problem had started. I checked the water pressure gauge which was at 0.5 bar and topped up to 1 bar but now the boiler make a banging noise as well, all radiators are bled.
JA: Anything else we should know to help you best?
Customer: Yes I'll try and fix myself
Submitted: 10 months ago.
Category: UK Plumbing
Customer: replied 10 months ago.
The boiler when switched on and reset and with no demand does not show any errors.
When hot water tap is turned on the boiler starts the water gets hot then goes cold and the boiler makes a loud banging noise.
With the central heating thermostat turned up the radiators upstairs get hot but downstairs they are cold, the radiator on the landing is hotter than the bedroom radiators. On the bedroom radiators the flow pipe appears hotter than the return pipe. I have not altered anything with the balancing before or after the fault occurred.
Prior to the fault happening I had been trying leaving the central heating on 24/7 with the boiler radiator water temp set at min and all seemed okay.
Expert:  Andrew Smith replied 10 months ago.

A 'u' code is usually poor circulation.

Can you check the pump is spinning, another clue is that the boiler is banging because the water is boiling inside the boiler

Customer: replied 10 months ago.
Hello, I tried the pump by removing the metal screw cover and using a screwdriver I twisted the pump around which was freely moving back and too, however with the mains switched on and the room stat demanding heat the screw on the pump was not spinning on its own.
With the water tap on the water initially comes out hot and then cold again and the boiler starts banging. Does hot water tap demand require the pump to work?
Expert:  Andrew Smith replied 10 months ago.

Both the hot water and heating require the pump to work to work properly, but on hw only then its a very short circuit through the plate heat exchanger and can quickly overheat.

Can you not get the pump to turn at all - try a flat blade bit in a electric screwdriver?

Customer: replied 10 months ago.
Is the screw that you turn the pump with using a srewdriver meant to spin around when the pump is functioning correctly? I assume it does as its what is used to free a stuck pump.
If it is not spinning when I have the heating on and the boiler just comes on for a couple of minutes then off again with the radiators still cold and the temp indicator on the boiler shows 75 how can i get the pump to spin?
Expert:  Andrew Smith replied 10 months ago.

The centre screw comes out and there is a slot on the inner shaft that is supposed to rotate inside as the pump turns. Try and free the insides and get them turning again.

If you can't then you will have to swap the pump or the head.

Customer: replied 10 months ago.
I have already moved the large chrome silver screw and put a screwdriver in and spun the pump back and too and it moved freely, while I was moving it back and too there was a very small trickle of water coming out..
Expert:  Andrew Smith replied 10 months ago.

Are you able to spin it while the boiler should be running?

Customer: replied 10 months ago.
can i ask, with the boiler on and under central heating demand I did not see the screw in the pump spinning and with the boiler heat exchange at 75 degrees as shown on the boiler and all the radiators cold what am I achieving by turning the pump screw again when I have already checked that is it not seized presviously?
Expert:  Andrew Smith replied 10 months ago.

Sometimes when the pump is energised you will find you can kick start it.

If you cannot then you will need to swap it (or just the head)

Customer: replied 10 months ago.
Is it possible to change the pump head without draining the boiler?
Expert:  Andrew Smith replied 10 months ago.

No, but if you turn off the valves underneath the boiler you can change the pump without draining the whole system.

Customer: replied 10 months ago.
I assume there is not much water to come out of the boiler when the valves to the central heating pipes are switched to off?
Is doing this method then suitable for just changing the pump head and how do I safely reduce the pressure in the boiler before changing the pump head?
Expert:  Andrew Smith replied 10 months ago.

If you shut off the valves and then let the pressure out through the pressure release valve until the boiler is at 0 bar, you just need an allen key to swap the pump head.

Customer: replied 10 months ago.
Good Morning
I'm going to phone around today to try and get a replacement pump head.
Going by what we have diagnosed so far, with the pump not working should I normally still be able to use hot water from the taps without the boiler making lots rumbling and banging noises? Hence, should the diverter valve be doing it job to allow hot water to the taps?
Expert:  Andrew Smith replied 10 months ago.

No, the hot water will not circulate through the plate heat exchanger, it will sit in the main heat exchanger and boil the water