Baxi 105e no DHW

[chillman]

Hi, I wonder if anyone can help with an ongoing problem I've had with my Baxi 105e combi boiler for the past 2 months?
History: Boiler is 12 yrs old & I have previously replaced several small parts, temp sensor diaphragm etc. so I have a good undertanding of my boiler.
Recent issue & work done;
CH was losing pressure (ejecting through PRV) & DHW was intermittent with temp reaching max then timing out.
Plumber replaced the rear section of the DHW Presure Differential Valve & added air into the expansion vessel.
From that moment on CH was fine and pressure remained constant but I had no DHW. When hot tap opened the pin in the Pressure Diff Valve was not moving and hence not connecting with the micro-switch. (With manual operation of the micro-switch and pulling the pin manually hot water can be achieved)
So plumber then replaced the front section of the DHW Pressure Diff Valve where the sensing pipes connect to rule out any diaphragm / bent pin issues. No Change.
I was told the sytem had a blockage as the sensing pipes going into the Pressure Diff Valve should sense the flow of water to move the pin?

I recently purchased a new Plate Heat Exchanger, as I'd read this is often the reason for overheating/blockages due to limscale build up and started the clean out job myself yesterday.
I took the whole of the lower part of the boiler apart, the pump, Inlet/Return manifold, DHW Pressure Diff Valve, Diverter Valve assembly and finally the Heat exchanger.
I cleaned everything thoroughly and due to living in a hard water area there was a lot of black flaky residue within all of those parts.
I checked that the sensing pipes were clear and the Inlet/Return manifold flow regulator was free from any gunk and after a couple of hours cleaning rebuilt the boiler.

Unfortunately there has been no change, the pin on the DHW Pressure Diff Valve still doesn't move when hot tap is opened and yes I have very good water presuure so it's not a low pressure fault.
Otherthing I have checked using the manual fault finding diagrams;
Diverter Valve Operating Head - 240v on cable and operates correctly

Only one other note that may have a bearing, the Pre-Heat part of the boiler hasn't been used for years as was really a waste of time, however when re-commissioning the boiler it was one of the tests and I realised that when the switch is turned to Pre-Heat ON nothing happens! I believe it is supposed to fire the boiler and heat water for the Expension Vessel/Pre-Heat Store even when you are not calling for hot water.

This problem has really messed with my head and three different plumbers have not been able to solve it, so any ideas would be great.
Thanks
Craig

 

[ianmcd]

The black flaky residue as you call it is Magnetite and nothing to do with living in a hard water area, hard water causes limescale you need to clean and inhibit your primary water

 

[FlyingFish]

First check would be to make sure that the water flowing out of the hot taps is actually coming from the boiler. Isolate the cold inlet under the boiler and then open a hot tap to make sure there is no water flow. I've seen faulty showers (non return valves) feed water into the hot system so effectively bypassing the boiler. Also make sure no other plumbing has reversed the flow of water through the boiler (seen a kitchen fitter do that). If that's ok then I would, (if the diverter valve has been repaired/replaced/assessed by a competent person) say your looking at a faulty dhw venturi. They are prone to collapsing at a certain age, and can be quite fiddly to remove the broken parts out of the cold inlet manifold. Baxi supply the part as a spare so it's repairable.

Mick

 

[Stadan20]

I would of just replaced the whole hydro block in the first place

 

[chillman]

Many thanks for the replies and advice.

Yes the water flow from the hot tap stops when the cold water is isolated from the boiler.
I haven't had the Diverter Valve repaired or replaced it was the DHW Pressure Diff Valve that was replaced. Is there something I should be looking at within that part that has a bearing on the DHW Diff Valve activating?

When I took the Hydro block apart I completely cleaned it, including the flow regulater (is this the same as the "Venturri" mentioned?) and apart from a lot of gunk removed, there were no blockages and the flow regulator with spring assembley was free and not broken.
As the Hydro block part (Inlet/Return manifold) is over £100 I really want to be sure that the fault is within this part.
Are they prone to fracturing internally so the flow is not detected by the sensing tubes? (I have no external leaks)

Is there anything else that can prevent the water flow being detected by the DHW Pressure Diff Valve?

Also, although I never use the pre-heat function, should I be concerned that the boiler doesn't fire up when the pre-heat dial is turned to the ON position?
Is this not highlighting a potential problem with the PCB?

Thanks,
Craig

 

[Groovejet]

iknow its old thread but rather than start all over what was the outcome?