Vokera Flowmatic 20-80RS Combi heating problems

[Johnyhp]

Having problems with the above boiler and British Gas.
Hot water is fine however central heating only heats upstairs radiators.If I isolate upstairs and close all but one of the downstairs ones I get heat in the one thats left.
The boiler does not lockout or discharge through the pressure release valve but seems to fire on low occacsionaly remaining at 80 degrees and 1.5bar.The pump seems to be running as the pressure increases from 1 bar to 1.5 when the boiler is turned on.It is also set at 3.
Have service contract with BG and called the nice people.
Man came and said it needed a powerflush.Strange as I flushed the system this time last year hiring a power flusher and using the appropriate chemicals.
I was out at the time but the misuss tells me that when she told him this the young chap phoned is supervisor to no avail.They maintain it needs pressure flushed.
I then fitted taps to the flow and return pipes and ran mains through them both forwards and backwards each rdiator in turn.Still no joy and not much crap either as would be expected in a sealed system freshly flushed.
A friend suggested microbore may be clogged,so today I cut and capped the downstairs flow and returns under the floor so that only the three downstairs radiators nearest the boiler (which are the ones we use anyway)were being fed by the boiler before replacing the microbore feeding them with 10mm plastic barrier pipe.The radiators are now fed by mini bore via reduction tees on the 22mm flow and returns.I also removed each radaitor and flushed them outside with a hose pipe and replaced lockshields and tvrs to each before I filled and bled the system including the dhw bleed and pump but the dowstairs radiators still dont heat up.There is no external bypass valve fitted to the system.
To my mind the symptoms suggest the water in the boiler must be cycling round the dhw manifold and not reaching the flow and returns to the radiators and that the raditors that are heating are doing so as a result of convection and not pumped water.
Could anyone confirm whether or not Im right and if so suggest what could be causing this effect and how I could prove it to BG to have it fixed.
Any help much appreciated
Thanks

 

[megawatt]

Sounds like a siezed diverter valve.

 

[AlanE]

To my mind the symptoms suggest the water in the boiler must be cycling round the dhw manifold and not reaching the flow and returns to the radiators and that the raditors that are heating are doing so as a result of convection and not pumped water.

So have you felt the DHW heat exchanger/pipes to check if this is the case?? A simple, and cheap, test before repiping the whole system!

 

[Johnyhp]

Thanks or the prompt replies. Ill check the pipes tomorrow.Although you'd think the BG engineer would have tried that and if memory serves they replaced that valve about three months ago.Still nothing to say its not broken again.
Ive managed to get the diagrams for the boiler and see it has an automatic by-pass built into the heating manifold.
Assuming its not the diverter any thoughts on the likelyhood of the by-pass valve letting by.

Johnyhp

 

[Agile]

I dont remember any bypass but if there is one its only a very small flow.

As suggested it sounds like the diverter valve manifold being jammed

OR

as one I saw recently the small spring at the front in the pin housing was broken and not pushing it back in properly.

However thats obvious because the pin was not retracting!

Whats yours doing?

Tony

 

[gas4you]

When you had this boiler fitted yonks ago did it replace an old standard gravity system. If so an outside chance could be that tere was/is an old dumbball valve that has now stuck closed, thereby preventing any flow to the downstairs rads.

I had this on a system about 3 years ago. The problem didn't manifest its self until about 2 years later. I had alwasys suspected there was a valve on the system but the then houseowner would not let me remove his nicely tiled boxing in in the kitchen to find it.

The next owner had to

 

[Johnyhp]

The story so far.
I felt the DHW pipes as suggested.They were warm so I removed and tested the three port valve blowing through it whilst blocking of the ports.
It seemed to allow some air into the DHW port but not much.
I stripped it down anyway and cleaned the internals which weren't that bad.
According to the exploded diagram all parts seem to be there but I note that when open there is only the rear of the metal valve contacting a metal locking ring to prevent water entering the DHW heat exchanger port,
I can only assume its designed this way to allow some water into the exchanger
Reassembled and bled as per manufacturers instructions and system performing much as before (one radiator heating after some time presumably by convection and/or weak pumping)
I think I can now rule out this valve as a possible cause.
The only other causes I can come up with are a faulty pump or internal by-pass valve.
Does anyone else have any suggestions or fault finding tips.
Incidentally the system was a fresh install 12 /13 years ago so there are no parts of an old system to go wrong such as a ball valve etc.
Again thanks for the responses so far any further help much appreciated.
Johnyhp

 

[DP]

If pump were defective, boiler would not fire.

To check internal bypass (cannot see how that can be a problem), check pipe above the pump. It should be colder than the flow pipe to the diverter.

I suspect there is a blockage especially if you have plastic pipes.

Are the isolating valves open?

 

[Johnyhp]

Eventually found the solution
Tried flushing out system with DS40.After about 2 hours water started pouring out of the joint between the pump and the heating manifold.
Disassembly of joint showed that the rubber washer was in real poor condition.
Couldnt find replacent rubber locally so a pair of fibre ones from B & Q took their place.
System now working better than it has for years with full flow to radiators and was even able to turn up DHW flow which had obviously been turned down to compensate for poor performance of circulatory system.Boiler also much quieter.
My best guess is that although this rubber washer didnt leak water due to deposits around it.It did allow enough air to be drawn into pump to seriously affect its performance and when ds 40 disolved all the deposits around it the washer it began to leak.
The alternative could be that the boiler was clogged and cleaned out by the Ds40 but that wouldnt explain the leaking washer
A lot of hastle but at least I was forced to study my boiler and now know enough about it to repair most common faults (exept the gas stuff of course)
Probably cancel my gas plan cover which they would be removing due to a lack of spares soon anyway.
Again thanks for all the help and suggestions received