Problem with Vokera Option combi Boiler

[paul34u]

Hi can anybody give me an idea's on my vokera option boiler the problem is when on central heating it only fire's up for 20 second's then shut's off apart from the pump then after about 45 seconds fire's up for 20 second's & keep doing this over & over but hot water fire's up ok with no problem's .
Please can anybody help.

 

[Agile]

Sounds like a diverter valve stuck in the DHW position!

Tony

 

[paul34u]

thanks for quick reply the system don't have a diverter valve that i'v seen in it as it a combi as two heating system on for HW & CH & the boiler stay's on when useing HW but shut's off when useing CH so don't stay on long to heat the radiators the two near to boiler get warm but boiler won't stay on longer then 20 seconds before shuting of then restarts after 45/60 later for 20 seconds more it do this cycle over & over but when useing hot water the boiler stays on all the time the hot water tap is on the problem is on the CH side as in it won't stay on just keep's restarting

 

[Agile]

If the rads start getting hot then either its a four pipe or the diverter valve is OK ( if it has one ).

In that case there are several things which may be causing the problem.

Can you watch the flame size when it goes off and tell me how bigh it is at the point that it goes out?

Tony

 

[paul34u]

hi Agile
thank's again for reply when i looked at flame on boiler before when useing HW the flame seem to be ok but if on CH side the flame seem to go down a bit just before it cut off but pump side still pushed water around & the green boiler light flash on & off till it restarts again about a minute later.hope this help in any way

 

[Agile]

Can you measure or even feel the temperature of the flow and return pipes to the boiler at the point it cuts off?

Can you measure the resistance of the sensors when the boiler is cold?

Tony

 

[Terrywookfit]

The option has a Bi-thermic heat exchanger.

 

[paul34u]

the flow pipe get's hot when it running for the 20 seconds then slowly cool's off when boiler cuts off till it re-start's about a minute later for 20 seconds more & so on & over time the closer radiator starts to get warm i'v swaped over the heating thermister from the HW to the CH to test it (these are the same value) but there no change but apart from that i'm unable to see any other sensor's that control heat shut off on CH side

 

[paul34u]

hi just a idea can there be a problem in the time limit control side on CH if this is the case is there a adjustment on control board to set it up as boiler ment to stay on for 3min

 

[Agile]

The situation is that the boiler comes on and gets the flow up to temperature within 20 seconds and the boiler correctly cuts the burner off and waits for the anti cycling delay before firing again.

This will either be caused by a restricted flow rate or an excessive power input on heating.

If it was working properly before, then the power setting should not have changed.

That then leaves the problem as caused by too low a flow rate. Thats most likely to be dirt in the eye of the pump or a blockage in the heating system.

I would suggest you check the pump impellor.

Tony

 

[paul34u]

thank's tony
after taking your advice i took the front of the pump off to check the pump impellor that was clear of any kind of blockage & free moving.
gave radiator's a flush thru as with the pipe's refilled bleed the air out again then on restarting the boiler stayed on for near on a minute not the normal 20 seconds heat got thru to the radiator's apart from one big one that got a little warm. so things looked good till today when the boiler went back to the 20 seconds & all radiator's are cold again

 

[DP]

Boiler is NOT fitted with a diverter valve.

OP, change the thermistor fitted on the left hand side. It will have a rubber boot covering it. Look for red and black wires. Will not cost a million bucks, but if no change, you will have a spare thermistor that will be useful sometime in the future.

 

[Agile]

Danny, can you safely explain to him how to turn the CH setting to minimum?

Including the rotation direction!

Tony

 

[paul34u]

hi DP
did change the thermistor but no change so even tho the pump spinning i suspect that mght be the cause. giving too low a flow rate?? but finding it hard to find one. is it possible to fit a new (standed) external pump on the out side of the boiler.

 

[weargas]

1. Is the pump definately spinning? The options have a terrible habit of the dhw flow switch sticking on which tells the boiler to fire but it stops the pump. Pull one of the wires off the dhw flow switch and turn the ch on to find out if this is the case.
2. Have you got thermostatic rad valves on all of your rads? It's a long shot but if there's no external bypass and you've got all trv's then they may all be stuck in the off position.