Vokera Mynute 20se - Boiler keeps turning on when not needed

[smashy]

Hi All,

I have Vokera Mynute 20se Boiler - its about 12 years old. I will describe the setup, and what I have done so far, in just a moment. The system works fine for HW and CH when we want it. The issue is that even every 15 minutes or so, day and night the boiler fires up for about 60 seconds, and then turns off. And this happens when the CH and HW are not on. The constant turning on is not only burning through my finances, also now kettling really badly. In fact the kettling is far worse when there is no draw on the system i.e. when it randomly turns on and off.

So the setup is this (and sorry if some irrelevant info here): It is set up as a 'system boiler' I believe (whatever that means). The boiler is in the kitchen, the thermostat is in the dining room, the digital timer control is in the airing cupboard, there is a megaflow in the loft, there is a hot water ring main (not sure its called that). In the airing cupboard is a small radiator connected to the HW system, not the CH system.

To attempt to resolve the situation, I have turned off the HW radiator in the airing cupboard. I have had the 'frost-stat' in the loft changed, and the heating engineer also changed a pump in the loft (I'm not sure that it does, but engineer says that it's constant on-off over many years, the mechanical arm under the plastic cover gets bunged up, and so doesnt fully operate). I've made sure there is enough water in the system (about 1.3 bar at the moment), and I have bled all the radiators multiple times. Also I have turned off the hot water ring.

I am now thinking about looking into each individual components inside the boiler to see if something is wrong. Could it be there is an internal thermostat that has deteriorated and keeps telling the boiler the water is freezing.

Does anyone have any ideas?

Thanks

 

[Boilerman2]

Sounds like a motorised valve actuator problem to me -
when the valve opens it makes a micro-switch which fires the boiler & pump, it could be that although the valve is opening and closing, the microswitch could have contacts that have become "welded" do you have 0ne Three port valve, or Two Two port valves??

 

[smashy]

Thanks Boilerman2.

I just looked "motorised valve actuator" up on the google, and that is the thingy in the loft that the heating engineer changed when he did the frost stat. So that can't be the problem as its brand new.

If I recall it has a sliding arm on it, and it moves OK without any problem.

As for how many ports, from memory its 3 port, but I'd need to check it out tomorrow.

 

[Boilerman2]

Is the pump flow switch diaphragm stuck in the "run" position, this may cause the burner to operate even when the pump is not running, that would explain the kettling as well!

 

[smashy]

Thanks again.

Can I just check with you, is the "diaphragm" the little rubber part you refer to the in this diagram?

Untitled

• smashy
• 6 Oct 2011
• 1

It is on page 29 of the service manual, http://vokera.co.uk/wp-content/uplo...e_installation_and_servicing_instructions.pdf

If so, it looks pretty easy to replace and I'm happy doing that. But how can it be stuck in the "run" position?

Thanks

 

[Boilerman2]

The pin can stick in the outward position, due to carp build up blocking the holes in the casting a failed diaphragm can also have the same result!

 

[Agile]

Are you sure of that?

Many ( all ? ) Vokera designs buck the trend and rest out and move inwards when there is a flow sensed!

Where is DP when he is wanted?

 

[nickso]

glazier is right, it moves in when the pump operates. No matter as it won't leave the boiler on as it needs a demand to do this IIRC. even if it sticks in the NC position, and it usually does, it just means the boiler will fire up quicker and if the pump is gubbed the limit stat will cut the boiler off instead.

 

[smashy]

Thanks guys. I've made some major progress. I got a new diaphragm and took the flow switch off and replaced. The old one was not too badly worn. After a lot of fiddling and working it through, I got to the bottom of the problem. As Agile said, the flow switch is backwards, it pulls in when there is a flow. And this is achieved with an air inlet (might be water, i'm not 100% sure) from the flow pipe behind the manifold. Not sure if that makes any sense to you guys, but the issue was the little hole was blocked and so the flow switch was stuck.

So the boiler works well now....in that it turns on correctly, and boils cleanly. The kettling is massively reduced. However it still does not turn off properly. So as you say Boilerman2, it is a motorised valve issue. The one I told you about in the loft that I replaced was for the hot water it seems - connected to the megaflow. I cannot find the one for the heating yet. But having worked it through and talking to my friendly generalist heating man, he tells me the valve is not opening and shutting properly when activated by the room thermostat.

If this logic is correct, I now have to hunt under foor boards and in a other odd places to see if I can find it.

Any thoughts on where it might be hidden?

 

[smashy]

Boilerman2 et al, I want to thank you for your posts.

A few months of a noisy boiler finally drove me to distraction and I did something about it.

So one problem was in fact that the zone valve was not closing properly. It took me ages to find this, it turned out to be directly behind the flue, which in turn was behind a disguised access panel - I can only think of one worse place to put it and that would be under the floor. Honestly what were they thinking.

The micro-switch inside the boiler was also faulty, it was not always turning off when there was no draw.

Finally, despite having already being changed, the frost-stat in the loft was faulty.

So thanks guys...all your tips were great.

Mike