Vokera Compact 28 Hot then cold then hot water......

[paulrushton]

Hi. We have a Vokera Compact 28 combi boiler which until about 2 months ago ran fine. Around 2 months ago every 1 in 10 showers the water would go hot then cold then hot etc. More recently however it is more like 99 out of 100 showers and is becoming a real pain!

I have also noticed that the hot water output pressure has fallen and no matter how open the black hot water valve it doesn't ever seem to take the skin off your back like it used to!

We had a water meter fitted around 2 months ago and wondering if dirt has got into it and blocking it. The cold water pressure is still excellant so don't think its that.

I've read about something to do a pipe going into the heat exchanger but am a beginner at boilers and don't want to break anything. Any help would be appreciatted. Thanks

 

[ChrisR]

Hard water area? Yorkshire isn't is it?
Does a tap turned half on do the same hot - cold -hot?

 

[paulrushton]

Yes it does. The majority of where we live is hard water but we are lucky and dont have hard water. The kettle never furs up so dont think its anything to do with limescale.

Could there be a build up of sludge somewhere caused when we had the water meter fitted? If so where should i look for it? Thanks

 

[clf-gas]

If your meter was fitted outside it is possible that dirt was ingressed, this could have got too the DHW flow switch, which i believe is on the cold feed within the boiler. Also attached to this is a flow restrictor, if memory serve's.
Dig out the manual and have a look. Good luck Paul.

 

[Gasguru]

Flow restictor to heat exchanger is blocked.

 

[paulrushton]

Thanks everyone for the quick replies.

Is it an easy job to clear the flow restrictor out and do i need any specialist tools?

The meter was fitted outside and the water was a bit murky until it was clear for 10-15 minutes.

 

[clf-gas]

Its not anything to major Paul, if you've got the manual follow the steps. tools wise, a decent selection spanners screwdrivers ect,

The important factor is to issolate electric, and dont spill water onto PCB, cover with dry rags and have some more handy.

If you cant find the manual, www.vokera.co.uk It should be there.
sorry dont know how to post links.

 

[paulrushton]

Thats great thanks for that. I have got the manual up already and having a look. I think i'll have a shower tommorow and try afterwards just in case!

Thanks again.

 

[Agile]

I would still expect the problem to be sludge on the primary side!

Is the flow from the hot tap still the same?

Can you check the resistance of the boiler sensors?

Tony

 

[paulrushton]

Hi

I managed to remove the flow restrictor on the input into the boiler but found that it looked clean and free of sludge. Inside it was a white shuttle with a magnet in it. (i assume i have removed the correct part!). I put it back togather and reconnected the two push fit connectors and it's still the same,.

I have noticed though that even with the water temp output turned all the way down the water temperature is still very hot for a short period of time before it then cuts out. Could this be a thermistor problem now? If so how would i check them i have a standard multimeter available. It seems as though the water is getting too hot and then cutting out for a few seconds before then restarting. Not sure though if this is due to the water passing through slower and being heated up for longer or if a sensor has died!

Help please!!!!!

 

[Agile]

It demonstrates to me that the primary flow to the plate HE is restricted.

It will get hot when DHW is not being used and gives a litre or so of hot water but is then unable to supply enough heat as a result of the blockage reducing the primary flow rate.

Tony

 

[paulrushton]

Tony thanks for the reply.

Is this something that I could clean. I have the manuel that gives details of removing it but unsure as to how exactly. Would i need someone corgi registered or does this not involve disrupting the gas.

Thanks

 

[Agile]

It does not strictly need a CORGI registered person.


However, it would be very unusual for someone who is not CORGI registered to have the necessary experience of working on boilers to do something a bit trickey like that.

Its not very difficult for a boiler engineer but perhaps a rather more advanced DIY job.

Tony

 

[nickso]

It demonstrates to me that the primary flow to the plate HE is restricted.

It will get hot when DHW is not being used and gives a litre or so of hot water but is then unable to supply enough heat as a result of the blockage reducing the primary flow rate.

Tony

a vokera compact does not have a plate h/e. it is a twin pass heatex.

I managed to remove the flow restrictor on the input into the boiler but found that it looked clean and free of sludge. Inside it was a white shuttle with a magnet in it. (i assume i have removed the correct part!). I put it back togather and reconnected the two push fit connectors and it's still the same,.

you have not checked the correct component. the part yo uhave removed sounds like the flow switch. follow the pipe from the flow switch up the way until it enters the heat exchanger. turn off the mains water and undo the connection where this pipe enters the heatex and in there you will find a little coloured disc with an o-ring inside it. either remove the o-ring or throw away the entire assembly then try the flow rate again.

be aware that removing the restrictor completely sometimes means it is difficult to reseal the connection as it leaves very little surface area for the washer to contact onto the heatex. you will see what i mean when you have it apart.

 

[Agile]

Right thanks for that Nick!

OK so its a single four pipe HE.

The flow through the DHW side seems to be correct but its not getting the heat transferred into it and worst it cuts out unexpectedly so the DHW goes cold.

I will assume that there is no diverter and the pump does not run in DHW. If thats the case then it would be either a DHW temperature control fault or again a sludged or scaled primary side.

Are you able to measure the cold resistance of the temperature sensors?

A CORGI would have measured the working and operating gas pressures but thats more difficult for a DIYer.

Tony