I have a Vokera Excell 80SP and the hot water goes on, heats up and then goes off. I have replaced the domestic diaphragm but that has not helped.

Keith R, December 2005

I have had new diaphram and return valves fitted but cannot get hot water on its own. When turning on the hot tap, everything fires up properly but radiators start heating up before water gets hot.

C Hawkins, January 2012

Just paid a corgi to sort the problem. All good. Diaphragm replaced and venturi cleaned. Charged me £70. Ding dong.

CJE, September 2009

Good readin your responses to this. Big help. Found apdi71 the right one for me. Rang a plumber friend and he said in his experience he's never had to replace the pressure differential switch on these boilers. To test it he said detach one of the silicone tubes from the flu venturi and when it fires up with the hot water running suck or blow slightly on the tube and keep the pressure on by crimping tube - this will activate the switch and should keep the burners going. If not the switch has probably gone. Ended taking the top off and giving everything a good clean including the fan which was pretty clogged. Without seeing clearly exactly what was that caused the problem. Put it back together and all seems to be well. Had a mountain of dead wasps to clean out of the the combustion chamber.

wundamanspanky, June 2009

I have had the same problem with the same boiler. By sharply tapping the small round plate on the top of the boiler, while a hot water tap is running cold water, the boiler would fire up and the water would then get hot. If this happens with your boiler you will have to get the pressure differential switch replaced. This switch is located beneath the round plate mentioned earlier. Note: tapping the boiler causes the switch contacts to function for a short while but they probably burnt and in need of replacement. approx £40.00 for the part.

David Munro, December 2007

Well, I have the same boiler, and if the symptom is hot water cycling, ie running a bath the water goes hot, boiler flame cuts out, water cools down to cold, boiler fires up, water warms, and repeats then I would put money on it being the flue venturi. The venturi is a pilot tube that sits in the flue gas exhaust stream after the fan, it connects to a differential air pressure switch via two silicone rubber tubes, if a deposit forms on the venturi pilot tube assembly it stops working properly. It is there to make sure the boiler doesn't fire with an obstructed flue/faulty fan. If you take the outer combustion chamber case front off you will see the tubes going from the venturi connecting pipes to the air pressure switch top right. You can not clean the flue by removing the rubber tubes and poking objects down the holes. Either you take the flue hood off and clean it out of the boiler (gently with a small needle file), or you might be able to clean it by removing the flue from the top from the boiler and gaining access from the top. The heat exchanger and fan impeller will probably need cleaning at the same time, and are most easily cleaned if the flue hood is removed. Remember gas appliances asre potenetially dangerous if improperly serviced, so take care and if unsure get a Vokera approved Corgi heating engineer to fix it. (www.vokera.co.uk)
Ian

apdi71nospam@letterafterfmail.com, December 2005

Got the same boiler and have had the same problem in the past..

Suggest you try the following;

1- switch to water only. That is heating turned off. Have someone turn on the hot tap and listen/look to see if boiler fires up. (Probably not from what you say)

2- observe the actuating pin that sticks out from the diaphragm assembly. This should move forward and push on the alloy lever that operates the small microswitch at the bottom left corner of the boiler case. If it isn't moving then the diaphragm is knackered.
3-If it is moving does it actually move forward enough to operate the alloy lever? If not again your diaphragm is stuffed.
4- If all of the above seem ok then you may have a faulty microswitch? This can be checked by operating manually with a suitably insulated screwdriver. There is also an adjusting nut at the microswitch end of the alloy lever.
From what you say however I suspect the diaphragm is split. The small quantity of hot water your getting is simply that stored in the heat exchanger/pipes. New ones are around £4. Do a google search for Vokera + diaphragm and make sure you order the correct size. As far as I know they come in small (around 2inch)and large (around 4inch) depending on date of manufacture.
Good idea to buy a couple as they seem to expire every few years.
All of the above of course assumes that, unlike myself, you are competent / trained to carry out the work and naturally if you scald yourself/gas yourself/electrocute yourself or monoxide your family then please don't blame me ;-). Better still call a Corgi engineer if you want to squander £50ish...

Stu, December 2005