this happens 4/5 times during 5 minute shower(trevi therm mk2) also happens when using only hot water tap in kitchen.
water pump and heat exchange plate changed, system is 5 years old and has been serviced each year by worcester. ch sytem no probs heats up instant, no cold spots, water in rads and system clean, rust inhibitor from new. worcester are going to replace heat exchange plate again. \i dont think they are finding correct solution

fred milburn, March 2007

when you run the hot water and the boiler cuts off this is the primary thermistor shutting down the gas to water heat exchanger becaise it is getting too hot, probably because the water to water plate heat exchanger is getting scaled or sludged.
so infact the c/h thermistor is protecing the boiler from overheating, solution descale/desludge water 2water heat ex or replace it

paul, March 2012

I had this happening after a service by B-Gas. The shower would cycle from hot to cold back to hot within a 1 min period. This only happened when the cold tap was used in conjuction with the hot. We had two engineers who couldn't sort it. The final one today did find the issue. The range of the boiler presser was belween 8.1 and 13.4. He changed the range to 1.5 and 13.4. when the cold water was on, the boiler pressure hit the original floor of 8.1, this is when the temperature started cycling. Moving the min pressure level down to 1.5, the actual boiler pressure never reaches this level, so no more temperature fluctuations. hope this helps

E, December 2011

i have a problem with my combi worcester cd35i ii the pressure of the unit is at just over 1 bar and when you either switch the hot water or the central heating on the relevant lights come on and the boiler kicks into action and the gas fires up, the pressure increases slowly to 1.5 bar then all of a sudden the pressure increases to just over 2bar and then the needle waves about and the boiler makes a banging noise as if there is an air lock and then switches reverting back to the standby mode and the pressure goes back to 1 bar, ive bled all the radiators, what else can i do or reset or change or is it an expensive plumber call out

andy, October 2010

hi, if your heating working fine, if boiler starts on off cycle every few secnds when called for hot water, banging noises from burner, flame changing colour to yellow and spreading out from sides when combustion chamber cover is off then there are few tests you need to carry out you will need a voltage tester or multy meter to mesure voltage which i can explain just ring me and i will help you out. I had same problem on a customer,s boiler and rang worcester technical 4 times spoke to 4 different engineers, they all told me different things to check and change which was really hopeless, time & money wasting. in the end i found the problem & rectified it. just call me and i will take you through testing and fixing the fault. Let me remind you all out there never attempt to repair a fault unless you are a competent person because this task requires special skills for testing. ring me on 07973 759 315.

UHL BIRMINGHAM, April 2010

Also having the same problem

had the heat exchanger changed last year Abd now the problems back again and the banging noises from the boiler can be heard from outside the house lol

any ideas

Mark, March 2010

I have the same problem , water hot, then cold through the shower , have tried everything and spent a fortune on plumbers, .Has anyone found a solusion, ( HELP )

Martin, February 2010

Hi, I have a 35cdi RSF (Mark I) Worcester bosch combi. The hot water temperature fluctuates, i.e. burner comes on and off and the shower is hot and cold all the time. I have had some guidance from Worcester tech support. I measure both thermistors and they seem to be within the required range when disconnected (about 12.5ohm at 21degC). As soon as you plug them in though (with boiler power off) the reading is much different (about 3ohm) and when you switch boiler power back on the reading is zero? Someone said this indicates a problem with the boiler control board? The flow swith is also working fine. When the water flow through the boiler is more than about 9 l/m, the burner stays on and the water temperature is constant (about 51degC at lowest setting and 61degC at highest setting). The problem is though that the flow rate through the shower head is much lower (to concerve energy), but it is still more than the min 3 l/m recommended. Please help.

Read more: http://www.justanswer.com/questions/1lcth-worcester-bosch-35-cdi-2-hot#ixzz0NCaapgFV

braamss, August 2009

were would i find the auto air vent

dilip parmar, February 2009

I have had this problem but with the central heating not the hot water, when set to low level of heating the pump cycles on-off repeatedly. I can "cure" it by turning the heat level control fully clockwise then anticlockwise a fwe times. I suspect the potentiometer that does the controlling is dirty inside, could be the same problem with your hot water control.

Simon, January 2009

I had the same problem but after reading about some people chasing money after plumbers who weren't sorting the problem, I decided to learn the system myself.
Yes there are various reasons for this automatic cut out of the burner but please try the basic one first.
Maybe the hot water is set too high.

Turn the temperature of the hot water down to 1 or even less, turn the hot tap on.
The boiler should still fire up.
If this doesn't cut out then turn the temperature up in small incriments untill the boiler starts cutting out again.
Then turn it back down a little bit.
It worked for me.

Jamie., September 2008

try the pc board.

Disconnect the DHW thermistor and put a meter across it take a note of the reading.

Now plug the thermistor back in and take a reading again they should be the same. if they are different then the fault is with the PC board

Steven, August 2008

no hot water is comming out

dil, August 2008

I have exactly the same problem and, like you, have had various plumbers in who have looked but failed to find a solution. In my case when there is a DHW demand the burner typically burns for 1 minute and then cuts out for 15-20 seconds before coming on again... and this cycle just repeats ad infinitum. I have experimented by dismounting the DHW temperature sensor from the hot DHW pipe but this has made no difference at all. (I had wondered whether the DHW sensor was faulty leading to a periodic over temperature shut down but this seems not to have been the case). Can anyone help?

Nick, January 2008

I have had the same problem for a long time (nearly 2 years). Several plumbers have tried to fix it - even spending lots of time on the phone with the worcester "tech support". They "suggested" several possible problems. Some were less expensive to fix than others. After trying the first few (each one under £30), the plumbers were pushing the expensive ones. I had NO confidence whatsoever in any of it (it seemed like simple guess work). SO, I still have not gotten this solved. It is a real pain (and, getting worse now that winter is here). Some more details: when using taps or showers the boiler comes on for about 30 seconds and then cuts out for about 15 seconds and then back on for 30 seconds. (sitting by the boiler, I can hear a little 'click' followed by the flame turning off... silence for 15 seconds... 'click' flame back on for 30 seconds... 'click' off, etc). It seems very regular. There are no cutouts whatsoever when radiators are getting their water. Any suggestions would be most appreciated.

James, December 2007

Hi, I have the same problem with the hot water on/and off, I have changed the hot water sensor and it hasn't worked, I was told it might be the hot water exchanger but this is £130, anyone have any other ideas.
Dave

dshaffer@dsecs.com, October 2007

poss flow switch

hi-spec plumbing, March 2007

try changing the hotwater temperature probe
cost about a tenner

At least this is what cures the problem on my old Worcester 350 combi (part nos 8.716.142.315.0)

good luck

me, March 2007