Firstly apologies for the long post but a lot has happened! I have a Vaillant Ecotec Plus 937 that was fitted in October 2008. It has been serviced annually and there have not been any issues until Feb this year when the boiler started making a vibrating noise when the pump was on. I have a service agreement with npower and called them out. The engineer diagnosed that the bearings had gone in the pump and ordered a replacement. This was fitted and it was then discovered that a 'connector' pipe was leaking (It's a copper pipe that looks like a stretched 'S'), this was also replaced and then replaced again when the initial one proved faulty (It was leaking due to a dent at one end). Whilst awaiting the second connector pipe the boiler started to develop the dreaded F81/F91 fault. Through looking at this forum I knew that this could be reset so I did this. I spoke to the engineer on his next visit. He didn't know much about this and said to call npower back if it did it again. Needless to say it has now been 3 months and npower are still trying to resolve this issue. They have replaced the following parts in this order - NTC sensor Divertor Valve PCB and today the heat exchanger/plate was removed and cleaned - the engineer said it was a bit dirty but not the amount he would be expecting to cause this problem and two thermistors were replaced. A couple of hours after the engineer left the fault occured again but whereas previously it has happened once or very rarely twice a day it happened three times in the space of an hour and even after I reset it there is still only a very small amount of hot water before it runs cold. When this happens I get the tap symbol on the display but no flame symbol which makes me think that the storage tank is partially filling and that this is the hot water that I am getting out. The boiler appears to be failing to ignite the burner when I am drawing hot water. When the small amount of hot water is exhausted the 'C' symbol flashes for ages to show that the storage tank is recharging? but I only get the flame symbol for a short while then code S20 is displayed. The engineer said today that if cleaning the heat exchanger didn't fix the problem then it was probably the APC pump that needs replacing which is the pump that feeds the storage tanks? I managed to get hold of a diagnostic flow chart from Vaillant via email which the engineer has worked through previously to eliminate any non-part related issues and last time he was here before today he spent some time on the phone with Vaillant technical who suggested cleaning the exchanger and/or replacing the APC pump. My questions are - Has anyone else experienced this and how was it resolved (All of the engineers that have previously visited have stated that they haven't worked on a 937 before so a bit of experience might be of benefit) Why would the DHW not be triggering the burner. Could this be linked to the fault codes or is this something else? No fault codes are displayed when the burner fails to ignite. And lastly (not sure I want to know the answer to this but) this has been going on for a while, what is npowers policy with issues that they can't resolve? I'm obviously being a bit pessimistic because this has been going on for so long now and there doesn't seem to be an end in sight. If you've made it to the end without falling asleep you have my thanks! Any advice / previous experience would be welcome.