Worcester Bosch 28CDi No Hot Water

I was asked by a lady to look at her Worcester Bosch 28CDi combination boiler because her hot water performance had been gradually declining before failing altogether. The decline occurred over a couple of months and first began to affect the upstairs taps. The lady had already called another engineer who gave her a price to replace the entire diverter valve, and asked me to give her a price as we had previously done work for her.

When I arrived the lady explained everything to me and so I tested the boiler to see what was happening. I ran the hot tap and nothing happened at the boiler. From below the boiler you can see the operation of the diverter valve and differential pressure valve. As I ran the hot tap the pin was hardly moving if at all to operate the hot water micro-switch. The lady explained that the hot water did work to some extent if the central heating was on. I put the central heating on and again opened a hot tap. Still the pin was not moving to an obvious degree. I asked about the temperature of the hot water as it was barely warm and the lady told me that it only came through hot once the heating had been on for some time.

It may have been that the boiler could have been repaired by replacing just the diaphragm inside the differential pressure valve. As the hot water had not been working at all and there was no visible movement of the hot water pin I decided that it would be best to replace the whole diverter valve as this is what I had quoted to do. I had brought the relevant diverter valve with me which are relatively inexpensive and fit a number of different boilers. Fortunately the valve also came with the associated o-rings and washers to make the job run smoothly.

I turned off the water supply and drained the water system as far as possible. I turned off both the valves to the boiler on the central heating circuit after warning the lady that they may require replacement. Operating the valves on a combination boiler is something I try to avoid doing as they often begin to leak due to their in-operation over the years. I chose isolate the boiler at the flow and return valves because the boiler was on the ground floor and only 18 inches off the floor which means it was one of the lowest points of the system. I then opened the white plastic drain points on the boiler which were completely blocked. Fortunately the boiler was high enough off the floor allowing me to get a bucket underneath and use a thin screwdriver to unblock the drain points.

With the boiler drained as far as possible the first task was to remove the filling loop. There are two screws at the left hand end that secure it to the heating manifold and it just pushes in to the water differential valve and seals using an o-ring. There is a metal plate which prevents the filling loop from moving to the left and coming out from its position. Once the two screws on the left hand end of the filling loop are undone the plate can be swung down and the filling loop gently twisted to get some movement before disengaging it from its location.

The next job is to remove the plate heat exchanger, which usually contains some water still and is worth putting a towel over the pcb housing for some extra protection. There is a clip on the top right of the plate heat exchanger which is held on by a screw which need to be removed. There is also a nut which needs to be undone and secures the plate heat exchanger to the diverter valve. The other two connections are just push-fit connections sealed by o-rings. So once the nut and the clip are undone the plate heat exchanger should be eased off the other two connection with a gentle twisting motion trying not to drop too much water over the pcb housing.

Now the diverter valve should be accessible.

The diverter valve needs to be removed in one piece with the differential pressure valve. Firstly, I disconnected the copper bypass pipe connected to the plastic flow manifold. I then undone the heat exchanger connection nut to the top of the diverter valve assembly. I pulled off the circlip securing the micro-switch to the right-hand end and pushed it up out of the way. There are a number of screws to be undone, I removed all the screws for the bracket on the right-hand end and four on the plastic flow manifold. The diverter valve should now be free to come out of the boiler. This diverter valve took some persuading as it was the original, I managed to add some leverage using a large screwdriver between the rear of the boiler and the diverter valve assembly.

Re-assembly is relatively straight forward if you replace and grease all the o-ring. The most stubborn o-ring to remove was the o-ring between the plastic flow manifold and the flow isolating valve. Due to its location it was difficult to get at and remove. I managed to get a thin screwdriver underneath it and snip it with the point of my Stanley knife. Fitting the bypass pipe back into the flow manifold before securing the flow manifold is also advisable. Don't forget to fit the new filter mesh to the cold water inlet. The metal locating bracket for the filling loop also needs to be slid over the diverter valve connection before refitting the plate heat exchanger. The plate heat exchanger wasn't lined up correctly the first time I fitted it and would advise to connect the nut to the diverter valve first.

Once everything was back together, I refilled the water and heating system. Fortunately there was no leaks. I tested the operation of the hot water which was working perfectly. I then tested the heating system and checked the radiators for air. Overall the job took 2 1/2 hours and I had only allowed 2. With the experience from this job I may be able to get it down to two hours but may quote for three in the future and take my time.

Disclaimer: The works described in this blog are a record of work undertaken and should not be taken as guaranteed to solve your problem. Work should only be carried out by persons qualified and competent to do so.