Baxi Combi 105e Losing Pressure

Went to have a look at a Baxi Combi 105e which was losing pressure regularly, the customer noticed that the pressured drops rapidly when the pump kicks in. He was also having to top it up regularly and there was a small leak coming from the underneath.

After removing the cover, I could see that there was some staining from the pin on top that activates the pump proving micro-switch. There was also water leaking from where the differential pressure diaphragm housing sits in the diverter valve. These were only minor leaks and did not fully explain the pressure loss.

I switched the heating on and the pressure dropped on the gauge, but I explained this is quite common on these particular boilers at the pressure fluctuations are due to the gauges proximity to the pump and the force of the pump affects the reading. The water pressure in the heating system was beginning to rise as the boiler was running, and I checked at the pressure relief valve outlet to see if there were any signs of water. There was no staining or running water, but a few droplets of water indicating that the PRV was not passing water continuously but had released some water which led me to suspect the pressure vessel was low on pressure.

I turned off the mains water to the boiler and opened the hot tap to drain off the water side of the boiler. I switched off the isolating valves on the flow and return to the boiler, and drained the boiler from it's drain point. I then undid the nuts on the differential pressure diaphragm housing which are 10mm, and released it from the diverter valve by undoing the grub screws which hold it onto the diverter valve. This could be removed and the leak could be seen from where the spindle exits the diverter valve to act upon the water differential pressure diaphragm. I removed both microswitches from the diverter valve body by undoing the single screw which holds each one on. I then undid, the nut underneath, nut the nut right hand side, the pressure gauge connection, removed the clip from the rear pipe and the pipe on the rear of the diverter valve which is held in by an 8mm nut. There is also a screw which holds the diverter valve in from underneath which I removed. The valve should then slide forward out of the plate heat exchanger and the pipe at the rear.

I then re-assembled the boiler with a new diverter valve in reverse adding some silicone grease to the o-rings and replacing the 3/4" fibre washers on the unions to the bottom and left hand side. When re-fitting the micro-switches it is important to replace them the correct way round or the boiler will fail to work properly. The pump proving micro-switch is connected to red wires and sits on top of the diverter valve. The hot water flow micro-switch sits in front of the differential pressure diaphragm housing and has red wires. Whilst the boiler was still  drained and the system was open I recharged the pressure in the pressure vessel which had lost the majority of it's pressure. After filling and testing everything looks to be working well again.

I have been led to understand that the pressure vessels on these boilers struggle to maintain their pressure because of the design of the boiler. This may be related to the fluctuating pressure when the boiler operates. I have been told that not using the hot water pre-heat extends the life of the pressure vessels.

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.