Cold water flow 18 l/min, Hot water 6 l/min. Combi

[Backboiler]

I just came across a right challenging problem that I'd sharewith you all. I went to a job where the customer complained of low hot water flow rates. They had a 30 kw Worcester Combi. I checked the cold and hot flow at all outlets and found consistent flows of 18 l/min on the cold and 6 l/min at the cold. Mmmm odd I thought. I checked the working pressure and gas rated the boiler and all was ok. I checked the cold flow into the boiler and it was 18 l/min there as well. I suspected there may be a scale buildup in the plate heat exchanger (expensive) or maybe a filter blocked in the cold supply or the hot outlet so I removed both pipes from the jig, but found no filters. I re-read the manual and noticed on the spare parts page, a reference to a flow restrictor. I dug deeper and found a meagre reference to a cartridge that was situated above the pump and prior to the diverter valve. The part was called a "flow sensor, flow restrictor housing and filter" (yeah I know). I removed the assembly and found all the three things inside the cartridge. The flow restrictor is a spoked disc and was partially blocked. When I tried to pull the debris out I noticed that it was an "o" ring! I had no choice but to pull the O ring out. I don't know why an O ring was inside a 3mm wide 15mm dia disc in the first place. having removed the restriction I made a decision to re-assemble the components in the cartridge, including the restrictor (minus the O ring) and refit the cartridge in the boiler. I checked the pressure and viola, hot water flow up to 12 l/minute. Just a word of warning when removing the cartridge; its brittle plastic and can be damaged with medium finger pressure. i just thought I'd share this in case anyone else has problems.