Combi boilers are a lot more reliable than they were ten years ago. Before that time (around 15 years ago) combi boilers were only just incorporating condensing technology and went through a period of change and fine tuning. Now, combi boilers are considered high-efficiency water heater and central heating units, which means that they can both heat your home and provide hot water, without the need for a separate water tank.
However, combi boilers are prone to the common problem of causing the hot water to go cold or lukewarm and then back to hot again. Symptoms like these are usually signs of a blockage in the hot water heat exchanger in the boiler. It usually blocks because of sludge in the heating system, and it really is a common problem with these systems, especially in installations where the fitter didn’t flush the heating system properly.
If this is the case, there are two potential options available:
- Remove the heat exchanger and descale it using a descaler such as Fernox DS4
- Replace the heat exchanger with a new one. However, you have to bear in mind that to access these heat exchangers; the entire hydraulic block has to come out. Dependant upon where your boiler is situated, this can be a real pain to do, especially with Worcester boilers.
If option 1 yields no results and keep it mind that this option doesn’t always work in the first place, option 2 would probably offer the favoured result, as with the heat exchanger all the washers and O-rings are included. However, it is highly recommended that you use an expert to carry out the needed work, such as DHS’ boiler maintenance in Bristol.
Unit 2 Septimus Buildings 
