In practice, the typical UK house has a heating load of 12-14Kw. However due to the fact that the boiler also provides hot water on demand a typical combi boiler is 24-30Kw in size. This represents a significant over capacity for space heating alone. The boiler will tend to run at low and less efficient part loads. The annual average load for space heating on an average domestic boiler is around 60%. Oversized boilers waste significant volumes of gas due to excessive energy dumps of heat that cause the boiler to come on and off repeatedly.
So in a nutshell, the 95% efficiency boiler you bought never gets a chance to run at its optimum efficiency. A bit like a big luxury car driving in the city. Your fuel consumption will be very high.
Hydromx will enable the boiler to output a greater amount of heat in a shorter period of time.
The boiler will burn gas at a higher rate for the time it is running but use it much more efficiently for a shorter period of time, creating significant part load efficiency savings.
In order for a boiler to condense (recover heat from exhaust gases) the return temperature has to be below 55°C. The return temperatures with Hydromx will be reduced enabling the boiler to run in condensing mode for longer, creating even more savings.