Conventional Boilers uses gas or oil to heat up the water. Water is stored in a large tank and it’s heated on a continuous basis. The water is passed through the pipes and through the radiators to heat up the room. Conventional boilers have an efficiency rating of 70-80%.
A combi boiler/condensing boiler, has a thermal efficiency rating up to 98%. Unlike a conventional boiler, which only heats the water, a condensing boiler, captures the water vapours in the flue gases and reuses it to give you additional heating energy. This process is referred to as the latent heat of vaporization.
In a Conventional Boiler, the unused heat is usually exhausted through the chimney. This heat source is wasted. A combi boiler/condensing boiler captures all the heat from the water plus the gases escaping from the chimney. It captures and utilizes it all, to give you additional heat.
The combi boiler/condensing boiler doesn’t use a traditional water tank. The system is based on an on-demand service model, where water is only heated when it’s required, such as when you turn on the hot water tap.
A condensing boiler/combi boiler is considered a closed-sealed system. The closed system doesn’t allow for the extra pressure to be pushed back to the city’s water system. Because of this, most municipalities will require an expansion tank to be installed with a combi heating system.
Expansion tanks reduces unnecessary water wastage. In a closed-sealed system, extra pressure will have nowhere to go, except for the fixtures such as your water taps and toilets. The additional pressure may cause your faucets to drip and your toilets to run. With an expansion tank, it gives the water a place to expand, so extra pressure isn't built up in your water pipes.