The boiler is the device where fuel (gas, oil, etc.) is burnt and the heat transferred via a heat exchanger to water which then circulates around the heating system. The key control on any gas boiler is the boiler thermostat which operates the gas valve. Modern boilers are much more sophisticated and include a wider range of controls.
Combi boilers produce hot water instantaneously. None of this water is stored. Virtually all heating systems which utilise a combi boiler are known as a sealed system. This term refers to a heating system whereby the expansion tank that the hot water expands into is sealed and not open as in a traditional open vented system.
Combi boilers generally have two heat outputs. 1 heat output is usually higher than the other. The DHW or Domestic Hot Water heat output is generally higher as more heat is required to product a larger amount of hot water per minute. For example a Vaillant EcoTec Plus produces 24 KW of heat for domestic hot water, this ensures that it can, where possible achieve a flow rate of 9.4 litres per minute. The CH or central heating output is however only 19 KW. This is simply physics as only a certain amount of heat can be injected into cold water per minute so the higher the DHW heat output the higher the amount of hot water the boiler can potentially produce.
I say potentially because combi boilers can produce an amount of hot water per minute equivalent to the amount of mains cold water coming into the boiler. So for instance if a combi boiler can produce 15 litres of hot water per minute, then the boiler requires at least 15 litres of cold mains water coming in to produce at best the same amount of hot water. If there is a demand for hot or cold water elsewhere in the house then the amount of hot water being produced by the boiler will be reduced, as this is effectively either splitting the DHW flowrate or less cold water is coming into the boiler.
It is worth remembering that combi boilers really hate hard water. If you live in a hardwater area it is always worth fitting a scale inhibitor to reduce that amount of limescale in the water before it reaches the boiler. Stainless steel heat exchangers generally last longer and are less prone to limescale. This is apparent with the Baxi Platinum which offers a 5 year parts and labour warranty.
Generally for a decent shower it is best to get a combi boiler which can produce at least 11.5 litres of DHW per minute. Remember that power showers cannot be used with combi boilers as this kind of shower uses a pump, but the boiler may not be able to produce enough hot water for this pump.
Also bare in mind that combi boilers are often installed in the wrong situations. Many plumbers/corgi engineers base their boiler choice on the DHW heat output. This is not how a survey for a suitable combi boiler should be carried out because of the details above about the two different heat outputs. Remember to always check that the CH output is suitable for your property.
System boilers have the components of a sealed system (expansion vessel, PRV, pump) built into them to simplify the installation. It follows that they are always used on sealed systems and do not require feed and expansion tanks.
Condensing or High Efficiency (HE) boilers are more efficient than Standard Efficiency (SE) boilers because they incorporate a second heat exchanger through which the flue gasses (products of combustion pass, this extracts more heat from the gasses before exhausing them to atmosphere. The second passing of the flue gasses results in them being cooler than standard efficiency boiler flue gasses so they condense in the flue & cause “pluming” (steam) to be seen coming from the flue terminal. The condense in the flue drains back through the boiler & passes to waste, the condense is slightly corrosive & should not drain through steel or copper pipework.