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There have been huge developments in the technology used in boilers over the last 30 years, and modern purpose-designed high-efficiency condensing boilers have practically nothing in common with floor-standing dinosaurs (still found in many older installations). However an evolution of design can be seen through the different generations of appliance which may help to understand aspects of their designs.
Thought to be extinct in the wild: not discussed here.
Low-efficiency open flue
- Cast iron heat exchanger
- Permanent pilot light (operates whenever the boiler is on standby, not just when the boiler is running).
- Thermocouple senses pilot light and stops flow of gas if flame not detected.
- Main burner operates when mains power is applied to boiler (from system controls)
- Built-in thermostat controlling boiler water temperature, via a rotary knob marked 0-6 with OFF position.
- Open (natural draught) flue: products of combustion are drawn up the flue by convection due the lower density of hot flue gases relative to cool air.
- Draught diverter: a downward-facing-fan- or cone-shaped duct in the flue allows air to be drawn up the flue along with the flue gases.
- Air for combustion is taken from the room in which the boiler installed.
Examples of this type of boiler:
- Potterton Kingfisher?
- Ideal Mexico
- Baxi Bermuda back-boilers (combination units with gas fires in front, fitted into a fireplace with a stainless-steel flue-liner up the chimney)
Low-efficiency balanced flue
Early boilers of the type above had open (conventional) flues. Newer models had balanced flues in which flue gases exited via a large rectangular duct at the back of the boiler and air for combustion was drawn in through a concentric duct. These types - also known as room sealed - are inherently safer than open flues as:
- there is no connection between the flue and the room in which the boiler is installed
- the boiler is not dependent on ventilation into the room for its combustion air
- fluff and dust in the room doesn't get into the boiler's burners (or filters) blocking them up.
Otherwise the technology of the boilers was similar except:
- the combustion chamber is fully enclosed preventing ready exchange of gases between the combustion side and the room side. Some boilers were available in both open and balanced flue versions, though the open flue types, whilst open to the room for the purpose of drawing in combustion air, typically do not have a draught diverter in the flue.
- piezo-electric spark ignition of pilot light. (Some later designs of purely open flue boilers had this too, particularly back boilers where the pilot light was hard to reach.)
- Ideal Mexico
Standard efficiency, pilot light
A change of building regulations obliged boiler manufacturers to improve the efficiency of their products. A typical boiler of this period has:
- Cast-iron heat exchanger
- Pilot light with electronic spark ignition, started only when controls call for boiler to run.
- Room-sealed design
- Fan-assisted flue
- Air pressure switch to ensure fan is running
- PCB with electrics/electronics
- Potterton Netaheat/Profile
- Baxi Solo2
Standard efficiency, no pilot light
Later standard efficiency models dispensed with the vestigial pilot light inherited from low-efficiency designs and featured direct ignition of the main burner. Consequently the boiler's electronics took over from the pilot light and gas valve the job of sensing that the burners were alight, and became more complex with flame-detection circuitry having to be included.
- Worcester CBi
- Potterton Suprima
High efficiency, secondary heat exchanger
With a move (enforced by a change of building regulations in 2006) to the higher efficiencies available through condensing the steam present in flue gases back into water boilers designs changed again. Some designs clearly adapted the technology of standard-efficiency boilers with the addition of a secondary heat exchanger operating on the flue gases of a cast-iron exchanger inherited from a standard-efficiency design. These have tended to be bulkier and/or more tightly-packed internally, sometimes with poor layout where the added components have obviously had to be shoe-horned into a package intended originally for fewer parts.
All condensing boilers of this type have:
- a condensate waste pipe
- plastic flue pipe (or flue portion of concentric flue assembly) angled back to boiler from outside (in horizontal flue arrangements)
- Baxi Barcelona
High efficiency, single heat exchanger
A typical modern condensing boiler features:
- purpose-designed heat exchanger made of stainless steel, aluminium or other materials
- pre-mix burner: where air and gas are mixed in the correct proportions for combustion and blown into the combustion chamber, often so the flame fires downwards
- modulating burner
- condensate trap storing and dispensing condensate waste in batches rather than a trickle
- Worcester Greenstar family