Boiler Evolution

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There have been huge developments in the technology used in gas 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. There are models which are 'links' between the 'stages' these stages are not hard and fast and hybrid models do exist; furthermore the same model name can span several generations of tech level.


No electrical connection to mains so non-electrically operated main gas valve/thermostat etc.

Thought to be extinct in the wild: not discussed here.

Low-efficiency open flue

  • Cast iron heat exchanger
  • Permanent, manually-ignited pilot light which is alight when the boiler is on standby as well as when it is running.
  • Thermocouple which senses pilot light and stops flow of gas if flame not detected.
  • Main burner which operates when mains power is applied to boiler (from system controls).
  • Built-in thermostat controlling boiler water temperature, via a rotary knob (often 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 (note that often the same models had both open and balanced flue variants):

  • Potterton Kingfisher
  • Baxi Bermuda back-boilers (combination units with gas fires in front, fitted into a fireplace with a flue-liner up the chimney)
  • Ideal Mexico.
  • Ideal Concord

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 or parallel compartment of the duct. These types - also known as room sealed - are inherently safer than open flues since:

  • 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: incomplete combustion can result in production of Carbon Monoxide
  • fluff and dust in the room doesn't get into the boiler's burners (or filters) which can block them up causing imcomplete combustion.

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
  • Glowworm Space Saver

Standard efficiency, fanned-flue, automatic 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.
  • Room-sealed design.
  • Fan-assisted flue which allowed for much more versatile flue length and positioning.
  • Air pressure switch to ensure fan is running before opening the pilot valve.
  • Pilot light ignited automatically by electronic spark ignition.
  • PCB with electrics/electronics.

Consequently the boiler's electronics took over from the pilot light and thermocouple the job of sensing that the burners were alight.


  • Potterton Profile & Netaheat Electronic
  • Potterton Kingfisher MFL
  • Baxi Solo2
  • Glowworm Fuel Saver and Micron

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. Such model usually have a modulating gas valve which can vary the gas output under the control of the PCB electronics. Some of the manufacturers began using digital electronics.


  • Worcester CBi
  • Potterton Suprima
  • Baxi Combi, Combi "Eco" (short for "Economy"), Combi Maxflow (storage combi)
  • Most Combination boilers without a permanent pilot light that aren't High Efficiency models.

High efficiency, secondary heat exchanger

With a move (enforced by a change of building regulations in 2005) to the higher efficiencies, available through condensing the steam present in flue gases back into water, boiler 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
  • Biasi
  • Potterton Suprima HE.

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 (by varying the speed of the premix fan).
  • syphonic condensate trap storing and dispensing condensate waste in batches rather than a trickle
  • Digital electronic controls.


  • Alpha CD range of boilers (CDC,CDS,CDX,CDR) brochure
  • Baxi Combi HE plus (upward firing burner) (brochure)
  • Baxi Platinum Combi HE (brochure manual)
  • Worcester Greenstar (web page)
  • Vaillant Ecomax/Ecotec
  • Potterton Promax Combi/System HE plus (a.k.a. "Gold") (brochure)
  • Potterton Promax HE plus Heating (upward firing burner) (brochure as above)
  • Ideal Isar / Icos / Istor
  • Keston Celsius