Guide to: Heating System Design

The following paragraphs provide an overview of how the majority of domestic heating systems work.  Key terms and definitions are highlighted to help you better understand the pipework and components that make up your system.  So make yourself a coffee, find a comfy chair and immerse yourself in a world of warm wonders……….

The core elements of any system are:

  • A boiler or source of heat
  • Pipework to distribute heated water
  • Emitters to release heat into living spaces (e.g. radiators or underfloor pipework)

Typically these are supplemented by:

  • A pump (or more accurately a circulator to get the water moving)
  • A programmer to set start and finish times
  • Thermostats to regulate the level of heat achieved
  • A hot water storage tank (see the Hot Water tab for more information on these)

Let’s consider these elements further, starting with:

 1. Boilers

There are two main types of boiler – heat only or combi (combination).  Heat only boilers typically heat stored hot water cylinders whilst combi boilers provide instantaneous hot water / hot water on demand.

A heat only Grant oil boiler


A combi Grant boiler incorporating hot water heat exchanger, circulator & expansion vessel (see below)


Whether heat only or combi, all boilers manufactured since 2005 are condensing boilers which are designed to be more efficient by extracting additional heat and reducing flue gas temperatures.  Condensing boilers generate a by-product of mildly acidic condensate which is removed to a waste pipe or drain for diluted disposal.

Enough on boilers for now – let’s consider:

2. Pipework, Emitters & Pumps

With occasional exception, pipework is typically arranged on a flow/feed and return basis with each radiator, cylinder or underfloor circuit passing hot water through it before heading back to the boiler.  A house will typically have an artery of large pipes branching off to emitters in a smaller bore, each emitter reducing the water temperature before sending it back to the boiler.

Every radiator receives hot water and returns cooler water back to the boiler (flows out, returns back)


Traditional ‘open-vented’ systems include a small cistern in the loft which rises and falls as the system water heats & expands then cools and contracts.  These cisterns incorporate a ball-cock valve to top-up water when required plus a vent pipe to safely discharge any excess heat.  Additionally, the cistern will have an emergency overflow to outside should the ball-cock valve fall.

Later installations (including all combi boilers) replace the cistern fed open design with a closed or sealed system design.  This incorporates an expansion vessel to absorb the increase in water volume and a filling loop to manually add water to the system.  A pressure relief valve will also be fired to ensure that were the expansion vessel to fail the excess pressure would be relieved by routing water to outside.

Expansion vessel with pressure gauge, filling connection and pressure relief valve


Older systems were typically installed with loops of larger bore pipes as they were designed to distribute heat without the assistance of a circulator pushing the water around.  These relied on gravity and can still be found heating hot water cylinders (albeit slowly and with inefficient heat transfer).  Modern building regulations require that both heating and hot water circuits be driven by circulators and controlled by motorised valves linked to time and temperature controls.

A Grundfos circulator typically connected to pipework via brass isolating valves


Which brings us to:

3. Controls

The normal sequence that determines whether a boiler fires up or not is as follows:

  • Does the householder want heat (communicated via a Manual or Programmed ON command on a programmer)?
  • What temperature is required (set via a room thermostat or cylinder thermostat)? Room ‘stats are typically dial controls in the hallway whilst cylinders have a sensor strapped to the side of the tank.
  • If ON is set and the room/cylinder is below target temperature then power is sent to the boiler to fire and the pump/circulator to spin.
Room thermostat which gives a reassuring ‘click’ when its microswitch is activated by turning









The hot water operation for a combination is a little different as the boiler is required to fire whenever heated water is required.  This is not strictly true for most oil combi boilers as they typically have a small store of heated hot water to use before firing is required.  However, for gas combi boilers a flow switch is activated whenever a hot tap is opened and the boiler fires to meet the target temperature set.

The final control items worth mentioning here are zone valves which are required on heat only boiler systems.  They are essentially motorised gates that allow independent control of central heating and hot water circuits.  Prior to their adoption there was no ability to just heat radiators or just heat hot water and their use is now mandatory to meet Building Regulations.

A Honeywell zone valve which opens when heat is required then sends power to fire up the boiler and circulator




Oil & Heating Services