Boiler Short Cycling

Short Cycling

Boiler short cycling occurs when a boiler subject to its own internal thermostat 'fires up' - starts operating when the thermostat detects that the water in the boiler has cooled down.

For example:
When your heating system does not require the boiler to fire, the heated water in the boiler's heat exchanger will begin to loose heat to the air around the boiler's casing and through the flue. This loss of heat is not related to any loss of heat in your home.

Short cycling is simply the boiler firing up to re-heat the water in it's own heat exchanger when that water has cooled down. Because this water heats up quickly, the temperature that the thermostat is set to is also quickly achieved and when this happens the boiler is again shut down.

The boiler can continue to fire, heating the water in the heat exchanger and shutting down for a prolonged period of time without actually producing any heat for your home.

When the boiler is running and not producing heat for your heating system this 'short cycling' is:

  1. Wasting energy - increasing your fuel bill - and
  2. Damaging to the boiler's heat exchanger from constant localized boiling

An essential part of a modern efficient central heating system is the prevention of boiler short cycling.


Preventing Short Cycling

In a fully automatic central heating system the short cycling of the boiler can be prevented by using the room thermostat/programmer and hot water cylinder thermostat to switch on the electrical supply to the boiler. This wiring arrangement is called a 'boiler interlock', meaning that the boiler can only fire when there is a demand for space heating, and/or for hot water.

A boiler interlock will:

  1. Protect the life of the boiler
  2. Save you money - and
  3. Save energy
wiring boiler interlock sequence

Typical heating interlock wiring control sequence

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