Boiler Short Cycling

Short cycling is when a boiler repeatedly cycles on and off. Whilst this is undesireable, it prevents the boiler from overheating when the heat cannot be dissipated quickly enough. It’s inefficient and imposes unnecessary stress on the boiler and heating system.

What causes Short Cycling?

Modern boilers generate heat quickly and efficiently, but that heat needs to dissipated just as quickly. This is typically through radiators, underfloor heating, hot water coil, etc.

However, if the heat output is higher than the system can handle, the excess heat will lead to overheating within the boiler. Therefore, to prevent damage, the boiler temporarily switches off for a period of time (typically several minutes), thereby allowing the boiler to cool down. Short Cycling may be caused by various reasons:

  • Oversized boiler (too large)
  • Poor system design
  • Restricted pipe work
  • Pump issues

Oversized boiler

This is extremely common, though not particularly surprising! Given that many customers, and even heating engineers, incorrectly assume a larger boiler will offer better performance. This is simply not true!

Most boilers operate within a fixed range determined by the output ratio of the particular boiler. Some are better than others and it may be as good as 1:10, or more typically 1:6 or 1:5. For example a 30kw boiler may modulate within the range 5-30kw (1:6), or 6-30kw (1:5).

Currently there is a boiler on the market with an impressive 1:17 modulation ratio. Though not available for technical design reasons, the perfect boiler would modulate from 1: infinity! (e.g. 0-30kw, in this example). The lower the boiler output, the more efficiently it can run all year round.

An average 3 bedroom house in the UK, would typically have a 15-24kw boiler. Compare this to the 6-8kw required to heat the average UK home on a very cold day in February!

Ideally, a boiler will fire-up, and run continiously for the whole heating period. The boiler output should modulate down as the room temperature approaches the desired set-point.

Poor System Design

Heating systems are very forgiving, to the extent that a poorly designed system will still emit some level of heat. Since the property eventually warms up, the occupier is none the wiser! Boiler cycling causes slow heat-up times and may indicate a poorly designed or installed system. For example all boilers require a minimum flow rate through the heat exchanger to dissipate the heat. If the circulation pump or flow and return pipes are under sized, there will be insufficient volume flow through the boiler causing it to overheat, shutdown and cycle!

Other Issues

As mentioned above, the heating system needs to dissipate the heat produced by the boiler. Anything distracting from this contributes to the boiler cycling. This includes undersized pipes, poor circulation or radiator sludge.

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