Michael McCrossan


Millions of Euro up in smoke due to deficient boiler control

What a waste!

Hot water boilers normally operate as the heat centre of our buildings. It is a widely overlooked fact that boilers which lack the proper control will nuisance cycle on their own losses. Every boiler rest cycle has an individual heat loss characteristic (i.e. boiler convection and radiation losses and primary circuit losses). The boiler can fire up to replace these predictable losses when there is no system demand for heat or hot water. There are periods during the day and during the heating season when this unwanted boiler activity intensifies. Short boiler cycling reduces the rated efficiency (steady state) of the boiler. Nailing this boiler nuisance activity is a massive energy conservation opportunity. GEM estimates are that circa 90% of commercial boilers are adversely affected i.e. it is wholesale. This problem can be easily detected but unfortunately it goes widely over looked in the industry at large. Typical savings ranging 6% – 22% can be achieved with additional retrofit on-boiler control. The elimination of the problem will insure optimum efficiencies, Co2 savings and the extension of boiler plant lifecycles with reduced maintenance costs.

A simple check!

A simple check can be completed on boiler plant to ensure that the boiler is not nuisance cycling. Check that when there is a ‘call for heat’ on the boiler the return water temperature at the boiler manifold is not ‘flat-lined’ or sitting at a higher temperature set point compared with when the boiler stopped firing. There simply cannot be a genuine demand for heat if this condition exists.

Boiler Sizing!

Boiler plant is typically sized to service mid-winter conditions and transient external or internal building loads which seldom appear on the boiler.

BMS control of boiler plant!

Because of the presence of boiler oversize, a significant reliance is placed on BMS to control boilers efficiently. It is widely assumed that the BMS will inhibit boiler nuisance activity. Conventional BMS control is based on common header set point logic. Once enabled through the BMS, boilers can still nuisance cycle on their own losses and waste energy. The opportunity to identify and correct this is often missed by energy managers. The problem can be compounded if off-line (lag) boilers are not hydraulically isolated in which case they become akin to large boiler house radiators causing parasitic loads to appear on the lead boiler through dilution. Typical savings in the range 6% – 22% can be achieved with additional retrofit on-boiler control.

Modern Boilers!

Modern condensing/modulating boiler units can also be prone to nuisance cycling due to their low water content, light weight steel alloy heat exchangers and the marked absence of any of thermal jacket. Condensing boilers are often not set up for optimum condensing efficiency. Modulating boilers will cycle on-off under low load conditions below their minimum turn down capacity. The presence in some, of in-circuit time delays to control the boiler rest cycle, alludes to the fact that a cycling problem has been identified by the manufacturers.

Boiler Specification:

The marketing of new boilers often alludes to >100% efficiencies and this is particularly unhelpful. It should be noted that the quoted instantaneous combustion efficiency of a boiler tells us nothing about its heat loss characteristics during boiler rest cycles. If the boiler nuisance cycles then it is wasting energy albeit efficiently! Pre and post purge losses which accompany excessive boiler cycling compound the losses. Excessive boiler cycling can lead to early failure of boilers and increased energy use in buildings. Source: Damianos, Day Ratcliffe 2007.

Complex problem …. Simple solution:

Do: Check your boiler is not nuisance cycling.

How: Temperature data log individual boiler flow/return and cross- reference against boiler stop/start activity

Fix: Introduce retrofit on- boiler optimisation to eliminate this nuisance activity


What is retrofit on-boiler optimisation:

Almost all (90%) of commercial boilers lack proper boiler inhibit. Proper boiler inhibit can deliver untapped energy savings. This is achieved by measuring the drop off in temperature every second (temperature gradient) on the flow and return pipework to each individual boiler using digital temperature sensors. A flow temperature and an independent return temperature dead band are introduced using on-boiler retrofit controls through the boiler interlock or fail safe relays in series with the BMS/boiler stat circuit. This dead band logic can operate at any temperature and so does not conflict with BMS fixed or variable set point strategies. The BMS header set points will be completely uncompromised. Typical savings in the range 6% – 22% can be expected.

Barriers to uptake of this intervention include the following …

  1. Design engineers reluctance to specify. There is a lack of awareness or acceptance of the existence and size of the problem. There is an over concentration on quoted boiler combustion efficiencies which do not match post installation seasonal efficiencies. Alternative and innovative control strategies need to be promoted to design teams assisted by Government Agencies and Professional Bodies.

‘Case study projects have demonstrated that savings available can range up to 50% improvement from a baseline design’ National Mitigation Plan July 2017


  1. A general lack of clear understanding as to exactly how the BMS controls the boilers: Energy and facility managers need to be more open to energy efficiency options outside the BMS. These options need opportunities to be promoted with backing from Professional Bodies and Government Agencies.


  1. Scaremongering: Current and established building control companies and energy control suppliers can undermine innovative approaches. This is counterproductive to best efficiencies. Typically no genuine inquiry or approach for information is conducted. Promotional/Educational materials from Government Agencies and Professional Bodies should assist in alleviating this ‘push back’.


‘The Programme for a Partnership Government commits us to chart a course towards achieving a low carbon and climate resilient future by 2050, while also recognising that no one has a monopoly on good ideas’ Leo Varadkar, T.D. National Mitigation Plan 2017.


  1. Categorisation: The lack of categorisation weakens the ability of new and innovative technologies to gain a foot hold in the market e.g. ACA listing. This needs to be fast tracked and broadened.


  1. Lack of supports: The lack of real supports to ensure that innovative technologies have a route to market.


Check if your boilers are nuisance cycling

Retrofit on-boiler optimising controls which will introduce dead band logic on the flow and return to each individual boiler






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