Realising the benefits of modern boilers in old installations
In selecting a replacement boiler for an existing heating/hot water system, the application of modern high efficiency boilers to old systems needs careful consideration to maximise the system efficiency, reliability and performance, to achieve expected savings, says Bob Walsh.
What’s the problem you ask? It’s just a boiler, and a small modification to the water connections and flue system should easily sort it.’
If only that were true!
The UK heating market is dominated by a boiler replacement, with over 80% of commercial boilers sold in the UK being replacements, and boilers over 20 years old forming the cornerstone of the market. If old boilers were replaced with modern high efficiency boilers as a priority, the carbon reduction programme would accelerate significantly. These old boilers probably consist of a cast-iron heat exchanger operating in an on/off mode at poor seasonal efficiency.
The Government raised the bar for boilers in replacement and new build through changes to Building Regulations Part L (most recently in 2006) to ensure the use of high-efficiency or condensing boilers in commercial heating. Older boilers would typically have a seasonal efficiency of 70 to 75% (gross) compared to the current minimum standard of 84% — with the norm for modern high-efficiency boilers being greater than 90% (gross). Over a 15-year cycle, fuel accounts for over 95% of the total outlay on heating plant, so savings gained from improving the building fabric, controls and boilers will have a direct benefit on fuel costs and carbon emissions.
The Boiler Efficiency Directive of 1997 moved the emphasis on boiler efficiencies from full load (gross) to minimum net efficiency and also recognised that standing losses were important, because all boilers cycle during operation. An old atmospheric boiler has large standby losses when not firing because the heat exchanger is open to the natural draught of the flue system, with heat losses of 1 to 2% of boiler output. Premix combustion and burner modulation considerably reduce cycling, and the combustion fan isolates the boiler from the flue system when it is not firing, so, for example, Hamworthy’s Wessex Modumax range has standby losses of less than 0.1% of output.
The terminology introduced to reflect the cyclic nature of operation was part-load efficiency, measured at 30% of full load. With the introduction of the Energy Using Products (EuP) Directive and UK Building Regulations (2006), the emphasis has changed to seasonal efficiency, recognising that the demand on the boiler is never constant, so performance is calculated in more realistic terms.
Benefits are derived from the significantly improved part-load performance of modern boilers, which typically modulate down to 20% of maximum firing rate. But a common trap presented by modern boilers is that they will only modulate under their own thermostat or under direct influence from system controls. If the demand regime remains unchanged, it will simply switch boilers on and off with no modulation!
Controls must therefore be considered in the upgrade. Applying improved system controls, such as direct weather compensation, 0 to 10 V BMS input or a suitable cascade-management strategy, and zoning of the building to reflect occupancy patterns or counter solar gain effects, maximises the potential of modern boilers to achieve desired comfort levels and realise fuel savings. The cost of an effective control system is relatively small (less than 5% of the outlay on heating plant) and will deliver tangible benefits greater than 30%.
Heat emitters are often overlooked.
Normal panel radiators designed to operate at 80/60°C are efficient in terms of heat output for a given wall space but do not maximise the potential of a condensing boiler. If a condensing boiler is to be considered at 50/30°C, the panel would have to be significantly larger for the same output, but is not always possible due to wall space restrictions. However, if fabric improvements have significantly reduced heat losses, existing panels may well be oversized, so they can deliver suitable performance at the lower temperatures.
Underfloor heating is perfectly suited for condensing boilers, operating at lower flow temperatures and maximising condensing operation.
Air-handling units and fan coils provide an alternative to heat emitters but require higher flow temperatures, again not maximising the potential of a condensing boiler. More recent developments in fan-coil batteries have acknowledged the lower temperature benefits of condensing boilers.
To generate domestic hot water, higher flow temperatures will be required to deal with storage and legionella issues, once again limiting the potential of a condensing boiler. This can be overcome by prioritising the regeneration of hot water, which when satisfied, allows the temperature regime to revert to condensing levels. In applications with large hot-water requirements, it may often be more effective to separate hot water from heating, and condensing storage water heaters can achieve excellent efficiencies. Separating out hot water will also improve the system efficiency when space heating is not required — such as during the summer.
Until recent years the norm in the UK was to install boilers in an open vented system, this being seen as the safest, least complicated option in comparison to a sealed (un-vented) system. Boilers were designed with low hydraulic losses to deal with the UK system design criteria of 11 K ∆T at that time. Modern boilers are designed around 20 K ∆T. The benefit of the reduced flow rate from modern boilers enables use of smaller pipes, valves and accessories and the delivery of reasonable flow temperatures whilst in condensing mode. More importantly, the higher hydraulic resistance of modern heat exchangers requires specific attention to be given to their application to open vented systems to prevent the all too common phenomenon of ‘duck and bounce’. If ignored, air and fresh water entering the system via the vent pipe and cold feed will ultimately cause the heat exchanger to fail.
When replacing a boiler, the condition of the system water must be considered and the quality of the water dealt with by flushing, chemical dosing and the use of dirt and air separators. This is particularly relevant in commercial systems, where the boiler, pipework and heat emitters have been predominantly steel/cast iron and subject to degradation. Dirty systems will foul the heat exchanger, reducing efficiency and ultimately causing failure. Systems that were sound with the old boiler and typically open-vented circuit, when subject to a new boiler on a pressurised system, can become problematic due to uncontrolled make-up water. A regime of water treatment, monitoring and management should be implemented to ensure a trouble-free system. Failure to do so will reduce the expected life and performance of any modern boiler.
The flue system characteristics for modern boilers require a significant change in approach to flue design. The burner fan on a pre-mix boiler can often drive the flue system. Consequently smaller, longer flue runs (sometimes in plastic) are possible, and may result in lower flue costs and flexibility on terminal location. It does not remove the need for flue system design if the installation is to be trouble free and quiet. The ‘Clean Air Act’ acknowledges that flue gases discharged at low level are a health risk. With modern boilers delivering higher efficiencies, the flue-gas temperature is significantly reduced, resulting in substantial pluming from the flue terminal. To avoid problems associated with pluming and to minimise risk to health, discharging at high level is important.
This problem is further exacerbated with condensing boilers generating significant amounts of condensate in the boiler or pressurised flue ducts. The slightly acidic condensate will exploit any weakness in the system, which will soon leak. Typically a 100 kW condensing boiler will generate 15 l/h of condensate. This must be dealt with by using fully sealed pressurised flue components in stainless steel, aluminium or plastic, and the condensate discharge system (plastic) must be designed to prevent condensate draining back through the boiler(s).
High-efficiency condensing boilers, applying the practices outlined here, will achieve significant energy savings and subsequent carbon reduction. There is then an opportunity to further reduce carbon emissions by the integration of renewables. Hamworthy can provide complete solutions to integrate condensing boilers and water heaters with a choice of technologies [solar thermal, biomass (wood chip/pellet), biofuel (liquid), and heat pumps]. Gas-fired boilers deal with seasonal and peak demands.
The use of controls is imperative to ensure that the renewable option is the ‘lead’ heat source. However, this opportunity can be limited due to the additional implementation costs associated with renewables.
Bob Walsh is technical director at Hamworthy Heating.
