BACK TO BASICS ON BOILER EFFICIENCY

Oct 9, 2019 | News & Events

Some might say that heating installers have never had it so good with the wide range of products from which to select and assemble a system that meets or even exceeds their customers’ needs and wants (or disappoints). David Iszchak, Technical Trainer at Vokèra by Riello looks at the basics of boiler efficiency in keeping homeowners comfortably warm at an affordable cost.

If the average homeowner does any research when considering a new boiler then it will probably be restricted to information from a brand they’ve heard of or whatever turns up on an internet search. They may often be more attracted by the presentation of a product, or the appearance of a control than the capability or potential of the appliance, or just as importantly, the appliance and control combined. Achieving comfort with energy efficiency at low cost is generally what most homeowners desire, but how are these three desires reconciled in a real world installation?

Certification efficiencies
Boiler efficiency is hard to assess as ‘test bench’ figures may not be representative of how a boiler will actually work in a property. Steady state test bed operation at two flow and return temperatures (80-60°C and 50-30°C) allow manufacturers to extrapolate test efficiencies of over 100% which, if the case is warm, the flue is warm, or there is water vapour at the terminal, could be mislead. These figures are steady state testing on certified test rigs and heating systems simply do not work in that way. The thermal performance of properties will also differ greatly depending on factors such as location and lifestyle: weather patterns are not consistent; occupation patterns vary, as do heat gains from solar, cooking and electrical appliances.

Reality Bites: The closest a real world system is likely to get to steady state testing is to be operated continuously at temperatures that do not allow the boiler to cycle on/off. For the majority of the heating season the maximum output of the boiler is not required (this is certainly the case with a combination boiler) so good modulation between maximum and minimum output to the heating system is important. Generally speaking, 1:5 is thought of as a minimum and a higher turndown ratio will improve matters if it does not entail too much additional cost or complication. So, keeping the boiler on longer seems to suggest this might bring the boiler closer to certification efficiencies, but at what cost? Running longer does not necessarily mean more fuel will be used as a boiler with a wide modulation range will be able to match output to the varying load of the property. However, during low load operation heat is still lost from the appliance through the flue. If the boiler cannot modulate down to meet the load then it will cycle off and on however if cyclical losses are low continual losses through the flue may exceed pre and post purge inefficiencies and standby losses. Range rating the boiler heating output to the likely maximum requirement of the property, particularly with combination boilers, is a simple way to reduce cyclical losses and boiler firing cycles.

Squaring the circle
Is there any way the boiler efficiency can be improved further? Well, yes there is, because we know condensing boiler efficiency is better with some heat exchangers if operated at lower temperatures to encourage harvesting of latent heat. This will require the type of control that can adjust the boiler flow temperature in line with system demand rather than simply turning the appliance on or off and operating up to a preset maximum temperature. Here the control is being integrated with the appliance and we introduce what I call a system efficiency which adds to the appliance efficiency. If the heating control adjusts the boiler in order it runs cooler and at condensing temperatures more often than the boiler efficiency will improve but will the efficiency gain be eroded by prolonged run time losses?

Advise responsibly
Whatever the type of boiler or control, comfort is the primary objective for most households but acceptable levels of comfort differ. Also, whilst an end-user’s preference may be 23⁰C if their budget only covers heating the house to 19⁰C then how can that circle be squared? It is often quoted that a 1⁰C reduction of the space heating setpoint is the equivalent of 8% saving in fuel for heating. If this is to be believed then a 3⁰C reduction should equate to a 24% reduction in the fuel cost –really? Almost a quarter?

Perhaps the most important thing to remember is that the homeowner is the one paying the fuel bill not the installer so any advice or claims on energy savings must be given responsibly. I know this may come under the heading of ‘stating the obvious’ to the reader of a trade magazine however, to use a phrase from the education sector, it’s also what’s known as ‘reinforcement.’ Sometimes the object of the exercise becomes hidden behind an avalanche of marketing information trumpeting worst to best case scenarios when that is rarely the case the installer is faced with. The object of the exercise is customer satisfaction with an installation and running cost they can afford that will win the installer the work and retain the customer; the homeowner is not a test bed for the latest marketed technological advance.

Finally, it is important not to lose sight of the absolute basics when it comes to optimising boiler and system efficiency:

• The boiler efficiency is the baseline, everything else is an incremental improvement to system efficiency.
• Running the space heating to a warmer air temperature will cost more – check the thermostat setting to see if it could be dropped a degree without compromising comfort and dress according to weather conditions.
• Running it longer will cost more – check if adjustments could be made to the time schedule.
• Running it too cool will cut costs but could mean discomfort – try not to adjust the thermostat too much, let it do its job.
• Not running the heating long enough may be a false economy and can lead to damp and mould in the house – don’t allow the property to become too cool and encourage condensation when unoccupied.
• Automatic adjustment of the boiler flow temperature to lower levels improves the efficiency of a condensing boiler – consider a control type that can adjust boiler temperature.
• Turning the heating up from a phone before arriving home could cost more than waiting till you get home; if the house contents are at a stable economy temperature during unoccupied periods it will not take long to warm the air.
• Turning the heating off may appear to save fuel but may cost more with the energy needed to reheat the property from an ambient temperature – heating the air only is quicker than heating the air, walls, furniture that have grown cold.
• Setting two levels, an occupied (comfort) level and an absence (economy) level is a good compromise – consider a control that offers choice of both time and temperatures.
• Firing the boiler is what costs the money – controls that regulate a space without interacting with the boiler are not as effective as devices that can – basic thermostatic radiator valves do not directly interact with the boiler and are often poorly positioned.
• A space heating control in one area is better than none at all – some sort of time/space heating boiler interlock is a mandatory requirement.
• A space heating control that can assimilate heat requirements from several areas, keep them at varying temperatures and control the boiler firing is better still – consider aggregated multi-zoning if the budget allows.
• A control that can adjust the boiler operating temperature while achieving the above is even better still – this will encourage condensing operation at the boiler.
• If the boiler is in a position that is difficult for the user to access, then a control that allows the user to monitor and adjust the boiler operation as well as control the heating at the room thermostat, phone or app is better still – the homeowner can monitor its operating condition from the room thermostat or internet connected device.
• Training is key to help installers understand the capabilities of what type of boiler and control is fitted. One of the most critical elements in the whole process is the installer’s ability to explain the control in a simple and concise manner avoiding confusion or repetition and encourage customers to use the controls for optimum heating efficiency and comfort.