Which uses less fuel:
radiators gently warm
all the time or hot
part of the time…?
I recently received in my inbox a striking e-mail from the Energy Saving Trust, claiming:
‘We bust popular energy myths this February.’ 
As I read on, one alleged myth particularly caught my eye:
‘Top 3 misconceptions … [Half] of all UK householders think it is cheaper to leave their home heating on all day rather than turning the heating on or off and up or down as required. …’
I am gratified that so many householders seem to have some common sense. As dwellings become increasingly well-insulated and draughtproofed, and as we change the methods used to heat them, it does become more sensible to operate a central heating system all the time with suitable controls instead of intermittently with time controls.
Some 85% of all UK dwellings are heated by gas and oil boilers. Since 2005 or 2007 respectively, nearly all new boilers using these fuels have had to be condensing ones . Condensing boilers are significantly more fuel-efficient if they are operated all the time with very cool return water; e.g., 25°C instead of part of the time with high return temperatures, typically 60°C .
The seasonal boiler efficiency might be raised this way from the mid 80s% into the mid or even high 90s%. Precise control of room temperatures is easier too if the water entering the radiators is just warm enough to heat the house in that weather.
Since time immemorial, the UK government has exhorted us to add insulation, draughtproofing and double glazing to our buildings. With a steadily lower heat loss, buildings cool more slowly if the heating is turned off. For a given comfort level, the saving from reducing the internal temperature for part of the time is lower than one might expect.
The most energy-efficient buildings that I know or helped design in the last 35 years have continuous heating and very low gas or oil bills. At a low building heat loss, it does not usually make sense to turn the space heating system on, off, on and off again all winter. The theoretical benefit of a lower average internal temperature with time controls is potentially outweighed by the higher boiler efficiency from operating the system on a trickle-charge, weather-compensated basis in winter .
A group of UK heating system installers have been trying to spread better knowledge and understanding of condensing boiler controls . They even go to the continent for training courses. On this side of the North Sea and the English Channel, they report relative ignorance and lack of knowledge.
So, could we all please assess whether we have inadvertently been spreading myths in the UK and seek instead to spread a better understanding of the subject? Thanks.
 However, it appears that the onus is on the householder, not the installer to comply with this law. Given that most householders do not know this, some of them might have installed non-condensing boilers after these dates, offering scope for a larger fuel saving if the boiler and controls are replaced.
 Figures from work at the University of Strathclyde, to be discussed further by the author in a forthcoming book. A retired UK mechanical engineer installed a condensing gas system which incorporates such controls in 2001. He has since measured a seasonal boiler efficiency of 96%.
 CHP systems or heat pumps are more sensitive than condensing boilers to the circulating water temperatures. The decrease in heat pump efficiency caused by on-off heating and resulting higher flow and return temperatures is already being recognised as a cause of some of the low COPs measured in the UK.