There is a class of product that claims to save energy by enhancing the output of central heating radiators. These are usually small fan units, but one product I have seen is just a metal plate stuck on with double-sided tape which passively increases the radiator’s surface area. Then of course we have those additives which improve heat transfer on the water side.
These devices are commonly sold on the basis that you will save energy because the room “will heat up faster”. In principle, there might be some truth in this, but savings would only arise if you optimised the start time of the heating to take advantage of the shorter warm-up period.
How big is this saving or loss likely to be? The picture on the right shows a simulation of the diurnal temperature variation, mid-day to mid-day, in an intermittently-heated space.
This profile is reasonably representative of a building with moderate thermal performance.
When the heating goes off at A, the temperature falls rapidly and first and then progressively more slowly until B, where the heating comes on again and boosts the temperature back up to point C. This simulation shows the effect of (a) boosting the heater output by 50% and (b) optimising the start time, in which case the heating now comes on later, at B’, because the radiator can raise the temperature faster and reach point C as before.
The height of the bars represents the difference between inside and outside temperature (which is held constant here to simplify the analysis) and because heat loss is proportional to the inside-outside temperature difference, the shaded area of the chart is a good proxy for daily heat loss and thus fuel consumption. The small triangular area CBB’ represents the daily energy saved by the change. It amounts in this case to 3.4% of the total, and this proportion will probably be the similar across a range of outside air temperatures; less in milder weather, and more when it is colder.
So if you would need to increase radiator output by half to save only around 3% on fuel, it is hard to see how the marginal increase in output from a stick-on booster is going to make a perceptible change. And remember, if you don’t optimise the start time, increasing the output will cause the room temperature to rise faster, achieving target temperature prematurely, leading not to a saving but to an equal, albeit negligible, loss.