Professor D.Sc. Jarek Kurnitski Helsinki University of Technology

Professor Dr. Jarek Kurnitski, one of the leading scientists in the HVAC field, is currently working as top energy expert in the Finnish Innovation Fund, Sitra. As a European REHVA awarded scientist, he has published close to 300 papers.

Bigger is certainly not better

Within the heating industry there is still the myth that within low temperature heating systems you need bigger radiators. Bigger however is certainly not better. During my comparative research into heat emitters, I found that even during the coldest winter period, rapidly changing heat output is needed to keep room temperature in the optimal comfort range. Both systems were set at 21°C, the lowest comfort limit and ideal indoor temperature. As you can see in Fig. A.1, when internal heat gains of not more than 0.5°C were detected, the radiator system with its small thermal mass reacted quickly and kept the room temperature close to the setpoint.

However, with the high thermal mass of underfloor heating, reaction time was much slower when heat gains were detected. This meant that underfloor systems kept emitting heat, taking the temperature far above the optimal, with strong uncomfortable fluctuations. In fact, in order to keep the room temperature closer to the optimal 21°C, my research shows that the only solution is to increase the setpoint for underfloor systems to 21.5°C.

For a lot of people 0.5°C may seem a small number. But when you apply that per hour, daily, across a whole Winter heating period, the numbers soon start to multiply and any hope for energy efficiency soon fades. A room temperature difference of one degree correlates to around 6% energy consumption. Fast response to heat gains and low system losses are key elements of energy efficient heating systems. Central control leads to overheating in some rooms with a consequent energy penalty, which is why my research recommends the use of low temperature systems to reduce system losses, as well as the use of heat emitters that can be individually controlled. This makes radiators the obvious choice.